The effects of affective picture stimuli on blink modulation in adults and children

The effects of affective picture stimuli on blink modulation in adults and children

Biological Psychology 68 (2005) 257–281 www.elsevier.com/locate/biopsycho The effects of affective picture stimuli on blink modulation in adults and ...

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Biological Psychology 68 (2005) 257–281 www.elsevier.com/locate/biopsycho

The effects of affective picture stimuli on blink modulation in adults and children Allison M. Watersa,b,*, Ottmar V. Lippa, Susan H. Spencea a

School of Psychology, University of Queensland, Brisbane, Qld, Australia School of Applied Psychology, Griffith University, Gold Coast Campus, PMB50 Gold Coast Mail Centre, Brisbane, Qld 9726, Australia

b

Received 2 December 2003; accepted 10 May 2004 Available online 30 July 2004

Abstract Two experiments examined blink modulation during viewing of pleasant, neutral and unpleasant picture stimuli in non-selected adults (N = 21) and children (N = 60) and children with anxiety disorders (N = 12). Blink reflexes were elicited by a white noise probe of 105 dB at lead stimulus intervals of 60, 240, 3500, and 5000 ms and during intertrial intervals. Blink modulation during unpleasant pictures was significantly different from blink modulation during neutral pictures at the 60 ms lead interval in children whereas adults showed no significant differences. Picture content had no differential effect on the extent of blink modulation for adults or children at the 240 ms lead interval. At the long lead intervals, blink modulation during unpleasant and pleasant pictures was significantly larger than during neutral pictures in adults. Picture valence did not differentially affect the extent of blink modulation at long lead intervals in children. Comparing the extent of blink modulation in anxious and non-selected children, blinks were significantly modulated during unpleasant pictures at the 60 ms lead interval for both groups. However, the extent of blink modulation was larger overall at this very short lead interval in anxious children. Children did not differ at other lead intervals. # 2004 Elsevier B.V. All rights reserved. Keywords: Affective picture stimuli; Startle blink modulation; Adults; Children; Childhood anxiety disorders

Numerous studies have shown that when an auditory startle-eliciting stimulus is presented shortly after a visual stimulus that itself does not elicit a startle response (i.e., a lead stimulus), the startle blink reflex is modulated differently depending on the duration of the lead interval. When the lead stimulus and the startle-eliciting stimulus are * Corresponding author. Tel.: +61 7 5552 8132. E-mail address: [email protected] (A.M. Waters). 0301-0511/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.biopsycho.2004.05.002

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presented to different sensory modalities, blink reflex magnitude is typically facilitated compared to reflexes elicited in the absence of lead stimuli at lead intervals between 0 and 60 ms (Graham, 1980), although the extent of blink facilitation for visual lead stimuli may depend on the type of stimuli employed (Aitken et al., 1999; Neumann et al., 2004). Very short lead interval blink facilitation is at least in part the result of a temporal summation of the facial motor nucleus activity generated by the lead stimulus and the blink-eliciting stimulus (Boelhouwer et al., 1991). However, blink facilitation at very short lead intervals has been used to index preconscious processing (Ford and Pfefferbaum, 1991) and studies have shown that blink modulation at 60 ms can be affected by the valence of the lead stimulus (Aitken et al., 1999; Waters et al., 2000). At intervals between 120 and 240 ms, the magnitude of the blink reflex is reduced or inhibited compared to baseline startle reflexes (Graham, 1980). Short lead interval blink inhibition has been said to reflect a short-lived gating process limiting responses to new stimuli (i.e., the acoustic startle-eliciting stimulus) while the processing of the lead stimulus is completed (Graham, 1992) and has, therefore, been used as an index of attentional processing. At long lead intervals greater than 2000 ms, startle blink reflexes are facilitated relative to baseline startle blinks with the extent of facilitation affected by a range of factors including the interest value of the lead stimulus (Lipp et al., 1998) and the valence of the lead stimulus (Bradley et al., 1993), with the strongest modulation occurring for blinks elicited during highly arousing affective stimuli (Cuthbert et al., 1996). In the latter case, blink facilitation while participants view unpleasant pictures is larger in comparison to blinks elicited during viewing of neutral and pleasant pictures (Cuthbert et al., 1996). Affective modulation at long lead intervals is thought to reflect the activation of defensive motivational processes by the match in affective valence between the unpleasant picture stimuli and the auditory startle-eliciting stimulus (Bradley et al., 1999). In a study of the time course of processing emotional picture stimuli, Bradley et al. (1993) used acoustic probes to elicit startle blink reflexes at lead intervals of 300, 800, 1300, and 3800 ms while participants viewed unpleasant, neutral and pleasant pictures. At the 300 ms lead interval, blink inhibition was larger during pleasant and unpleasant pictures than during neutral pictures, suggesting that more attention was selectively allocated to these lead stimuli due to their motivational significance. At the 800 ms lead interval, startle blinks were larger during unpleasant than during pleasant pictures but these blinks were smaller than those during neutral pictures, suggesting that startle reflex differentiation due to affective valence had at least partially been initiated by this time. At lead intervals of 1300 and 3800 ms, the effect of affective valence was more pronounced with larger startle blinks during unpleasant pictures than during pleasant or neutral pictures. Bradley et al. (1993) concluded that these results provide support for attentional startle modulation at short lead intervals and affective startle modulation at longer lead intervals. Studies of startle reflex modulation at very short lead intervals during processing of visual lead stimuli are few in number. However, in a recent study of the time course of processing threat and neutral words, it was found that startle blink modulation was larger at a 60 ms lead interval during viewing of threat words than during neutral words (Aitken et al., 1999). Thus, very short lead interval blink modulation may index biased processing of unpleasant/fear-related stimuli at a very early and possibly preattentive stage of information processing.

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Studies of affective blink modulation in children lag behind the extensive research conducted with adults. Moreover, in the studies that have been conducted, the focus has primarily been on blink modulation at long lead intervals and the results have not been entirely consistent with the affective account documented for adults. For instance, McManis et al. (1995) were unable to demonstrate startle facilitation to pictures with negative affective valence in 7- to 10-year-old children. It was only when responses were analysed separately by gender that girls were found to show the expected (adult-like) facilitation of startle while viewing unpleasant pictures whereas boys showed inhibition of startle. Cook et al. (1995) also were unable to demonstrate the adult pattern using affectively valent script-induced imagery in school-age children. Almost identical startle magnitude was found during imagery of pleasure, joy, sadness, fear, and anger. Similar to earlier findings, McManis et al. (2001) reported that girls showed the expected increase in blink magnitude when viewing unpleasant pictures compared to pleasant pictures at long lead intervals whereas boys tended to show the opposite effect. Of the even fewer studies that have examined anxiety-related differences in children’s processing of affective information using the startle blink modification paradigm, the results have been equally inconsistent compared with the findings obtained from studies with adults. For instance, Cook et al. (1995) found that startle responses of children who scored higher on a fear survey schedule were smaller during unpleasant than during pleasant imagery and were smaller in ‘high fear’ than in ‘low fear’ children. This significant affective valence by fear interaction was opposite to results obtained with adults for whom there are numerous studies demonstrating a robust relationship between affective startle modulation and high fearfulness (e.g., Cook et al., 1991; Hawk et al., 1992). Moreover, in another study of 128 children (67 girls) aged between 10 and 12 years who were categorised as behaviourally inhibited at 4 months of age, i.e., displayed a temperamental characteristic shown to predispose to anxiety, McManis et al. (1996) reported no effect for behavioural inhibition or gender on children’s startle responses. In the most comprehensive review to date on the literature regarding affective startle modulation in children, Ornitz (1999) concluded that it is possible that unpleasant pictures or imagery capture children’s attention to a greater degree, resulting in smaller startle responses to these stimuli, particularly in boys (McManis et al., 1995) and more fear-prone children (Cook et al., 1995). It is notable, however, that cognitive theories of anxiety argue that biases for threatening/ unpleasant stimuli should occur at a very early and possibly preattentive stage of information processing in anxious individuals (e.g., Beck and Clark, 1997; Williams et al., 1997; Mogg and Bradley, 1998). Given that studies conducted to date with children have only examined startle blink modulation at lead intervals greater than 2000 ms (e.g., McManis et al., 1995, 2001), it is possible that the effect that anxiety may have on children’s selective processing of unpleasant/fear-related stimuli has subsided by this late stage of stimulus processing. In extension of previous research in our laboratory in which it was found that high trait-anxious adults displayed larger startle blinks at a 60 ms lead interval while viewing threat words than neutral words compared to low trait-anxious adults (Experiment 2; Aitken et al., 1999), we conducted a small study examining the time course of processing threat and neutral words in clinically anxious children (Waters et al., 2000). We found that the onset latency of anxious children’s blink responses was significantly shorter during viewing of threat words than neutral words at the 60 ms lead interval. Thus, although preliminary in

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nature, these results can suggest that anxiety in children may affect the processing of emotional material at a much earlier stage of information processing. A notable limitation of studies examining the effects of affective picture stimuli on blink modulation in children is that the affective stimuli employed have not been as arousing as those used in studies with adults, primarily because these stimuli may be distressing when viewed by young children (e.g., McManis et al., 2001; for further discussion, see Grillon et al., 1999). However, it is notable, that of the few studies that have examined children’s valence and arousal ratings of affective picture stimuli, the results have been somewhat inconsistent. For example, children have been found in one experiment to rate unpleasant and pleasant pictures as moderate to high in arousal, whereas in a second experiment, children’s arousal ratings were significantly higher for pleasant pictures than for unpleasant pictures, and the arousal ratings of unpleasant pictures were only within the moderate range and not significantly different to neutral pictures (McManis et al., 2001). Given that the arousal value of affective stimuli is important for the observation of affective blink modulation (Cuthbert et al., 1996), children’s inconsistent evaluations of the arousal level of affective picture stimuli, particularly when these stimuli were already lower in arousal than those used in studies with adults, may be important for understanding the inconsistent results observed in studies of affective blink modulation in children. The present study had three major aims. The first aim was to examine children’s appraisal of the valence and arousal level of affective stimuli given that arousal has been shown to influence the extent of affective startle modulation (e.g., Cuthbert et al., 1996). The second aim was to extend previous research by examining whether the patterns of startle blink modulation in children differed from those of adults during early and late stages of viewing unpleasant, neutral and pleasant picture stimuli. The third aim was to examine whether the processing of unpleasant stimuli differed as a function of anxiety status in children, and more specifically, whether anxiety influenced children’s processing of unpleasant stimuli during the early stages of picture processing.

1. Experiment 1 In Experiment 1, we obtained children’s affective valance and arousal ratings for a range of unpleasant, neutral and pleasant pictures selected from the International Affective Picture System (IAPS: Center for the Study of Emotion and Attention [CSEA], 1999). As Experiment 1 was exploratory in nature given the few studies that have examined children’s ratings, no specific hypotheses were formulated.

2. Method 2.1. Participants Participants in Experiment 1 were 89 children aged between 6 and 12 years (49 girls) who were recruited from a local primary school following approvals from the Queensland Education Department, the principal of the primary school, and written informed consent

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from parents. Some children participated in Experiments 1 and 2, however, the time delay of approximately 14 months between the two experiments reduces the likelihood of carry over effects. 2.2. Procedure Children completed the rating task on a group basis during regular class time. A slide projector was set-up in the classroom and 38 pictures (12 neutral, 12 pleasant, and 14 unpleasant)1 selected from the IAPS (CSEA, 1999) were projected onto a white screen. Children were given a rating booklet containing scales relating to affective valence, arousal and dominance. An adapted version of the Self-Assessment Manikin was used for children to provide their ratings. The dimensions of affective valence, arousal, and dominance were explained in a manner appropriate for children and several practice trials were completed. Children were told to take as long as necessary to rate each picture, that it was not a test, and that there were no right or wrong answers. Children raised their hand when they finished rating a picture. The next picture was presented after all children raised their hand. The session lasted approximately 40 min for each class. 2.3. Response definitions and statistical analyses Analyses were based on children’s affective valence and arousal ratings using the ninepoint scale of the IAPS (CSEA, 1999). Higher scores on the affective valence scale indicate higher levels of perceived pleasantness. The higher the score on the arousal scale, the more arousing the picture was rated. Analyses were performed using t-tests. The critical values for these t-tests were derived from Sidak’s tables to control against the accumulation of alpha error (Rohlf and Sokal, 1981). The significance level was set at .05 for all analyses.

3. Results 3.1. Rating data Table 1 presents the mean affective valence and arousal ratings from all participants reported in the IAPS and from the Australian children in Experiment 1. As is evident, children’s affective valence ratings were similar to those reported in the IAPS, whereas their arousal ratings for unpleasant and neutral pictures were markedly lower than those reported in the IAPS. Analyses confirmed that children’s ratings of the pleasantness of neutral and unpleasant pictures were not significantly different to the IAPS ratings (t-values both less than 1.59). However, their ratings of the pleasant pictures were higher than those reported in the IAPS (t(20) = 4.01). 1 IAPS picture numbers: 2210, 1710, 1050, 1670, 5450, 7330, 2280, 7430, 2410, 9584, 7150, 9050, 1750, 7560, 1201, 7570, 7460, 7500, 1300, 6260, 8490, 5480, 9592, 1463, 9911, 2510, 8420, 6230, 1740, 6370, 5950, 1930, 8200, 7502, 6250, 7004, 3530, 7050.

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Table 1 Affective valence and arousal ratings from the IAPS and for children in Experiment 1 Valence IAPS

Children

Boys

Girls

Arousal IAPS

Children

Boys

Girls

Unpleasant 3.12 (1.02) 3.82 (0.75) 4.63 (1.62) 3.17 (1.36) 6.37 (0.64) 2.32 (0.72) 2.72 (1.54) 2.00 (1.39) Neutral 5.23 (0.90) 5.54 (0.74) 5.53 (0.68) 5.55 (0.82) 3.57 (1.09) 2.14 (0.94) 2.14 (1.35) 2.14 (1.44) Pleasant 7.59 (0.46) 8.26 (0.30) 8.10 (0.80) 8.38 (0.74) 5.39 (0.74) 6.70 (0.81) 6.40 (1.83) 6.94 (1.79)

Analyses of children’s arousal ratings confirmed that children rated the neutral and unpleasant pictures much lower compared to the values reported in the IAPS (t(22) = 3.45; t(25) = 15.28, respectively). Children also rated the pleasant pictures as significantly more arousing compared to the values reported in the IAPS (t(20) = 3.92). Children’s valence and arousal ratings were also subjected to analyses of variance with gender as a between-subjects factor. As shown in Table 1, girls and boys gave similar valence ratings for neutral and pleasant pictures, but girls rated unpleasant pictures as more unpleasant than boys. These observations were confirmed by a significant Valence by Gender interaction, F(2,86) = 8.96, P < .001. The Valence and the Gender main effects were also significant, F(2,86) = 550.80, P <.001 and F(1,87) = 9.76, P = .001, respectively. Subsequent t-tests substantiated that girls rated unpleasant pictures as more unpleasant than boys (t(86) = 6.95). Girls’ and boys’ ratings of neutral and pleasant pictures were not significantly different (t-values both less than 1.34). Supporting observations in Table 1 that girls and boys rated the pictures differently in terms of arousal, the analysis of variance revealed a significant Arousal by Gender interaction, F(2,86) = 3.46, P = .036. The Arousal main effect was also significant F(2,86) = 344.68, P <.001. Although children rated the arousal level of unpleasant pictures to be lower than the ratings reported in the IAPS, follow-up t-tests of the Valence by Gender interaction confirmed that girls rated unpleasant pictures as less arousing (t(86) = 3.30) and pleasant pictures as more arousing (t(86) = 2.48) than boys did. Boys’ and girls’ arousal ratings of neutral pictures did not differ.

4. Discussion The results of Experiment 1 demonstrated important differences in the way children rated the pleasantness and arousal level of picture stimuli. The results revealed that children rated pleasant pictures to be more pleasurable and arousing, and unpleasant and neutral pictures to be less arousing compared to the ratings for all participants reported in the IAPS (CSEA, 1999). While one could see how children might find pictures of ice-creams, candy, and cute animals to be more pleasurable than adults might find them (from whom the IAPS ratings are primarily based), of greater interest was the finding that children rated the unpleasant and neutral pictures as less arousing than the values reported in the IAPS. Although this was a surprising finding, it is notable that a similar pattern of arousal ratings was reported in one experiment by McManis et al. (2001)—children rated pleasant pictures as more arousing than either unpleasant or neutral pictures whereas the arousal ratings for neutral and unpleasant pictures did not differ and were only in the moderate range. One

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explanation similar to that proposed by McManis et al. (2001) is that children ‘played down’ their ratings given that the task was completed in a group setting with children typically seated next to their friends. However, if the group setting had an impact, one would expect children’s valence ratings to have been similarly affected. An alternative explanation may be that children did not completely understand the concept of arousal as it applies to negative and neutral stimuli. Further studies are required to determine the reliability of children’s evaluative responses to affective stimuli. Of further interest in the present study was evidence that although girls and boys rated unpleasant pictures lower in comparison to the arousal ratings reported for all participants in the IAPS (CSEA, 1999), girls rated the unpleasant pictures as more unpleasant and less arousing than boys did. Girls also rated pleasant pictures as more pleasurable compared to boys. Although McManis et al. (2001) did not find gender differences in rated arousal, they did find as we did, that girls rated unpleasant pictures as more unpleasant and pleasant pictures as more pleasant compared to boys. Moreover, the present findings are consistent with a considerable body of literature which demonstrates differences in the reactivity of boys and girls to emotional material. For instance, girls have less difficulty than boys in shifting attention from one affective state to another, i.e., either positive or negative (Wilson, 2003), and with improvement in the ability to read emotion in facial expressions, girls report feeling less social anxiety and a greater sense of self-worth whereas boys’ selfconcept is negatively related to improvement (Grinspan et al., 2003). Girls are also more likely than boys to report feeling sad or scared in response to videos showing children in sad or scary scenarios (Strayer, 1993), and in children’s accounts of anger and sadness, girls are more likely to refer to specific agents of sadness and anger (i.e., peers and siblings) and to specific themes of loss and control than boys are (Hughes and Dunn, 2002). Thus, the present findings are consistent with the literature suggesting that there are important differences in the way boys and girls subjectively evaluate and respond to affective information, and they suggest that girls from as young as 6 years of age may be more reactive to negative emotional stimuli. 4.1. Experiment 2 In Experiment 2, we measured the magnitude of startle blink reflexes elicited by an acoustic startle stimulus while adults and children viewed pleasant, neutral and unpleasant pictures. Blink reflexes were elicited at lead intervals of 60, 240, 3500 and 5000 ms and during intertrial intervals. On the basis of previous research assessing long lead interval blink modulation (e.g., Bradley et al., 1993; McManis et al., 2001), one might expect that adults and children would show larger blink modulation during viewing of unpleasant pictures than during neutral pictures and smaller blink modulation during pleasant pictures than during neutral pictures. However, since eight of the 12 unpleasant pictures used in the present experiment were rated by children in Experiment 1 as being low in arousal and given that affective modulation has been shown to be strongest in adults with highly arousing stimuli (Cuthbert et al., 1996), a possibility was that affective modulation may not be observed in Experiment 2. Moreover, given evidence of gender differences in the extent of affective blink modulation at long lead intervals in children (e.g., McManis et al., 2001), we examined whether patterns of blink modulation differed between girls and boys at the

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3500 and 5000 ms lead intervals. Although the effect of picture content on blink modulation at short lead intervals has not been assessed in children, previous research has demonstrated that the mature pattern of short lead interval blink inhibition is stable by about 8 years of age (e.g., Ornitz et al., 1986, 1991). Therefore, based on evidence of an attentional account of short lead interval modulation in adults (Bradley et al., 1993), we examined whether the extent of blink modulation would be smaller during unpleasant pictures and pleasant pictures than during neutral pictures at the 240 ms lead interval. Extrapolating from previous studies in our laboratory (e.g., Aitken et al., 1999), we examined whether startle blink modulation would be larger during unpleasant pictures than during neutral and pleasant pictures at the 60 ms lead interval.

5. Method 5.1. Participants 5.1.1. Adults Participants in Experiment 2 were undergraduate psychology students (16 females) aged between 17 and 37 years (M = 20.38; S.D. = 5.89) who participated on an informed consent basis to obtain course credit. Data from six participants (three females) were rejected due to high rates of missing eyeblink responses. Therefore, analyses were based on data from 21 participants (13 females). Participants’ mean score of 34.91 (S.D. = 5.61) on the Trait scale of the State-Trait Anxiety Inventory (STAI; Spielberger et al., 1970) indicated that the present sample reported anxiety levels that were within the average range. 5.1.2. Children Ninety-five children aged between 9 and 12 years participated in Experiment 2. Participants were recruited from a local primary school following approvals from the Queensland Education Department, the principal of the primary school, and written informed consent from parents. Only children who were reported by parents to be free from intellectual impairment or a learning disorder and who spoke English as their first language were included in the experiment. Analyses for the startle blink task were based on data from 60 children (41 girls) aged between 9 and 12 years. Data were not recorded from one child due to equipment failure and data from 34 children (15 girls) were discarded due to unscorable data resulting from unstable baselines and missing blink responses. For the children from whom data were not included in the analyses, the mean percentage of trials rejected due to unstable baselines was 40.46% for boys and 47.89% for girls. The mean percentage of trials that were not included due to zero responses was 6.76% for boys and 4.67% for girls. By contrast, the mean percentage of trials rejected due to unstable baselines for children included in the final dataset was 9.36% for boys and 8.51% for girls, and the mean percentage of trials scored as zero responses was 0.47% for boys and 0.76% for girls. Thus, the high rate of rejected trials that resulted in the exclusion of data from 34 children was primarily the result of unscorable data due to unstable baselines. On the basis of observations of children

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completing the task, the high rate of unstable baselines may have been due to children often moving and glancing away during intertrial intervals and during picture presentations. Higher rates of unusable data have been reported in other studies with children and may be due to a greater frequency of eye movements (Obrist et al., 1973), blinks (Shapiro, 1973), and off-task glancing (Krupski and Boyle, 1978). Based on socio-demographic information obtained from children’s parents, children tended to come from average-income Australian families with over half the sample living with parents who were married. Parents and children completed the Spence Children’s Anxiety Scale (SCAS; Nauta et al., 2004; Spence, 1998), a psychometrically-sound parentand child-report measure assessing children’s anxiety in accordance with the symptom clusters specified in the Diagnostic and Statistical Manual of Mental Disorders-Fourth Edition (American Psychiatric Association, 1994). A mean score of 24.22 (S.D. = 13.49) on the child-report version of the SCAS (SCAS-C), indicated that children reported anxiety levels that were within the normal range for non-clinical children aged 7 to 12 years (M = 20.51; S.D. = 10.69; Muris et al., 2000). Parents’ mean total score of 17.27 (S.D. = 9.48) on the parent-report version (SCAS-P) also was within the normal range for non-clinical children (M = 14.20; S.D. = 9.7; Nauta et al., 2004). 5.2. Apparatus/stimuli The pictorial stimuli were 36 colour pictures selected from the IAPS (CSEA, 1999) on the basis of affective valence ratings.2 The pictures were selected across three valence categories of unpleasant, neutral, and pleasant, with each category containing 12 exemplar pictures. Eight of the 12 unpleasant pictures, eight of the 12 neutral pictures and nine of the 12 pleasant pictures used in the startle blink task were rated by children in Experiment 1. The pictures were presented via a slide projector (Pradovit 153) fitted with a tachistoscope shutter (Gerbrands G1166(S)). The blink-eliciting stimulus was a burst of white noise set at 105 dB A that lasted for 50 ms and had an instantaneous rise time. The white noise was produced by a custom-built tone and noise generator and was presented through stereophonic headphones (Sennheisser, Model HD25-1-70). Intensity of the white noise was calibrated using an audiometer (Bruel and Kjaer, Model 2603). An IBM-compatible computer controlled the blinkeliciting stimuli, the picture presentations and the trial sequencing. Blink reflex was measured by recording the electromyographic (EMG) activity of the orbicularis oculi muscle by placing two miniature Ag/AgCl electrodes (IVM, Model E220N-LP) filled with a standard electrolyte directly beneath the left eye approximately 1 cm apart. A ground electrode was attached to the forearm (impedance level less than 10 kV). Raw EMG was amplified with a wide-band AC pre-amplifier (Grass, Model 7P3C), with low and high amplitude cut-offs of 10 Hz and 3 kHz, respectively. The signal was calibrated at 100 mV/cm of pen deflection and displayed on a polygraph (Grass, Model 7D). Raw EMG was digitised and sampled on-line using the IBM-compatible 2 IAPS picture numbers: 1050, 9584, 9050, 1201, 1300, 9592, 9911, 1930, 3530, 1302, 3230, 3210, 7009, 7150, 7170, 7705, 7050, 7004, 7100, 7140, 7500, 7560, 1670, 2190, 1710, 7330, 8461, 5480, 1999, 7430, 1463, 1750, 7460, 8490, 7450, 8420.

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computer. A sampling rate of 1000 Hz was used for the raw EMG signals in a samplingtime window of 100 ms before the onset of the blink-eliciting stimulus to 400 ms after stimulus onset. The equipment used to assess adults’ and children’s responses was the same with the exception that the Grass (Model 7P3C) pre-amplifier was replaced with a Coulbourn Instruments (Model S75-01) bio-amplifier with low and high amplitude cut-offs of 90 Hz and 1 kHz, respectively for the assessment of children. The Grass pre-amplifier was replaced because it could not be relocated to the school. However, comparisons of data acquired through the different amplifiers were possible because data from each participant were expressed as the percentage of change in the magnitude of startle blinks elicited during picture viewing compared to during intertrial intervals. 5.3. Procedure Data collection was conducted with adults first followed by children. Both groups completed the startle blink task along with four other experimental tasks, including a dot-probe detection task, an emotional Stroop task and two visual search tasks, all of which were the same for adults and children (see Waters et al., submitted for publication-a, in press). The tasks were completed in a counterbalanced order across participants. Adults were assessed in an experimental laboratory at the University of Queensland. They completed the consent form and the STAI (Spielberger et al., 1970) upon arrival at the laboratory and then completed the five experimental tasks according to the scheduled order. Because the arousal level of picture stimuli has been shown to affect the magnitude of startle blink reflexes (Cuthbert et al., 1996), participants viewed each of the pictures and rated them on the three dimensions of the IAPS (CSEA, 1999) using an adapted version of the Self Assessment Manikin. The pictures were projected through a glass screen set into the wall of the experimental room and were projected over participants’ right shoulder onto the wall in front of them. After electrode placement, participants were requested to focus on the screen, to keep their eyes open, and to refrain from moving during the experiment. The lighting was dimmed to enable sufficient illumination of the pictures. Participants were in constant contact via a video camera and an intercom installed in the experimental room. Average temperature was 21 8C and humidity was 81%. For children, the study was explained to classes within each grade at the primary school commencing with Grade 7 classes (i.e., 11–12 years old) and terminating with Grade 4 classes (i.e., 9–10 years old). Children gave parents a research study information sheet, a research participation consent form, a family environment information sheet, and the SCAS-P (Nauta et al., 2004), and returned the completed forms to their classroom if parental consent was given. Children eligible to participate completed the five experimental tasks individually in a spare resource room at the primary school. The tasks were completed across two sessions set a week apart with the SCAS-C (Spence, 1998) completed at the beginning of the first session. The order of the tasks within and across the two sessions was counterbalanced across participants. Each session lasted approximately 40 min and data collection was completed within 5 months. Children in Experiment 2 did not provide affective valence and arousal ratings of the picture stimuli used in the

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startle blink task due to constraints placed on the amount of time children could be out of class for each session. As laboratory facilities were not available at the primary school, a section of a resource room was converted into a viewing room in which children could complete the task without distraction. Four black curtains were draped from the ceiling at one end of the room to create a dimly lit cell. The lens of the slide projector was placed through a small hole cut into a curtain at one end of the cell and the pictures were projected on to a white screen placed at the opposite end inside the cell. The equipment used to control the startle blink task was located on a table outside the cell. As air conditioning facilities were not available, the temperature and humidity were not controlled. However, the temperature in the resource room was comfortable since the testing phase was conducted during Australian autumn and winter. The startle task involved the presentation of 36 pictorial stimuli one at a time. The size of the projection for both adults and children was approximately 80 cm high by 100 cm wide and participants were seated approximately 1.5 m from the picture display. Each picture presentation lasted 6 s and was accompanied by an auditory blink-eliciting stimulus with picture-to-probe onset intervals of either 60, 240, 3500, or 5000 ms. Three sets of 12 trials were developed to create a total of 36 picture trials. Within each set of 12 trials, four pictures from each valence category were paired with a blink-eliciting stimulus at each of the four lead intervals. Then each picture and each probe position were rotated using a Latin square design so that each appeared at every serial position within the set of 12 trials. This procedure resulted in a total of 12 sequences of 36 picture trials. Intertrial intervals (ITIs) lasted for 15, 20 or 25 s with nine ITIs containing a blink-eliciting stimulus balanced across the three sets of 12 picture trials to form the baseline condition. Only one blinkeliciting stimulus was presented during an ITI. No blink-eliciting stimulus occurred within 7 s after the offset or prior to the onset of a pictorial stimulus. The 12 sequences of 45 startle trials (36 during picture viewing and 9 during intertrial intervals) were rotated across participants. 5.4. Response definitions, data screening and statistical analyses Digitised EMG data were rectified off-line and filtered and integrated with a Butterworth low-pass filter (time-constant of 80 ms). Startle latency and magnitude were derived from the integrated response signal. As the analyses of startle latency data revealed no significant effects of interest in Experiment 2 or 3, only startle magnitude data are reported. Blink magnitude, in analog/digital (A/D) units, was defined as the difference between the foot-point and maximum-point of the integrated response signal within 200 ms after the onset of the startle-eliciting stimulus. If no blink response was detectable, magnitude was scored as zero and latency as missing. A trial was discarded if (a) the baseline EMG was not stable and varied by more than 50 A/D units prior to the onset of the startle-eliciting stimulus, or (b) the onset of a blink response occurred within 10 ms of the onset of the startle-eliciting stimulus. Data from a participant were rejected if the number of zeroresponses and/or discarded trials exceeded one-third of all the trials. Startle magnitude data were converted to percentage change scores by subtracting the average ITI startle magnitude score from startle magnitude scores averaged over the three

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trials for each valence condition at each lead interval. These values were then divided by the average ITI startle magnitude score and multiplied by 100. These scores reflected the change in startle blink modification relative to the baseline condition and reduced the effect of individual differences in blink responses. Startle magnitude data were analysed using SAS version 8.2 (SAS Corporation, Cary, NC). A mixed model analysis of variance (i.e., SAS PROC MIXED) was performed in order to account for the unequal variance due to the different sample sizes (Wilcox et al., 1986). Several covariance patterns across time were considered testing issues relating to sphericity. An unstructured covariance matrix, an autoregressive covariance matrix and a compound symmetric covariance matrix were compared, and the compound symmetric covariance structure yielded the best fit. The decimal values in the degrees of freedom reflect the Satterthwaite method used in the PROC MIXED procedure to calculate the denominator degrees of freedom. Because we were interested in assessing the effect of picture valence at specific time points during early and late stages of information processing rather than the overall pattern of blink modulation which is now wellestablished (i.e., inhibition at short lead intervals; facilitation at long lead intervals; for a review see Bradley et al., 1999), we examined the effect of valence at each lead interval separately. Thus, the effect of Valence was examined at lead intervals of 60, 240, 3500 and the 5000 ms with Group (adults, children) as a between-subjects factor. Differences in boys’ and girls’ blink modulation due to picture content were examined across both long lead intervals. Planned comparisons relating to our hypotheses at each lead interval were made with t-tests using Sidak’s adjustment to control for the accumulation of alpha error. The significance level was set at .05 for all analyses.

6. Results 6.1. Adults’ rating data Table 2 presents the mean affective valence and arousal ratings of adult participants in Experiment 2 and from the IAPS (CSEA, 1999). As can be seen, the mean valence ratings of the present sample were not significantly different from the IAPS means for pleasant, neutral and unpleasant pictures (t-values all less than 1.97). These results indicate that the present sample rated the unpleasant and pleasant pictures as similarly negative and positive as all participants reported in the IAPS (CSEA, 1999). However, as is evident in Table 2, the unpleasant, neutral and pleasant pictures were rated as less arousing by the present sample compared with the arousal values reported in the IAPS (t(22) = 2.87; t(22) = 3.92; t(22) = 3.66, respectively). Table 2 Affective valence and arousal ratings from the IAPS and for adults in Experiment 2

Unpleasant Neutral Pleasant

Valence IAPS

Adults

Arousal IAPS

Adults

3.19 (0.86) 5.03 (0.40) 7.31 (0.64)

2.57 (0.67) 5.27 (0.55) 7.15 (0.60)

6.03 (0.67) 3.04 (0.79) 4.94 (0.80)

5.01 (0.70) 1.77 (0.80) 3.81 (1.09)

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6.2. Startle blink magnitude Fig. 1 presents the startle magnitude percentage change scores for adults (left panel) and children (right panel) during unpleasant, neutral and pleasant pictures at each lead interval. As can be seen, at the 60 ms lead interval, the percentage of change in blink modulation during unpleasant pictures was different from that observed for neutral and pleasant pictures for children, whereas very little difference in blink modulation due to picture content was evident for adults. These observed differences were confirmed by a significant Group by Valence interaction, F(2,121) = 4.42, P = .01. The Group and Valence main effects were not significant (both F-values less than 1.16). Planned comparisons indicated that in children, differences between percentage magnitude change during unpleasant and neutral pictures was significant (t(118) = 2.82, P = .015), whereas the difference between unpleasant and pleasant pictures was marginally significant (t(118) = 2.10, P = .08). In adults, the extent of blink magnitude modulation did not differ across the picture contents (all t-values less than 1.38). Analyses of differences in the percentage of change in blink modulation due to picture content at the 240 ms lead interval revealed no significant results (all F-values less than 1.83). At the 3500 and 5000 ms lead intervals, adults and children displayed notably different patterns of blink modulation due to picture content (see Fig. 1). As is shown, adults

Fig. 1. Mean blink magnitude percentage change scores (+S.E.) for adults and children during viewing of unpleasant, neutral and pleasant pictures as a function of the lead interval. The symbol (*) indicates significant differences between adults and children at the 60 ms lead interval. (#) indicates significant differences between adults and children at the 3500 and 5000 ms lead interval.

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displayed a greater percentage of change in blink modulation during unpleasant and pleasant pictures than during neutral pictures at both lead intervals. Differences in the percentage of change in blink modulation due to picture content were less pronounced at both lead intervals for children. Analyses at the 3500 ms lead interval, revealed a significant Group by Valence interaction, F(2,139) = 8.45, P < .001, and a significant Group main effect, F(1,78.9) = 17.31, P < .001. Planned comparisons confirmed that in adults, blink modulation during neutral and unpleasant pictures and during neutral and pleasant pictures was significantly different (t(40) = 3.83, P < .001 and t(40) = 3.08, P < .008, respectively). Analyses of children’s responses revealed no significant differences (tvalues less than 1.95). Analyses at the 5000 ms lead interval also revealed a significant Group by Valence interaction, F(2,112) = 4.81, P <.001. Planned comparisons revealed that the percentage of change in adults’ blink modulation during unpleasant pictures and neutral pictures was significantly different (t(40) = 3.03, P < .009). The other comparisons were not significant (t-values less than 1.85). Differences in the extent of blink modulation due to picture content were not significant in children (all t-values less than 1.22). The analyses with children’s gender as the between-subjects factor revealed no significant results (F values less than 1.37).

7. Discussion The pattern of blink modulation in Experiment 2 suggests that adults and children were differentially affected by emotional content during the time course of picture processing. The differential effect of unpleasant pictures at the 60 ms lead interval in children but not in adults was an unexpected result and may indicate differential processing of unpleasant pictorial stimuli in children at a very early and possibly preattentive stage of information processing. At the 240 ms lead interval, picture content did not significantly affect the extent of blink modulation in either adults or children. Although Bradley et al. (1993) found a differential effect of picture content at the 300 ms lead interval, it is unlikely that the lack of an effect in the present experiment reflected that pictures were incapable of being differentially processed at 240 ms, particularly given evidence that controlled attentional processes can be observed as early as 120 ms (Filion et al., 1993). Moreover, Bradley et al. (1993) found that startle blink reflexes were equally modified by both unpleasant and pleasant pictures at the 300 ms lead interval, suggesting that foreground stimulus arousal rather than valence underlies short lead interval blink modulation. Therefore, viewed in light of the lower arousal ratings reported for both adults and children (Experiment 1), a more likely explanation for the lack of a differential effect at the 240 ms lead interval is that unpleasant and pleasant pictures at relatively lower levels of foreground activation were not more motivationally significant than neutral pictures to elicit greater processing protection (Graham, 1980). At the long lead intervals, the typical pattern of affective blink modulation was not observed for adults with these relatively low arousing pictures. These findings are similar to those reported by McManis et al. (2001) in which the affective pattern of blink

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modulation was not found for either male or female adults with low arousing picture stimuli. Thus, the data are consistent with the hypothesis (Cuthbert et al., 1996) and subsequent findings (e.g., McManis et al., 2001) that low to moderately arousing pictures do not strongly or reliably activate defensive emotional responses in adults. On the other hand, the lack of a significant effect of picture content in children at the long lead intervals was perhaps more unexpected given evidence of affective modulation in children, particularly in girls, using relatively low arousing pictures (e.g., McManis et al., 2001). Although differences in blink modulation due to picture content were not significant, it is notable that the pattern of smaller blink modulation during pleasant pictures than during neutral pictures is consistent with the affective account, whereas the tendency for blink modulation to be smaller during unpleasant pictures than during neutral pictures is not (e.g., Bradley et al., 1993). It is notable, however, that the finding of smaller blink reflexes during unpleasant pictures is similar to the finding that children tended to show smaller blink reflexes during unpleasant imagery (e.g., Cook et al., 1995). One explanation for the unexpected results for the unpleasant pictures is that these stimuli were insufficiently arousing to children to elicit defensive motivational processes (e.g., enhanced startle blink responses during unpleasant pictures than during neutral pictures; Cuthbert et al., 1996). Although it is a limitation of the present experiment that affective valence and arousal ratings were not obtained from children meaning that the relationship between arousal level and blink responses could not be assessed, children’s arousal ratings in Experiment 1 (in which 8 of the 12 unpleasant pictures used in the present experiment had been rated) suggested that children appraised unpleasant pictures as relatively low in arousal activation. Moreover, given that the unpleasant pictures were rated to be relatively low in arousal by both boys and girls in Experiment 1, the lack of gender differences in blink modulation in the present experiment may indicate that at relatively low levels of perceived arousal activation, girls as well as boys (e.g., Ornitz, 1999), may respond with greater interest rather than with defensive activation to unpleasant picture stimuli (i.e., smaller startle blinks during unpleasant pictures than during neutral pictures). On the other hand, children’s blink reflexes tended to be smaller during pleasant pictures than during neutral pictures even though data from Experiment 1 suggested that children found these pictures to be highly arousing. Although one possibility is that children’s subjective appraisals of affective stimuli are not reliably related to patterns of blink modulation, the present results accord with the view that for strongest affective blink modulation to be observed, unpleasant and pleasant picture stimuli must be high in arousal activation (Cuthbert et al., 1996). Future research with children would benefit from assessing children’s subjective evaluations of affective picture stimuli in conjunction with their startle blink responses and from selecting unpleasant and pleasant picture stimuli that are matched on arousal level. Although very short lead interval startle modulation has received considerably less attention than blink modulation effects at short and long lead intervals (Filion et al., 1999), it is notable that we have found evidence for a differential effect of threat and neutral word lead stimuli at the 60 ms lead interval in other studies with both adults and children (e.g., Aitken et al., 1999; Waters et al., 2000). Although the processing of word and picture stimuli presumably differs in important ways, previous research has shown that the threshold at which participants can correctly identify visual stimuli at better than chance

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rate is on average 34.8 ms with a range of 16.7 to 83.3 ms (Cheesman and Merikle, 1986). Thus, it is plausible that some initial processing of visual stimuli can be completed within a 60 to 90 ms interval associated with the shortest lead interval in the present experiment. Nevertheless, given that this study is the first to the authors’ knowledge to compare very short lead interval blink modulation in children and adults using the picture-viewing paradigm, speculations about the psychological significance of the finding that unpleasant stimuli had a differential impact on children are made cautiously. One possibility is that initial elevated reactivity to unpleasant stimuli in children may serve an adaptive function. Consistent with contemporary theories of emotion that argue that the initial appraisal of a situation or object (as benign, positive, or negative) is one of the major determinants of the emotional response to that situation (e.g., Oatley and Jenkins, 1996), it is conceivable that with less prior knowledge and experience with unpleasant stimuli, an initial heightened reactivity to unpleasant stimuli would help protect children from situations of potential danger and threat to well-being. In support, it is well established that a considerably greater number of fears are experienced during childhood than they are during adulthood (for a review, see Gullone, 2000)—fears which are thought to be developmentally appropriate and adaptive since they protect children from danger and harm (King, 1993). Moreover, the nature and focus of these normative fears of childhood are related to children’s age and developmental level and emerge and subside at identifiable points in development due to increasing cognitive capability and maturity (Gullone, 2000). It is notable that whereas some studies of attentional bias for threatrelated information in children have shown that such a bias is restricted to anxious children (e.g., Vasey et al., 1995, 1996; Taghavi et al., 1999, 2003), other studies have revealed that a bias toward threat stimuli is common to both anxious and non-anxious children up to 12 years of age (e.g., Ehrenreich and Gross, 2002; Kindt et al., 1997a, 1997b, 2000; Waters et al., submitted for publication-a, in press). Moreover, evidence from some of these studies suggests that anxiety-related differences in an attentional bias toward threat-related stimuli may be stronger in older youths (e.g., Kindt et al., 1997a, 1997b; Vasey et al., 1995; but see Taghavi et al., 1999 for a different result). Thus, it has been proposed that children up to about 12 years of age may initially display a bias toward threat-related information, and that with increasing development, this bias may subside in normally developing children but persist in anxious children (e.g., Kindt et al., 1997). Thus, although the present results are preliminary, they suggest that the examination of whether initial elevated reactivity to unpleasant stimuli in childhood may subside with development would be a useful focus of future research. It may be argued that a limitation of the present results is that adults’ prior viewing of the pictures before completing the task may have affected the magnitude of blink responses. However, since the blink reflex modulation of adults and children did not differ at the 240 ms lead interval and given that adults’ results at the long lead intervals were similar to the results observed by McManis et al. (2001) in which adults did not view the pictures beforehand, there is little evidence to support that the prior viewing had an effect. Moreover, given that the effect of controlled attentional processes can be observed at a lead interval of 120 ms but not as early as a 60 ms lead interval (Filion et al., 1993), it seems unlikely that the prior viewing could have diminished the differential effect of picture content at the 60 ms lead interval in adults.

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7.1. Experiment 3 Experiment 3 was designed to examine whether the processing of unpleasant stimuli differed as a function of anxiety status in children, and more specifically, whether anxiety influenced children’s processing of unpleasant stimuli during the early stages of picture processing. Given that Experiment 2 revealed larger blink modulation during unpleasant pictures compared to neutral pictures at the 60 ms lead interval in nonselected children, it was hypothesised in Experiment 3 that this difference would be accentuated in clinically anxious children (Aitken et al., 1999). No specific group differences were predicted for the 240 ms lead interval in absence of prior research with children, or at the 3500 and 5000 ms lead intervals given that previous studies did not find evidence of anxiety-related differences in children’s processing of affective stimuli during these later stages of information processing (e.g., Cook et al., 1995; McManis et al., 1996).

8. Method 8.1. Participants Participants in the anxiety group were 23 clinically anxious children (9 girls) aged between 9 and 12 years (M = 10.14; S.D. = 1.07). Data from the 60 children in Experiment 2 formed the non-selected group. The inclusion criteria was the same as Experiment 2, and only children who were reported by parents to have no pre-existing psychological diagnoses and to have not received psychological or psychopharmacological treatment for anxiety were included in the anxiety group. Participants for the anxiety group were recruited via a media campaign offering an anxiety disorders diagnostic interview at no charge to families with referral to appropriate treatment services if required. In exchange, children participated in the experimental session. Information about the study was circulated via newspaper articles, radio announcements and advertisements in school newsletters. Diagnostic interviews were performed using the parent version of the Anxiety Disorders Interview Schedule for DSM-IV: Child Version (ADIS-C for DSM-IV; Albano and Silverman, 1997). The ADIS-C for DSM-IV is the most commonly used semi-structured diagnostic instrument for assessing childhood anxiety disorders and possesses sound psychometric properties (e.g., Silverman and Eisen, 1992; Silverman and Nelles, 1988; Silverman and Rabian, 1995). Only children with a severity rating of 4 or greater for at least their primary diagnosis were included in the anxiety group. Of 25 children initially assessed, two children (1 girl) did not meet the clinical cut-off level. However, similar to Experiment 2, data from 11 children (6 boys) was discarded due to high rates of missing blink responses and unstable baselines. Thus, the analyses were based on startle blink data obtained from 12 children (8 boys) in the anxiety group. Diagnostic information for the 12 children, including the number of children with each of the diagnoses as their primary diagnosis, the mean severity of each primary diagnosis and the mean number of diagnoses per child is presented in Table 3.

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Table 3 Diagnostic summary of the anxious children in Experiment 3 Primary Diagnosis Severity = 0 (low) to 8 (high)

N

Mean Severity Rating

Primary Primary Primary Primary

4 2 5 1

5.5 6.5 6.4 7.0

Generalised Anxiety Disorder Separation Anxiety Disorder Social Anxiety Disorder Specific Phobia

Mean number of diagnoses per child = 2.08

Additional information about the psychological status of children in the anxiety group was obtained through a battery of questionnaires. Their mean total score of 12.17 (S.D. = 7.15) on the Revised Children’s Manifest Anxiety Scale (RCMAS; Reynolds and Richmond, 1978) is comparable to the mean of 14.55 reported for clinically-anxious children on this measure (Spence, 1998), and their average T-score of 70.64 (S.D. = 4.27) on the Internalising Scale of the Child Behaviour Check List (CBCL; Achenbach, 1991) is within the clinical range. Furthermore, a mean total score of 27.33 (S.D. = 14.75) on the SCAS-C and a mean total score of 35.50 on the SCAS-P are comparable to the mean total scores reported for clinically-anxious children on these measures (SCAS-C: M = 32.20; S.D. = 21.97; Spence, 1998; SCAS-P: M = 31.80; S.D. = 14.0; Nauta et al., 2004). Based on socio-demographic information obtained from parents, children in both groups tended to come from middle-income Australian families, with over 70% of children living in families in which parents were married. Univariate analyses of variance to examine differences in socio-economic status between the groups, as measured by the Daniel Prestige Scale (Daniel, 1983), revealed no significant differences in mothers’ or fathers’ socio-economic status (F-values less than 2.87). 8.2. Apparatus and procedures Upon parents contacting the research team, an explanation about the study was provided and if parents wished to proceed, a semi-structured screening interview was conducted addressing the inclusion criteria and assessing the nature of the child’s anxiety problems. If children met criteria for participation in the study, an appointment was arranged for the diagnostic interview and experimental session. At the commencement of the session, families were given a research information sheet and signed the research consent form. Children completed the experimental session while the ADIS-C for DSM-IV was administered with parents. Experimenters administering the experimental tasks with children were blind to the reason children were taking part in the study. Children completed the tasks individually in the same laboratory as adults in Experiment 2. The SCAS-C (Spence, 1998) and the RCMAS (Reynolds and Richmond, 1978) were completed first followed by the startle blink task and the four other tasks described in Experiment 2 (Waters et al., submitted for publication-a, in press). The tasks were completed in a counterbalanced order across participants. Because the experimental session with anxious children needed to be completed within the time taken to complete the diagnostic interview with parents, anxious children did not rate the affective valence and arousal levels of the picture stimuli.

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After the diagnostic interview, feedback was provided to parents on the outcomes of the interview and questions were answered. The psychological approach to the treatment of childhood anxiety problems was overviewed and appropriate treatment referral options were discussed with relevant contact details provided if necessary. Parents completed the questionnaire measures and were advised they would receive an assessment report within a week of the interview. 8.3. Response definitions, data screening, and response definitions The parameters of the startle blink task, the response definitions and the statistical analyses were the same as those described in Experiment 2 with Group (anxious; nonselected) as the between-subjects factor.

9. Results Startle magnitude percentage change scores for both groups of children are presented in Fig. 2. As can be seen at the 60 ms lead interval, the percentage of change in startle blink modulation was larger during unpleasant pictures than during neutral and pleasant pictures for both groups. Fig. 2 (left panel) also demonstrates that the extent of blink modulation at

Fig. 2. Mean blink magnitude percentage change scores (+S.E.) for anxious children and non-selected children during viewing of unpleasant, neutral and pleasant pictures as a function of the lead interval. The symbol (*) indicates significant differences between anxious and non-selected children at the 60 ms lead interval.

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this very short lead interval was larger in anxious children during viewing of all three picture types. The analysis confirmed these observations by revealing main effects for Valence and Group, F(2,31.3) = 3.92, P = .03 and F(1,24.8) = 16.98, P <.001, respectively. The interaction was not significant (F-value less than 0.3). Follow-up analyses of the Valence main effect revealed that the percentage of change in blink modulation was larger during unpleasant pictures than during neutral pictures, t(140) = 3.31, P < .001. The difference in the percentage of change during unpleasant and pleasant pictures was marginally significant (t(140) = 2.25, P = .068). The analyses at the remaining lead intervals revealed no significant results (F-values less than 1.33).

10. Discussion The results of Experiment 3 extend the findings obtained in Experiment 2 and in previous research (e.g., McManis et al., 2001; Waters et al., 2000) by demonstrating that children, regardless of anxiety status, showed more pronounced blink modulation during unpleasant pictures than during neutral pictures at the 60 ms lead interval. The results also revealed that the impact of lead stimuli on blink modulation at this very short lead interval was more pronounced as a function of children’s anxiety status. Differences in blink modulation due to picture content at the other lead intervals were not significant. In accordance with the discussion provided in Experiment 2, the results at the 60 ms lead interval add weight to the argument that all children, irrespective of anxiety level, may initially be reactive to unpleasant stimuli at a very early stage of information processing (Kindt et al., 1997). However, a limitation to this interpretation that should first be addressed is to do with participant selection for the anxious group. Previous research demonstrates that startle is differentially potentiated in anxiety disorders to the extent that the situation-specificity of startle potentiation matches the situation-specificity of the anxiety (Cook, 1999). It may be argued, therefore, that the lack of a differential effect of anxiety on children’s blink responses during unpleasant stimuli is the result of the weak match between children meeting criteria for a range of anxiety disorder diagnoses and the broad assortment of pictorial exemplars depicting unpleasant scenes. Although a systematic investigation of such a specificity match is required in studies with children, results from studies employing other experimental procedures with children have yielded mixed results. For example, studies with a more specific match between the stimuli employed and children’s anxiety concerns have found evidence of a general threat bias among children (e.g., spider stimuli and spider-fearful and non-fearful children; Kindt et al., 1997), whereas studies with a less specific match have found evidence for an anxiety-specific threat bias (e.g., mixed anxiety-disordered children and test-anxious children with general threat word stimuli; Vasey et al., 1995, 1996, respectively). One reason a bias specifically for unpleasant stimuli may not be enhanced in anxious children is that the processes underpinning heightened reactivity to stimuli depicting anxious concerns may not be as selectively sensitised given that these children are likely to be much closer to the onset of their disorder than older anxious youths and adults (Mayer et al., 1999). Thus, given some evidence of a common bias for threat-related information in children up to 12 years of age, and stronger evidence of anxiety-specific biases for threat-related information in older

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youth (e.g., Kindt et al., 1997a, 1997b; Vasey et al., 1995), the present finding of larger blink modulation during unpleasant pictures for all children at the 60 ms lead interval can be said to accord with the argument that children up to about 12 years of age may initially be reactive to unpleasant stimuli (Kindt et al., 1997). Longitudinal research is required to track the developmental progression of biased responding to affective stimuli from childhood to adulthood as a function of anxiety status. Of interest was the observation that at the very short lead interval of 60 ms, the impact of picture stimuli on blink modulation was more pronounced in anxious children. The fact that this difference was evident at the 60 ms lead interval accords with cognitive theories to the extent that anxiety-related biases are thought to occur early, during preattentive and attentional stages, rather than during the later stages of information processing (e.g., Beck and Clark, 1997; Williams et al., 1997). However, the fact that more pronounced blink modulation for anxious children was not specific to unpleasant pictures is discordant with the notion of mood congruent biases proposed by these theories. Together with the observation that the groups did not differ at the other lead intervals, larger blink modulation overall just 60 ms into picture processing can suggest that picture stimuli produced an initial rapid elevation in arousal in anxious children which then quickly dissipated during the time course of picture processing. Thus, although the small sample of anxious children is a limitation to the present experiment, the results may suggest that anxious children have a lower threshold for arousal activation in that they reacted with increased vigilance to any distinctive foreground stimulus during a very early stage of information processing. Stronger support for this contention could have been gained if the affective valence and arousal ratings of the picture stimuli were obtained from children in the present experiment. Comparisons of the ratings from anxious children and controls would have permitted the investigation of whether anxious children rated the picture stimuli to be more arousing. It is notable, however, that we have found evidence in other studies in our laboratory of enhanced reactivity to stimuli in general in anxious children. For instance, we found that anxious children were more vigilant in detecting both fear-relevant and non-fear relevant target pictures in a visual search task (Waters et al., submitted for publication-a). Anxious children also were found to be faster overall than non-anxious peers in locating dot-probes that replaced affective pictures than neutral pictures in a probe detection task (Waters et al., in press). These findings may suggest that anxious children have a lower threshold for arousal activation in that they responded with increased vigilance to distinctive foreground stimuli. Thus, although the present results do not address the issue of whether heightened arousal activation may be a cause or consequence of anxiety in children, they suggest that theoretical models that emphasise differences in arousal activation may provide useful frameworks for guiding future investigations with children and those with anxiety disorders. On a broader note, the present study highlights some of the practical constraints confronting researchers in the field of clinical child experimental psychology, particularly with regard to the elicitation and measurement of children’s responses to highly arousing affective stimuli. One such issue is the high rate of discarded data from children in the present experiment, which highlights the difficulty one can face in keeping children focused during a passive task such as a picture viewing paradigm. The present experimental

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design could be improved by the inclusion of short breaks during the experimental session and by presenting a fixation cross seconds before the onset of a picture stimulus in order to regain children’s focus. A second issue concerns the nature of the stimuli that can be used in studies of children’s responses to emotional stimuli. By way of example, the picture stimuli in the present study were screened by the university ethics committee, the Queensland Education Department, the school principal and by students’ teachers. McManis et al. (2001) reported similar screening procedures in their study which, like in our study, resulted in the elimination of the most highly arousing unpleasant and pleasant pictures that are typically used in studies of affective perception in adults (e.g., Bradley et al., 1993; Cuthbert et al., 1996). Moreover, the results from Experiment 1 indicated that children did not evaluate the unpleasant pictures that could be used with children to be as arousing as adults rated them. Since other procedures typically used with adults, such as threat of shock, are also inappropriate for use with children, recent attempts to overcome these limitations have included the investigation of darkness-induced potentiation of the startle reflex in children and adolescents (e.g., Grillon et al., 1999). However, if we are to better understand the emotional and attentional processes of children, and those with psychopathology, then a challenge will be to develop experimental procedures that safely yet thoroughly permit the investigation of children’s responses to affective stimuli. In summary, the present experiments revealed that non-selected children, particularly boys, subjectively evaluated negative affective stimuli to be less arousing than adults rated them. It also was found that unpleasant picture stimuli had a larger impact than neutral or pleasant stimuli on the reactivity of non-selected children during a very early and possibly preattentive stage of information processing. Finally, the impact of distinctive picture stimuli in general was more pronounced in anxious children during this very early stage of stimulus processing.

Acknowledgement This research was supported by ARC Grant A10027227. References Achenbach, T.M., 1991. Manual for the Child Behavior Checklist/4-18 and 1991 Profile, Department of Psychiatry, University of Vermont, Burlington. Aitken, C.J., Siddle, D.A.T., Lipp, O.V., 1999. The effects of threat and non-threat word lead stimuli on blink modification. Psychophysiology 36 (6), 699–705. Albano, A.M., Silverman, W.K., 1997. Clinician Manual for the Anxiety Disorders Interview Schedule for DSMIV—Child Version, The Psychological Corporation, New York. American Psychiatric Association, 1994. Diagnostic and Statistical Manual of Mental Disorders, 4th ed. Author, Washington, DC. Beck, A.T., Clark, D.A., 1997. An information processing model of anxiety: automatic and strategic processes. Behaviour Research and Therapy 35, 49–58. Boelhouwer, A.J.W., Teurlings, R.J.M.A., Brunia, C.H.M., 1991. The effect of an acoustic warning stimulus upon the electrically elicited blink reflex in humans. Psychophysiology 28, 133–139.

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