Psychophysiology, 43 (2006), 498–503. Blackwell Publishing Inc. Printed in the USA. Copyright r 2006 Society for Psychophysiological Research DOI: 10.1111/j.1469-8986.2006.00420.x Effects of perceptual load on startle reflex modification at a long lead interval

GARY L. THORNE,a MICHAEL E. DAWSON,a and ANNE M. SCHELLb aDepartment of Psychology, University of Southern California, Los Angeles, California, USA bDepartment of Psychology, Occidental College, Los Angeles, California, USA

Abstract Inhibition of the startle eyeblink response at long lead intervals has been hypothesized to occur when attention is directed away from the modality of the startle stimulus, particularly if attention is directed to a stimulus of high perceptual load. In a test of this hypothesis, participants performed a delayed-matching-to-sample task. On each trial a pattern of dots (the sample) was followed by a second pattern of dots (the target). The task was to say whether the sample and target patterns matched. Perceptual load was manipulated by varying the number of dots in the sample. Auditory startle stimuli were presented 1200 ms after onset of the samples. A linear increase in startle magnitude was found as the number of dots increased. The results are not consistent with the hypothesis that startle inhibition occurs when the lead and startle stimuli are in different modalities under conditions of high perceptual load. Descriptors: Startle eyeblink, Attention, Attentional modulation, Task demand, Perceptual load, Arousal

A stimulus with an abrupt onset and sufficient intensity, such as a modification, see Putnam & Vanman, 1999). Specifically, it has brief loud burst of white noise, elicits a reflexive startle response. been proposed that long lead interval startle modification is mo- In humans this response is most commonly measured by the dality specific. Tasks that require attention to the modality of the magnitude of the eyeblink (for reviews of startle elicitation, re- startle stimulus are hypothesized to increase (facilitate) startle cording, and quantification, see Berg & Balaban, 1999; magnitude, whereas tasks that require attention to a modality Blumenthal et al., 2005). If the startling stimulus is preceded by other than that of the startle stimulus are hypothesized to de- a nonstartling stimulus, such as a soft tone, the magnitude of the crease (inhibit) startle magnitude (Putnam, 1990). startle response is modified. The nonstartling stimulus is called a Numerous studies have reported reliable long lead interval lead stimulus or prepulse and the interval between the onset of startle facilitation when attention is directed toward the modality the lead and startle stimuli is called a lead interval. The effect of the startle stimulus (e.g., Bo¨hmelt, Schell, & Dawson, 1999; of the lead stimulus on startle magnitude depends on the lead Filion, Dawson, & Schell, 1993; Jennings, Schell, Filion, & interval. At short lead intervals (15–400 ms), the magnitude of Dawson, 1996; Lipp & Hardwick, 2003). Other studies have the startle response is typically decreased compared to the mag- reported reliable long lead interval startle inhibition when atten- nitude of the response without the lead stimulus, a phenomenon tion is directed away from the modality of the startle stimulus called prepulse inhibition (for a review of short lead interval (e.g., Anthony, Butler, & Putnam, 1978; Anthony & Graham, startle modification, see Blumenthal, 1999). At long lead inter- 1985; Hazlett, Dawson, Schell, & Nuechterlein, 2001; Lipp & vals (greater than 800 ms), the magnitude of the startle response Neumann, 2004; Neumann, 2002; Putnam & Meiss, 1980; is typically increased, although the exact effect may depend on Simons & Zelson, 1985; Zelson & Simons, 1986). Both of these the direction of attention (for a review of long lead interval startle findings are consistent with the modality-specific hypothesis. However, contrary to the modality-specific hypothesis, a large number of studies have reported reliable long lead interval startle We gratefully acknowledge the assistance of William C. Williams for facilitation even when attention is directed away from the providing computer software for off-line scoring of the eyeblink data and Devon Prewitt for help in data collection. modality of the startle stimulus (e.g., Bo¨hmelt et al., 1999; Lipp This research was conducted as part of a doctoral dissertation by & Hardwick, 2003; Lipp, Siddle, & Dall, 1997, 1998, 2000; G. L. Thorne under the direction of M. E. Dawson. It was supported in Neumann, Lipp, & McHugh, 2004; Vanman, Bo¨hmelt, Dawson, part by NIMH grants R01 MH46433 and K02 MH01086 to Michael E. & Schell, 1996). These studies used visual lead stimuli with Dawson. auditory or tactile startle stimuli and found long lead interval Address reprint requests either to: Michael E. Dawson, Department facilitation, with greater facilitation generally during attended of Psychology, Seeley G. Mudd Building, Room 501, University of Southern California, Los Angeles, CA, 90089-1061, USA; e-mail: task-relevant stimuli than during task-irrelevant stimuli. These [email protected]; or Gary L. Thorne, N5307-5 Argonne Lane, studies suggest that startle facilitation will be observed with any Spokane, WA 99212, USA; e-mail: [email protected]. attended stimulus, regardless of modality match or mismatch 498 Perceptual load and startle modification 499 with the startle stimulus. The reasons for the inconsistent cross- observe startle inhibition relative to unmodified baseline at the modality findings are not yet clear. highest levels of perceptual load. Lipp and colleagues proposed that modality-specific startle inhibition might occur only when task demand is high enough to require a strategy of strong localized orienting toward the loca- Method tion and modality of an expected stimulus (Lipp & Hardwick, 2003; Lipp et al., 1998). Neumann et al. (2004, experiment 3) Participants tested this hypothesis using a stimulus duration judgment task in Participants were 61 volunteers (48 women and 13 men) from which participants counted the number of longer duration lights undergraduate psychology classes at the University of Southern of one color (attended lead stimuli) while ignoring lights of a California who received course credit for participation. There different color (ignored lead stimuli). Task demand was manip- were no a priori selection criteria. All participants were informed ulated by altering the difference in the durations of the attended about the experiment and signed consent forms before partici- lights. In the low demand condition, the longer duration was 7 s pating. Research procedures and methods of obtaining informed and in the high demand condition the longer duration was 5.2 s, consent were approved by the University of Southern California both compared to 5 s standard duration. The eyeblink response Institutional Review Board. The data for 4 participants were to auditory startle stimuli at lead intervals of 3.5 and 4.5 s was excluded from analysis due to technical problems during collec- larger for attended than ignored lead stimuli; however, there tion. The data for 3 participants were excluded because their was no reliable effect of task demand. Startle response magnitude baseline startle responses were too small (median baseline was facilitated relative to the intertrial interval baseline for both response less than 1 mV) for dependable assessment of startle attended and ignored lead stimuli in both task demand condi- modification. This left a final sample of 54 participants (46 tions, with facilitation apparently being highly significant for women and 8 men) with usable data. the attended stimuli (see Figure 3 of Neumann et al., 2004). Neumann et al. concluded that the results did not support the Stimuli and Timing hypothesis that increased task demand in cross-modality condi- The visual stimuli and the sequence of presentation are illustrated tions will elicit a decrease in startle response. They noted, how- in Figure 1. All stimuli were presented against a dark gray back- ever, that their procedure increased task demand by increasing cognitive demand, and hypothesized that it might be necessary to increase task demand by increasing perceptual load (e.g., Lavie, 1995; Lavie & Tsal, 1994) in order to obtain cross-modality startle inhibition. Perceptual load refers to the complexity of the operations required for stimulus identification by a limited capacity early stage of perceptual processing (Lavie & De Fockert, 2003). Per- ceptual load is most commonly defined operationally in terms of display set size, the number of independent visual objects that must be perceptually processed in order to perform a task (Lavie & Tsal, 1994). Perceptual load is low when set size is small and perceptual load is high when set size is large. Low perceptual load tasks are hypothesized to use only part of available early perceptual processing capacity; therefore, all stimuli, including task-irrelevant stimuli, are processed, and selection occurs at a later cognitive stage. In contrast, high perceptual load tasks are hypothesized to exceed available early perceptual processing ca- pacity; therefore, selection must occur at an early stage and only task-relevant stimuli are processed. Lavie and colleagues have accumulated a substantial body of research consistent with this hypothesis. The purpose of the present experiment was to test the Neumann et al. (2004) hypothesis that high task demand, spe- cifically high perceptual load, will elicit cross-modality startle inhibition. We investigated the effect of four levels of perceptual load on long lead interval cross-modality startle modification in a within-subjects design. We used a visual delayed-matching-to- sample task in which subjects reported whether the pattern of dots in a target display matched the pattern of dots in a previous sample display. We manipulated perceptual load by varying set size, the numbers of dots in the displays. On some trials an au- ditory startle stimulus was presented 1200 ms following onset of the sample displays. If the hypothesis that long lead interval cross-modality startle inhibition requires high perceptual load is correct, we expected to obtain progressively decreasing response Figure 1. Sequence of visual displays presented on each trial. The to auditory startle stimuli as the number of dots increased, and to response display, a blank screen, is not illustrated. 500 G.L. Thorne, M.E. Dawson, and Anne M. Schell ground (luminance 5.76 cd/m2) on a computer monitor posi- Procedure tioned 1 m in front of the participant. There were four phases in this experiment. In the first phase, On each trial, there was a sequence of six stimulus displays. participants read and signed the consent form and listened to The first display, the cue display, presented a number in light gray recorded instructions regarding electrode placements and the need (luminance 122.3 cd/m2), indicating the number of dots in the to avoid unnecessary movement. In the second phase, the exper- upcoming sample and target. Thus, the cue display allowed par- imenter attached the electrodes for recording EMG and checked ticipants to anticipate the difficulty of the upcoming trial. The impedances. In the third phase, participants listened to additional second display, the warning display, consisted of 45 middle-gray recorded instructions describing the task, watched a demonstra- dots (luminance 43.65 cd/m2) in a tight circular cluster subtend- tion of the task, and performed a series of 12 practice trials, in- ing a visual angle of about 51. The third display, the sample cluding 2 startle stimuli. Participants were instructed to compare display, was identical to the warning display except that the dots the pattern of dots in the samples and targets and to say ‘‘same’’ if representing the sample were presented in a lighter gray tone the patterns were the same and ‘‘different’’ if the patterns were (luminance 122.3 cd/m2). Perceptual load was manipulated by different. In the fourth phase, the experiment proper was con- varying the number of lighter gray dots in the sample. There were ducted, which ran continuously without breaks until completed. 3 dots in the low load condition, 6 dots in the medium-low load As an incentive for good performance, participants were paid condition, 9 dots in the medium-high load condition, and 12 dots a $5.00 reward if they made no more than 15 wrong judgments or in the high load condition. The numbers of dots were determined a $3.00 reward if they made no more than 20 wrong judgments by pilot work to obtain a mean accuracy of about 90% in the low on the task. Participants who made more than 20 wrong judg- load condition and about 55%, slightly better than chance, in the ments were not paid. All participants received course credit re- high load condition. gardless of performance. The fourth display, the delay display, was identical to the There were six sets of eight trials per set. Within each set, there warning display. The fifth display, the target display, consisted of were two trials at each of the four levels of perceptual load in a cluster of the same number of dots presented at the same lu- random order. One trial at each level of perceptual load was minance as in the sample. On one-half of the trials, the pattern in probed with a startle stimulus during each set. Two additional the sample was altered by moving one of the dots to a different startle stimuli were presented randomly between trials in each set. position. The final display, the response display (not shown in Thus, there were a total of 48 trials and 36 startle stimuli. Figure 1), consisted of the dark gray background without any dots. During the response display participants reported whether Data Scoring, Reduction, and Analysis the pattern of dots in the target display was the same as, or The recorded EMG data were scored off-line by a computer pro- different from, the pattern of dots in the sample display. As gram that digitally integrated the raw EMG data at a 10-ms time shown in Figure 1, warning displays were presented for 500 ms, constant and calculated a startle amplitude score for each trial in sample displays were presented for 3000 ms, delay displays were which blink onset occurred within 21–120 ms of startle stimulus presented for 1000 ms, target displays were presented for 500 ms, onset and peak magnitude occurred within 150 ms of blink onset. and response displays were presented for 2000 ms. Scores for trials with an unstable prestimulus EMG baseline were Startle stimuli (50-ms bursts of 105 dBA white noise with a replaced with the score for the nearest trial in the same condition. nearly instantaneous rise time) were presented 1200 ms after the A total of five scores in the entire data set were replaced. Scores of onset of the sample display on half of the trials in each condition. zero were not replaced. The median of the baseline magnitude scores Thus, startle response was measured during the interval when was computed for each participant as a measure of baseline startle participants were processing the pattern of dots needed to per- response and was used to compute percentage change scores for form the task. each trial using the following formula: Percentage change 5 [(Mag- Intertrial intervals, during which the cue display for the next nitude during task – Median baseline)/Median baseline)] Â 100. trial was presented continuously, were 6 s in duration without, Using this formula, positive percentage change scores indicate and 12 s in duration with, a baseline startle stimulus. Startle startle facilitation and negative percentage change scores indicate stimuli used to measure baseline (unmodified by task perfor- startle inhibition. The median of the individual percentage mance) response were presented on randomly chosen trials at the change scores was computed for each participant, for each con- midpoints of the 12-s intertrial intervals. dition. Medians were used for these calculations because they are robust, unaffected by extreme scores (for a discussion of robust Measurement of Dependent Variables statistics, see Wilcox, 2005). The analyses described below were The startle eyeblink reflex was measured by the magnitude of the performed using these medians. Means of the median scores were electromyographic (EMG) response of the orbicularis oculi computed by condition across all participants for descriptive muscle, which controls the movement of the eyelid. This response purposes, and medians for each condition for each participant was recorded from two miniature (4 mm) silver-silver chloride were used in the statistical analysis. (Ag-AgCl) electrodes placed below the lower eyelid of the left eye Overall statistical analyses were performed using linear, and one large (8 mm) Ag-AgCl electrode placed behind the left quadratic, and cubic contrasts. Specific comparisons were per- ear for the common connection. Skin preparation and electrode formed using t tests. An alpha level of .05 was used for all sta- attachment followed standard procedures to obtain an imped- tistical analyses. For each specific comparison, an estimate of ance of not more than 10 kO, preferably less than 5 kO. The effect size (d; Cohen, 1988) was also computed. EMG signal was amplified, filtered (low-pass 500 Hz, high-pass 10 Hz), digitally sampled at 1000 Hz, and stored on a computer Results by Contact Precision Instruments equipment and software. The same equipment and software also controlled stimulus presen- Mean baseline startle response was 14.32 mVandmaximum tation and timing. baseline startle response was 88.04 mV. Because baseline startle Perceptual load and startle modification 501 stimuli were presented during the visual cue displays, and because and the 6-dot, t(53) 5 2.13, po.05, d 5 .29, but not the 3-dot, visual stimuli and their anticipation are known to suppress t(53) 5 0.73, p4.40, d 5 .10, conditions. The linear trend, endogenous activity of the orbicularis occuli muscle (e.g., F(1,53) 5 9.31, po.005, but not the quadratic, F(1,53) 5 2.29, Bernstein, Taylor, Weinstein, & Riedel, 1985; Goldstein, Bauer, p4.10, and cubic, F(1,53) 5 .08, p4.70, trends, was statistically & Stern, 1992; Van Boxtel, Damen, & Brunia, 1996), we com- reliable. pared the mean absolute EMG amplitude for the 500-ms seg- ment prior to the baseline startle stimulus to an identical 500 segments in each task condition. There were no reliable differ- Discussion ences between the baseline EMG mean during the ITI and the means prior to the 3-, 6-, 9-, and 12-dot conditions, nor were The accuracy results are consistent with the conclusion that task there reliable differences between the different dot conditions. demand, manipulated by perceptual load, increased as the Thus, any differential startle reactivity under the different ex- number of dots increased. The startle modification results are perimental conditions cannot be due to differences in ongoing consistent with the conclusion that, in this procedure, cross- EMG activity at the time of the startle stimulus. modality startle response at a long 1200-ms lead interval increases as perceptual load increases, rather than decreases as predicted. Accuracy The startle modification results are not consistent with the The accuracy data revealed the expected decrease in accuracy as modality-specific hypothesis prediction of startle inhibition for the number of dots increased (see Figure 2A). Accuracy was cross-modality procedures (Putnam, 1990). Startle facilitation reliably greater than chance in the 3-dot, t(53) 5 27.77, po.001, above baseline was statistically reliable in the 6-, 9-, and 12-dot d 5 3.78, 6-dot, t(53) 5 8.47, po.001, d 5 1.15, and 9-dot, conditions. There was no evidence of startle inhibition at any t(53) 5 5.50, po.001, d 5 .75, but not the 12-dot, t(53) 5 1.14, level of perceptual load. p4.20, d 5 .15, conditions. The linear trend was statistically re- The startle modification results are also not consistent with liable, F(1,53) 5 272.43, po.001. Accuracy for trials during the the hypothesis that high perceptual load will elicit long lead in- first half of the experiment did not differ from accuracy for trials terval cross-modality startle inhibition (Neumann et al., 2004). during the second half of the experiment for any condition, all ts Contrary to this hypothesis, progressive increases in perceptual o1.5, all ps 4.10. load elicited progressive increases, instead of decreases, in startle response. There was a slight decrease in mean startle magnitude Startle Modification between the 9-dot and 12-dot conditions, but this was not sta- The startle modification data revealed an increase in startle mag- tistically reliable. It might be argued that this decrease represents nitude as the number of dots increased (see Figure 2B). Startle the start of a progressive decrease in startle facilitation, which magnitude was reliably greater than baseline in the 12-dot, would, with additional increases in perceptual load, lead to star- t(53) 5 2.85, po.01, d 5 .39, 9-dot, t(53) 5 2.74, po.01, d 5 .37, tle inhibition; however, this seems unlikely. Accuracy in the 12-dot condition was not reliably above chance, which indicates that the task had become essentially impossible. We suggest that increasing perceptual load in the present ex- periment, which would have increased task demand, might have also increased arousal, which in turn might have elicited an in- crease in startle response above that in the baseline condition. Increased task demand is known to increase nonspecific arousal in a variety of tasks (Kahneman, Tursky, Shapiro, & Crider, 1968; Kramer & Weber, 2000). Graham (1975) suggested that increased arousal facilitates the startle response at long lead in- tervals. In the present experiment participants were able to an- ticipate the number of dots, therefore the difficulty, of each upcoming trial from the numbers in cue displays. Thus, the easy conditions (few dots) might have elicited less arousal and a smaller startle response, and the difficult conditions (more dots) might have elicited more arousal and a larger startle response. Therefore, in this discrete trial paradigm, the effects of increasing perceptual load may have been offset by the effects of increased arousal. The task in the 12-dot condition was so difficult that at least some participants might not have tried as hard as they did when the task was less difficult; therefore, arousal might have declined slightly. Most cross-modality studies, including the present one, have attempted to increase focused attention by increasing task de- mand; however, increased task demand is also likely to increase Figure 2. A: Mean percentage of correct task responses for each of the arousal, which facilitates the startle response. Thus, task diffi- conditions. Error bars indicate 95% confidence intervals. B: Mean culty manipulations may elicit an effect opposite to that intend- percentage change in startle magnitude from unmodified baseline for ed. This is consistent with the finding that, in the present study, each of the conditions. Error bars indicate one standard error of the startle response decreased as task demand, due to perceptual mean. load, decreased and was not reliably different from baseline in the 502 G.L. Thorne, M.E. Dawson, and Anne M. Schell easiest (3-dot) condition, which presumably elicited the lowest Simons, 1986). All of these studies obtained long lead interval level of arousal. cross-modality startle inhibition. These results are consistent with It is unlikely that the results of the present experiment can be the suggestion that long lead interval startle modification is mo- attributed to differential memory load across the task conditions. dality specific during sustained continuous tasks and fast-paced In the present experiment, startle stimuli were presented shortly discrete trial tasks, but not slow-paced discrete trial tasks, which is after sample display onset while the sample displays were still consistent with the suggestion of Lipp, Neumann, Pretorius, and visible; therefore, participants did not have to maintain the pat- McHugh (2003) and the results of Lipp and Neumann (2004). terns of dots in memory without reference to an actual stimulus at The results of the present experiment are consistent with the the time the startle stimulus was presented. hypothesis that, during a discrete trial task, long lead interval Vigilance tasks in which participants must maintain constant startle modification is affected by perceptual load but, contrary attention, either because the task is continuous (e.g., Neumann, to Neumann et al. (2004), the effect is facilitory rather than in- 2002) or because of a fast pace of discrete trial stimuli (e.g., hibitory. Our results are consistent with previous findings that Hazlett et al., 2001), may be qualitatively different in their effect long lead interval startle magnitude is facilitated during discrete on startle than tasks with discrete trials with slower pacing. Weare trial procedures when the lead and startle stimuli are in different aware of five studies that investigated the effect of continuous or modalities. However, long lead interval cross-modality startle fast-paced visual tasks on the startle response to auditory stimuli inhibition may increase as perceptual load increases in contin- (Hazlett et al., 2001; Lipp & Neumann, 2004; Neumann, 2002; uous or fast-paced performance tasks when early selection is Rissling, Dawson, Schell, & Nuechterlein, 2005; Zelson & more critical for task performance.

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