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Attentional blink modulation in a reaction time task: performance feedback, warning stimulus modality, and task difficulty
Biological Psychology 62 (2003) 115 /132 www.elsevier.com/locate/biopsycho
Attentional blink modulation in a reaction time task: performance feedback, warning stimulus modality, and task difficulty Ottmar V. Lipp *, Sascha A. Hardwick School of Psychology, University of Queensland, St. Lucia, Qld 4072, Australia Received 1 May 2002; accepted 25 July 2002
Abstract The present research investigated the effect of performance feedback on the modulation of the acoustic startle reflex in a Go/NoGo reaction time task. Experiment 1 (n /120) crossed warning stimulus modality (acoustic, visual, and tactile) with the provision of feedback in a between subject design. Provision of performance feedback increased the number of errors committed and reduced reaction time, but did not affect blink modulation significantly. Attentional blink latency and magnitude modulation was larger during acoustic than during visual and larger during visual than during tactile warning stimuli. In comparison to control blinks, latency shortening was significant in all modality conditions whereas magnitude facilitation was not significant during tactile warning stimuli. Experiment 2 (n /80) employed visual warning stimuli only and crossed the provision of feedback with task difficulty. Feedback and difficulty affected accuracy and reaction time. Whereas blink latency shortening was not affected, blink magnitude modulation was smallest in the Easy/No Feedback and the Difficult/Feedback conditions. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Attention; Blink startle modulation; Reaction time task
* Corresponding author. Tel.: /61-733-65-6385; fax: /61-733-65-4466 E-mail address: [email protected] (O.V. Lipp). 0301-0511/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 5 1 1 ( 0 2 ) 0 0 1 1 5 - 1
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1. Introduction Modulation of the human blink reflex has become increasingly popular as a methodology for the assessment of psychological processes such as emotion or attention (for a review see Dawson et al. 1999). This development reflects upon the unique advantages of this methodology, which is said not to interfere with the psychological process in question and can be employed in participants like criminal psychopaths or schizophrenic patients. Moreover, the blink modulation methodology can be employed in non-human animal subjects which has permitted a detailed study of the neural pathways involved in mediating startle modulation phenomena (Koch and Schnitzler, 1997). To date, these advantages have been exploited mainly in studies of emotional blink modulation (Bradley et al., 1990) and in studies of sensory gating deficits in schizophrenia (Cadenhead and Braff, 1999) whereas less is known about attentional blink modulation effects, in particular those observed at lead intervals of more than 2 s. Two basic procedures have been employed in the investigation of the effect of attention on blink modulation. In one type of experiment, participants are instructed to attend to the blink-eliciting stimulus itself or to a non-reflex eliciting stimulus that is presented simultaneously in a different sensory modality (e.g. Hackley and Graham, 1984). Attending to the blink-eliciting stimulus enhances blink reflexes whereas directing attention towards a different modality reduces reflex magnitudes. In the lead stimulus paradigm, blink reflexes are elicited during the processing of a lead stimulus, which captures attention due to task instructions or its inherent interest value. In these studies, changes in blink latency and magnitude relative to blinks elicited during a control stimulus or during stimulus free intervals are taken as indicators of the extent of attentional processing of the lead stimulus. The extent and direction of the blink latency and magnitude modulation observed is dependent on the time interval between lead stimulus onset and blink stimulus onset, the lead interval, as well as on the modality of both stimuli (for reviews see Filion et al., 1998; Dawson et al., 1999). The latency of blinks elicited during attended stimuli is usually shortened relative to control blinks whereby the shortening is largest at lead intervals of more than 2 s. Blink magnitude is inhibited relative to control blinks at lead intervals of less than 400 ms, and facilitated at lead intervals of several seconds (Graham, 1975). The directional influence of attention to the lead stimulus on blink magnitude modulation is currently a matter of debate, with a particular emphasis on the role of stimulus modality. Studies that employed lead and reflex stimuli from the same modality found enhancement of both, magnitude inhibition and facilitation during attended relative to ignored lead stimuli (e.g. Filion et al., 1994). Results from studies assessing attentional blink modulation during lead stimuli from different modalities have yielded less consistent results. Findings by Anthony and Graham (1985) and Putnam and colleagues (for a review see Putnam, 1990) that attending to a lead stimulus in a different sensory modality reduced blink magnitude at lead intervals longer than 2 s led to the notion that attentional blink magnitude modulation is modality specific. Attention will enhance blink magnitude if directed towards the
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modality in which the reflex eliciting stimulus is presented and will reduce blink magnitude if directed to a different modality. More recent research, however, has produced results that are not consistent with this interpretation. Lipp et al. (2000b) found enhancement of the magnitude of acoustic blinks during visual and acoustic change stimuli in a habituation paradigm (see also Bohlin et al., 1981). Using the same counting and discrimination paradigm as employed by Filion et al. (1994), Lipp et al. (1998) and Bo¨hmelt et al. (1999) found larger acoustic blink magnitude during to-be-attended than during to-be-ignored visual and acoustic lead stimuli. However, Lipp et al. (1998) failed to find differential blink modulation during tactile lead stimuli. Finally, Lipp et al. (2000a), Experiment 3 failed to replicate the modality specific blink modulation effects that had been reported by Putnam and colleagues in a reaction time task (Putnam, 1990). Rather, they found facilitated acoustic blink reflexes during visual and acoustic warning stimuli. The question of whether attentional blink modulation at long lead intervals is modality specific is significant for two reasons. First, modality specificity is invoked as the criterion to distinguish emotional from attentional blink modulation phenomena (Bradley et al., 1990). Blink startle is enhanced during unpleasant and inhibited during pleasant lead stimuli regardless of whether the lead stimuli are visual (Lang et al., 1990), acoustic (Bradley and Lang, 2000), or olfactory (Miltner et al., 1994) and whether the reflex stimuli are acoustic or visual (Bradley et al., 1990). Assuming that attentional modulation is modality specific, any enhancement of blink startle in a cross modality paradigm has to reflect emotion rather than any other characteristic of the lead stimulus. Demonstrations of modality unspecific attention effects, however, render this simple rationale invalid. Second, modality specificity provides information as to the nature of the attentional process involved in a particular experimental procedure. If stimulus modality affects the extent of attentional blink modulation, then the point within the neural network at which the effect of the reflex eliciting stimulus is modulated has to be located before the junction of inputs from different sensory modalities, an example for an early selection process. If, however, increased attentional processing facilitates reflexes regardless of stimulus modality, then the location at which attention affects reflexive responding has to be after the point of cross-sensory integration (Graham, 1992). Both issues seem sufficient to justify further studies of the effect of stimulus modality on attentional startle modulation. Among the procedures used to investigate attentional blink modulation at long lead intervals, the reaction time paradigm seems to have provided rather equivocal results. Putnam and her colleagues (Putnam, 1990) reported blink magnitude data that are consistent with a modality specific account. In these experiments, participants were presented with acoustic, visual, or tactile warning stimuli in a simple reaction time task or a Go/NoGo discrimination task. Blinks were elicited by acoustic stimuli in all experiments. Relative to control blinks elicited during nowarning stimulus conditions, blink magnitude was facilitated during acoustic warning stimuli, but inhibited during tactile and visual warning stimuli. Moreover, the extent of blink modulation, facilitation or inhibition respectively, increased towards the offset of the warning stimulus. Lipp et al. (2000a), Experiment 3 aimed
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to replicate these findings by presenting participants with stimulus compounds that consisted of two visual or two acoustic stimuli. Participants were instructed to press a button as quickly as possible if one element of the stimulus compound was terminated before the second, but not in the reverse case. In addition, task difficulty was varied by using different lengths of stimulus offset asynchrony. Contrary to the predictions from a modality specific account, acoustic blinks were facilitated during acoustic and visual warning stimuli and were larger in the Difficult than in the Easy condition regardless of warning stimulus modality. This discrepancy across studies may reflect procedural differences. First, Putnam and colleagues employed trial-by-trial response feedback in order to maintain the attention and co-operation of their participants. No feedback was provided in the Lipp et al. (2000a) study, although a decrease in reaction time across trials seemed to indicate that participants stayed on task. Second, Lipp et al. employed visual stimuli as non-acoustic warning signals, whereas tactile warning signals were used in the modality mismatch conditions of the majority of the studies conducted by Putnam and colleagues. Using tactile lead stimuli, Lipp et al. (1998) have failed to find attentional blink modulation in a discrimination and counting task whereas acoustic blink was facilitated during acoustic and visual to-be-attended lead stimuli. This pattern of result may indicate differences across lead stimulus modalities that, however, do not follow the pattern predicted by modality specific accounts of attentional startle modulation. Experiment 1 was designed to replicate and extent the findings reported by Lipp et al. (2000a), Experiment 3 and assessed attentional startle modulation during tactile as well as acoustic and visual warning stimuli. Provision of trial-by-trial performance feedback was varied in a 2 /3 factorial between subject paradigm. A modality specific account of attentional blink startle modulation would predict facilitation of the acoustic blink during acoustic warning stimuli and inhibition during visual and tactile warning stimuli. Facilitation and inhibition are expected to be larger in the Feedback than in the No Feedback conditions. A modality non-specific account predicts blink facilitation during warning stimuli regardless of stimulus modality with larger facilitation in the Feedback condition.
2. Method 2.1. Participants One-hundred-thirty-eight undergraduate students aged 16 /40 years (Mean /19.7 years) participated for course credit and provided informed consent. Participants were assigned upon arrival at the laboratory to one of six groups until 20 participants in each group provided complete data. The data of 18 participants, 1 from Groups Acoustic/No Feedback and Visual/Feedback, 3 from Group Acoustic/ Feedback, 4 from Groups Visual/No Feedback and Tactile/No Feedback and, 5 from Group Tactile/Feedback were rejected due to an excessive number of missing data. The proportion of women and men varied slightly across groups (Group
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Acoustic/No Feedback: 14:6, Group Acoustic/Feedback: 12:8, Group Visual/No Feedback: 14:6, Group Visual/Feedback: 14:6, Group Tactile/No Feedback: 12:8, Group Tactile/Feedback: 11:9). 2.2. Apparatus Orbicularis oculi electromyogram (EMG) was recorded with two domed Ag/AgCl miniature electrodes 4 mm in diameter filled with a standard electrolyte (Surgicon E10). One electrode was placed under the pupil of the left eye and the second was placed lateral at a distance of 1 cm edge-to-edge. A ground electrode was attached to the participants’ left forearm. Raw EMG was amplified with a Grass 7P3C AC preamplifier (0.5 amplitude high-pass cut-off of 10 Hz; low-pass cut-off of 3000 Hz; calibration: 100 mV/cm pen deflection) and displayed on the polygraph. The raw EMG was digitised and sampled on-line with an IBM (486)-compatible computer. A sampling rate of 1000 Hz was used in a time window from 100 ms prior to onset to 400 ms after offset of blink-eliciting stimuli. Pure tones of 800 and 1200 Hz served as warning stimuli in Group Acoustic. Tones were presented at an intensity of 70 dBA with an instantaneous rise-time. The visual warning stimuli in Group Visual were generated with a red and a green frosted bulb (Osram, 40 W) placed behind a small screen (15 /20 cm) set into the wall of the experimental room. The screen was located 140 cm from the participant at eye level. If switched on, the bulbs diffusely illuminated parts of the screen’s surface in a manner that a red and a green patch were visible. The vibration units of two Mowat sensor that were taped 3 cm apart to the back of participants’ left hand generated the tactile stimuli. Each stimulator was driven by a custom-built power supply at a frequency of 50 Hz, thus, the tactile sensation experienced at the two locations was identical. The tone, light, and tactile stimuli have been used previously in experiments investigating habituation of the orienting response and were found to elicit equivalent electrodermal responses. This was taken as an indication that the stimuli did not differ in salience. A burst of white noise of 105 dBA, a duration of 50 ms and an instantaneous rise-time served as the blink-eliciting stimulus. Acoustic stimuli were produced by a custom-built tone and noise generator and presented via stereophonic headphones (Sennheisser HD25). The stimulus sequence, stimulus durations, and intertrial intervals were controlled by the computer, which also sampled the EMG. All participants were asked to operate a micro switch with their right thumb on target trials. Participants in the Feedback condition were informed that response feedback would be provided on a computer screen, which was located on the left side of the projection screen at eye level. For the first 3 reaction time trials, feedback consisted of the message ‘Well done/Your reaction time was XXX ms/You have earned 30 c/Keep up the good work’. For the remaining reaction time trials, the nature of the message depended on the reaction time performance. If the reaction time was slower than the average reaction time across the three preceding trials, the message ‘Your reaction time was XXX ms/Your reaction time has not improved/No reward on this trial/Your total earnings so far are YYY c’. If the reaction time was
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by less than 20 ms faster than the average on the preceding three trials the message read ‘Well done/Your reaction time was XXX ms/It has improved by XXX ms/You have earned 10 c/Your total earnings total so far YYY c’. If the improvement was more than 20, but less than 40 ms, the phrase ‘Well done’ was substituted with ‘Very well done’ and a 30 c reward was given whereas the phrase ‘Excellent’ was displayed and a reward of 50 c was given if the improvement was more than 40 ms. Response feedback commenced 2 s after the participant’s response and was presented for 4 s. Participants were not informed about the relationship between performance and rewards. 2.3. Procedure Participants were seated in a semi-reclining chair and the experiment was monitored from an adjoining room. After electrode attachment, participants received three presentations of the blink-eliciting stimulus to check the placement of the EMG electrodes and one example of a target and of a non-target stimulus. Each participant was presented with 45 compound stimuli consisting either of two pure tones (Groups Acoustic), two lights (Groups Visual), or two vibrations (Groups Tactile). One stimulus was switched off after 6 s whereas the second component of the compound stayed on for another 500 ms (500 ms offset asynchrony). On 23 of the trials one component was longer, e.g. the high tone lasted 6500 ms and the low tone 6000 ms, whereas the second was longer on the remaining trials, e.g. the high tone lasted 6000 ms and the low tone 6500 ms. Half the participants in each modality group were asked to press the micro switch as quickly as possible whenever the first component was longer than the second, e.g. whenever the high tone outlasted to low tone, but not if the second component was longer than the first. Instructions were reversed for the other half. Participants were presented with 63 startle eliciting stimuli, 45 during lead stimuli and 18 during intertrial intervals. Stimulus probes were presented at 5 different lead intervals, 3.5, 4.5, 5.0, 5.3, and 5.6 s after stimulus onset. The warning stimuli were arranged into 9 blocks of 5 trials. Within each block, each lead interval was used once, and two intertrial interval startles were presented. Intertrial interval probes were not presented within 6 s after stimulus offset or prior to stimulus onset. A different random order of lead intervals was used within each block. By rotating the lead intervals in a Latin square, five different trial sequences were created. Intertrial intervals varied between 15, 20, and 25 s. 2.4. Scoring, response definition and statistical analysis Digitised EMG activity was rectified offline and filtered and integrated with a Butterworth low pass filter (time constant 80 ms). Measures of latency and magnitude were derived from the integrated signal. Response latency was the time between blink stimulus onset and response onset, defined as the point at which 10% of the maximum slope of the integrated curve was reached. Response magnitude was defined as the difference in mV between the maximum of the integrated response curve within 200 ms after blink stimulus onset and the level seen at response onset. If
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no response was detectable, a magnitude of 0 was recorded and latency was scored as missing. A trial was discarded if (1) the baseline EMG was not stable within 100 ms prior to the startle stimulus, i.e. varied by more than 50 A/D units, or (2) the onset of a response was not within 20/60 ms after the onset of the blink stimulus. The data from a participant were rejected if (1) more than half the responses at any lead interval were discarded trials, or if (2) the number of discarded trials or 0 responses exceeded one third of all trials. Using this set of criteria the blink data from 18 participants were rejected due to insufficient data. The main reason for rejection was the presence of an excessive number of discarded responses. Magnitude and latency of blinks elicited during stimuli were averaged across blocks of three trials at each lead interval and intertrial interval startle magnitude and latency were averaged into three trial blocks. Magnitude and latency of blink startle elicited during warning stimuli were converted into change scores prior to statistical analyses, i.e. difference values (stimulus/intertrial interval) were calculated as an estimate of latency modulation and percentage change scores were calculated as an estimate of magnitude modulation. Percentage change scores were employed to reduce the impact of individual differences in startle magnitude. The number of trials containing errors, i.e. premature responses on target trials or any response on non-target trials, was subjected to a 3 /2 (Modality /Feedback) ANOVA. Response times shorter than 100 ms were rejected and scored as missing. Reaction time to target stimuli was arranged into 6 blocks, 5 blocks of 4 trials and one block that contained 2 or 3 trials, and analysed in a 3 /2 /6 (Modality / Feedback /Block) ANOVA. The 95% confidence intervals of the means were inspected to determine whether blink magnitude and latency during stimuli were modified significantly. Blink modulation was regarded as significant if 0 was outside the confidence interval. Differences in blink latency and magnitude change scores were assessed in 3/2 /5 /3 (Modality /Feedback /Lead interval /Block) factorial ANOVAs. Multivariate F -values are reported for all main effects and interactions that involved within participant factors with more than two levels. Multiple comparisons of means were made with t-tests. The critical values for these t-tests were derived from Sidak’s tables to protect against the accumulation of a-error (Rohlf and Sokal, 1981). Level of significance was set at 0.05 for all statistical analyses.
3. Results 3.1. Response time Participants in Groups Acoustic and Tactile committed more errors in the Feedback condition than in the No Feedback condition (Acoustic: 10.8 [SEM: 1.0] vs. 4.8 [1.0], t (114) /3.62; Tactile: 17.2 [1.8] vs. 7.0 [0.8], t(114) /6.16), whereas there was no difference in Group Visual (4.2 [1.1] vs. 2.9 [1.1], t B/1.). This
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impression was confirmed by the ANOVA, which yielded main effects for Modality, F (2,114) /27.02, P B/0.001, and Feedback, F (1,114) /37.29, P B/0.001, as well as a Modality /Feedback interaction F (2,114) /7.24, P B/0.001. Reaction time was shorter in the presence of performance feedback, 439 ms, than without feedback, 581 ms, F (1,114) /21.33, P B/0.001. Stimulus modality affected reaction time in that it was slower in the tactile condition than in the visual and the acoustic condition, all t (218) /3.0, and slower in the acoustic condition than in the visual condition on Blocks 1, t(218) /4.98, and 4, t(218) /3.08. The analysis yielded main effects for Modality, F (2,114) /45.36, P B/0.001, and Block, F (5,110) /29.31, P B/0.001, and a Modality /Block interaction, F (10, 218) / 2.61, P B/0.01.
3.2. Control blinks Latency of intertrial interval blinks increased across blocks, F (2,113) /6.23, P / 0.003, and was longer on Blocks 2, 41.2 ms, and 3, 41.3 ms, than on Block 1, 39.9 ms. Magnitude of intertrial interval blinks decreased across all three blocks, F (2,113) / 82.92, P B/0.001, (Block 1: 177 mV; Block 2: 128 mV; Block 3: 112 mV, all t/4.5). Moreover, it differed across experimental conditions, Modality /Feedback interaction, F (2,114) /4.83, P /0.01. Post hoc comparisons revealed that intertrial interval blink magnitude was larger in the Tactile/Feedback group than in the Tactile/No Feedback group, 194 vs. 111 mV, t(114) /3.81, but there was no difference between feedback groups in the other Modality conditions, all t B/2.0.
3.3. Blink magnitude modulation The extent of blink magnitude modulation is shown in the upper panel of Fig. 1. In comparison to intertrial interval blinks, blink was facilitated at all lead intervals in the Acoustic/Feedback, Acoustic/No Feedback, and Visual/No Feedback conditions, and at the 4.5, 5.3, and 5.6 s lead intervals in the Visual/Feedback conditions. No significant blink magnitude facilitation was found in either group trained with tactile stimuli. Blink magnitude modulation differed across the three Modality conditions, F (2,114) /47.58, P B/0.001, and across lead intervals, F (4,111) /3.47, P /0.01. Post hoc tests confirmed that magnitude modulation in the acoustic condition exceeded modulation in the visual condition, t(114) /6.47, which in turn was larger than in the tactile condition, t(114) /3.08. Across modality and feedback conditions, blink magnitude modulation was larger at the 5.6 s position, t(111) /3.88, and at the 5.3 s position, t(111) /2.93, than at the 3.5 s position. Although suggested in Fig. 1, there was no differential effect of response feedback on blink magnitude modulation across the three Modality conditions. All interactions involving the factors Feedback and Modality were non-significant, all F B/2.0.
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Fig. 1. Mean blink magnitude modulation (upper panel) and blink latency shortening (lower panel) during Experiment 1 as a function of Group and lead interval (Vertical bars represent standard errors of the means. A/N, Acoustic/No Feedback; A/F, Acoustic/Feedback; V/N, Visual/No Feedback; V/F, Visual/ Feedback; T/N, Tactile/No Feedback; T/F, Tactile/Feedback).
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3.4. Blink latency modulation Blink latency shortening is depicted in the lower panel of Fig. 1. In comparison to the latency of intertrial interval blinks latency shortening was significant at each lead interval in all of the six experimental groups. The extent of blink latency shortening differed among Modality conditions, F (2,114) /37.44, P B/0.001, and across lead intervals, F (4,111) /7.45, P B/0.001, and there was also a Modality /Lead interval interaction, F (8,224) /2.21, P B/0.05. Post hoc comparison confirmed that the extent of blink latency shortening was larger in the acoustic than in the visual condition and larger in the visual than in the tactile condition at each lead interval, all t(224) /3.30. The interaction reflects the fact that in the acoustic condition, latency shortening was larger at the 5.6 s position than at the 3.5 s position, t (224) / 3.99, and in the visual condition, was larger at the 5.6 s position, t(224) /3.57, and at the 5.3 s position, t(224) /4.48, than at the 3.5 s position and was larger at the 5.3 s position than at the 4.5 s position, t(224) /3.20.
4. Discussion The provision of performance feedback and monetary reward resulted in a reduction of the reaction time across modality conditions and an increase in the number of errors committed, at least in Groups Acoustic and Tactile. The increase in errors, i.e. button presses to blink-eliciting stimuli or on non-target trials, may reflect an increased readiness to respond. Alternatively, one may argue that the increase of incorrect presses reflects a speed accuracy trade-of in that increased error rates had no negative consequences. This seems unlikely as such a change in response strategy would result in an increase in button presses on non-target trials, however, most of the errors consistent in presses to the startle probes. Moreover, it is difficult to conceive as to why this change in response strategy was restricted to participants in the acoustic and tactile conditions. Taken together, the behavioural measures indicate that the provision of response feedback affected participants’ involvement in the task. There was, however, no clear evidence for an effect of performance feedback on blink latency or magnitude modulation. Consistent with previous findings blink modulation was affected by the modality of the warning signal. Blink latency shortening and magnitude facilitation were larger during acoustic than during visual and larger during visual than during tactile warning stimuli. Blink latency shortening increased towards the offset of the warning stimulus during acoustic and visual, but not during tactile warning stimuli. A similar increase in blink magnitude facilitation across lead intervals was not affected by stimulus modality. As the acoustic, visual and tactile warning stimuli employed in Experiment 1 were not matched for intensity, one might argue that the difference in blink modulation observed across modality conditions reflects intensity effects. This seems unlikely, however, as more recent studies from our laboratory, which employed the same warning stimuli at intensities matched in a psychophysical procedure yielded the same pattern of results in a discrimination and counting task (Lipp, Neumann,
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Pretorius and McHugh, in press) and during passive attention (Lipp et al., 2003). The observation that in a reaction time task background orbicularis oculi EMG is reduced more during a visual than during an acoustic warning stimulus may provide an alternative explanation for the difference found between Groups Visual and Acoustic (Van Boxtel et al., 1996) 1. This may indicate that the overall larger blink reflex modulation during acoustic than during visual warning signals observed here and in previous studies reflects differences on the level of motor output rather than effects of attention. Such an interpretation is consistent with the observation by Lipp et al. (1998), Experiments 1 and 2 who relative to visual task stimuli, found enhanced blink facilitation during acoustic task-relevant and task-irrelevant stimuli. However, more research exploring the relationship between changes in background EMG and blink magnitude seems required before firm conclusions can be drawn. The present findings resemble the attentional blink modulation results reported previously (Lipp et al., 1998, Lipp et al., 2000a, Experiment 3). Similar to the results observed during a discrimination and counting task (Lipp et al., 1998), there was no evidence for attentional modulation of the magnitude of an acoustic blink reflex during tactile lead stimuli. There was, however, significant blink latency shortening during tactile warning stimuli in the present study whereas there was no such latency reduction in the discrimination and counting task. This discrepancy across studies may reflect the larger number of participants presented with tactile stimuli in the present study (40 vs. 24) as the same stimuli were used in both experiments and the overall extent of latency shortening was similar across experiments. Moreover, the finding of significant latency shortening during tactile warning stimuli together with evidence that the tactile stimuli can support blink magnitude modulation when used as conditional stimuli in an aversive Pavlovian conditioning procedure (Lipp et al., 1998) indicates that the failure to find blink magnitude modulation in the present study does not indicate that tactile stimuli are ineffective as prepluse stimuli. Similar to the results of a previous reaction time experiment (Lipp et al., 2000a, Experiment 3), there was no evidence for inhibition of acoustic blink reflexes during visual attention demanding stimuli. Rather, blinks were facilitated during visual stimuli in particular at the late lead intervals at which blink inhibition had been largest in previous reports of modality specific attentional blink modulation (Putnam, 1990). Similarly, no blink magnitude inhibition was found during tactile warning stimuli. The present results deviate, however, from those reported by Lipp et al. (2000a), Experiment 3 in that no significant effect was found for a manipulation thought to enhance attention to the warning stimuli. Lipp et al. (2000a), Experiment 3 had varied task difficulty and found larger acoustic blinks during visual warning signals in the Difficult than in the Easy condition. In the present experiment, no significant difference emerged between the feedback conditions and blinks during visual warning stimuli were numerically smaller in the Feedback than in the No Feedback condition, a result consistent with a modality non-specific account of attentional startle modulation. On the other hand, this difference across experiments may reflect 1
We are grateful to one anonymous reviewer for pointing us towards this explanation.
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the fact that blink magnitude modulation in Group Visual/No Feedback (mean across lead intervals: 41%) was somewhat larger, although not significantly so, than in the corresponding group in Lipp et al. (2000a), Experiment 3; Group Light/Easy; mean across lead intervals 26%, although both groups had been presented with the same stimuli. In order to resolve this apparent discrepancy Experiment 2 was designed to assess the effects of task difficulty and performance feedback on attentional modulation of acoustic blinks during visual warning stimuli.
5. Experiment 2 Experiment 2 assessed the effects of task difficulty and performance feedback on modulation of the acoustic blink reflex in a 2 /2 factorial design. The manipulation of task difficulty was the same as that used by Lipp et al. (2000a), Experiment 3 in that stimulus offset asynchrony was 500 ms in the Easy condition and 50 ms in the Difficult condition. Thus, Group Easy/No Feedback replicated Group Light/Easy from Lipp et al. (2000a), Experiment 3 and Group Visual/No Feedback from Experiment 1. Group Difficult/No Feedback replicated Group Light/Diff from Lipp et al. (2000a), Experiment 3 and Group Easy/Feedback replicated Group Visual/ Feedback from Experiment 1.
6. Method 6.1. Participants Eighty undergraduate students (Mean age 19.3 years; range 17/33) volunteered participation in exchange for course credits and provided informed consent. Participants were assigned to four groups upon appearance at the laboratory until each group contained 20 participants who provided complete data. This procedure resulted in a slightly unbalanced sex distribution across groups (Easy/No Feedback: 10:10 [female: male]; Easy/Feedback: 16:4; Diff/No Feedback: 9:11; Diff/Feedback; 12:8). Data from an additional six participants were rejected prior to the analyses due to insufficient data (2 in Group Easy/Feedback and 4 in Group Difficult/No Feedback). 6.2. Apparatus and procedure Experiment 2 was conducted in a laboratory similar to the one used for Experiment 1. The recording apparatus and the scoring procedures were the same as used in Experiment 1. Only visual warning stimuli were used and the length of the stimulus offset asynchrony was varied across groups. It was 500 ms in the easy condition and 50 ms in the difficult condition. The statistical analyses were conducted analogous, adapted for a 2/2 (Difficulty /Feedback) between subject design.
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7. Results 7.1. Response time Participants in Group Diff/Feedback committed more errors (8.1 [SEM/1.4]) than did the participants in the other three groups (Easy/No Feedback: 1.5 [0.2]; Easy/Feedback: 3.2 [0.7]; Diff/No Feedback: 2.5 [0.5]), all t(76) /4.20, as indicated by main effects for Difficulty, F (1,76) /12.97, P B/0.01, and Feedback, F (1,76) / 19.85, P B/0.001, and a Difficulty /Feedback interaction, F (1,76) /5.96, P B/0.05. Reaction time was faster in the Feedback than in the No Feedback conditions, 392 vs. 537 ms, F (1,76) /10.80, P B/0.01, and faster in the Easy than in the Difficult condition, 380 vs. 549 ms, F(1,76) /14.69, P B/0.001. Reaction time declined across blocks of trials, F (5,72) /11.84, P B/0.001, at rates that differed between the two difficulty conditions, Difficulty /Block interaction, F(5,72) /2.82, P B/0.05. The interaction reflects that in the Difficult condition, reaction time during Block 1 was slower than during all other blocks, all t (72) /3.90, whereas in the Easy condition reaction time during Block 1 was slower only than reaction time during the last block. 7.2. Control blinks The latency of control blinks elicited during interstimulus intervals was longer on Block 3 than on Block 1, 42.3 vs. 40.5 ms, F (2,75) /7.13, P B/0.01, and control blink magnitude decreased across blocks of trials, F (2,75) /33.38, P B/0.001, Block 1: 170 mV; Block 2: 128 mV; Block 3: 105 mV, all t(75) /3.50. 7.3. Blink magnitude modulation The extent of blink magnitude modulation across the five lead intervals is shown in Fig. 2, upper panel. Blink magnitude was facilitated significantly at all lead intervals in Groups Easy/No Feedback, Easy/Feedback, and Difficult/No Feedback, but at none of the lead intervals in Group Difficult/Feedback. Blink magnitude facilitation did not seem to differ across the four groups at the three early lead intervals, but seemed larger in Group Difficult/No Feedback than in the other groups at the 5.3 and 5.6 s lead intervals. This impression was confirmed by the analysis which yielded Difficulty /Position, F (4,73) /2.69, P B/0.05, and Difficulty /Feedback /Block interactions, F (2,75) /5.24, P B/0.01, as well as a marginal interaction Difficulty /Feedback /Position, F (4,73) /2.03, P /0.099. The Difficulty /Feedback /Block interaction reflects the fact that blink magnitude modulation in Group Difficult/Feedback was smaller than modulation in Groups Easy/Feedback and Difficult/No Feedback on Block 3, all t(75) /3.20, but not on Blocks 1 and 2. Although not significant on the set level, the Difficulty / Feedback /Position interaction was followed up with post hoc t-tests as it might be of theoretical interest. Multiple comparisons revealed no differences at lead intervals 3.5, 4.5, and 5.0 s. At the 5.3 s lead interval, blink modulation in Group
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Fig. 2. Mean blink magnitude modulation (upper panel) and blink latency shortening (lower panel) during Experiment 2 as a function of Group and lead interval (Vertical bars represent standard errors of the means. E/N, Easy/No Feedback; E/F, Easy/Feedback; D/N, Difficult/No Feedback; D/F, Difficult/ Feedback).
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Difficult/No Feedback was larger than blink modulation in Groups Difficult/ Feedback, t (73) /4.26, and Easy/No Feedback, t(73) /3.81, whereas at the 5.6 s lead interval blink modulation in Group Difficult/No Feedback was larger than blink modulation in Group Difficult/Feedback, t(73) /4.58 and there was a trend towards larger blink modulation in Group Difficult/No Feedback than in Group Easy/No Feedback, t(73) /3.18, P B/0.10. 7.4. Blink latency modulation Blink latency shortening was significant at all lead intervals in all four groups. As can be seen in Fig. 2, lower panel, the extent of blink latency shortening did not differ across groups, but increased towards the offset of the warning stimulus, F (4,73) /4.68, P B/0.01. Post hoc comparisons confirmed that blink latency shortening at the 5.3, 4.9 ms, and 5.6 s lead intervals, 5.2 ms, was larger than at the 3.5 s lead interval, 3.8 ms, both t (73) /3.10.
8. Discussion and general discussion As in Experiment 1, the reaction time data confirm the validity of the between groups manipulations. Reaction time was longer during the Difficult condition and shorter in the presence of performance feedback. Blink latency modulation was enhanced toward the offset of the warning stimulus, but did not vary as a function of the between group manipulation. This result is consistent with the findings of Experiment 1, in which latency shortening in Groups Visual did not differ across feedback conditions. Blink magnitude, however, was affected by the task conditions. Replicating the results of Lipp et al. (2000a), Experiment 3 blink magnitude facilitation at the two lead intervals closest to stimulus offset was larger in Group Difficult/No Feedback than in Group Easy/No Feedback. Moreover, there was no difference between Groups Easy/Feedback and Easy/No Feedback confirming the results of Experiment 1. Both results are consistent with a modality non-specific account of attentional blink modulation. The finding of smaller blink magnitude modulation in Group Difficult/Feedback than in Group Difficult/No Feedback, however, does not fit with such an account. Rather it seems in support of a modality specific account. The results of Experiment 2 seem to suggest that the effect of stimulus modality on attentional blink magnitude modulation is mediated by the demands that a particular task poses. Borrowing from Sokolov (1963) distinction between generalised and localised orienting, one may argue that in a procedure that requires passive attention, such as a habituation paradigm, or during a task that is simple or does not involve the participants, generalised orienting predominates that results in a modality unspecific enhancement of stimulus input. Increasing the task demands, either by enhancing task difficulty or by providing motivational incentives, may initially enhance generalised orienting until a level of processing demand is reached that requires the more focussed, selective stimulus processing seen during localised
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orienting. This processing style is associated with an attenuation of stimulus input from modalities other than that of the target stimulus in order to avoid impairment of task performance. Thus, the differences in attentional startle modulation reported by Putnam (1990) and in previous studies from our laboratory may reflect the predominance of localised or generalised orienting respectively. The question remains, however, as what determines the transition from one mode of orienting to the other. The present and previous data seem to indicate that the level of task difficulty alone is not sufficient to determine the mode of processing. In the present study, there was no significant correlation between reaction time and the extent of blink magnitude modulation for any lead interval, all r2 B/0.05. Anthony and Graham (1985) found results indicative of a modality specific, localised processing mode in a habituation procedure in which participants were presented with interesting or dull acoustic and visual lead stimuli without any task requirement. One may argue that this finding reflects characteristics of the stimuli used, music box melodies vs. tones and pictures of faces vs. white light. Alternatively, the finding may reflect on the sample employed in that the pattern was significant only in 16-week-old infants, but not in adults. However, there are other reports of results consistent with a modality specific processing mode from procedures that seemed to impose a low demand on participants. Anthony and Putnam (1985) reported inhibition of acoustic blink during a tactile warning signal at the offset of which participants were required to press a button. The reaction times observed in this task averaged at about 340 ms, a performance level that is compatible with the one seen in the Easy conditions of the present experiment. Thus, it seems that a simple reaction time task may be sufficient to trigger a modality specific processing mode. The reaction time studies reviewed by Putnam (1990) have in common that they employed performance feedback, although the feedback procedure was less elaborate than the one used in the present research, at least in some studies. Anthony and Putnam (1985), for instance, employed two light stimuli as feedback, one indicating slow reaction time and the other premature responses. It may be that enhanced task involvement due to the provision of performance feedback, or an enhanced arousal mediated by it, is a prerequisite for engaging in a modality specific processing style. This contention can be tested in future studies by employing a wider range of levels of task difficulty while varying the extent of performance feedback provided. It should be noted, however, that even in Group Difficult/Feedback, which seemed to be most conducive for engagement in a localised processing mode, no inhibition of blink magnitude was observed. Moreover, contrary to the results reported by Anthony and Putnam (1985), blink latency was reliably shortened even though there was no significant magnitude facilitation. The second interesting finding to emerge from the present study is the lack of attentional blink startle magnitude modulation observed during tactile warning stimuli at the long lead intervals used here (for evidence of attentional blink modulation during tactile lead stimuli at short lead intervals see Flaten and Elden, 1999). This result replicates findings in the discrimination and counting task where no difference in probe reflex magnitude was found between task-relevant and task-
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irrelevant tactile stimuli (Lipp et al., 1998). This lack of differential blink magnitude modulation coincided with a level of performance on the discrimination and counting task and an extent of differential electrodermal responding that did not differ from the one observed with visual or acoustic task stimuli. Moreover, differential blink magnitude modulation was observed when the tactile stimuli were employed as conditional stimuli in an aversive Pavlovian conditioning procedure. These findings imply that tactile stimuli were suitable signals and stimulated the circuits involved in orienting as indexed by autonomic responses. In addition, tactile stimuli can activate the neural circuitry that underlies emotional blink modulation, but not the circuitry that underlies attentional startle modulation. Thus, the present results suggest that further studies including tactile warning stimuli can teach us about the dissociation of attentional and emotional blink startle modulation.
Acknowledgements This work was supported by Grant A10027218 from the Australian Research Council.
References Anthony, B.J., Graham, F.K., 1985. Blink reflex modification by selective attention: evidence for the modulation of ‘automatic’ processing. Biological Psychology 21, 43 /59. Anthony, B.J., Putnam, L.E., 1985. Cardiac and blink reflex concomitants of attentional selectivity: a comparison of adults and young children. Psychophysiology 22, 508 /516. Bo¨hmelt, A.H., Schell, A.M., Dawson, M.E., 1999. Attentional modulation of short- and long-leadinterval modification of the acoustic startle blink response: comparing auditory and visual prestimuli. International Journal of Psychophysiology 32, 239 /250. Bohlin, G., Graham, F.K., Silverstein, L.D., Hackley, S.A., 1981. Cardiac orienting and startle blink modification in novel and signal situations. Psychophysiology 18, 603 /611. Bradley, M.M., Cuthbert, B.N., Lang, P.J., 1990. Startle reflex modification: emotion or attention? Psychophysiology 27, 513 /522. Bradley, M.M., Lang, P.J., 2000. Affective reactions to acoustic stimuli. Psychophysiology 37, 204 /215. Cadenhead, K.S., Braff, D.L., 1999. Schizophrenia spectrum disorders. In: Dawson, M.E., Schell, A.M., Bo¨hmelt, A.H. (Eds.), Startle Modification: Implications for Neuroscience, Cognitive Science, and Clinical Science. Cambridge University Press, Cambridge, pp. 231 /244. Dawson, M.E., Schell, A.M., Bo¨hmelt, A.H., 1999. Startle Modification: Implications for Neuroscience, Cognitive Science, and Clinical Science. Cambridge University Press, Cambridge. Filion, D.L., Dawson, M.E., Schell, A.M., 1994. Probing the orienting response with startle modification and secondary reaction time. Psychophysiology 31, 68 /78. Filion, D.L., Dawson, M.E., Schell, A.M., 1998. The psychological significance of human startle eyeblink modification: a review. Biological Psychology 47, 1 /43. Flaten, M.A., Elden, A., 1999. Caffeine and prepulse inhibition of the acoustic startle reflex. Psychopharmacology 147, 322 /330. Graham, F.K., 1975. The more or less startling effects of weak prestimulation. Psychophysiology 12, 238 / 248.
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Graham, F.K., 1992. Attention: the heartbeat, the blink, and the brain. In: Campbell, B.A., Hayne, H., Richardson, R. (Eds.), Attention and Information Processing in Infants and Adults: Perspectives from Human and Animal Research. Erlbaum Associates, NJ, pp. 3 /29. Hackley, S.A., Graham, F.K., 1984. Early selective attention effects on cutaneous and acoustic blink reflexes. Physiological Psychology 11, 235 /242. Koch, M., Schnitzler, H.U., 1997. The acoustic startle response in rats */circuits mediating evocation, inhibition and potentiation. Behavioural Brain Research 89, 35 /49. Lang, P.J., Bradley, M.M., Cuthbert, B.N., 1990. Emotion, attention, and the startle reflex. Psychological Review 97, 377 /395. Lipp, O.V., Siddle, D.A.T., Dall, P.J., 1998. Effects of stimulus modality and task condition on blink startle modification and on electrodermal responses. Psychophysiology 35, 452 /461. Lipp, O.V., Siddle, D.A.T., Dall, P.J., 2000a. The effect of warning stimulus modality on blink startle modification in reaction time tasks. Psychophysiology 37, 55 /64. Lipp, O.V., Siddle, D.A.T., Dall, P.J., 2000b. The effects of change in lead stimulus modality on the modification of acoustic blink startle. Psychophysiology 37, 715 /723. Lipp, O.V., Neumann, D.L., McHugh, M.J., 2003. Lead stimulus modality change and the attentional modulation of the acoustic and electrical blink reflex. Biological Psychology 62, 27 /48. Lipp, O.V., Neumann, D.L., Pretorius, N.R., McHugh, M.J., in press. Attentional blink modulation during sustained and after discrete lead stimuli presented in three sensory modalities. Psychophysiology. Miltner, W., Matjak, M., Braun, C., Diekman, H., Brody, S., 1994. Emotional qualities of odors and their influence on the startle reflex in humans. Psychophysiology 31, 107 /110. Putnam, L.E., 1990. Great expectations: anticipatory responses of the heart and brain. In: Rohrbaugh, J.W., Parasuraman, R., Johnson, R. (Eds.), Event Related Potentials. Oxford University Press, Oxford, pp. 109 /129. Rohlf, F.J., Sokal, R., 1981. Statistical Tables. Freeman, San Francisco. Sokolov, E.N., 1963. Perception and The Conditioned Reflex. Pergamon, Oxford. Van Boxtel, A., Damen, E.J.P., Brunia, C.H.M., 1996. Anticipatory EMG responses of pericranial muscles in relation to heart rate during a warned simple reaction time task. Psychophysiology 33, 576 / 583.
Attentional blink modulation in a reaction time task: performance feedback, warning stimulus modality, and task difficulty Ottmar V. Lipp *, Sascha A. Hardwick School of Psychology, University of Queensland, St. Lucia, Qld 4072, Australia Received 1 May 2002; accepted 25 July 2002
Abstract The present research investigated the effect of performance feedback on the modulation of the acoustic startle reflex in a Go/NoGo reaction time task. Experiment 1 (n /120) crossed warning stimulus modality (acoustic, visual, and tactile) with the provision of feedback in a between subject design. Provision of performance feedback increased the number of errors committed and reduced reaction time, but did not affect blink modulation significantly. Attentional blink latency and magnitude modulation was larger during acoustic than during visual and larger during visual than during tactile warning stimuli. In comparison to control blinks, latency shortening was significant in all modality conditions whereas magnitude facilitation was not significant during tactile warning stimuli. Experiment 2 (n /80) employed visual warning stimuli only and crossed the provision of feedback with task difficulty. Feedback and difficulty affected accuracy and reaction time. Whereas blink latency shortening was not affected, blink magnitude modulation was smallest in the Easy/No Feedback and the Difficult/Feedback conditions. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Attention; Blink startle modulation; Reaction time task
* Corresponding author. Tel.: /61-733-65-6385; fax: /61-733-65-4466 E-mail address: [email protected] (O.V. Lipp). 0301-0511/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 5 1 1 ( 0 2 ) 0 0 1 1 5 - 1
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1. Introduction Modulation of the human blink reflex has become increasingly popular as a methodology for the assessment of psychological processes such as emotion or attention (for a review see Dawson et al. 1999). This development reflects upon the unique advantages of this methodology, which is said not to interfere with the psychological process in question and can be employed in participants like criminal psychopaths or schizophrenic patients. Moreover, the blink modulation methodology can be employed in non-human animal subjects which has permitted a detailed study of the neural pathways involved in mediating startle modulation phenomena (Koch and Schnitzler, 1997). To date, these advantages have been exploited mainly in studies of emotional blink modulation (Bradley et al., 1990) and in studies of sensory gating deficits in schizophrenia (Cadenhead and Braff, 1999) whereas less is known about attentional blink modulation effects, in particular those observed at lead intervals of more than 2 s. Two basic procedures have been employed in the investigation of the effect of attention on blink modulation. In one type of experiment, participants are instructed to attend to the blink-eliciting stimulus itself or to a non-reflex eliciting stimulus that is presented simultaneously in a different sensory modality (e.g. Hackley and Graham, 1984). Attending to the blink-eliciting stimulus enhances blink reflexes whereas directing attention towards a different modality reduces reflex magnitudes. In the lead stimulus paradigm, blink reflexes are elicited during the processing of a lead stimulus, which captures attention due to task instructions or its inherent interest value. In these studies, changes in blink latency and magnitude relative to blinks elicited during a control stimulus or during stimulus free intervals are taken as indicators of the extent of attentional processing of the lead stimulus. The extent and direction of the blink latency and magnitude modulation observed is dependent on the time interval between lead stimulus onset and blink stimulus onset, the lead interval, as well as on the modality of both stimuli (for reviews see Filion et al., 1998; Dawson et al., 1999). The latency of blinks elicited during attended stimuli is usually shortened relative to control blinks whereby the shortening is largest at lead intervals of more than 2 s. Blink magnitude is inhibited relative to control blinks at lead intervals of less than 400 ms, and facilitated at lead intervals of several seconds (Graham, 1975). The directional influence of attention to the lead stimulus on blink magnitude modulation is currently a matter of debate, with a particular emphasis on the role of stimulus modality. Studies that employed lead and reflex stimuli from the same modality found enhancement of both, magnitude inhibition and facilitation during attended relative to ignored lead stimuli (e.g. Filion et al., 1994). Results from studies assessing attentional blink modulation during lead stimuli from different modalities have yielded less consistent results. Findings by Anthony and Graham (1985) and Putnam and colleagues (for a review see Putnam, 1990) that attending to a lead stimulus in a different sensory modality reduced blink magnitude at lead intervals longer than 2 s led to the notion that attentional blink magnitude modulation is modality specific. Attention will enhance blink magnitude if directed towards the
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modality in which the reflex eliciting stimulus is presented and will reduce blink magnitude if directed to a different modality. More recent research, however, has produced results that are not consistent with this interpretation. Lipp et al. (2000b) found enhancement of the magnitude of acoustic blinks during visual and acoustic change stimuli in a habituation paradigm (see also Bohlin et al., 1981). Using the same counting and discrimination paradigm as employed by Filion et al. (1994), Lipp et al. (1998) and Bo¨hmelt et al. (1999) found larger acoustic blink magnitude during to-be-attended than during to-be-ignored visual and acoustic lead stimuli. However, Lipp et al. (1998) failed to find differential blink modulation during tactile lead stimuli. Finally, Lipp et al. (2000a), Experiment 3 failed to replicate the modality specific blink modulation effects that had been reported by Putnam and colleagues in a reaction time task (Putnam, 1990). Rather, they found facilitated acoustic blink reflexes during visual and acoustic warning stimuli. The question of whether attentional blink modulation at long lead intervals is modality specific is significant for two reasons. First, modality specificity is invoked as the criterion to distinguish emotional from attentional blink modulation phenomena (Bradley et al., 1990). Blink startle is enhanced during unpleasant and inhibited during pleasant lead stimuli regardless of whether the lead stimuli are visual (Lang et al., 1990), acoustic (Bradley and Lang, 2000), or olfactory (Miltner et al., 1994) and whether the reflex stimuli are acoustic or visual (Bradley et al., 1990). Assuming that attentional modulation is modality specific, any enhancement of blink startle in a cross modality paradigm has to reflect emotion rather than any other characteristic of the lead stimulus. Demonstrations of modality unspecific attention effects, however, render this simple rationale invalid. Second, modality specificity provides information as to the nature of the attentional process involved in a particular experimental procedure. If stimulus modality affects the extent of attentional blink modulation, then the point within the neural network at which the effect of the reflex eliciting stimulus is modulated has to be located before the junction of inputs from different sensory modalities, an example for an early selection process. If, however, increased attentional processing facilitates reflexes regardless of stimulus modality, then the location at which attention affects reflexive responding has to be after the point of cross-sensory integration (Graham, 1992). Both issues seem sufficient to justify further studies of the effect of stimulus modality on attentional startle modulation. Among the procedures used to investigate attentional blink modulation at long lead intervals, the reaction time paradigm seems to have provided rather equivocal results. Putnam and her colleagues (Putnam, 1990) reported blink magnitude data that are consistent with a modality specific account. In these experiments, participants were presented with acoustic, visual, or tactile warning stimuli in a simple reaction time task or a Go/NoGo discrimination task. Blinks were elicited by acoustic stimuli in all experiments. Relative to control blinks elicited during nowarning stimulus conditions, blink magnitude was facilitated during acoustic warning stimuli, but inhibited during tactile and visual warning stimuli. Moreover, the extent of blink modulation, facilitation or inhibition respectively, increased towards the offset of the warning stimulus. Lipp et al. (2000a), Experiment 3 aimed
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to replicate these findings by presenting participants with stimulus compounds that consisted of two visual or two acoustic stimuli. Participants were instructed to press a button as quickly as possible if one element of the stimulus compound was terminated before the second, but not in the reverse case. In addition, task difficulty was varied by using different lengths of stimulus offset asynchrony. Contrary to the predictions from a modality specific account, acoustic blinks were facilitated during acoustic and visual warning stimuli and were larger in the Difficult than in the Easy condition regardless of warning stimulus modality. This discrepancy across studies may reflect procedural differences. First, Putnam and colleagues employed trial-by-trial response feedback in order to maintain the attention and co-operation of their participants. No feedback was provided in the Lipp et al. (2000a) study, although a decrease in reaction time across trials seemed to indicate that participants stayed on task. Second, Lipp et al. employed visual stimuli as non-acoustic warning signals, whereas tactile warning signals were used in the modality mismatch conditions of the majority of the studies conducted by Putnam and colleagues. Using tactile lead stimuli, Lipp et al. (1998) have failed to find attentional blink modulation in a discrimination and counting task whereas acoustic blink was facilitated during acoustic and visual to-be-attended lead stimuli. This pattern of result may indicate differences across lead stimulus modalities that, however, do not follow the pattern predicted by modality specific accounts of attentional startle modulation. Experiment 1 was designed to replicate and extent the findings reported by Lipp et al. (2000a), Experiment 3 and assessed attentional startle modulation during tactile as well as acoustic and visual warning stimuli. Provision of trial-by-trial performance feedback was varied in a 2 /3 factorial between subject paradigm. A modality specific account of attentional blink startle modulation would predict facilitation of the acoustic blink during acoustic warning stimuli and inhibition during visual and tactile warning stimuli. Facilitation and inhibition are expected to be larger in the Feedback than in the No Feedback conditions. A modality non-specific account predicts blink facilitation during warning stimuli regardless of stimulus modality with larger facilitation in the Feedback condition.
2. Method 2.1. Participants One-hundred-thirty-eight undergraduate students aged 16 /40 years (Mean /19.7 years) participated for course credit and provided informed consent. Participants were assigned upon arrival at the laboratory to one of six groups until 20 participants in each group provided complete data. The data of 18 participants, 1 from Groups Acoustic/No Feedback and Visual/Feedback, 3 from Group Acoustic/ Feedback, 4 from Groups Visual/No Feedback and Tactile/No Feedback and, 5 from Group Tactile/Feedback were rejected due to an excessive number of missing data. The proportion of women and men varied slightly across groups (Group
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Acoustic/No Feedback: 14:6, Group Acoustic/Feedback: 12:8, Group Visual/No Feedback: 14:6, Group Visual/Feedback: 14:6, Group Tactile/No Feedback: 12:8, Group Tactile/Feedback: 11:9). 2.2. Apparatus Orbicularis oculi electromyogram (EMG) was recorded with two domed Ag/AgCl miniature electrodes 4 mm in diameter filled with a standard electrolyte (Surgicon E10). One electrode was placed under the pupil of the left eye and the second was placed lateral at a distance of 1 cm edge-to-edge. A ground electrode was attached to the participants’ left forearm. Raw EMG was amplified with a Grass 7P3C AC preamplifier (0.5 amplitude high-pass cut-off of 10 Hz; low-pass cut-off of 3000 Hz; calibration: 100 mV/cm pen deflection) and displayed on the polygraph. The raw EMG was digitised and sampled on-line with an IBM (486)-compatible computer. A sampling rate of 1000 Hz was used in a time window from 100 ms prior to onset to 400 ms after offset of blink-eliciting stimuli. Pure tones of 800 and 1200 Hz served as warning stimuli in Group Acoustic. Tones were presented at an intensity of 70 dBA with an instantaneous rise-time. The visual warning stimuli in Group Visual were generated with a red and a green frosted bulb (Osram, 40 W) placed behind a small screen (15 /20 cm) set into the wall of the experimental room. The screen was located 140 cm from the participant at eye level. If switched on, the bulbs diffusely illuminated parts of the screen’s surface in a manner that a red and a green patch were visible. The vibration units of two Mowat sensor that were taped 3 cm apart to the back of participants’ left hand generated the tactile stimuli. Each stimulator was driven by a custom-built power supply at a frequency of 50 Hz, thus, the tactile sensation experienced at the two locations was identical. The tone, light, and tactile stimuli have been used previously in experiments investigating habituation of the orienting response and were found to elicit equivalent electrodermal responses. This was taken as an indication that the stimuli did not differ in salience. A burst of white noise of 105 dBA, a duration of 50 ms and an instantaneous rise-time served as the blink-eliciting stimulus. Acoustic stimuli were produced by a custom-built tone and noise generator and presented via stereophonic headphones (Sennheisser HD25). The stimulus sequence, stimulus durations, and intertrial intervals were controlled by the computer, which also sampled the EMG. All participants were asked to operate a micro switch with their right thumb on target trials. Participants in the Feedback condition were informed that response feedback would be provided on a computer screen, which was located on the left side of the projection screen at eye level. For the first 3 reaction time trials, feedback consisted of the message ‘Well done/Your reaction time was XXX ms/You have earned 30 c/Keep up the good work’. For the remaining reaction time trials, the nature of the message depended on the reaction time performance. If the reaction time was slower than the average reaction time across the three preceding trials, the message ‘Your reaction time was XXX ms/Your reaction time has not improved/No reward on this trial/Your total earnings so far are YYY c’. If the reaction time was
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by less than 20 ms faster than the average on the preceding three trials the message read ‘Well done/Your reaction time was XXX ms/It has improved by XXX ms/You have earned 10 c/Your total earnings total so far YYY c’. If the improvement was more than 20, but less than 40 ms, the phrase ‘Well done’ was substituted with ‘Very well done’ and a 30 c reward was given whereas the phrase ‘Excellent’ was displayed and a reward of 50 c was given if the improvement was more than 40 ms. Response feedback commenced 2 s after the participant’s response and was presented for 4 s. Participants were not informed about the relationship between performance and rewards. 2.3. Procedure Participants were seated in a semi-reclining chair and the experiment was monitored from an adjoining room. After electrode attachment, participants received three presentations of the blink-eliciting stimulus to check the placement of the EMG electrodes and one example of a target and of a non-target stimulus. Each participant was presented with 45 compound stimuli consisting either of two pure tones (Groups Acoustic), two lights (Groups Visual), or two vibrations (Groups Tactile). One stimulus was switched off after 6 s whereas the second component of the compound stayed on for another 500 ms (500 ms offset asynchrony). On 23 of the trials one component was longer, e.g. the high tone lasted 6500 ms and the low tone 6000 ms, whereas the second was longer on the remaining trials, e.g. the high tone lasted 6000 ms and the low tone 6500 ms. Half the participants in each modality group were asked to press the micro switch as quickly as possible whenever the first component was longer than the second, e.g. whenever the high tone outlasted to low tone, but not if the second component was longer than the first. Instructions were reversed for the other half. Participants were presented with 63 startle eliciting stimuli, 45 during lead stimuli and 18 during intertrial intervals. Stimulus probes were presented at 5 different lead intervals, 3.5, 4.5, 5.0, 5.3, and 5.6 s after stimulus onset. The warning stimuli were arranged into 9 blocks of 5 trials. Within each block, each lead interval was used once, and two intertrial interval startles were presented. Intertrial interval probes were not presented within 6 s after stimulus offset or prior to stimulus onset. A different random order of lead intervals was used within each block. By rotating the lead intervals in a Latin square, five different trial sequences were created. Intertrial intervals varied between 15, 20, and 25 s. 2.4. Scoring, response definition and statistical analysis Digitised EMG activity was rectified offline and filtered and integrated with a Butterworth low pass filter (time constant 80 ms). Measures of latency and magnitude were derived from the integrated signal. Response latency was the time between blink stimulus onset and response onset, defined as the point at which 10% of the maximum slope of the integrated curve was reached. Response magnitude was defined as the difference in mV between the maximum of the integrated response curve within 200 ms after blink stimulus onset and the level seen at response onset. If
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no response was detectable, a magnitude of 0 was recorded and latency was scored as missing. A trial was discarded if (1) the baseline EMG was not stable within 100 ms prior to the startle stimulus, i.e. varied by more than 50 A/D units, or (2) the onset of a response was not within 20/60 ms after the onset of the blink stimulus. The data from a participant were rejected if (1) more than half the responses at any lead interval were discarded trials, or if (2) the number of discarded trials or 0 responses exceeded one third of all trials. Using this set of criteria the blink data from 18 participants were rejected due to insufficient data. The main reason for rejection was the presence of an excessive number of discarded responses. Magnitude and latency of blinks elicited during stimuli were averaged across blocks of three trials at each lead interval and intertrial interval startle magnitude and latency were averaged into three trial blocks. Magnitude and latency of blink startle elicited during warning stimuli were converted into change scores prior to statistical analyses, i.e. difference values (stimulus/intertrial interval) were calculated as an estimate of latency modulation and percentage change scores were calculated as an estimate of magnitude modulation. Percentage change scores were employed to reduce the impact of individual differences in startle magnitude. The number of trials containing errors, i.e. premature responses on target trials or any response on non-target trials, was subjected to a 3 /2 (Modality /Feedback) ANOVA. Response times shorter than 100 ms were rejected and scored as missing. Reaction time to target stimuli was arranged into 6 blocks, 5 blocks of 4 trials and one block that contained 2 or 3 trials, and analysed in a 3 /2 /6 (Modality / Feedback /Block) ANOVA. The 95% confidence intervals of the means were inspected to determine whether blink magnitude and latency during stimuli were modified significantly. Blink modulation was regarded as significant if 0 was outside the confidence interval. Differences in blink latency and magnitude change scores were assessed in 3/2 /5 /3 (Modality /Feedback /Lead interval /Block) factorial ANOVAs. Multivariate F -values are reported for all main effects and interactions that involved within participant factors with more than two levels. Multiple comparisons of means were made with t-tests. The critical values for these t-tests were derived from Sidak’s tables to protect against the accumulation of a-error (Rohlf and Sokal, 1981). Level of significance was set at 0.05 for all statistical analyses.
3. Results 3.1. Response time Participants in Groups Acoustic and Tactile committed more errors in the Feedback condition than in the No Feedback condition (Acoustic: 10.8 [SEM: 1.0] vs. 4.8 [1.0], t (114) /3.62; Tactile: 17.2 [1.8] vs. 7.0 [0.8], t(114) /6.16), whereas there was no difference in Group Visual (4.2 [1.1] vs. 2.9 [1.1], t B/1.). This
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impression was confirmed by the ANOVA, which yielded main effects for Modality, F (2,114) /27.02, P B/0.001, and Feedback, F (1,114) /37.29, P B/0.001, as well as a Modality /Feedback interaction F (2,114) /7.24, P B/0.001. Reaction time was shorter in the presence of performance feedback, 439 ms, than without feedback, 581 ms, F (1,114) /21.33, P B/0.001. Stimulus modality affected reaction time in that it was slower in the tactile condition than in the visual and the acoustic condition, all t (218) /3.0, and slower in the acoustic condition than in the visual condition on Blocks 1, t(218) /4.98, and 4, t(218) /3.08. The analysis yielded main effects for Modality, F (2,114) /45.36, P B/0.001, and Block, F (5,110) /29.31, P B/0.001, and a Modality /Block interaction, F (10, 218) / 2.61, P B/0.01.
3.2. Control blinks Latency of intertrial interval blinks increased across blocks, F (2,113) /6.23, P / 0.003, and was longer on Blocks 2, 41.2 ms, and 3, 41.3 ms, than on Block 1, 39.9 ms. Magnitude of intertrial interval blinks decreased across all three blocks, F (2,113) / 82.92, P B/0.001, (Block 1: 177 mV; Block 2: 128 mV; Block 3: 112 mV, all t/4.5). Moreover, it differed across experimental conditions, Modality /Feedback interaction, F (2,114) /4.83, P /0.01. Post hoc comparisons revealed that intertrial interval blink magnitude was larger in the Tactile/Feedback group than in the Tactile/No Feedback group, 194 vs. 111 mV, t(114) /3.81, but there was no difference between feedback groups in the other Modality conditions, all t B/2.0.
3.3. Blink magnitude modulation The extent of blink magnitude modulation is shown in the upper panel of Fig. 1. In comparison to intertrial interval blinks, blink was facilitated at all lead intervals in the Acoustic/Feedback, Acoustic/No Feedback, and Visual/No Feedback conditions, and at the 4.5, 5.3, and 5.6 s lead intervals in the Visual/Feedback conditions. No significant blink magnitude facilitation was found in either group trained with tactile stimuli. Blink magnitude modulation differed across the three Modality conditions, F (2,114) /47.58, P B/0.001, and across lead intervals, F (4,111) /3.47, P /0.01. Post hoc tests confirmed that magnitude modulation in the acoustic condition exceeded modulation in the visual condition, t(114) /6.47, which in turn was larger than in the tactile condition, t(114) /3.08. Across modality and feedback conditions, blink magnitude modulation was larger at the 5.6 s position, t(111) /3.88, and at the 5.3 s position, t(111) /2.93, than at the 3.5 s position. Although suggested in Fig. 1, there was no differential effect of response feedback on blink magnitude modulation across the three Modality conditions. All interactions involving the factors Feedback and Modality were non-significant, all F B/2.0.
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Fig. 1. Mean blink magnitude modulation (upper panel) and blink latency shortening (lower panel) during Experiment 1 as a function of Group and lead interval (Vertical bars represent standard errors of the means. A/N, Acoustic/No Feedback; A/F, Acoustic/Feedback; V/N, Visual/No Feedback; V/F, Visual/ Feedback; T/N, Tactile/No Feedback; T/F, Tactile/Feedback).
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3.4. Blink latency modulation Blink latency shortening is depicted in the lower panel of Fig. 1. In comparison to the latency of intertrial interval blinks latency shortening was significant at each lead interval in all of the six experimental groups. The extent of blink latency shortening differed among Modality conditions, F (2,114) /37.44, P B/0.001, and across lead intervals, F (4,111) /7.45, P B/0.001, and there was also a Modality /Lead interval interaction, F (8,224) /2.21, P B/0.05. Post hoc comparison confirmed that the extent of blink latency shortening was larger in the acoustic than in the visual condition and larger in the visual than in the tactile condition at each lead interval, all t(224) /3.30. The interaction reflects the fact that in the acoustic condition, latency shortening was larger at the 5.6 s position than at the 3.5 s position, t (224) / 3.99, and in the visual condition, was larger at the 5.6 s position, t(224) /3.57, and at the 5.3 s position, t(224) /4.48, than at the 3.5 s position and was larger at the 5.3 s position than at the 4.5 s position, t(224) /3.20.
4. Discussion The provision of performance feedback and monetary reward resulted in a reduction of the reaction time across modality conditions and an increase in the number of errors committed, at least in Groups Acoustic and Tactile. The increase in errors, i.e. button presses to blink-eliciting stimuli or on non-target trials, may reflect an increased readiness to respond. Alternatively, one may argue that the increase of incorrect presses reflects a speed accuracy trade-of in that increased error rates had no negative consequences. This seems unlikely as such a change in response strategy would result in an increase in button presses on non-target trials, however, most of the errors consistent in presses to the startle probes. Moreover, it is difficult to conceive as to why this change in response strategy was restricted to participants in the acoustic and tactile conditions. Taken together, the behavioural measures indicate that the provision of response feedback affected participants’ involvement in the task. There was, however, no clear evidence for an effect of performance feedback on blink latency or magnitude modulation. Consistent with previous findings blink modulation was affected by the modality of the warning signal. Blink latency shortening and magnitude facilitation were larger during acoustic than during visual and larger during visual than during tactile warning stimuli. Blink latency shortening increased towards the offset of the warning stimulus during acoustic and visual, but not during tactile warning stimuli. A similar increase in blink magnitude facilitation across lead intervals was not affected by stimulus modality. As the acoustic, visual and tactile warning stimuli employed in Experiment 1 were not matched for intensity, one might argue that the difference in blink modulation observed across modality conditions reflects intensity effects. This seems unlikely, however, as more recent studies from our laboratory, which employed the same warning stimuli at intensities matched in a psychophysical procedure yielded the same pattern of results in a discrimination and counting task (Lipp, Neumann,
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Pretorius and McHugh, in press) and during passive attention (Lipp et al., 2003). The observation that in a reaction time task background orbicularis oculi EMG is reduced more during a visual than during an acoustic warning stimulus may provide an alternative explanation for the difference found between Groups Visual and Acoustic (Van Boxtel et al., 1996) 1. This may indicate that the overall larger blink reflex modulation during acoustic than during visual warning signals observed here and in previous studies reflects differences on the level of motor output rather than effects of attention. Such an interpretation is consistent with the observation by Lipp et al. (1998), Experiments 1 and 2 who relative to visual task stimuli, found enhanced blink facilitation during acoustic task-relevant and task-irrelevant stimuli. However, more research exploring the relationship between changes in background EMG and blink magnitude seems required before firm conclusions can be drawn. The present findings resemble the attentional blink modulation results reported previously (Lipp et al., 1998, Lipp et al., 2000a, Experiment 3). Similar to the results observed during a discrimination and counting task (Lipp et al., 1998), there was no evidence for attentional modulation of the magnitude of an acoustic blink reflex during tactile lead stimuli. There was, however, significant blink latency shortening during tactile warning stimuli in the present study whereas there was no such latency reduction in the discrimination and counting task. This discrepancy across studies may reflect the larger number of participants presented with tactile stimuli in the present study (40 vs. 24) as the same stimuli were used in both experiments and the overall extent of latency shortening was similar across experiments. Moreover, the finding of significant latency shortening during tactile warning stimuli together with evidence that the tactile stimuli can support blink magnitude modulation when used as conditional stimuli in an aversive Pavlovian conditioning procedure (Lipp et al., 1998) indicates that the failure to find blink magnitude modulation in the present study does not indicate that tactile stimuli are ineffective as prepluse stimuli. Similar to the results of a previous reaction time experiment (Lipp et al., 2000a, Experiment 3), there was no evidence for inhibition of acoustic blink reflexes during visual attention demanding stimuli. Rather, blinks were facilitated during visual stimuli in particular at the late lead intervals at which blink inhibition had been largest in previous reports of modality specific attentional blink modulation (Putnam, 1990). Similarly, no blink magnitude inhibition was found during tactile warning stimuli. The present results deviate, however, from those reported by Lipp et al. (2000a), Experiment 3 in that no significant effect was found for a manipulation thought to enhance attention to the warning stimuli. Lipp et al. (2000a), Experiment 3 had varied task difficulty and found larger acoustic blinks during visual warning signals in the Difficult than in the Easy condition. In the present experiment, no significant difference emerged between the feedback conditions and blinks during visual warning stimuli were numerically smaller in the Feedback than in the No Feedback condition, a result consistent with a modality non-specific account of attentional startle modulation. On the other hand, this difference across experiments may reflect 1
We are grateful to one anonymous reviewer for pointing us towards this explanation.
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the fact that blink magnitude modulation in Group Visual/No Feedback (mean across lead intervals: 41%) was somewhat larger, although not significantly so, than in the corresponding group in Lipp et al. (2000a), Experiment 3; Group Light/Easy; mean across lead intervals 26%, although both groups had been presented with the same stimuli. In order to resolve this apparent discrepancy Experiment 2 was designed to assess the effects of task difficulty and performance feedback on attentional modulation of acoustic blinks during visual warning stimuli.
5. Experiment 2 Experiment 2 assessed the effects of task difficulty and performance feedback on modulation of the acoustic blink reflex in a 2 /2 factorial design. The manipulation of task difficulty was the same as that used by Lipp et al. (2000a), Experiment 3 in that stimulus offset asynchrony was 500 ms in the Easy condition and 50 ms in the Difficult condition. Thus, Group Easy/No Feedback replicated Group Light/Easy from Lipp et al. (2000a), Experiment 3 and Group Visual/No Feedback from Experiment 1. Group Difficult/No Feedback replicated Group Light/Diff from Lipp et al. (2000a), Experiment 3 and Group Easy/Feedback replicated Group Visual/ Feedback from Experiment 1.
6. Method 6.1. Participants Eighty undergraduate students (Mean age 19.3 years; range 17/33) volunteered participation in exchange for course credits and provided informed consent. Participants were assigned to four groups upon appearance at the laboratory until each group contained 20 participants who provided complete data. This procedure resulted in a slightly unbalanced sex distribution across groups (Easy/No Feedback: 10:10 [female: male]; Easy/Feedback: 16:4; Diff/No Feedback: 9:11; Diff/Feedback; 12:8). Data from an additional six participants were rejected prior to the analyses due to insufficient data (2 in Group Easy/Feedback and 4 in Group Difficult/No Feedback). 6.2. Apparatus and procedure Experiment 2 was conducted in a laboratory similar to the one used for Experiment 1. The recording apparatus and the scoring procedures were the same as used in Experiment 1. Only visual warning stimuli were used and the length of the stimulus offset asynchrony was varied across groups. It was 500 ms in the easy condition and 50 ms in the difficult condition. The statistical analyses were conducted analogous, adapted for a 2/2 (Difficulty /Feedback) between subject design.
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7. Results 7.1. Response time Participants in Group Diff/Feedback committed more errors (8.1 [SEM/1.4]) than did the participants in the other three groups (Easy/No Feedback: 1.5 [0.2]; Easy/Feedback: 3.2 [0.7]; Diff/No Feedback: 2.5 [0.5]), all t(76) /4.20, as indicated by main effects for Difficulty, F (1,76) /12.97, P B/0.01, and Feedback, F (1,76) / 19.85, P B/0.001, and a Difficulty /Feedback interaction, F (1,76) /5.96, P B/0.05. Reaction time was faster in the Feedback than in the No Feedback conditions, 392 vs. 537 ms, F (1,76) /10.80, P B/0.01, and faster in the Easy than in the Difficult condition, 380 vs. 549 ms, F(1,76) /14.69, P B/0.001. Reaction time declined across blocks of trials, F (5,72) /11.84, P B/0.001, at rates that differed between the two difficulty conditions, Difficulty /Block interaction, F(5,72) /2.82, P B/0.05. The interaction reflects that in the Difficult condition, reaction time during Block 1 was slower than during all other blocks, all t (72) /3.90, whereas in the Easy condition reaction time during Block 1 was slower only than reaction time during the last block. 7.2. Control blinks The latency of control blinks elicited during interstimulus intervals was longer on Block 3 than on Block 1, 42.3 vs. 40.5 ms, F (2,75) /7.13, P B/0.01, and control blink magnitude decreased across blocks of trials, F (2,75) /33.38, P B/0.001, Block 1: 170 mV; Block 2: 128 mV; Block 3: 105 mV, all t(75) /3.50. 7.3. Blink magnitude modulation The extent of blink magnitude modulation across the five lead intervals is shown in Fig. 2, upper panel. Blink magnitude was facilitated significantly at all lead intervals in Groups Easy/No Feedback, Easy/Feedback, and Difficult/No Feedback, but at none of the lead intervals in Group Difficult/Feedback. Blink magnitude facilitation did not seem to differ across the four groups at the three early lead intervals, but seemed larger in Group Difficult/No Feedback than in the other groups at the 5.3 and 5.6 s lead intervals. This impression was confirmed by the analysis which yielded Difficulty /Position, F (4,73) /2.69, P B/0.05, and Difficulty /Feedback /Block interactions, F (2,75) /5.24, P B/0.01, as well as a marginal interaction Difficulty /Feedback /Position, F (4,73) /2.03, P /0.099. The Difficulty /Feedback /Block interaction reflects the fact that blink magnitude modulation in Group Difficult/Feedback was smaller than modulation in Groups Easy/Feedback and Difficult/No Feedback on Block 3, all t(75) /3.20, but not on Blocks 1 and 2. Although not significant on the set level, the Difficulty / Feedback /Position interaction was followed up with post hoc t-tests as it might be of theoretical interest. Multiple comparisons revealed no differences at lead intervals 3.5, 4.5, and 5.0 s. At the 5.3 s lead interval, blink modulation in Group
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Fig. 2. Mean blink magnitude modulation (upper panel) and blink latency shortening (lower panel) during Experiment 2 as a function of Group and lead interval (Vertical bars represent standard errors of the means. E/N, Easy/No Feedback; E/F, Easy/Feedback; D/N, Difficult/No Feedback; D/F, Difficult/ Feedback).
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Difficult/No Feedback was larger than blink modulation in Groups Difficult/ Feedback, t (73) /4.26, and Easy/No Feedback, t(73) /3.81, whereas at the 5.6 s lead interval blink modulation in Group Difficult/No Feedback was larger than blink modulation in Group Difficult/Feedback, t(73) /4.58 and there was a trend towards larger blink modulation in Group Difficult/No Feedback than in Group Easy/No Feedback, t(73) /3.18, P B/0.10. 7.4. Blink latency modulation Blink latency shortening was significant at all lead intervals in all four groups. As can be seen in Fig. 2, lower panel, the extent of blink latency shortening did not differ across groups, but increased towards the offset of the warning stimulus, F (4,73) /4.68, P B/0.01. Post hoc comparisons confirmed that blink latency shortening at the 5.3, 4.9 ms, and 5.6 s lead intervals, 5.2 ms, was larger than at the 3.5 s lead interval, 3.8 ms, both t (73) /3.10.
8. Discussion and general discussion As in Experiment 1, the reaction time data confirm the validity of the between groups manipulations. Reaction time was longer during the Difficult condition and shorter in the presence of performance feedback. Blink latency modulation was enhanced toward the offset of the warning stimulus, but did not vary as a function of the between group manipulation. This result is consistent with the findings of Experiment 1, in which latency shortening in Groups Visual did not differ across feedback conditions. Blink magnitude, however, was affected by the task conditions. Replicating the results of Lipp et al. (2000a), Experiment 3 blink magnitude facilitation at the two lead intervals closest to stimulus offset was larger in Group Difficult/No Feedback than in Group Easy/No Feedback. Moreover, there was no difference between Groups Easy/Feedback and Easy/No Feedback confirming the results of Experiment 1. Both results are consistent with a modality non-specific account of attentional blink modulation. The finding of smaller blink magnitude modulation in Group Difficult/Feedback than in Group Difficult/No Feedback, however, does not fit with such an account. Rather it seems in support of a modality specific account. The results of Experiment 2 seem to suggest that the effect of stimulus modality on attentional blink magnitude modulation is mediated by the demands that a particular task poses. Borrowing from Sokolov (1963) distinction between generalised and localised orienting, one may argue that in a procedure that requires passive attention, such as a habituation paradigm, or during a task that is simple or does not involve the participants, generalised orienting predominates that results in a modality unspecific enhancement of stimulus input. Increasing the task demands, either by enhancing task difficulty or by providing motivational incentives, may initially enhance generalised orienting until a level of processing demand is reached that requires the more focussed, selective stimulus processing seen during localised
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orienting. This processing style is associated with an attenuation of stimulus input from modalities other than that of the target stimulus in order to avoid impairment of task performance. Thus, the differences in attentional startle modulation reported by Putnam (1990) and in previous studies from our laboratory may reflect the predominance of localised or generalised orienting respectively. The question remains, however, as what determines the transition from one mode of orienting to the other. The present and previous data seem to indicate that the level of task difficulty alone is not sufficient to determine the mode of processing. In the present study, there was no significant correlation between reaction time and the extent of blink magnitude modulation for any lead interval, all r2 B/0.05. Anthony and Graham (1985) found results indicative of a modality specific, localised processing mode in a habituation procedure in which participants were presented with interesting or dull acoustic and visual lead stimuli without any task requirement. One may argue that this finding reflects characteristics of the stimuli used, music box melodies vs. tones and pictures of faces vs. white light. Alternatively, the finding may reflect on the sample employed in that the pattern was significant only in 16-week-old infants, but not in adults. However, there are other reports of results consistent with a modality specific processing mode from procedures that seemed to impose a low demand on participants. Anthony and Putnam (1985) reported inhibition of acoustic blink during a tactile warning signal at the offset of which participants were required to press a button. The reaction times observed in this task averaged at about 340 ms, a performance level that is compatible with the one seen in the Easy conditions of the present experiment. Thus, it seems that a simple reaction time task may be sufficient to trigger a modality specific processing mode. The reaction time studies reviewed by Putnam (1990) have in common that they employed performance feedback, although the feedback procedure was less elaborate than the one used in the present research, at least in some studies. Anthony and Putnam (1985), for instance, employed two light stimuli as feedback, one indicating slow reaction time and the other premature responses. It may be that enhanced task involvement due to the provision of performance feedback, or an enhanced arousal mediated by it, is a prerequisite for engaging in a modality specific processing style. This contention can be tested in future studies by employing a wider range of levels of task difficulty while varying the extent of performance feedback provided. It should be noted, however, that even in Group Difficult/Feedback, which seemed to be most conducive for engagement in a localised processing mode, no inhibition of blink magnitude was observed. Moreover, contrary to the results reported by Anthony and Putnam (1985), blink latency was reliably shortened even though there was no significant magnitude facilitation. The second interesting finding to emerge from the present study is the lack of attentional blink startle magnitude modulation observed during tactile warning stimuli at the long lead intervals used here (for evidence of attentional blink modulation during tactile lead stimuli at short lead intervals see Flaten and Elden, 1999). This result replicates findings in the discrimination and counting task where no difference in probe reflex magnitude was found between task-relevant and task-
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irrelevant tactile stimuli (Lipp et al., 1998). This lack of differential blink magnitude modulation coincided with a level of performance on the discrimination and counting task and an extent of differential electrodermal responding that did not differ from the one observed with visual or acoustic task stimuli. Moreover, differential blink magnitude modulation was observed when the tactile stimuli were employed as conditional stimuli in an aversive Pavlovian conditioning procedure. These findings imply that tactile stimuli were suitable signals and stimulated the circuits involved in orienting as indexed by autonomic responses. In addition, tactile stimuli can activate the neural circuitry that underlies emotional blink modulation, but not the circuitry that underlies attentional startle modulation. Thus, the present results suggest that further studies including tactile warning stimuli can teach us about the dissociation of attentional and emotional blink startle modulation.
Acknowledgements This work was supported by Grant A10027218 from the Australian Research Council.
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