برونگرایی، توجه و واکنش پذیری پاسخ وحشت زدگی

Abstract The present study investigated the impact of directed attention tasks performed by introverts and extraverts, using the brainstem startle eyeblink response as a measure of automatic reactivity. Participants (23 introverts and 24 extraverts) were instructed to either attend to a startle-eyeblink-eliciting acoustic noise pulse (90 or 105 dB) or to a drawing of animals, or to ignore all stimuli, while their eyeblink reflex to the noise pulse was measured. In the no-task condition, introverts were generally more reactive than extraverts. When attention was directed to the startle-eliciting stimulus, startle response amplitude was potentiated in both introverts and extraverts. When attention was directed to a visual task, acoustic startle reactivity was reduced for introverts, but not for extraverts. Therefore, introverts were more able to focus their attention, and were less distractible than extraverts. These data support previous findings of a narrower focus of attention in introverts than in extraverts, based on differences in arousal as a function of extraversion.

Introduction The purpose of this study was to investigate variations in attentional processing in introverts and extraverts, using the startle response to probe early stages of information processing. Introverts show more pronounced differences in reactivity as a function of directed attention instructions than do extraverts (Eysenck & Eysenck, 1985). Stenberg, Rosen and Riseberg (1990) measured ERPs (event related potentials) (N120 and P200) and found that attention made more of a difference for introverts than for extraverts, both for directing attention towards a stimulus and for distracting attention away from a stimulus. That is, instructing participants to attend to the ERP-eliciting stimulus resulted in greater reactivity in introverts than extraverts. This pattern was reversed when participants were instructed to attend to a stimulus in a different modality, illustrating the fact that introverts were more able to focus attention to the task, and were less distracted by the ERP-eliciting stimulus. Stenberg et al. explained this in terms of higher arousal in introverts (Eysenck, 1967), and narrower attentional focus as arousal increases (Easterbrook, 1959). That is, as arousal increases, the range of cue utilization becomes more restricted, possibly due to arousal-dependent lateral inhibition (Walley & Weiden, 1973). Koelega (1992) reported a meta-analysis of several dozen studies published over a 30-year period in which the relationship between extraversion and vigilant attention was examined. Although these differences are seen in studies utilizing tasks requiring vigilant attention, variations in arousal explain differences between introverts and extraverts in visual selective attention tasks as well, as seen in assessments of sustained attention and susceptibility to distraction (Szymura & Necka, 1998). Introverts also show greater P3 amplitude than do extraverts, based on differences in attentional allocation (Daruna, Karrer & Rosen, 1985). Blumenthal, Chapman and Muse (1995) showed that a social encounter reduces startle eyeblink reactivity in introverts, but not in extraverts. The social encounter situation is arousing, and it also draws attention away from the startle stimulus (Bovelsky & Blumenthal, 1997), decreasing startle reactivity. Therefore, this finding supports the hypothesis that introverts focus their attention more effectively on the social encounter, and are less reactive to distracting stimuli, than is the case for extraverts. Brebner and Cooper (1978) hypothesized that introverts are “geared to inspect”, such that increased excitation is associated with cognitive processing to a greater degree than is the case for extraverts. Therefore, reactivity in directed attention tasks should be more pronounced in introverts when attending to a target stimulus, but also more attenuated when attention is focused on stimuli in other sensory modalities. The startle response is a rapid and automatic response to a sudden stimulus, and one of the motor components of this reflexive response is the eyeblink (Berg & Balaban, 1999). This startle eyeblink response has been used to investigate a wide variety of cognitive, neurophysiological, and clinical phenomena (Dawson, Schell & Boehmelt, 1999). Stelmack (1997) reviewed literature that shows that introverts are more reactive than extraverts to moderate intensity punctate stimuli (75–90 dB), using measures of SCR (skin conductance response) and ERP. Blumenthal et al. (1995) have also found this for startle amplitude, in support of Eysenck’s (1967) theory of higher arousal in introverts. Directing attention towards a startle stimulus generally increases the effectiveness of that stimulus. Distracting attention away from the startle stimulus generally results in reduced reactivity. This attentional modulation of startle has been seen in studies using a variety of stimuli and situations (Anthony & Graham, 1985, Blumenthal, 1999, Hackley & Graham, 1983 and Schicatano & Blumenthal, 1998). Bovelsky and Blumenthal (1997) showed that startle is reduced by distracting attention away from the startle stimulus, when this distraction involves directing attention towards a different sensory modality. These data make it clear that instructing participants to direct their attention influences stimulus processing at a fairly early point in the pathway. Subcortical stimulus processing is affected by directions to attend to one stimulus modality or another, as shown by the ability of these instructions to modulate a response with an average latency of 40–50 ms. This also suggests that the impact of attention on startle modulation is modality specific, providing support for an early selection theory of attention. In the present study, introverts and extraverts were presented with startle stimuli in several attention conditions, including a no-task condition, a condition in which attention was directed towards the startle-eliciting stimulus, and a condition in which attention was directed towards a stimulus in a different sensory modality. More variation in startle reactivity as a function of attention task was expected in introverts than in extraverts. Further, this difference was expected to be more pronounced at the higher of two stimulus intensities, based on the fact that optimal levels of arousal are reached at lower stimulation levels in introverts than in extraverts (Eysenck & Eysenck, 1985 and Geen, 1984). That is, arousal increases as stimulus intensity increases, up to a point of reversal of this curve, after which increased stimulus intensity results in decreased arousal, illustrating transmarginal inhibition. The introverts were expected to be more able to attend, and more resistant to distraction, as arousal (driven by stimulus intensity) increased, with both effects leading to a narrower focus of attention (Stenberg et al., 1990).

3. Results 3.1. Stimulus and Task effects Startle response amplitude showed a significant Task effect, F(3,132)=19.96, P<0.001, a Stimulus Intensity effect, F(1,44)=103.50, P<0.001, and a Task by Stimulus Intensity interaction, F(3,132)=3.54, P<0.054. A Task by Stimulus Intensity by Extraversion interaction, F(3,132)=4.16, P<0.05, was also found ( Fig. 1 and Fig. 2). This interaction will be explored in the following paragraphs, by comparing certain Task levels, with Stimulus Intensity and Extraversion included in the analyses. Startle response amplitude as a function of task instructions and extraversion ... Fig. 1. Startle response amplitude as a function of task instructions and extraversion group for 90 dB noise stimuli. Figure options Startle response amplitude as a function of task instructions and extraversion ... Fig. 2. Startle response amplitude as a function of task instructions and extraversion group for 105 dB noise stimuli. Figure options Habituation of startle response amplitude was tested by comparing the Baseline block of trials to the No Task trial block. Since the three tasks were assigned in random order, habituation would be seen if reactivity in the No Task block was significantly lower than that in the Baseline block. Habituation was found, in that reactivity decreased from the Baseline block to the No Task block, F(1,44)=41.72, P<0.001. However, comparing these two task conditions also revealed a significant interaction between Task, Stimulus Intensity, and Extraversion, F(1,44)=4.91, P<0.05. The amount of habituation was somewhat smaller in extraverts at 90 dB than in introverts at 90 dB, or than that in either group at 105 dB ( Fig. 1 and Fig. 2). For the Visual Task and the Acoustic Task, the appropriate comparison is to the No Task condition, since habituation would be assumed to be equivalent in these three conditions (due to random ordering). If the task had no effect, then habituation should be equal in these conditions. When comparing reactivity in the Visual Task with that in the No Task condition, a significant Task by Extraversion interaction was found, F(1,44)=4.70, P<0.05. Distracting attention away from the startle stimulus by directing attention towards a visual task tended to decrease reactivity in Introverts, but increase reactivity slightly in Extraverts. When comparing reactivity in the Acoustic Task with that in the No Task condition, significant effects of Task, F(1,44)=15.59, P<0.001, and Stimulus Intensity, F(1,44)=94.12, P<0.001, were found. A marginal difference between groups was found, F(1,44)=3.15, P=0.083, but the Extraversion variable did not interact significantly with any other variable. Startle response latency showed a Task effect, F(3,132)=6.17, P<0.001, a Stimulus Intensity effect, F(1,44)=60.76, P<0.001, and a marginal Task by Stimulus Intensity interaction, F(2,132)=2.61, P<0.062. Response latency increased (from 41.48 to 42.78 ms) when attention was directed to the visual stimulus, and decreased (from 41.48 to 40.52 ms) when attention was directed to the acoustic stimulus. Latency was also shorter for more intense stimuli (39.53 vs. 41.15). No effect of Extraversion was seen for response latency. Startle response probability showed a Task effect, F(3,135)=4.46, P<0.01, a Stimulus Intensity effect, F(1,45)=38.27, P<0.001, and a Task by Stimulus Intensity interaction, F(3,135)=3.19, P<0.05. This interaction was due to the enhancement of response probability when attending to the acoustic stimulus, but only for the less intense stimuli (78.9–86.3). For more intense stimuli, response probability was above 96% in all cases, suggesting a ceiling effect. No effect of Extraversion was seen for response probability.