| The present study used psychophysiological methods to investigate discrepancies between basic brain-stem reactivity, as measured by the eyeblink startle response, and cortical (i.e. attentional) responses, as measured by event-related potentials (ERPs), in high and low defensive individuals. Sixty undergraduate volunteers (19 males) participated in the present study. The EPQ-L scale was used to assess defensiveness; those scoring within the top quartile of the sample were assigned to the high defensive group (M = 11.06, SD = 2.05, n = 17), while the remaining participants were assigned to the low defensive group (M = 4.84, SD = 2.61, n = 43). Participants received startle probes consisting of 50 msec white noise blasts at low (74db) and high (104db) intensity. Eyeblink data were collected by measuring electromyographic (EMG) activity at the orbicularis oculi muscle, while ERP data (N1 and P2) were collected from the Fz site. Relative to low defensive individuals, high defensive individuals were expected to show greater levels of startle reactivity, even while they showed evidence of stimulus attenuation at the level of the cortex as reflected by smaller P2 amplitudes. Low defensive individuals were expected to show congruence at the levels of the brain stem and cortex.;Contrary to expectations, no group differences emerged on startle measures, as both groups showed comparable amounts of startle reactivity and comparable stimulus-dependent increases in startle magnitude. However, as expected, low defensive individuals showed stimulus-dependent increases in N1 and P2 ERPs, while high defensive individuals showed comparable increases in N1, but no such increases in P2. This suggests that high and low defensive individuals show similar levels of brain-stem reactivity (as assessed by eyeblink responses) and stimulus registration (as assessed by the N1). However, high defensive engage in (putatively automatic) attentional processes (as reflected by P2 magnitude) that serve to attenuate the processing of unpleasant stimuli at the level of the cortex. These findings point to a possible physiological basis for the discrepancies between self-report and objective measures of distress that are often observed in high defensive individuals. |
