HABITUATION OF P300 AND REFLEX MOTOR (STARTLE BLINK) RESPONSES TO REPETITIVE STARTLING STIMULI IN CHILDREN

Positive EEG deflections with the latency and scalp distribution of th e P300 accompany startle in response to loud auditory stimuli in a non -task context. The purpose of this investigation is to determine if ha bituation would have effects on the P300 similar to those on the start le blink. Thirty-four normal 7 to Ii-year-old boys from a startle habi tuation study had EEG recordings of sufficient quality to provide data for the current study. Startle was measured both as orbicularis oculi EMG and vertical EOG and P300 was recorded at Pt in response to 40 10 4 dB bursts of white noise presented at 23-s intervals. Both the start le response and the P300 habituated toward asymptotic levels after the first 28 trials, suggesting that both startle and the subsequent cogn itive evaluation of the startling stimulus, reflected in the P300 resp onse, are modulated by a common neurophysiological mechanism extrinsic to the direct startle pathway. A modest significant correlation betwe en the P300 and the vertical EOG peak latencies for the initial trials suggests that the cognitive evaluation of the startling stimulus may also include evaluation of the reflex response to that stimulus. Analy ses of the within-subject associations between startle and P300 initia l amplitudes and rates of habituation showed that these parameters var ied independently within the individual subject, suggesting that the P 300 is not a component of the startle response. Rather, it reflects an evaluation of the startling stimulus, decreasing in amplitude as the surprising value of the startling stimulus decreases with habituation.