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Abstract
Polich & Kok (1995) (Biological Psychology, 41, 103-146) have recently argued that
P300 is not only sensitive to specific 'cognitive' variables, but also to non-specific
biological processes such as arousal. Fluctuations in arousal are said to be indexed
by an inverse relationship between latency and amplitude. We tested this hypothesis
with a drug that decreases arousal-the barbiturate secobarbital sodium. Twelve subjects
performed a visual 80-20% oddball task at two levels of stimulus quality and after
ingesting the drug (2.9 mg/kg body weight) or a placebo. Reaction time (RT) and P300
were collected simultaneously and the latter was analyzed on both a single trial and
average basis. The RT results confirmed that secobarbital interacts with stimulus
quality. Secobarbital slowed single trial P300 by about half as much as RT, and this
slowing was additive with stimulus quality. Thus the two measures dissociated. Secobarbital
did not influence P300 amplitude. Average P300 revealed the same pattern of results,
although the size of the latency effects was somewhat attenuated. RT and P300 latency
were more strongly correlated than P300 latency and amplitude. We propose that P300
latency reflected the slowing of stimulus evaluation produced by the depressant properties
of the drug, and that fluctuations in arousal are not necessarily associated with
a simple inverse relationship between P300 latency and amplitude.