STIMULUS AND RESPONSE PROBABILITIES: EFFECTS ON THE AMPLITUDE OF P300

Evoked potentials elicited by tones of high and low probability of
presentation requiring the suppression of a frequent response (counting
or not-counting) or its maintenance were studied. Low probability
(Novel) tones requiring response (counting) suppression elicited P300
amplitudes of the event-related potential comparable to those elicited
by low probability tones (Target) which did not require suppression of
an ongoing frequent response. This finding was not in agreement with
those reported in a similar study by Karlin and Martz (1973). It was
advanced that situations in which subjects maintain a high level of
response readiness enhance the effects of stimulus probability and
response probability on P300 amplitude values. In agreement with the
current literature, low probability stimuli (Novel and Target) were
found to consistently elicit larger P300 amplitudes than high
probability (Background) stimuli regardless of response frequency. N100
amplitudes were also larger for low probability tones than for high
probability Background tones. These results were attributed to either
habituation, attention or a combination of both. P200-N250 peak-to-peak
amplitude values were also shown to be larger in response to Target and
Novel tones than in response to Background tones.