Intertarget interval does not affect P300 within oddball series

The long and the short of it: influence of interstimulus interval on auditory P300 abnormalities in schizophrenia

Event-related brain potentials were recorded from 10 patients with DSM-IV schizophrenia (9 men) and 10 healthy control subjects (9 men) during the performance of two auditory oddball tasks, one using a 1.5 second interstimulus interval (ISI), the other using an 8 second ISI. P300 amplitude to target tones (.20 probability) and standard tones (.80 probability) were measured from midline electrodes Fz, Cz, and Pz. Results showed different effects of ISI in the two groups. Controls showed a slight decrease in P300 amplitude to targets but a marked increase in P300 to standards with the increase in ISI. In contrast, schizophrenic patients showed no change in the P300 to targets and a relatively small increase in P300 to standards with the ISI increase. Moreover, relative to the controls, P300 amplitude to targets was reduced in the schizophrenic patients at the short but not the long ISI. Implications for the cognitive significance of the P300 and its reduction in schizophrenia are discussed.

Effect of the relevance and position of the target stimuli on P300 and reaction time

The relationships among stimulus relevance and the position of target stimuli in a sequence with the P300 component of event related brain potentials and reaction time were investigated. An auditory oddball series was presented to 42 healthy, young, right-handed female participants. In the series, participants were to ignore the standard stimuli and count the deviants from 1 to 6. When reaching the count of 6, they had to restart their count from 1 again. While counting, half of the sample had to press a key after the deviants, numbers 1, 2 and 3, but not after the numbers 4, 5 and 6. The other half of the sample had to press the key when numbers 4, 5 and 6 rather than when numbers 1, 2, 3 appeared. The P300 amplitude of the more relevant (count and press) stimuli was higher than that of the P300 amplitude of the less relevant (count only) stimuli. This is associated with a greater allocation of cognitive resources to the more relevant stimuli. An interaction was also found between the position of the target stimuli in a sequence and the stimulus relevance, which is attributed to the cognitive preparation. Furthermore, the reaction time tended to increase along the sequence of the more relevant stimuli, due perhaps to an increase in physical fatigue.