Effects of certainty, modality shift and guess outcome on evoked potentials and reaction times in chronic schizophrenics.

Evoked potentials and reaction times were obtained from chronic schizophrenics and normal controls to light and sound stimuli presented in random order. In the 'certain' condition subjects were told what the next stimulus would be, in the 'uncertain' condition they were asked to guess. Amplitudes were usually larger for normals than for schizophrenics, for 'uncertain' than for 'certain' conditions, and in cross- than in ipsimodal stimulus-sequences. The effect of certainty was stronger in normals across 4 leads; so was the effect of modality shift at vertex. While these findings replicate earlier results from acute schizophrenics, no condition X group interactions could be found in the reaction time measures. Two additional results were interpreted as showing basically different attitudes with respect to the predictability of events: (1) there was a slow positivity between the verbal information and the following stimuli which was largest for schizophrenics in the conditions of certainty; (2) while normals showed long-term habituation only in N1- but not in P3-amplitudes, the reverse was true for schizophrenics.