AbstractSchizophrenic and alcoholicSs judged stimulus durations of 0.50 and 0.55 seconds. Stimulus was 1, 3 or 5 dark dots and was preceded by variable foreperiod duration of 1, 3 or 5 seconds. Judgement of stimulus duration was found to be a monotonically increasing function of foreperiod duration. When pitch of sound was correlated with foreperiod duration so that it functioned as prior information, such monotonic function was eliminated for schizophrenics only when the numerosity of dots was constant from trial to trial. The effect of prior information as to foreperiod duration was negligible for both schizophrenics and alcoholics when the numerosity of dots was variable from trial to trial. Uniqueness of schizophrenia was shown to be associated with prior time uncertainty reduction of foreperiod duration in the context of minimal event uncertainty of stimulus.