Altered 'three-flash' illusion in response to two light pulses in schizophrenia

Background: Disorganization is a core dysfunction in schizophrenia. Coherent thought and behavior rely on the interactive neural responses to temporally discrete external events. Previous studies have demonstrated that a single visual stimulus (event) is abnormally affected by another (as in backward masking), but the integration (or 'synthesis') of temporally discrete events remains largely unexplored in schizophrenia. We examined the perceived interaction of two elementary visual events in schizophrenia patients, using a psychophysical approach. Methods.- Two brief, spatially-coincident foveal light pulses (5 ms) were presented with different inter-stimulus intervals (ISls). At ISIs around 100 ms, a substantial proportion of the light pulse pairs was paradoxically perceived as three flashes, a known phenomenon in normal subjects. The subjects reported the number of flashes perceived ('one', 'two' or 'three'). Results.- Schizophrenia patients (n=28) reported fewer 'three flashes' than normal controls (n=26) at the ISIs where 'three flash' reports were greatest in normal subjects (90 to 110 ms). On the other hand, at longer ISls (130-310 ms) patients reported 'three flashes' in more trials than did normal subjects. The perception of three flashes in patients was correlated with certain aspects of clinical status, including the positive and general subscales of the PANSS. Discussion.- The alteration of the 'three-flash' illusion in schizophrenia suggests that the synthesis of discrete visual events is temporally 'dilated' or distorted, which might contribute to disorganized thought and behavior. (C) 2008 Elsevier B.V All rights reserved.