Reduced multisensory integration in individuals with schizophrenia : evidence from psychophysical studies

The studies in this dissertation investigate multisensory integration in a sample of patients with schizophrenia using three different paradigms. Chapter 2 evaluates the integrity of visual-tactile integration in this population using the size-weight illusion (SWI), in which the smaller of two objects of identical mass feels substantially heavier compared to a larger object. The result show a reduced SWI in schizophrenia patients which cannot be explained by observed differences in weight discrimination sensitivity. These results support the idea of reduced multisensory integration, as well as dysfunctional efference copy mechanisms, in this population. These integration deficits are correlated with reduced sensorimotor gating, as assessed by prepulse inhibition (PPI). Chapter 3 investigates auditory and visual integration of congruent cues for target detection using an intersensory facilitation of reaction time (RT) paradigm, on which non-psychiatric individuals are known to be faster to detect bimodal targets, with cues from two sensory modalities, compared to detection times for unimodal targets. Though patients with schizophrenia do show some RT speeding when detecting bimodal targets, it is not to the same extent as non-psychiatric individuals, indicating reduced automatic multisensory integration of these cues. The amount of RT facilitation is related to the symptoms of schizophrenia, as well as the modality of the patient's hallucinations. Finally, Chapter 4 presents a version of the McGurk illusion, in which auditory speech perception is biased by the presentation of an incongruent visual speech cues. This McGurk paradigm has yielded inconsistent patterns of results in previous studies with schizophrenia patients, and this study design includes many experimental conditions designed to address methodological differences between these earlier investigations. This study finds no evidence of a difference between schizophrenia patients and non- psychiatric participants on the incidence of the McGurk illusion; however, there is evidence for impaired lip- reading ability in this population. Generally, these results support the idea of a specific impairment in multisensory integration in patients with schizophrenia, above any beyond deficits within individual sensory modalities. As these integration deficits correlate with the symptoms and characteristics of schizophrenia, this is an important area for future research