Cerebral lateralization in schizophrenics as explained by a lateralized semantic priming paradigm

There is empirical research that has demonstrated that semantic priming facilitates lexical decision making in non-schizophrenics (Neely, 1991) as well as schizophrenics (Manschreck et al., 1988). Furthermore, lexical decision tasks that utilize semantic primes and targets presented to either to the left or right visual field have allowed researchers to speculate about right and left hemispheric language processing capabilities. Priming effects generally diminish when the semantic distance between the prime and target is great; however, indirect priming has been demonstrated to be robust in thought-disordered schizophrenics and minimal in other groups. In the only study of lateralized indirect semantic priming, indirect priming effects occurred in thought disordered schizophrenics bilaterally and were minimal or absent in non-thought disordered schizophrenics and normal controls (Weisbrod et al., 1998). The study by Weisbrod et al. contained a critical oversight; primes were presented centrally and targets laterally, not allowing for a direct measure of interhemispheric processes (Chiarello, et al., 1990). The current study presented primes and targets to the same visual field as well as to opposite visual fields at two different SOAs (215 ms and 750 ms) to 21 thought-disordered schizophrenics, 26 non-thought schizophrenics, and 23 controls to better measure hemispheric priming differences. Results indicated that the thought-disordered schizophrenics displayed robust priming for direct and indirect primes presented to the same hemisphere at both SOAs. Non-thought disordered schizophrenics initially inhibited to indirect primes in the right hemisphere and primed to direct primes in the left hemisphere; however, at the long SOA the opposite pattern was observed. Non-schizophrenics demonstrated initial left hemisphere inhibition for indirect primes and broad overall right hemisphere activation, whereas long SOA right hemisphere inhibition and slight left hemisphere activation for indirect primes was observed. No cross priming effects were observed at the short SOA. Long SOA overall left hemisphere inhibition of right hemisphere priming was observed in thought-disordered schizophrenics, whereas the other groups displayed left hemisphere inhibition of right hemisphere direct priming only. Results indicated that the nature of thought disorder in schizophrenia may be related to dysfunctional lateral inhibition in the left cerebral hemisphere.