Reduced top-down influences in contour detection in schizophrenia

Abstract

These data are further evidence that perceptual organization impairments in schizophrenia are illness severity-related, and that schizophrenia patients as a whole are less sensitive to top-down manipulations in this type of task.

Attentional modulation of perceptual organisation in schizophrenia

“…The basic finding was that in controls the order in which the stimuli were presented influenced the performance, with higher scores achieved when the stimuli were presented in increasing order of difficulty. This contextual top-down modulation was less effective in schizophrenia patients (Silverstein et al, 2006).…”

“…However, in the Silverstein et al study, high-contrast stimuli were used, and the stimulus parameters did not fit the physiological properties of early visual areas. In another study, Silverstein et al (2006) used a contour integration task in which participants had to detect circular shapes made of a closed path of Gabor patches that were embedded in randomly oriented patches (noise). This task is thought to be related to the functioning of the early visual cortex, similarly to our paradigm.…”

Are oscillatory brain responses generally reduced in schizophrenia during long sustained attentional processing?

“…However, the possibility that a reduction in alpha activity was only detected for non-targets because the larger number of epochs investigated for non-targets compared to targets led to a better signalto-noise ratio cannot be ruled out. Variation in oscillatory alpha activity should be assessed in more detail, as neuropsychological studies implicate an imbalance between sensory and cognitive processing in schizophrenia (Mathes et al, 2005;Silverstein et al, 2006) and paradigms with a low cognitive load may lead to an increased fronto-central alpha response and better intertrial phase coherence in patients than in healthy controls (Basar-Eroglu et al, in press).…”

Effects of stimulus structure and target-distracter similarity on the development of visual memory representations in schizophrenia

“…For example, all published studies of perceptual organisation in schizophrenia in which strong top-down input was required for perceptual grouping to occur have found that patients demonstrated impairments (e.g., Silverstein et al, 1996aSilverstein et al, , 1996bSilverstein et al, , 1998a. In contrast, when the stimuli to be processed consisted of closed, geometric forms consisting of visual primitives (e.g., Chey & Holzman, 1997;Knight & Silverstein, 1998), or even when these forms consist of noncontiguous elements but the overall shape was a``good form'' (Silverstein et al, 1998a(Silverstein et al, , 1998bSilverstein, Kova Âcs, Corry, & Valone, 2000;Silverstein, Wong, Schenkel, Kova Âcs, Feher, Smith et al, 2003), patients performed relatively normally. We view these and similar results as indicating that when processing mainly relies on prespecified feature hierarchies, patients perform relatively normally, whereas when dynamic grouping is involved, performance deteriorates.…”