Schizophrenic feature recognition deficits are independent of task criterion

Dysregulation of thalamic sensory "transmission" in schizophrenia: neurochemical vulnerability to hallucinations

Cholinergic arousal mechanisms predispose thalamic and cortical neurons to fire action potentials at gamma rhythms, which have a tendency to resonate in thalamocortical networks, thereby forming coherent assemblies under constraints of sensory input to specific thalamic nuclei, on the one hand, and prefrontal and limbic attentional mechanisms, on the other. Perception may be based on sustained assemblies of coherent gamma oscillations in thalamocortical circuits. In schizophrenia, the impact of sensory input on self-organization of thalamocortical activity may be generally reduced. As a result, processes underlying perception can become uncoupled from sensory input, particularly at times of hyperarousal, leading to domination of attentional mechanisms and the emergence of hallucinations. Evidence is reviewed that implicates excessive neuronal noise in specific thalamic nuclei in the generation of hallucinations in schizophrenia. Nicotinic receptor abnormalities, dopaminergic hyperactivity and glutamate-receptor hypofunction are reconciled within a model of psychotic symptom generation that places crucial emphasis on dysfunction of the reticular thalamic nucleus.

Self-initiated encoding facilitates object working memory in schizophrenia: implications for the etiology of working memory deficit

BACKGROUND

Working memory (WM) deficit is present in a majority of patients with schizophrenia but it is unclear which components of WM are impaired. Past studies suggest that encoding may be compromised. One important determinant of encoding is the deployment of selective attention to the target stimulus. In addition, attention and encoding are modulated by motivational factors. In this study, we investigated the effects of self-initiated encoding (i.e., voluntary attention) on WM.

METHODS

19 patients with schizophrenia and 19 matched control subjects participated in visual WM and control tasks. Encoding was manipulated by asking subjects to select from two face targets and memorize 1) one of the two identical faces (Non-preference condition), 2) one that is marked (Non-choice condition), and 3) one they prefer (Preference condition). WM accuracy for both location (spatial) and identity (object) was measured.

RESULTS

Overall, patients with schizophrenia were less accurate and slower than the control subjects but the deficit was greater for object WM. However, patients were more accurate in object WM when they selected a preferred face as their target during encoding (preference condition) compared with the other two conditions. This effect was not significant for spatial WM.

CONCLUSIONS

These results suggest that voluntary, self-initiated attention may facilitate object encoding especially if the selection of the target involves affective choice, and that attention may play different roles in encoding 'what' versus 'where' in WM. Since encoding affects all forms of memory, these results may have a more general implication for memory.

Rapid visual information processing in schizophrenic patients: the impact of cognitive load and duration of stimulus presentation. A pilot study

The inability to sustain attention has been proposed as a core deficit in schizophrenia. The Continuous Performance Task (AX-CPT) and the Rapid Visual Information Processing Task (RVP) are widely used neuropsychological tasks to measure sustained attention. The RVP displays numbers as stimuli, whereas the AX-CPT uses letters. Ten patients with chronic schizophrenia and 18 healthy control subjects were studied using four different versions of the RVP. The versions differed with regard to stimulus presentation time (600 vs. 1,200 ms) and the number of target sequences to be memorized: either one sequence (low cognitive load) or two sequences (high cognitive load). Schizophrenic patients showed a reduced number of hits only on the task version with 600 ms stimulus duration coupled with high cognitive load. The combination of high cognitive load and short stimulus duration created a critical performance breaking point for schizophrenic patients. This finding supports the hypothesis that patients have an impaired ability to coactivate different cognitive performances; thus the results favor the theory of impaired functional connectivity in schizophrenia.

Hallucinations: synchronisation of thalamocortical gamma oscillations underconstrained by sensory input

What we perceive is the product of an intrinsic process and not part of external physical reality. This notion is consistent with the philosophical position of transcendental idealism but also agrees with physiological findings on the thalamocortical system. gamma-Frequency rhythms of discharge activity from thalamic and cortical neurons are facilitated by cholinergic arousal and resonate in thalamocortical networks, thereby transiently forming assemblies of coherent gamma oscillations under constraints of sensory input and prefrontal attentional mechanisms. Perception and conscious experience may be based on such assemblies and sensory input to thalamic nuclei plays merely a constraining role in their formation. In schizophrenia, the ability of sensory input to modulate self-organisation of thalamocortical gamma activity may be generally reduced. If during arousal thalamocortical self-organisation is underconstrained by sensory input, then attentional mechanisms alone may determine the content of perception and hallucinations may arise.

Perceptual and conceptual information processing in schizophrenia and depression

Schizophrenic patients (n = 20), depressive patients (n = 20), and normal adults (n = 20) were compared on global vs local analyses of perceptual information using tachistoscopic tasks and on top-down vs bottom-up conceptual processing using card-sort tasks. The schizophrenic group performed more poorly on tasks requiring either global analyses (counting lines when distracting circles were present) or top-down conceptual processing (rule learning) than they did on tasks requiring local analyses (counting heterogeneous lines) or bottom-up processing (attribute identification). The schizophrenic group appeared not to use conceptually guided processing. Normal adults showed the reverse pattern. The depressive group performed similarly to the schizophrenic group on perceptual tasks but closer to the normal group on conceptual tasks, thereby appearing to be less dependent on a particular information-processing strategy. These deficits in organizational strategy may be related to the use of available processing resources as well as the allocation of attention.

Lateralized visual-field inattention in schizophrenia

Numerous studies have shown impaired verbal functioning in schizophrenic patients as compared with normals. The verbal deficits are generally attributed to damage of the left cerebral hemisphere. This attribution is based on literature which suggests that verbal processing is primarily mediated by the left hemisphere in right-handed humans. This study explored left-hemispheric integrity directly by assessing sustained attention in both the left and right hemispheres of 40 schizophrenic patients with the Weintraub Cancellation Tasks. Patients made significantly more errors of omission on the right visual field than on the left. These results are consistent with cognitive research in schizophrenia by demonstrating selective left-hemispheric impairment relative to right-hemispheric functioning.