A.4 Discussion
B.4.7 Disturbed semantic networks in schizophrenia?
The contrast scores can be interpreted as a measure of associative strength of objects within super-ordinate categories and as a measure of distinction between the cortical representations of these cat-egories. In the schizophrenic group these measures were significantly reduced, indicating a less dis-tinct cortical organization of the semantic system in schizophrenia. The localization of these differ-ences along the visual ventral stream as the major cortical substrate of object representation further supports this view. The loosening of associative strength within categories was also reflected in the
7 To assess the signal quality, the SNR was computed for each object-specific MNE for each subject. The SNR was defined as the ratio of the root mean square of the time window from 170-210 ms after stimulus onset to the standard deviation of the 100 ms prestimulus baseline. Analysis of variance with the between-factor GROUP indicated that groups did not differ in terms of there SNR (F<1). Additionally, in an identical way the SNR for the time interval
B.4.7 : Discussion - Disturbed semantic networks in schizophrenia?
outcome of the hierarchical clustering where patients showed a less well-defined representation of objects. Categorical representations already develop during infancy (Eimas, 1994) and formation of these categories is based on correlations between attributes of individual objects (Bhatt et al., 2004).
On a cortical level this might correspond to the formation of neural networks by synchronous neural activity through Hebbian learning. As categorical representations are formed long before the onset of schizophrenia, one might wonder about the causes of semantic dysfunction in adult schizophren-ics. Recent views have suggested that schizophrenia may be a disorder of dysfunctional neuroplas-ticity and deficits in GABAergic (Costa et al. 2004), glutamatergic (McCullumsmith et al., 2004), cholinergic (Hyde & Crook, 2001) and dopaminergic (Winterer & Weinberger, 2004) neurotrans-mitter systems involved in neuroplasticity mechanisms have been found in schizophrenia. These deficits might result in the arbitrary spreading of activation within and the formation of disordered semantic neural networks. In that sense, disturbed semantic networks caused by altered neurotrans-mitter systems can be a plausible explanation for the outcome of this study.
92
C : Concluding remarks and outlook - Concluding remarks and outlook
C Concluding remarks and outlook
The present study examined object categorization from a neuroscience perspective. Spatiotemporal correlations and unsupervised clustering were applied to the magnetoencephalographic recordings.
While previous research has shown difference between superordinate categories, this is the first study that reveals associations of exemplars within categories with a clear distinction across super-ordinate categories. It is also noteworthy that the found effects were rather localized and possibly would not have been detected with a less dense array of sensors. The methods used in this study re-quire sufficient spatial resolution, which can only be achieved with high-density recordings. In ad-dition, this study illustrates how methods of analysis that go beyond the traditional approach of comparing mean amplitudes at certain time windows and recording sites, can provide new insights into cognitive mechanisms and their underlying brain architecture.
The possibility to look at categorical organization and associative strengths within semantic net-works is not limited to control subjects and can also be applied to clinical samples. Here, weaker as-sociations were discerned in a group of schizophrenic patients. It is conceivable that in the future di-agnosis and classification of disease will be based less exclusively on ‘surface’ measures and rely more upon direct indices of brain function. Indeed, recent research has shown that abnormal mag-netic brain activity differs between schizophrenic and depressed patients (Wienbruch et al, 2004) and is helpful in distinguishing between schizophrenic subgroups (Fehr et al, 2003). Possible appli-cations of the findings might not be limited to assess the status quo - measuring changes across time could allow to monitor or evaluate therapeutical improvements. Therapeutical approaches should include systematic training of base level concepts to restore their represenation as well as proper medication addressing deficits or imbalances of transmitter that potentially cause the dysfunctions within semantic neural networks.
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