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The present study consists of two parts: The first part is made up of questions concerning the cognitive underpinnings of auditory verbal hallucinations in schizophrenia. As this thesis framed schizophrenia as a multivariate problem, neural correlates to auditory verbal and visual hallucinations were investigated in the second part. The main finding is that vividness of mental imagery was increased in all putative high-risk groups as well as the patients themselves, compared with low-schizotypy controls. Therefore, it seems that vivid imagery is a trait rather than a state marker, and may be related to the genetic liability to develop schizophrenia. However, no evidence was found for a linear relationship between vividness of mental imagery and predisposition to hallucinate. Self-reported imagery vividness and predisposition to hallucinate did not depend on psychomotor speed or intelligence. In addition, individual psychopathology ratings did not correlate significantly with the mental imagery scores. Furthermore, the analysis of the control orientation and the degree of dysfunctional psychopathological status across the schizophrenia spectrum, showed an independence of control orientation and dysfunctional status from each other, as well as from other markers of schizophrenia or schizophrenic-like individuals. As a conclusion, external control orientation seems to be a symptom or a trait marker of schizophrenia. The results lead to the assumption that, beside schizophrenic individuals, first-degree relatives and schizotypy controls have some impairments and visible signs without suffering from the illness directly. This would lead to the further assumption that the illness schizophrenia is not only genetic but also dependent on environmental factors. In the second part of the study, we investigated anatomical and functional brain abnormalities in the schizophrenia patients compared with first-degree relatives and healthy controls. Here, the results followed the continuum of healthy controls, first-degree relatives and schizophrenic patients in the functional and anatomical data sets, and in the language lateralization. The decrease of lateralisation correlated with the severity of symptoms in the patient group. The investigation of visual hallucinations showed activity in higher visual areas during the experience of visual hallucinations in a schizophrenia patient and in a blindfolded subject. The activity in higher visual areas followed the boundaries of category-selective areas in both subjects. In contrast to the memory-related areas found in the schizophrenic patient experiencing visual hallucinations, we did not observe memory-related areas during visual hallucinations induced by blindfolding. This suggests that the neural mechanisms underlying hallucinations in schizophrenia are at least partly distinct from those operational in cortical deafferentation. It is proposed that individual differences in psychopathology, as well as neuropsychological and psychosocial functioning may provide further means to understand the complex and highly dynamic aspects of hallucinations specifically and schizophrenia in general. The enlargement of the subject sample to high-schizotypy controls and first-degree relatives of patients allowed new insights into the mental imagery debate and the dysfunctional connectivity pattern known to be responsible for psychotic symptoms. Further topics of research are discussed.
Visual working memory (WM) and selective attention are fundamental cognitive mechanisms, both operating at the interface between perception and action. They are related because both are postulated to have limits with respect to how much information can be processed. Specifically, selective attention has been implicated as a limiting factor for the storage capacity of visual WM. However, visual WM and attention have been largely studied in isolation and interactions between the two have rarely been addressed. This dissertation aimed at investigating interactions between selective attention and the encoding of information into visual WM in the context of one common characteristic feature, namely their limitation in capacity. An experimental task was used that combined visual search with delayed discrimination and the demands on selective attention and WM encoding were manipulated orthogonally. In each trial participants were presented with a search array consisting of nine different grey geometric shapes. A small L-shaped item that appeared in one of four different orientations and that was coloured either blue or red was placed in the centre of each shape. Participants were instructed to search for predefined target items (Ls oriented 90°) and to memorise the shapes associated with these target items. After a delay phase a probe was presented and participants decided whether it did or did not match one of the memorised shapes. Attentional demand was manipulated by changing the search efficiency in the visual search component of the task (easy vs. difficult search) and WM load was manipulated by the number of targets (1 to 5). A behavioural study was conducted to isolate the processes that allowed participants to successfully encode complex shapes into WM while engaging spatial attention for a visual search task. The data provided evidence for a two-step encoding strategy. In the first step participants selected and memorised only the locations of all target items and only then they encoded the associated shapes at a later step. This strategy allowed them to cope with the interference between WM and attention that would otherwise take place. In the second part of this dissertation interference between visual attention and the encoding into visual WM was investigated on the level of neural activation using functional magnetic resonance imaging (fMRI). Specifically, the hypothesis was tested that the capacity limitation of visual WM is due to common limited-capacity neural resources shared by visual WM and attention. Two separate fMRI experiments were conducted that combined visual search and delayed visual discrimination for either objects (experiment 1) or locations (experiment 2). The results revealed overlapping activation for attention-demanding visual search and object WM encoding in distributed posterior and frontal regions. In the right prefrontal cortex and bilateral insula BOLD activation additively increased with increased WM load and attentional demand. Conversely, the analysis revealed an interaction effect in several visual, parietal, and premotor areas. These regions showed overlapping activation for the two task components and were severely reduced in their WM load response under the condition with high attentional demand. This interaction effect was found in similar frontal and posterior regions when combining visual search and spatial WM encoding in experiment 2. In contrast, regions in the prefrontal cortex were selectively responsive to WM load and differed to some degree depending on the WM domain. Here, activation associated with increased WM load was delayed rather than reduced under high attentional demand. The fMRI results provide convergent evidence that visual selective attention and the encoding of information into WM share, to a high degree, common neural resources. The findings indicate that competition for resources shared by visual attention and WM encoding can limit processing capabilities in distributed posterior brain regions but not the prefrontal cortex. The findings support the view that WM evolves from the recruitement of attentional mechanisms (Cowan, 2001; Wheeler und Treisman, 2002) the very same that act upon perceptual representations as well (Slotnick, 2004; Jonides et al., 2005; Pasternak and Greenlee, 2005; Postle, 2006; Ranganath, 2006). The similarity in the effects of interference between attention and the encoding of objects or locations into WM indicates that the attention-based model of WM encoding is valid across different WM domains. The capacity of visual WM can be limited at various stages of processing. The behavioural and fMRI data presented in this dissertation illustrate that one major bottleneck of information processing arises from the common demands on neural and cognitive resources shared between visual WM and selective attention during the encoding stage.
Synchronized neural activity in the visual cortex is associated with small time delays (up to ~10 ms). The magnitude and direction of these delays depend on stimulus properties. Thus, synchronized neurons produce fast sequences of action potentials, and the order in which units tend to fire within these sequences is stimulusdependent, but not stimulus-locked. In the present thesis, I investigated whether such preferred firing sequences repeat with sufficient accuracy to serve as a neuronal code. To this end, I developed a method for extracting the preferred sequence of firing in a group of neurons from their pair-wise preferred delays, as measured by the offsets of the centre peaks in their cross-correlation histograms. This analysis method was then applied to highly parallel recordings of neuronal spiking activity made in area 17 of anaesthetized cats in response to simple visual stimuli, like drifting gratings and moving bars. Using a measure of effect size, I then analyzed the accuracy with which preferred firing sequences reflected stimulus properties, and found that in the presence of gamma oscillations, the time at which a unit fired in the firing sequence conveyed stimulus information almost as precisely as the firing rate of the same unit. Moreover, the stimulus-dependent changes in firing rates and firing times were largely unrelated, suggesting that the information they carry is not redundant. Thus, despite operating at a time scale of only a few milliseconds, firing sequences have the strong potential to provide a precise neural code that can complement firing rates in the cortical processing of stimulus information.