- The neurophysiological correlates of working memory dysfunction in schizophrenia (2010)
- The pathophysiology of schizophrenia is still poorly understood. Investigating the neurophysiological correlates of cognitive dysfunction with functional neuroimaging techniques such as electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) is widely considered to be a possible solution for this problem. Working memory impairment is one of the most prominent cognitive impairments found in schizophrenia. Working memory can be divided into a number of component processes, encoding, maintenance and retrieval. They appear to be differentially affected in schizophrenia, but little is known about the neurophysiological disturbances which contribute to deficits in these component processes. The aim of this dissertation was to elucidate the neurophysiological underpinnings of the component processes of working memory and their disturbance in schizophrenia. In the first study the the neurophysiological substrates of visual working memory capacity limitations were investigated during encoding, maintenance and retrieval in 12 healthy subjects using event-related fMRI. Subjects had to encode up to four abstract visual shapes and maintain them in working memory for 12 seconds. Afterwards a test stimulus was presented, which matched one of the previously shown shapes in fifty percent of the trials. A bilateral inverted U-shape pattern of BOLD activity with increasing memory load in areas closely linked with selective attention, i.e. the frontal eye fields and areas around the intraparietal sulcus, was observed already during encoding. The increase of the number of stored items from memory load three to memory load four in these regions was negatively correlated with the increase of BOLD activity from memory load three to memory load four. These results point to a crucial role of attentional processes for the limited capacity of working memory. In the second study, the contribution of early perceptual processing deficits during encoding and retrieval to working memory dysfunction was investigated in 17 patients with schizophrenia and 17 healthy control subjects using EEG and event-related fMRI. A slightly modified version of the working memory task used in the fist study was employed. Participants only had to encode and maintain up to three items. In patients the amplitude of the P1 event-related potential was significantly reduced already during encoding in all memory load conditions. Similarly, BOLD activity in early visual areas known to generate the P1 was significantly reduced in patients. In controls, a stronger P1 amplitude increase with increasing memory load predicted better performance. These findings indicate that in addition to later memory related processing stages early visual processing is disturbed in schizophrenia and contributes to working memory dysfunction by impairing the encoding of information. In the third study, which was based on the same data set as the second study, cortical activity and functional connectivity in 17 patients with schizophrenia and 17 to healthy control subjects during the working memory encoding, maintenance and retrieval was investigated using event-related fMRI. Patients had reduced working memory capacity. During encoding activation in the left ventrolateral prefrontal cortex and extrastriate visual cortex was reduced in patients but positively correlated with working memory capacity in controls. During early maintenance patients switched from hyper- to hypoactivation with increasing memory load in a fronto-parietal network which included left dorsolateral prefrontal cortex. During retrieval right ventrolateral prefrontal hyperactivation was correlated with encoding-related hypoactivation of left ventrolateral prefrontal cortex in patients. Cortical dysfunction in patients during encoding and retrieval was accompanied by abnormal functional connectivity between fronto-parietal and visual areas. These findings indicate a primary encoding deficit in patients caused by a dysfunction of prefrontal and visual areas. The findings of these studies suggest that isolating the component processes of working memory leads to more specific markers of cortical dysfunction in schizophrenia, which had been obscured in previous studies. This approach may help to identify more reliable biomarkers and endophenotypes of schizophrenia.
- Attentional demand influences strategies for encoding into visual working memory (2007)
- Visual selective attention and visual working memory (WM) share the same capacity-limited resources. We investigated whether and how participants can cope with a task in which these 2 mechanisms interfere. The task required participants to scan an array of 9 objects in order to select the target locations and to encode the items presented at these locations into WM (1 to 5 shapes). Determination of the target locations required either few attentional resources (“popout condition”) or an attention-demanding serial search (“non pop-out condition”). Participants were able to achieve high memory performance in all stimulation conditions but, in the non popout conditions, this came at the cost of additional processing time. Both empirical evidence and subjective reports suggest that participants invested the additional time in memorizing the locations of all target objects prior to the encoding of their shapes into WM. Thus, they seemed to be unable to interleave the steps of search with those of encoding. We propose that the memory for target locations substitutes for perceptual pop-out and thus may be the key component that allows for flexible coping with the common processing limitations of visual WM and attention. The findings have implications for understanding how we cope with real-life situations in which the demands on visual attention and WM occur simultaneously. Keywords: attention, working memory, interference, encoding strategies