Thesis #841

In this PhD project, the functions of cortical regions that control smooth pursuit eye movements (SPEM) and visual attention were investigated. Combining behavioural (eye movement) measurements and functional magnetic resonance imaging (fMRI) the cortical areas participating in the processing of visual information and motor information were investigated. Overlapping cortical Blood Oxygen Level Dependency (BOLD) activations might indicate where the transformation of visual input information into a motor output response takes place. Furthermore, the influence of visual attention on these mechanisms was studied. In a third step, the location and function of subregions of the motion sensitive MT+ complex – which plays a crucial role in the control of SPEM – was explored in more detail. In the final experiment, functional differences between regions of the SPEM network during the processing of visual motion by varying the amount of coherently moving target dots were investigated. In the first study it was shown that visual information processing takes place in the posterior parietal cortex (PPC) and MT+ and that oculomotor output processing takes place in the frontal eye fields (FEF), the supplementary eye fields (SEF), the cingulate gyrus and precuneus in addition to the above mentioned areas. Possible transformation sites were found in MT+ and within the PPC. In the second study it was shown that processing of visual attention during SPEM is fully integrated in the SPEM network, but certain aspects of the control of attention like the dissociation of attention from gaze are especially processed in the PPC. Furthermore it was shown that the ‘premotor theory’ of Rizzolatti (1984) is also valid for SPEM. In the third study two subregions of the motion sensitive MT+ complex, MST (medial superior temporal) and MT (middle temporal), were identified on group level. In contrast to monkey studies in the current study the eccentricity of the flow field relative to the midline played a minor role for the location of the MT+ subregions. These results question the assumed size of MT receptive fields in humans. The fourth study revealed that the visual input signal is modulated by retinal information whereas the oculomotor output is modulated by the eye movement signal or a mixture of visual and oculomotor information. Integration of visual and oculomotor information seems to take place in MST and visual areas V7/LOP. Processing of differential motion of eye and background appears to take place in the PPC. Surprisingly PPC hardly reacted if eye and background moved in phase. Primary visual area V1 probably receives eye movement signals. Its functional connections and exact functional role need further investigation.