The experiments reported in this thesis aimed at investigating how voluntary control is exerted through the modulation of neural circuitry mediating reflexive behaviours. Eye movements were used as a model system to study how two primitive collicular reflexes, the fixation reflex and inhibition of return are
integrated, for use in visual cognition. I observed that automaticity and control independently influence the FOE (Chapter 2). Automaticity was studied by systematically varying the foreperiods within a block of trials. Voluntary control was examined by inducing different task sets (either pro-, or anti-saccades). I showed that both influence the FOE independently. In chapter 3, I examine whether patients with PD can voluntarily control their oculomotor reflexes. I showed that dopamine deficiency due to Parkinson's disease impairs voluntary control of the FOE (Chapter 3). I explain this in terms of the role of the basal
ganglia in oculomotor behaviour. In Chapter 4 and 5, I asked how inhibitory tags (IOR), which are used by visual cognition to aid visual search, are remapped and represented by the parietal cortex. In order to investigate this, Transcranial Magnetic Stimulations (TMS) was applied over the parietal cortex just after the onset of an eye movement. I found that when subjects were
required to make horizontal eye movements to the right or left, TMS over the right parietal cortex prevented remapping of IOR for both saccade directions (Chapter 4). However, TMS over the left parietal cortex did not influence remapping of IOR. When subjects were required to make vertical eye movements (Chapter 5), TMS over the right parietal cortex prevented remapping of IOR in both the right and left visual field. However, TMS over the left parietal cortex prevented remapping of IOR in only the right visual field.
These findings suggest a different role for the right and left parietal cortex. More precisely, the findings suggest that the right hemisphere does not only remap the inhibitory tag, but also maintains a stable representation across eye movements. The reported findings provide further support for the theoretical
models which were discussed in the introduction.