Human frontal eye fields and visual search
This thesis tested whether the human frontal eye fields (FEFs) have visuospatial functions that are dissociable from FEF oculomotor functions. Functional magnetic resonance imaging (fMRI) was used to localize the FEFs, and transcranial magnetic stimulation (TMS) was applied in a series of experiments to transiently disrupt information processing in the FEFs. It was shown that TMS applied over the right FEFs degrades subjects' performance on a visual conjunction search task in which eye movements were not required and were not made. A TMS timing protocol subsequently showed that computations in the FEFs that occur between 40 and 80ms after the onset of a visual search array are critical for accurate performance. This suggests that, as in the monkey, the human FEFs may accumulate and use visual evidence from extrastriate cortex, which forms the basis for accurate visuospatial discrimination. A training protocol showed that the right FEFs are no longer critical for accurate visuospatial discrimination performance once a search task has been extensively practised. This study further suggested that the FEFs may have a previously unknown role in the perception of left-right rotated shapes. A study on feature and spatial priming indicated that these two phenomena have distinct causal mechanisms. The left FEFs appear to access a spatial memory signal during the process of saccade programming. When TMS is applied during this period, the spatial priming benefit is abolished. Altogether, this thesis presents evidence that visuospatial and oculomotor functions can be dissociated in the human FEFs. The data on timing and the effects of learning correspond well with results reported in monkeys. The priming experiment offers the first evidence that the left FEFs are crucial for spatial priming, while the learning study suggests the novel hypothesis that the FEFs are crucial for left-right rotated shape perception.