ON HEMISPHERIC SPECIALISATION AND VISUAL DIRECTION SENSE

In this thesis the hypothesis is advanced that the right cerebral hemisphere of man, in particular the posterior parietal cortex, is specialised for representing egocentric space.

It is demonstrated that the majority of human subjects are more accurate in directing saccades to the remembered position of small targets that are briefly flashed in the left visual (hemi)field (LVF) than in the right (RVF). Furthermore, most individuals are also more accurate in localising perceptually the position of small visual stimuli in the LVF than in the RVF. Both these results suggest that the right hemisphere is superior to the left in localisation. Furthermore, the finding that in both motor and perceptual tests subjects appeared consistently to underestimate the direction of the visual stimulus suggests that similar representations may subserve both these forms of localisation.

In two further experiments averaged event-related potentials (ERPs)--both visual and ocularmotor--were recorded over the parietal cortex of each hemisphere whilst subjects made saccades to either remembered or to continuously lit targets. I found no evidence of differences in activity between the parietal cortex of each hemisphere. However, components of ERPs related either to the directing of attention, the programming of saccades or the predictability of target location were found over both hemispheres.

In the both of these experiments I found that the reaction time of saccades (SRT) may be related to the covert directing of attention. In particular there was a cost in SRT if attention first had to be disengaged from a visual stimulus. Furthermore, there is evidence to suggest that attention may selectively be oriented to detect events either in peripheral or central vision.

The data presented in this thesis are consistent with the hypothesis that the right hemisphere of man is specialised for representing egocentric space. Although the parietal cortex appears to have an important role in directing attention and gaze, the parietal cortex of the right hemisphere may not exclusively be specialised for those functions.