Date of Award


Degree Type


Degree Name

Doctor of Philosophy


The effects of retinal eccentricity on prehension and on the perception of object dimensions was investigated. Human subjects reached out and grasped different sized objects viewed at different peripheral positions along the horizontal and vertical meridians. The sensitivity of visuomotor grip scaling to object width was compared with visuoperceptual judgements about object width.;When retinal eccentricity was varied in the temporal field, along the horizontal meridian, it was found that reaches in peripheral vision were slower and exhibited longer deceleration periods relative to reaches in central vision. Further, the amplitude of the grasp, though scaled to object width, increased as object views became more eccentric. Despite these changes in reach kinematics, prehension was, in general, just as reliable in the far periphery as in the near periphery. Retinal eccentricity exerted opposite effects on visuomotor and visuoperceptual measures of sensitivity to object width. Unlike grasping, which overestimated object width in the visual periphery, perceptual judgements underestimated object width and became more variable.;Examination of prehension and perception responses in the peripheral upper and lower visual fields produced results similar to those observed in the peripheral temporal field. In addition, prehension responses were more efficient and grasps were better scaled, in the lower visual field compared to the upper field. Perceptual judgements did not show a performance asymmetry between the upper and lower fields.;Retinal eccentricity affected prehension even when subjects were grasping an object with which they were highly familiar and it was concluded that subjects' 'perceptions' of object width in the visual periphery do not influence grasp calibration. It was posited that the changes observed in prehension kinematics reflect a compensatory strategy for dealing with the closing phase of the reaching response under conditions in which detailed visual input is not available.;The findings are consistent with research demonstrating that the cortical visual areas mediating the control of skilled action have an extensive representation of the visual periphery; whereas, the cortical visual areas subserving object perception are largely restricted to the central field. The findings are also consistent with the possibility that the lower visual field may be more intimately linked with the visual areas subserving visuomotor prehension.



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