HUMAN SACCADIC EYE-MOVEMENTS IN THE ABSENCE OF THE GENICULOCALCARINE PROJECTION

This paper describes the results of an investigation into the spatial and temporal properties of the saccadic eye-movement system in a subject with a ''blind'' hemifield caused by accidental damage to the left geniculostriate projection. The results show that blind hemifield stimulation can elicit goal-directed saccades of short latency provided the contrast of the visual stimulus is above the threshold required for saccade generation at the corresponding stimulus eccentricity. Signal processing associated with blind hemifield pathways shows reduced sensitivity by comparison with the normal hemifield and greater dependence on stimulus size, eccentricity and mode of presentation. Higher contrast levels are always needed for blind field stimulation and larger variance in mean saccadic amplitudes and latencies is normally observed. Large stimulus eccentricities which would normally require corrective saccades exhibit a different response pattern to that observed in normal vision. The results show that the centre of gravity effect and the generation of rapid, ''express-saccades'' observed in normal vision are not inevitable outcomes of midbrain processing of visual information. The subject''s performance reveals strategic patterns of eye-movement responses which maximize the use of his normal hemifield. Visual performance in the control of saccadic eye-movement responses appears to be optimum when the detection of the visual stimulus is associated with both the geniculostriate and the midbrain pathways. Experiments designed to test for the effects of light scatter into the normal hemifield show that the subject''s saccadic eye-movement performance following visual stimulation of the blind hemifield cannot be attributed either to the amount or the pattern of light scatter into the normal hemifield. The implications of present findings on the results of other studies of eye-movements in subjects with scotomas associated with cortical lesions are also examined.