Nasotemporal overlap in the human retina investigated by means of simple reaction time to lateralized light flash

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Abstract
When light flashes are presented laterally simple vocal and manual responses are faster to stimuli in the visual half-field having direct access to the responding hemisphere (an “uncrossed” reaction) than stimuli which go initially to the nonresponding hemisphere (a “crossed” reaction). In the latter case an interhemispheric crossing is presumably necessary and so the crossed-minus-uncrossed difference (CUD) can be identified with interhemispheric transmission time. This paradigm was used to investigate the problem of whether or not there is an overlap of ipsi- and contralaterally projecting ganglion cells at the border between nasal and temporal areas of the human retina, resulting in dual representation of the midline in the brain. If such an overlap does exist then presenting stimuli on this region ought to result in an abolition of the CUD since information would be equally available to both hemispheres. Accordingly vocal and manual reaction times to flashes presented at 1/2, 1, 2, and 4 deg of visual angle were measured. In both cases a consistent CUD was found and this was present at all four points. These results are interpreted as arguing against the existence of overlap in man though some alternative possibilities are discussed.