A neuropsychological investigation into nasotemporal overlap in the human retina.

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 non-responding hemisphere (a 'crossed' reaction) . In the latter case an interheraispheric crossing is presumably necessary and so the crossed-minus-uncrossed difference (CUD) can be tentatively 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 and there would be no need for any interhemispheric crossing. Four experiments failed to confirm this prediction in that a CUD was found to be present with stimuli presented down to an eccentricity of 1/2 deg in conditions of: vocal (Experiment 1) and manual (Experiment 2) responding in partially dark adapted subjects; manual finger release and thumb press responding in light adapted subjects (Experiment 3); manual responding in an acallosal subject (Experiment A). The results are interpreted as arguing against the existence of overlap in man though some possible reasons why this conclusion may be premature are discussed.