Organization of the visual projection upon the optic tectum of a reptile (Alligator mississippiensis)

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Abstract
Electrical responses were recorded from the optic tectum of the alligator with steel microelectrodes, and the receptive field for each of a series of systematically spaced recording sites was delineated on a transparent plastic hemisphere positioned in front of the contralateral eye. A precise retinotopic relationship was determined with the inferior portion of the retina projecting onto the dorsal surface of the tectum, and the superior portion of the retina represented on the lateral and ventral aspects of the tectum. The equatorial line of the hemisphere is situated laterally and runs obliquely across the tectum from the rostrolateral pole to the caudomedial corner. The center of the visual field occupies a disproportionate amount of tectal space providing for functional magnification. Receptive field size and spacing diminish for the representation of the central region implying a retinal area for increased visual acuity. It is suggested that this alinearity between the visual field and the tectum is expressed in its fiber architecture and is probably a consequence of specialization of the eye rather than an evolutionary modification within the reptilian central nervous system.

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