Cortical blindness after overlapping retinal-striate lesions: a limit to plasticity in the central visual system.

Lesions in cats, rats, and monkeys that spare more than 2% of the optic tract or visual cortex cause trivial deficits on most measures of vision. Overlap in the topography of the visual system may allow the spared remnant to 'see' a wider field of vision than the physiological map predicts. We tested whether monkeys left with only the lower retinal-field portion of their striate map could see with information coming from the upper half of the retina. In 6 rhesus monkeys the ganglion fibers exiting from the lower half of both retinae were cut with a photocoagulator. Later, the portion of area 17 which, according to the electrophysiological map, controls upper retinal vision, was ablated bilaterally. The combined retinal and striate lesions overlapped to include the entire visual field. Together they produced cortical blindness. The monkeys' performance of two pattern and object tasks remained at chance throughout the survival period. A previous study has described considerable sparing of vision after combined optic tract and visual cortex lesions in cats. Differences in the lesion methods and in the anatomy of the cat and monkey visual system may explain the disagreement.