Visual deficits and recovery following monocular lid closure in a prosimian primate

The behavioral consequences of long-term monocular lid closure in 6 deprived and 2 normal prosimian primates (Galago crassicaudatus) were examined using a visual field perimetry technique and a variety of visuomotor tests. Deprived galagos had their right lid sutured either within 8 (n = 4) or 38 (n = 2) days postpartum and were reared under these conditions for at least 8 months. Normal galagos responded briskly on all visuomotor tests including tracking, response to visual threat, ability to descend from a platform, and obstacle avoidance. Perimetry tests of these animals showed that the visual field with both eyes open was 180 degrees while each eye tested individually had a 135 degrees field. Deprived galagos tested with their experienced eye exhibited normal visuomotor behavior and normal visual fields. Following reverse suture all deprived animals behaved initially as if blind on all tests. Over the next 4-8 weeks these animals showed some limited recovery on the visuomotor tasks. Tests of perimetry indicated that by the end of four weeks of testing, vision was limited to the deprived monocular segment where it remained stable to the end of the testing period. With both eyes open, deprived galagos exhibited a normal 180 degrees visual field. Curiously, however, when stimuli were introduced simultaneously in corresponding points in both monocular segments of the field, the majority of orienting responses of deprived subjects favored the right deprived eye. Under similar testing conditions one normal subject favored its right eye and the other its left eye. The reversibility of monocular deprivation was explored further either by extending the period of forced use of the deprived eye in one animal or by removing the non-deprived eye in another animal. No further improvement in visual field perimetry was seen following a longer period of forced use of the deprived eye. In contrast, a substantial increase in the visual field could be demonstrated one month following removal of the originally experienced eye: the deprived eye field expanded to include the entire 90 degrees ipsilateral hemifield. The contralateral field of the deprived eye, however, continued to remain blind even after one year. These behavioral results suggest that although the original binocular competitive disadvantage suffered by the deprived eye during development is relatively permanent, it can partially be ameliorated by late removal of the non-deprived eye.