NEURAL AND BEHAVIORAL-EFFECTS OF EARLY EYE ROTATION ON THE OPTOKINETIC SYSTEM IN THE WALLABY, MACROPUS-EUGENII

Total optokinetic responses measured by electro-oculography and single unit recordings from the nucleus of the optic tract and torsal termin al nucleus (NOT-DTN) pf the. accessory optic system were taken from yo ung adult wallabies, whose one eye had been rotated about the optic ax is at birth, and were compared with those from normal controls. 2. The velocities of the horizontal component of the slow phases of optokine tic nystagmus were measured in the horizontal plane as a function of t he direction of stimulus motion. In normal animals the overall gain du ring monocular stimulation was greatest for horizontal temporonasal mo vement, with a lesser response to movement in the opposite, nasotempor al, direction. Upward or downward vertical stimulus motion did not eli cit horizontal responses. In animals where one eye was removed at birt h and the other eye was normal, the characteristic bidirectional respo nse was retained; the response was identical with that elicited from o ne eye of a normal animal. 3. After surgical rotation (extorsion) of t he left eye by similar to 90 degrees on or within a few days of birth, the animals were grown to adulthood. The visual streak of the retina of the operated eye was then found, in individual cases, to be between 30-100 degrees from horizontal with the head held in the standard res ting position. This angle was taken as the definitive degree of cyclot orsion resulting from the operation in each animal. The extraocular mu scles connected with regions of the eye adjacent to the location of th eir outgrowth in the orbit and not with the normal point of attachment on the globe, Animals were tested as young adults after 8-11 mo of no rmal visual experience. 4. When the animal had one normal and one rota ted eye, monocular stimulation of the normal eye led to optokinetic ny stagmus that was not obviously different to that obtained from monocul ar stimulation in animals with two normal eyes. Monocular stimulation of the rotated eye induced the largest horizontal slow phase velocity when the stimulus was moved in a direction that was dose to the estima ted angle of rotation of the eye. The bidirectional response character istics of the rotated eye were retained but the response became more a symmetric than normal; the response in what would have been the tempor onasal direction was considerably larger than the response in the oppo site direction. The directional properties of the optokinetic response s with both eyes open were the sum of the responses from each eye alon e. 5. When one eye was rotated at birth and the other removed at the s ame time, the slow phases of horizontal eye movements had a maximum ve locity when the stimulus moved at an angle similar to that of the esti mated rotation. The response remained bidirectional although the veloc ity of the response in what would have been the nasotemporal direction was greatly reduced. Thus the directional characteristics of the opto kinetic nystagmus appeared not to be influenced by the rotated positio n of the eye, but there were quantitative changes in the magnitude of the responses. 6. Physiological recording revealed that the average di rectional tuning of neurons in the NOT-DTN contralateral to the rotate d eye was similar in each case to the definitive angle of the eye rota tion when that eye was stimulated monocularly. That is, cells showed n o trace of adaptation to, or compensation for, the rotated position of the contralateral eye. 7. In animals where one eye was extorted and t he other was normal, the directional tuning plots of neurons in the NO T-DTN ipsilateral to the normal eye showed a small but significant ext orsion of their directional tuning suggesting that the tuning of motio n detectors in the ipsilateral visual pathway may be influenced by the rotation of the opposite, contralateral eye.