Early visual deprivation: strategies to prevent visual deficits and factors that promote recovery when deficits occur

The role of experience in the development of the central visual pathways has been a controversial issue that has been explored in the past through examination of the consequences of imposed periods of continuously abnormal visual input. I will report on a different approach whereby kittens were provided daily with separate periods of normal and abnormal visual exposure (Mitchell et al., 2003). For a period of 4 weeks beginning at 4 weeks of age, the total visual experience each day of kittens was restricted to 7 hours split unevenly between periods of monocular (ME) and binocular (BE) exposure. For the former exposure, the kittens wore an opaque mask that covered one eye for either 4 to 7 hrs while for the remaining time the mask was removed to allow binocular exposure. Visual acuity of the deprived eye for square-wave gratings was measured by use of a jumping stand immediately after termination of the 4 — week period of mixed visual exposure. The results indicated that two hours of normal binocular visual input each day allowed the development of normal visual acuity in both eyes even when imbedded in far longer periods of abnormal visual exposure that alone would lead to blindness in the deprived eye. The beneficial effects of BE were not observed in animals that wore dissociating prisms during the period when both eyes were open indicating that it was critical that the visual input to the two eyes be both simultaneous and concordant for visual acuity in the deprived eye to develop to normal levels. The results are not what would be expected by a passive process whereby neural connections are continually shaped in response to the integrated visual input during the sensitive period, but instead suggest a process which places greater weight on normal or certain preferred input that matches closely the anatomical and functional organization that develops prior to birth through programmed gene expression. In particular, concordant binocular visual input may be privileged with respect to its influence on visual development. On the other hand, changes to the visual pathway in response to visual input that favors one eye may require more extensive exposure because of the time necessary to mediate a different or more extensive set of synaptic or hetereocellular events than those that occur in response to concordant binocular input. A special role for binocular visual input was also observed in studies of the recovery from an early period of monocular deprivation. Although the resulting visual deficits in the deprived eye are reduced or eliminated by a period of reverse occlusion (analogous to patching therapy of human amblyopes), the improved vision in the deprived eye may not persist after the period of occlusion of the non-deprived eye is terminated. Moreover, the vision of the non-deprived eye may be permanently compromised as a result of occlusion. Experiments on animals that received periods of part-time occlusion of the non-deprived eye that allow a daily period of binocular visual exposure indicate a route by which deleterious effects of full-time occlusion can be avoided.