Cortical recovery from effects of monocular deprivation caused by diffusion and occlusion

Two forms of visual deficit were induced in 12 pairs of kittens (4-6 weeks of age) by monocular lid suture combined with either an opaque soft contact lens ('occlusion' amblyopia) or a clear lens ('diffusion' amblyopia) which had been kept behind the sutured eyelids for 5-9 weeks. The kittens were then reverse-sutured at the age of 9-15 weeks, and the previously open eye was occluded with an opaque lens for the next 5-7 weeks. We compared across the two groups the proportion of binocularly driven cells (group 2-6) and cells predominantly activated by stimulation of the initially deprived eye (groups 1-3). Both values were significantly higher in the 'occlusion' kitten, though the difference was small, compared to the 'diffusion' kitten. In addition, we measured visual acuity and peak contrast sensitivity for the two eyes in 4 of the 12 pairs, using the sweep VEP method applied under anesthesia and paralysis. The 'occlusion' kitten showed consistently better acuity and higher peak contrast sensitivity than the 'diffusion' kitten, when the initially deprived eye was tested. Taken together, the present results suggest that there is a difference in the depth of amblyopia caused by monocular 'occlusion' and monocular 'diffusion'. The cortical effects of the latter are more difficult to reverse than those of the former.