Optic nerve fibre lesions in adult cats: Pattern of recovery of spatial vision

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Abstract
Partial deafferentation in the pre-geniculate optic pathways of the adult cat was produced by micro-injection of diphtheria toxin. In this lesion a certain proportion of fibres was damaged while neighbouring fibres remained unaffected. Behavioural methods were used to study the time course, pattern, and extent of recovery of spatial vision after such lesions. Cats were trained to discriminate square-wave gratings from blank fields. A range of grating sizes (0.13–2.61 cycles/degree) at high contrast were presented each day and when performance was consistently 100%, the lesion was placed. At 24 h post-lesion and serially thereafter grating discrimination was tested. Contrast sensitivity was also measured before and after the lesion. All cats recovered to pre-lesion performance levels. The time course of recovery was characterized by an early rapid phase followed by a longer slower phase. The early recovery was temporally coincident with dispersal of oedema at the site of the lesion, while the later slower recovery probably represented a functional reorganization at the synapse. Optic nerve fibre analyses at one year post-lesion showed reduction in fibre content ranging from 33 to 77% in different cats without shift in the fibre size histograms. The length of recovery time was directly related to the magnitude of fibre loss. The return of spatial frequency perception was hierarchical — first medium (1–4 days), then low (1–2 months) and finally high spatial frequencies (5–8 months). Possible mechanisms are discussed and it is suggested that such a pattern of psychophysical results could be accounted for by different rates of recovery for X and Y pathways, the Y-type recovering at a faster rate than the X-type.