Visual influences on the development and recovery of the vestibuloocular reflex in the chicken

Whenever the head turns, the vestibuloocular reflex (VOR) produces compensa tory eye movements to help stabilize the image of the visual world on the r etina. Uncompensated slip of the visual world across the retina results in a gradual change in VOR gain to minimize the image motion. VOR gain changes naturally during normal development and during recovery from neuronal dama ge. We ask here whether visual slip is necessary for the development of the chicken VOR (as in other species) and whether it is required for the recov ery of the VOR after hair cell loss and regeneration. In the first experime nt, chickens were reared under stroboscopic illumination, which eliminated visual slip. The horizontal and vertical VORs (h- and vVORs) were measured at different ages and compared with those of chickens reared in normal ligh t. Strobe-rearing prevented the normal development of both h- and vVORs. Af ter 8 wk of strobe-rearing, 3 days of exposure to normal light caused the V ORs to recover partially but not to normal values. In the second experiment , 1-wk-old chicks were treated with streptomycin, which destroys most vesti bular hair cells and reduces hVOR gain to zero. In birds, vestibular hair c ells regenerate so that after 8 wk in normal illumination they appear norma l and hVOR gain returns to values that are normal for birds of that age. Th e treated birds in this study recovered in either normal or stroboscopic il lumination. Their hVOR and vVOR and vestibulocollic reflexes (VCR) were mea sured and compared with those of untreated, age-matched controls at 8 wk po sthatch, when hair cell regeneration is known to be complete. As in previou s studies, the gain of the VOR decreased immediately to zero after streptom ycin treatment. After 8 wk of recovery under normal light, the hVOR was nor mal, but vVOR gain was less than normal. After 8 wk of recovery under strob oscopic illumination, hVOR gain was less than normal at all frequencies. VC R recovery was not affected by the strobe environment. When streptomycin-tr eated, strobe-recovered birds were then placed in normal light for 2 days, hVOR gain returned to normal. Taken together, the results of these experime nts suggest that continuous visual feedback can adjust VOR gain. In the abs ence of appropriate visual stimuli, however, there is a default VOR gain an d phase to which birds recover or revert, regardless of age. Thus an 8-wk-o ld chicken raised in a strobe environment from hatch would have the same ga in as a streptomycin-treated chicken that recovers in a strobe environment.