Avian vestibuloocular reflex: adaptive plasticity and developmental changes.

1. This study demonstrates plasticity of the vestibuloocular reflex (VOR) in chickens and compares it to that of other species and to that of newly hatched chicks. Adaptive changes in the VOR were induced by subjecting the animals to combinations of visual and vestibular stimuli that simulated the effect of the VOR being either too low in gain or reversed in phase. 2. The VOR of chickens resembles that of mammals, but the curve of phase lead versus frequency seems shifted toward higher frequencies. The VOR of newly hatched chicks has extremely low gain (less than 0.1). 3. In both the older and newly hatched animals, the VOR gain increased substantially after 2 h in an environment in which the imposed en bloc rotations produced increased retinal image slip in the normal directions. Similarly, 2 h of reversed retinal image slip produced decreased VOR gain and, in some cases, reversal fo the phase of the VOR. The gain changes were largest at the "training" frequency. The phase changes were in the direction of increased phase lead. Changes in the gains of the optokinetic responses and of the combination of VOR and optokinetic responses were also seen, especially in the newly hatched animals. 4. The newly hatched birds showed the largest VOR changes in the increased-gain situation, whereas the older birds showed the largest changes in the reversed-phase situation, as assessed by the changes in the average retinal slip velocity experienced. These results may well not be a consequence of differences in age, per se, but of differences in average retinal slip experienced in the two experimental situations at the start of the trial because of the lower VOR gain of the newly hatched animals. There seems to be no dramatic difference in VOR plasticity between newly hatched and older birds. 5. Our results with reversed visual motion are substantially different from those obtained in similar studies on rabbits, suggesting that these two species use different error signals to control the adaptive adjustment of the VOR.