Optokinetic and vestibulo-ocular reflexes in dark-reared rabbits

Rabbits were raised in complete darkness for 7 months after birth. Eye movements were measured at the end of this period and in the next 3 months of normal light exposure with chronically implanted scleral coils. Horizontal optokinetic nystagmus (OKN) was tested inside a large drum which was rotated at velocities between 0.06 and 60 degrees/sec. Vestibuloocular reflexes (VOR) were elicited by sinusoidal horizontal oscillation on a torsion swing at frequencies between 0.11 and 2.13 Hz and amplitudes up to 10 degrees. At the end of the light-deprived period (with the eyes still covered) a VOR could be elicited consisting of a normal mixture of smooth and saccadic components and normal phase relations, but the amplitude of the smooth (compensatory) component was reduced to about 1/3 of normal control values. At the first exposure to light an OKN could be immediately elicited which was normal in most respects, except for a reduction of the ratio slow phase eye speed/drum speed to about 2/3 of the value in normal controls. The preference of each eye for anterior motion and the quasiconjugate character of nystagmus in monocular stimulation were unaffected. Also the improvement of the VOR by vision was normal. No abnormal habituation or fatigeability were observed. In the next 3 months of normal light exposure about half of the amplitude defects in both systems were restored, largely in the first week. The remaining defects were apparently permanent.