Optokinetic and vestibulo-ocular reflexes in dark-reared rabbits

Abstract

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 °/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 °. 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 quasi-conjugate 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.