Adaptation of the vestibulo-ocular reflex in amblyopia

Adaptation of the vestibulo-ocular reflex (VOR) is demonstrated by changes in gain in response to discrepancies between visual and vestibular stimulation. The authors have investigated the effect of monocular asymmetries of OKN in amblyopia upon adapting the gain of the VOR. Adaptation was investigated by modifying the horizontal balance of the VOR. While monocularly fixating a head referenced spot for 2 min, subjects were sinusoidally oscillated on a chair inside an optokinetic drum that rotated in one direction (left or right) at the peak velocity of sinusoidal chair rotation. The VOR was then measured during continued sinusoidal body oscillation in darkness for 1 min. Imbalance of the horizontal VOR gain equalled the ratio of slow phase velocities in the rightward and leftward directions. After rotating the drum in the nasalward direction, an increase was observed in slow phase gain of the VOR in the nasalward direction for either eye of our amblyopes that was significantly greater than similar changes in gain for the normals. Increased VOR gains for the amblyopic group following temporalward stimulation were significantly less than the nasal aftereffect. Gain changes of the VOR in normals had similar magnitudes following nasal or temporal stimulation. These results suggest that disturbances of OKN in amblyopia are common to the pathways that modify the slow phase gain of the VOR.