Initiation and cancellation of the human heave linear vestibulo-ocular reflex after unilateral vestibular deafferentation.

The effect of unilateral vestibular deafferentation (UVD) on the linear vestibulo-ocular reflex (LVOR) was studied in 11 humans an average of 52 months following surgical UVD. Controls consisted of seven healthy age-matched subjects. The LVOR was evoked by directionally random, transient whole body interaural (heave) translation with a peak acceleration of 0.5 g while subjects viewed earth-fixed (LVOR) and head-fixed (cancellation) targets 15, 25, and 200 cm distant. The magnitude of the LVOR slow phase was inversely proportional to target distance for both subject groups. Neither latency nor the magnitude of the LVOR significantly differed in the ipsi- vs contralesional directions (P>0.1) in UVD. When the target disappeared at heave onset, subjects with UVD had LVOR slow phase displacement 100 ms later that was 5% of ideal at 15 cm, 6% at 25 cm, and 16% at 200 cm. This was significantly less than corresponding control values of 41, 43, and 50%. During cancellation the LVOR magnitude 100 ms from heave onset was reduced at all target distances by an average of 40+/-4%, and the relative reduction did not significantly differ between controls and subjects with UVD (P>0.1). Cancellation latency did not vary significantly among target distances or subject groups. It is concluded that after UVD, the LVOR is bilaterally and symmetrically reduced but remains modulated by viewing distance and cancellation effort.