THE LATENCY OF THE CAT VESTIBULOOCULAR REFLEX BEFORE AND AFTER SHORT-TERM AND LONG-TERM ADAPTATION

Latencies of normal and adapted feline vestibulo-ocular reflex (VOR) w ere studied in five cats by applying +/- 20-degrees/s horizontal head velocity steps (4000-degrees/s2 acceleration) and measuring the elicit ed horizontal or vertical reflex eye responses. Normal VOR latency was 13.0 ms +/- 1.9 SD. Short-term adaptation was then accomplished by us ing 2 h of paired horizontal sinusoidal vestibular stimulation and pha se-synchronized vertical optokinetic stimulation (cross-axis adaptatio n). For long-term adaptation, cats wore x 0.25 or x 2.2 magnifying len ses for 4 days. The cats were passively rotated for 2 h/day and allowe d to walk freely in the laboratory or their cages for the remainder of the time. The latency of the early (primary) adaptive response was 15 .2 ms +/- 5.2 SD for cross-axis adaptation and 12.5 ms +/- 3.9 SD for lens adaptation. This short-latency response appeared within 30 min af ter beginning the adaptation procedure and diminished in magnitude ove rnight. A late (secondary) adaptive response with latency of 76.8 ms /- 7.0 SD for cross-axis adaptation and 68.1 ms +/- 8.8 SD for lens ad aptation appeared after approximately 2 h of adaptation. It had a more gradual increase in magnitude than the primary response and did not d iminish in magnitude overnight. These data suggest that brainstem VOR pathways are a site of learning for adaptive VOR modification, since t he primary latency is short and has a similar latency to that of the n ormal VOR.