Cervico-ocular reflex enhancement in labyrinthine-defective and normal subjects

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Abstract
The cervico-ocular reflex (COR) was elicited in 12 normal and 30 labyrinthine-defective (LD) subjects, lacking a vestibulo-ocular reflex (VOR) in the usual laboratory tests, by sinusoidal horizontal rotation of the body at 30° amplitude and frequencies of 0.1, 0.2 and 0.4 Hz. The head was fixed in space and the eyes were open in total darkness. The gain of the COR was measured by relating the maximum slow phase velocity (MSPV) of the eyes to the maximum input velocity (19, 38 or 75°/s) and by relating the cumulative eye displacement per hemicycle to the peak-to-peak amplitude of 60°. The COR gain was below 0.25 in the normal subjects. In the LD subjects, a clear COR was observed with a gain of about 0.7 (SD 0.3) at 0.1 Hz, decreasing to about 0.4 (SD 0.2) at 0.4 Hz. There was no appreciable phase lag in 11 of the LD subjects. In 17 other LD subjects, the phase lag was close to 0° at 0.1 Hz and increased to about 20–30° at 0.4 Hz. Testing 4 additional normal subjects with the special instruction to attempt fixation of the knee during trunk rotation in the dark, resulted in an enhanced COR showing many “catch-up” saccades with a gain close to unity; the smooth component was also enhanced. The pattern with catch-up saccades was different from the pattern observed in most LD subjects, which closely resembled the normal VOR elicited by sinusoidal rotation. The special instruction to attempt fixation of the knee was also given to one LD subject during stimulation at 0.2 Hz. Previously at this frequency, this subject had a smooth sinusoidal response with a gain of 0.33; after the instruction, the gain rose to a value of 0.92. The response continued to be smoothly sinusoidal during some entire hemicycles and only a few catch-up saccades occurred in other hemicycles. We conclude that: 1. The passive COR has a low gain in normal subjects and is enhanced in LD subjects. 2. The frequency behaviour of the smooth component of the enhanced passive COR suggests a low-pass system and, also in view of its phase behaviour, some involvement of the smooth pursuit system. 3. The occurrence of zero phase responses suggests that prediction may play an important part. 4. The passive COR can also be enhanced in normal subjects when given an appropriate instruction. The pattern, however, is different from that of the enhanced COR in LD subjects.