OPTICALLY INDUCED PLASTICITY OF THE CERVICOOCULAR REFLEX IN PATIENTS WITH BILATERAL ABSENCE OF VESTIBULAR FUNCTION

The horizontal cervico-ocular reflex (COR) was examined in five labyri nthine-defictive subjects (LDS), during both passive oscillations of t he head on the trunk (HTexam) and of the trunk under the earth-fixed h ead (THexam) at 0.1-0.5 Hz, peak angular displacement +/-30 degrees. S ubjects were tested in the dark, before and immediately after adaptati on to binocular magnifying (x1.9) and reducing (x0.6) lenses. During l ong-term adaptation, the LDS were exposed to the normal environment fo r 5 h while wearing lenses. Short-term adaptation experiments (15-20 m in) consisted of sustained ocular following of a small LED in an other wise dark room and in full-room illumination. This LED was either stat ionary in space whilst the subjects moved their head actively, or fixe d on the chair and rotating with the trunk during head-fixed stimulati on. In all five patients, magnifying lenses increased COR gain (peak s low-phase eye velocity/peak stimulus velocity), whereas reducing lense s reduced the gain. Under HTexam the gain changes were greater, more c onsistent and the phases approximately compensatory to head displaceme nt, whereas during THexam the gain decreased and phase increased at hi gher frequencies. COR adaptation was observed during foveal stimulatio n alone, but the effects were stronger with added background illuminat ion. Results during an imaginary target task showed that the gain can be influenced strongly by mental set. Our findings indicate a highly m odifiable COR in subjects with loss of vestibular function. Both perip heral and foveal retinal information contribute to the plastic changes in COR gain. Somatosensory cues from the trunk as well as cognitive/p erceptual factors may be involved in the modification of the COR, by p roviding information about the relevance of eye movements, and contrib ute to the stabilisation of gaze in space.