Oculomotor response to rapid head oscillation (0.5–5.0 Hz) after prolonged adaptation to vision-reversal

Summary This study examines long-term (up to 27 days) effects of maintained vision reversal on (i) smooth visual tracking with head still, (ii) oculomotor response to actively generated head oscillation and (iii) spontaneous saccades. Dove prism goggles produced horizontal, but not vertical (sagittal plane), vision reversal. Eye movements were recorded by EOG; head movements by an electro-magnetic search coil.Both visual tracking and saccade dynamics remained unchanged throughout. In contrast, both the ocular response to active head osculations (goggles off and subject looking at a stationary target) and associated retinal image blur showed substantial and retained adaptive changes, akin to those previously found in the vestibulo-ocular reflex as tested in darkness at 0.17 Hz.However, several additional unexpected results emerged. First, in the fully adapted state smooth eye movements tended to be of reversed phase in the range 0.5–1.0 Hz (in spite of normal vision during tests), but of normal phase from about 2 Hz and above (in spite of negligible visual tracking in this upper range). Second, after permanent removal of the inverting goggles, this peculiar frequency response of the fully adapted state quickly (36 h) reverted to a dynamically simpler condition manifest as retained (2–3 weeks) attenuation of gain (eye vel./ head vel.) which, as in control conditions, was monotonically related to frequency. From these two findings it is inferred that the fully adapted state may have comprised two separate components: (i) A simple element of monotonic and long-lasting gain attenuation and (ii) a complex, frequency labile, element which could be quickly rejected. Dynamic characteristics of the putative complex element were estimated by vectorial subtraction of the simple one from that of the fully adapted condition. The outcome suggests that the inferred complex condition might represent a predictive element.Two further findings are reported: (i) Substantially different vectors of the adapted response were obtained with normal and reversed vision at 3.0 Hz head oscillation, indicating a novel visual influence acting above the cut-off frequency for visual tracking. (ii) During head oscillation in the vertical sagittal plane (in which vision was not reversed) there was never any image blur, indicating high geometric specificity in the adaptive process.Supported by Canadian Medical Research Council Operating Grant No. MT5630