SACCADIC EYE-MOVEMENTS AND THE HORIZONTAL VESTIBULOOCULAR AND VESTIBULO-COLLIC REFLEXES IN THE INTACT GUINEA-PIG

The guinea-pig is an attractive model for investigating gaze stabiliza tion because it is suitable for in vitro and in vivo studies. However, few data are available on its oculomotor performance. We therefore in vestigated spontaneous eye movements, horizontal vestibulo-ocular (HVO R) and vestibulo-collic reflexes (HVCR) in the alert head-fixed guinea -pig using the magnetic search coil method. First the characteristics of the spontaneous saccades in the light were analysed. They occurred with a mean frequency of 4.6/min and with a mean amplitude and duratio n of 7.41 +/- 3.57 deg and 30.9 +/- 9.5 ms, respectively (n = 340). Sa ccadic duration and velocity were linearly related to the amplitude of the eye movement. The HVOR was studied in response to sinusoidal rota tions (0.01 Hz to 2 Hz, peak head velocity of 40 deg/s) in the dark. V estibular responses were linear at 0.5 and 0.05 Hz for peak head veloc ities between 40 and 80 deg/s. As in other species, the gain increased and the phase lead decreased with increasing frequencies. The number of fast phases per second increased with peak head velocity and with i ncreasing frequencies from 0.01 to 0.5 Hz, with a plateau between 0.2 and 0.5 Hz. The HVOR time constant, when measured in response to veloc ity steps, was 7.0 +/- 1.5 s and the latency of the vestibular respons es averaged 21 +/- 4 ms. Finally, the HVCR was assessed in unrestraine d guinea-pigs subjected to horizontal sinusoidal rotation in the frequ ency range of 0.05-2 Hz. Exploratory behaviour was prevalent and there were few head stabilization episodes. However, when it occurred, the HVCR gain and phase were relatively flat over a frequency range from 0 .1 to 2 Hz, reaching values close to 0.9 and 12 deg, respectively. In summary, the saccadic eye movements, the HVOR and the HVCR in the guin ea-pig appear to be sufficiently similar to those of other vertebrates , including humans, to allow this species to be used as a model for st udies of new pharmacological agents for vestibular disorders and post- lesional plasticity.