Early components of the human vestibulo-ocular response to head rotation: latency and gain

To characterize vestibulo-ocular reflex (VOR) properties in the time window in which contributions by other systems are minimal, eye movements during the first 50-100 ms after the start of transient angular head accelerations (similar to 1000 degrees/s(2)) imposed by a torque helmet were analyzed in normal human subjects. Orientations of the head and both eyes were recorde d with magnetic search coils (resolution, similar to 1 min arc; 1000 sample s/s). Typically, the first response to a head perturbation was an anti-comp ensatory eye movement with zero latency, peak-velocity of several degrees p er second, and peak excursion of several tenths of a degree. This was inter preted as a passive mechanical response to linear acceleration of the orbit al tissues caused by eccentric rotation of the eye. The response was modele d as a damped oscillation (similar to 13 Hz) of the orbital contents, appro aching a constant eye deviation for a sustained linear acceleration. The su bsequent compensatory eye movements showed (like the head movements) a line ar increase in velocity, which allowed estimates of latency and gain with l inear regressions. After appropriate accounting for the preceding passive e ye movements, average VOR latency (for pooled eyes, directions, and subject s) was calculated as 8.6 ms. Paired comparisons between the two eyes reveal ed that the latency for the eye contralateral to the direction of head rota tion was, on average, 1.3 ms shorter than for the ipsilateral eye. This hig hly significant average inter-ocular difference was attributed to the addit ional internuclear abducens neuron in the pathway to the ipsilateral eye. A verage acceleration gain (ratio between slopes of eye and head velocities) over the first 40-50 ms was similar to 1.1. Instantaneous velocity gain, ca lculated as Veye(t)/Vhead(t-latency), showed a gradual build-up converging toward unity (often after a slight overshoot). Instantaneous acceleration g ain also converged toward unity but showed a much steeper build-up and larg er oscillations. This behavior of acceleration and velocity gain could be a ccounted for by modeling the eye movements as the sum of the passive respon se to the linear acceleration and the active rotational VOR. Due to the lat ency and the anticompensatory component, gaze stabilization was never compl ete. The influence of visual targets was limited. The initial VOR was ident ical with a distant target (continuously visible or interrupted) and in com plete darkness. A near visual target caused VOR gain to rise to a higher le vel, but the time after which the difference between far and near targets e merged varied between individuals.