The influence of target distance on eye movement responses during vertical linear motion.

Studies of the linear vestibulo-ocular reflex (LVOR) suggest that eye movement responses to linear head motion are rudimentary. This may be due to inadequate control of target distance (D). As D approaches infinity, eye movements are not required to maintain retinal image stability during linear head displacements, but must become increasingly large as D shortens. The LVOR in the presence of visual targets (VLVOR) was tested by recording human vertical eye and head movements during self-generated vertical linear oscillation (averaging 2.7 Hz at peak excursion of 3.2 cm) while subjects alternately fixated targets at D = 36, 142, and 424 cm. Response sensitivity rose from 0.10 deg/cm (5.8 deg/s/g) for D = 424 cm to 0.65 deg/cm (37.5 deg/s/g) for D = 36 cm. Results employing optical manipulations, including spherical lenses to modify accommodation and accommodative convergence, and prisms to modify fusional vergence without altering accommodation, imply that the state of vergence is the most important variable underlying the effect. Trials in darkness (LVOR) and with head-fixed targets (visual suppression of the LVOR) suggest that, while visual following and perhaps "mental set" influences results, the major proportion of the VLVOR response is driven by vestibular (presumably otolith) inputs.