The effect of accommodative hysteresis on apparent stationarity

The potential contribution of accommodation to the ability to perceive whether an object is moving in space during lateral movement of the head was examined in 16 visually-normal young adults. The target, which provided a highly effective accommodation stimulus, was viewed monocularly in an otherwise completely darkened room. Self-generated head movements of controlled amplitude and frequency were used to produce lateral target motion of continuously variable amplitude either in-phase or counter-phase to the lateral translation of the head. Tonic accommodation and the target/head displacement ratio needed for apparent target stationarity were measured before and after sustained focus at either the nearpoint or farpoint. Tonic accommodation shifted inward under the nearpoint condition and outward under the farpoint condition. The tonic aftereffects induced under the nearpoint and farpoint conditions were accompanied by increases and decreases, respectively, in the degree of positive (same direction as the head) concomitant motion required for apparent target stationarity. These results suggest that the altered accommodative effort required to compensate for the tonic aftereffects influenced the magnitude of the absolute visual parallax ascribed to self-motion. The accommodative system therefore appears to contribute to position constancy, at least in a relatively impoverished visual environment.