Eye fixation and attention as modifiers of perceived distance.

An error in the perceived distance of a physically stationary object results in an apparent horizontal motion of the object if the head is moved horizontally. Procedures have been developed to use this apparent motion to measure the apparent distance of the object. Research also has indicated that the apparent distance of an object will tend to be displaced toward the distance at which the eyes are fixated. In the present study head-motion procedures were used to measure the effect of eye fixation and attention upon the apparent distance of a point of light in two experiments. Substantial errors in perceived distance occurred in the predicted direction as a function of fixation distance. Attending to an object at a distance other than the distance of the fixated point of light had a much less effect upon the apparent distance of the point. It is concluded that the apparent conomitant motion which occurs in a variety of situations as a function of the distance of fixation indicates that substantial errors in perceived distance are common in most environments.