Induced esophoric shifts in eye convergence and illusory distance in reduced and structured viewing conditions

Two experiments measured distance judgments in reduced and structured viewing conditions before and after induced phoria. Experiment 1 induced phoria shifts with three 5-min. intervals of close handwork (i.e., needle threading) that had naturalistic characteristics of observers (a) choosing their own viewing distance, (b) moving their heads freely, and (c) viewing in a well-illuminated, structured environment. The handwork induced a 1.20-diopter esophoric shift and a 1.22-cm overestimation of distance under reduced test conditions. The handwork induced no measurable illusion under structured viewing conditions. Experiment 2 induced phoria shifts with 10 min. of fixation on a target 11 cm away. This procedure induced a 4.65-diopter esophoric shift, a 6.34-cm overestimation of distance in reduced conditions, and a 2.31-cm overestimation of distance in structured conditions. We argue that visual and motor information interact to determine perceptions and that induced esophoric shifts in everyday situations may shape the nature of that interaction.