The relationship between eye position and egocentric visual direction.

Hering's model of egocentric visual direction assumes implicitly that the effect of eye position on direction is both linear and equal for the two eyes; these two assumptions were evaluated in the present experiment. Five subjects pointed (open-loop) to the apparent direction of a target seen under conditions in which the position of one eye was systematically varied while the position of the other eye was held constant. The data were analyzed through examination of the relationship between the variations in perceived egocentric direction and variations in expected egocentric direction based on the positions of the varying eye. The data revealed that the relationship between eye position and egocentric direction is indeed linear. Further, the data showed that, for some subjects, variations in the positions of the two eyes do not have equal effects on egocentric direction. Both the between-eye differences and the linear relationship may be understood in terms of individual differences in the location of the cyclopean eye, an unequal weighting of the positions of the eyes in the processing of egocentric direction, or some combination of these two factors.