The computation of binocular visual direction: a re-examination of Mansfield and Legge (1996)

Mansfield and Legge (1996) reported recently that a target's perceived binocular direction is dependent on the ratio of contrasts presented to the two eyes. Although their main conclusion concerned the dependence of perceived direction on interocular contrast, they also argued that the change in perceived direction is due to a shift in the position of the cyclopean eye and that the relative directions of binocular targets are unaffected by eye position. We take issue with both of these arguments. With regard to the former, their task was an alignment task, not an egocenter task, so it did not provide information relevant to the position of the cyclopean eye. Indeed, their data can be explained by the conventional theory of binocular visual directions with a fixed cyclopean eye (e.g., Hering, 1879; Ono, 1981) once a simple, but important modification is added. With regard to their conclusion concerning eye position, we show that the vergence of the eyes has a clear and systematic effect on perceived relative directions in the setup used by Mansfield and Legge.