LUMINANCE CONTRAST AND SPATIAL-FREQUENCY TUNING OF THE TRANSIENT-VERGENCE SYSTEM

Vergence has transient components that are stimulated by brief present ations of stimuli at large disparities (up to several degrees), The qu estion that we have addressed is what stimulus features are encoded by this system, A competition paradigm [Jones & Kerr, (1972), Vision Res earch, 12, 1425-1430) was used in which three gabors were presented, A single Gabor was presented to the fovea of one eye and two gabors, 2. 5 deg to either side of the fovea, to the other; one of which, when pa ired with the single Gabor defined a convergent direction, the other a divergent direction. When these Gabors are identical, observers have a tendency to respond in one particular direction, First we determined if increasing the luminance contrast of the Gabor pair whose disparit y was opposite to the observer's response-bias direction (variable-con trast pair) relative to the remaining Gabor (reference) could alter th e observer's response direction, Secondly, we determined if the contra st required for such a change in response was affected by the relative spatial frequency of the convergent and divergent Gabors, The referen ce Gabor was held at 2 cpd and the variable Gabor pair was varied betw een 5.6 and 0 (a gaussian) cpd, Results demonstrated that increasing t he luminance contrast of the variable pair relative to the reference G abor could alter the observer's response direction, even when the cont rast of only one of the variable-pair Gabors was increased, The lumina nce contrast required for this change to occur was directly related to the spatial frequency of the variable pair over the entire frequency range tested, Vergence responses were preferentially made to lower spa tial frequencies, even when a low spatial frequency was paired with a high one, We conclude that transient-vergence responses are not reduce d by mixed contrasts (i.e. no contrast-paradox effect) and appear to b e mediated by a system that employs a single lowpass sensitive channel , (C) 1998 Elsevier Science Ltd. All rights reserved.