Orientation and luminance polarity tuning of the transient-vergence system

Previously, Edwards, Pope & Schoor, Vision Research, 38, 705-717, demonstrated that transient disparity vergence appears to be mediated by a system that employs a single low-pass sensitive spatial channel whose performance is not reduced by dichoptic mixed contrasts (no contrast paradox) or dichoptic mixed spatial frequencies. This broadband tuning to both contrast and spatial frequency may be indicative of a second-order or non-linear envelope extraction system. The current study tests for lack of tuning to orientation and luminance polarity which are typically taken as evidence of a second-order system. We found that when the transient vergence system was simultaneously presented with both convergent and divergent disparities, there was a small but distinct bias in favor of responding in the direction defined by matched orientations or luminance polarities over unmatched pairs. Although less frequent, responses to orthogonal carriers or opposite luminance polarities were possible. The vergence system could match a horizontal with a vertical carrier, or a light gaussian with a dark gaussian. The degree of orientation or luminance polarity tuning varied inversely with the disparity magnitude over the range of 2.5-5 degrees, and the orientation tuning peaked at a spatial frequency about 2 cpd. At all disparities tested, however, the tuning was very broad, and other candidate features for mediating transient-vergence need to be investigated.