Binocular capture: the effects of spatial frequency and contrast polarity of the monocular target.

The influence of binocular disparate targets on the perceived visual direction of adjacent monocular targets has been termed "binocular capture". The magnitude of capture increases significantly with increasing vertical separation between monocular targets. This study sets out to elucidate the interaction between spatial frequency content, contrast polarity and vertical separation between monocular targets to establish the roles of the monocular target's positional uncertainty and the underlying position-encoding mechanism in the production of binocular capture. Relative alignment thresholds and bias were measured separately for a pair of vertically separated monocular Vernier spatial frequency ribbons and a pair of monocular Gaussian bars presented across a random dot stereogram. Ribbon pairs comprised carrier frequencies that were either matched (8 cpd or 1 cpd) or mismatched (top ribbon 1 cpd, bottom ribbon 8 cpd, and vice versa). The Gaussian bars were presented with either matched contrast (bright/bright) or opposite polarity (bright/dark) contrast. Capture magnitudes increased significantly with vertical separation for all ribbon conditions and for both contrast polarity conditions. In these conditions, capture magnitude co-varied with relative alignment threshold. The matched 1 cpd ribbons showed a significant effect of separation and relative alignment threshold on capture magnitude for low contrast stimuli but not for high contrast stimuli. The results are consistent with the hypothesis that perceived visual direction of a monocular target becomes increasingly dependent on the surround visual direction when its relative position signals are poor. Furthermore, its vulnerability to capture by the surround visual direction seems to be dependent on the underlying position encoding mechanism employed to compute relative position information of the monocular target.