title: Linear binocular combination of responses to contrast modulation: contrast-weighted summation in first- and second-order vision.
abstract: Binocular combination for first-order (luminance-defined) stimuli has been widely studied, but we know rather little about this binocular process for spatial modulations of contrast (second-order stimuli). We used phase-matching and amplitude-matching tasks to assess binocular combination of second-order phase and modulation depth simultaneously. With fixed modulation in one eye, we found that binocularly perceived phase was shifted, and perceived amplitude increased almost linearly as modulation depth in the other eye increased. At larger disparities, the phase shift was larger and the amplitude change was smaller. The degree of interocular correlation of the carriers had no influence. These results can be explained by an initial extraction of the contrast envelopes before binocular combination (consistent with the lack of dependence on carrier correlation) followed by a weighted linear summation of second-order modulations in which the weights (gains) for each eye are driven by the first-order carrier contrasts as previously found for first-order binocular combination. Perceived modulation depth fell markedly with increasing phase disparity unlike previous findings that perceived first-order contrast was almost independent of phase disparity. We present a simple revision to a widely used interocular gain-control theory that unifies first- and second-order binocular summation with a single principle--contrast--weighted summation-and we further elaborate the model for first-order combination.
  
  
  CONCLUSIONS
  Second-order combination is controlled by first-order contrast.
authors.affiliation: McGill Vision Research, Department of Ophthalmology, McGill University, Montreal, QC, Canada.
authors.forename: Jiawei
authors.initials: J
authors.lastname: Zhou
authors.affiliation: School of Life & Health Sciences, Aston University, Birmingham, UK.
authors.forename: Mark A
authors.initials: MA
authors.lastname: Georgeson
authors.affiliation: McGill Vision Research, Department of Ophthalmology, McGill University, Montreal, QC, Canada.
authors.forename: Robert F
authors.initials: RF
authors.lastname: Hess
mesh_terms: D000328:Adult; D015350:Contrast Sensitivity / Q000502:physiology*; D003867:Depth Perception / Q000502:physiology; D006801:Humans; D008962:Models, Theoretical; D010775:Photic Stimulation; D015348:Vision, Binocular / Q000502:physiology*
keywords: binocular combination; binocular disparity; contrast modulation; contrast-gain control; first-order; interocular suppression; rivalry suppression; second-order