title: Spatial phase dependence and the role of motion detection in monocular and dichoptic forward masking.
abstract: Contrast thresholds for briefly flashed gratings were measured by the QUEST procedure, under conditions of forward masking by gratings of the same spatial frequency (usually 1 c/deg). Low-contrast masks reduced threshold at short onset asynchronies (0 to 50 msec), while higher contrasts raised threshold over a broader temporal range (0 to 100-140 msec). Both effects depended on the spatial phase relation, but in different ways. Threshold reduction at 0- +/- 90 deg phases probably arises from spatio-temporal filtering by direction-selective mechanisms. This conclusion was supported by computer simulation of a motion detector model. The direction-selective stage of motion analysis may be entirely monocular, since facilitation at 90 deg was abolished by dichoptic presentation. Threshold elevation was phase-dependent at short SOA's (20-50 msec), with a minimum at +/- 90 deg, but was not phase-dependent at longer SOA's (70-140 msec). In-phase masking (0 deg) was about equally strong monocularly and dichoptically, but dichoptic threshold elevation showed no phase-dependence at any SOA. Threshold elevation at longer SOA's, and with dichoptic presentation, may reflect a purely suppressive binocular masking effect, unselective for spatial phase, and its basis may be the same as contrast adaptation. At short SOA's, monocular and binocular masking data apparently reflect a mixture of this phase-independent suppression and phase-selective facilitation.
authors.affiliation: Department of Psychology, University of Bristol, U.K.
authors.forename: M A
authors.initials: MA
authors.lastname: Georgeson
mesh_terms: D015350:Contrast Sensitivity / Q000502:physiology; D005556:Form Perception / Q000502:physiology*; D006801:Humans; D008297:Male; D008954:Models, Biological; D009039:Motion Perception / Q000502:physiology*; D010364:Pattern Recognition, Visual / Q000502:physiology*; D010470:Perceptual Masking / Q000502:physiology*; D012684:Sensory Thresholds / Q000502:physiology