Binocular unmasking with unequal interocular contrast: The case for multiple Cyclopean eyes

Under certain conditions, the detection threshold for a sinusoidal grating embedded in a noisy background may be an order of magnitude lower when binocular cues are available than when monocular cues only are present. Such binocular unmasking occurs only when the degree of interocular disparity for the target differs from that of the background. Two classes of models have been advanced to account for such unmasking. The first assumes that orientation-specific, spatial frequency channels in each eye encode the amplitude and phase of the spatial frequency component of the pattern the channel is tuned to detect. Thus, a difference in interocular disparity between target and background could result in interocular amplitude and/or phase differences in left- and right-eye spatial frequency channels. When, however, there are no disparity differences between target and background, there will be no interocular differences in amplitude and phase in the left- and right-eye channels. In this model, then, binocular unmasking reflects the binocular system’s ability to respond to interocular amplitude and/or phase differences in the patterns presented to the two eyes. In the second class of models, it is assumed that the left and right-eye patterns are first summed to form a “Cyclopean” eye. In these models, detection depends on the effect this summation process has on the power spectrum of the summated patterns. To decide between these two classes of models, we observed the occurrence of binocular unmasking when (1) the contrast of masker and signal was varied identically in both eyes and (2) the contrast of masker and signal was varied in one eye only. Consistent with our previous research, we found that the results can be accounted for in terms of a linear summation model of binocular unmasking; the alternative interocular phase detection model was disproved. The implications of these findings for binocular contrast summation in the absence of visual noise are discussed.     

Binocular vision enhances phase discrimination by filtering the background

Previous studies have shown that the detectability of a noise-masked target can be enhanced under stereoscopic viewing when the target’s interocular disparity differs from that of the noise. This enhanced detectability can be accounted for by a model postulating that the binocular system linearly sums the left-eye and right-eye views of a visual scene. This model also predicts enhanced phase discrimination under specifiable interocular disparities of target and noise. Two experiments were conducted in which subjects were asked to discriminate between two luminance patterns (target and foil) that differed only in phase. The target patterns were constructed by summating two vertical sinusoidal gratings in which the phase difference between the higher and the lower spatial frequency gratings was 45°. The foils contained the same two component frequencies, with a phase difference of ?45°. Thus, targets and foils were mirror images of one another. The ability of subjects to Discriminate between these stereoscopically viewed mirror-image patterns was investigated under two sets of interocular disparities: those that, according to our model, would unmask one or both spatial frequency components, and those that would leave both components masked by the noise. Phase discrimination was enhanced only when both component frequencies of the target and foil were unmasked. The implications of these findings for template-matching and phase-discrimination models of pattern discrimination are considered.     

A comparison of higher order auditory processes in younger and older adults

The authors conducted 3 experiments investigating the effects of aging on higher order auditory processes. They compared younger and older adults with respect to (a) their auditory channel capacity, (b) the extent of their top-down control over auditory gain, and (c) their ability to focus attention on a narrow band of frequencies. To ensure that subclinical cochlear processing deficits in older adults (e.g., higher thresholds, poorer discrimination of frequency and intensity differences) did not limit performance, the authors used only stimuli that were perfectly discriminable by all participants. No age differences were found in any of these experiments, suggesting that some higher order auditory processes (e.g., top-down control over auditory gain, auditory attention) are preserved in normal aging, despite numerous age-related declines in peripheral auditory functionality.     

Monocular unmasking of noise-embedded patterns.

Binocular disparity cues may help an observer "unmask" a target in a background, thereby enhancing its detectability (e.g., Moraglia & Schneider, 1992). Here, we sought to determine whether similar effects could be produced by monocular displacement cues resulting from a two-frame sequential presentation of a Gabor pattern (a sinusoidal modulation of luminance combined with a Gaussian modulation of local contrast) embedded in an unvarying field of two-dimensional Gaussian noise. The Gabor in the second frame was spatially displaced relative to its location in the first frame; the horizontal displacement corresponded to a phase shift of the peak spatial frequency of the Gabor of 0 degree, 90 degrees, 180 degrees, 360 degrees, or 540 degrees. Monocular detection thresholds for the Gabor were appreciably lower for the 90 degrees, 180 degrees, and 540 degrees shift, than for the 0 degree and 360 degrees values. We explain these findings in terms of a model that constitutes the monocular analog of our summation model of binocular unmasking.     

Effects of direction and magnitude of horizontal disparities on binocular unmasking

Conditions under which binocular unmasking (BU), as an analogue of binaural unmasking, occurs have been explored. Observers were to detect through a stereoscope a Gabor signal in patches of two-dimensional broadband gaussian noise surrounded by a frame of uniform noise. The right-eye gaussian field was displaced relative to the left eye so that it appeared either in front of or behind the frame. Performance when signal disparity was equal to that of the noise--a condition functionally equivalent to monocular processing--was compared to that obtained when signal disparity was zero--a case in which BU should occur. Enhanced signal detectability of up to 12 dB and of nearly constant magnitude was observed in the latter condition when uncrossed disparities of up to 67.60 min visual angle and display durations of 1 s were employed. Signal detectability declined appreciably with increasing disparity (both crossed and uncrossed) when display duration was reduced to 90 ms, thus preventing the occurrence of compensatory vergence eye movements. It is suggested that BU effects may result from a process of linear summation of monocular inputs.