Alexander I. Cogan

Depth in anticorrelated stereograms: Effects of spatial density and interocular delay

Disparity-based depth is not perceived in densely textured, anticorrelated random-dot stereograms (RDSs) whose elements carry opposite signs of brightness contrast on corresponding loci, as extant data show. We observed global depth in anticorrelated RDSs flashed repetitively with an interocular delay. During the delay time, a dot array in one eye was paired with a gray frame in the other eye and thus could interact with the negative afterimage of the contralateral dot array. A correlated RDS (e.g. 8 min arc dots, 50% density, 15-msec flash duration) lost depth with delays > 45 msec. An anticorrelated RDS, that was otherwise identical, showed robust depth when flashed with an interocular delay of some 60 msec. A delay was not always necessary to produce depth. At low dot density (1-2%), anticorrelated RDSs showed disparity-dependent local depth even when displayed continuously, or flashed simultaneously; as dot density alone was increased, depth was progressively lost. To make global depth visible in a dense RDS flashed with an interocular delay, the internal response had to be strongly biphasic. Our results support the generally held notion that cyclopean depth signals emerge exclusively from same-sign binocular cortical filters. However, the exclusionary rule may be invalid with respect to the processing of coarse local depth with figural stimuli. Relative depth between a pair of small dots was easily perceived when one of the dots was in opposite contrast, but the depth threshold was then about 0.5 log unit higher than with the same-contrast pair of dots indicating that the internal effects of contrast have not all lost their sign prior to binocular disparity processing. It remains to be determined whether depth can be perceived from edges of opposite contrast.

The relationship between the apparent vertical and the vertical horopter.

Abstract A luminous line subtending 31 at l m observation distance was set to appear vertical. For five observers with normal vision, in the absence of biasing factors, systematic errors were small. In a dichoptic condition (red-green separation) the line, at objective vertical, was seen as an x and had to be tilted, top away from the observer, for the red and the green images to be seen fully coincident: this “collinearity tilt”, taken to represent the tilt of the vertical horopter in the median plane, averaged 27 (S.E. 2.1) in the same five observers. Then, meridional magnification of 5.1% was introduced at 135° axis in one eye and at 45° axis in its fellow eye. The resulting inclinations of the apparent vertical and the vertical horopter were always in the direction predicted by the optical declination of the two retinal images, and the spatial locus of the apparent vertical always remained some 20 in front of the horopter, in the upper hemifield, and behind the horopter in the lower hemifield. The Helmholtzian declination of the main vertical retinal meridians is confirmed. A new scaling effect is postulated to account for veridicality of the stereoscopic vertical.

Human binocular interaction: Towards a neural model

A new model for binocular processing is described. (i) In the bilateral either-eye channel, summation of the excitatory monocular responses is preceded by partial, reciprocal inhibition between each eyes responses. (ii) In the fused channel, the response is binocular, purely excitatory, multiplicative. (iii) The net binocular response of the system is a sum of the outputs of (i) and (ii). The model contains no independent monocular contributions to the net binocular response. All stimuli on corresponding retinal loci are processed in the either-eye channel; in addition, the fused channel responds to stimuli that are near 0-phase interocularly. The model is sufficiently general to account for binocular performance at the differential luminance threshold and in suprathreshold contrast matching, and it also offers a novel explanation for interocular transfer of adaptation. Plausibility of the model is briefly considered with regard to visual neurophysiology.

ADAPTATION TO APPARENT MOTION

A spot alternating between two positions can produce apparent motion (AM). Following prolonged inspection, the AM degenerates into flicker. This adaptation effect was found to depend on spacing and timing; the probability of seeing motion during a 30-sec inspection period declined linearly with log spatial separation (over a range from 0.1 to 1 deg), and with log alternation rate (over a range from 2 to 4.5 Hz). Cross-adaptation, in which subjects were adapted to one alternation rate and tested at another, showed that low alternation rates gave stronger motion signals than high rates did. Adaptation to real motion (RM) strongly suppressed AM, which suggests that AM must be stimulating the same neural pathways as RM. Flickering spots (i.e. in-phase flicker) produced less adaptation than did a spot alternating between two positions (i.e. counterphase flicker), so the adapting mechanism must be responding to relative temporal phase. Embedding the adapting spots in configurations of other spots, which altered the pattern of perceived adapting motion without altering the local retinal stimulation, minimized the adaption, so the adapting mechanism must be responding to the path of seen motion. Adaptation can be used to measure the strength of AM and shows that AM is strongest for small separations, low alternation rates and high luminance contrast.

Binocular summation in detection of contrast flashes

We studied monocular and binocular detection of foveal flashes of different contrast. When background contours were binocularly fused, detectability (d’) of binocular test flashes was, on the average, twice the detectability of monocularly presented flashes. The precise amount of binocular advantage varied with test contrast: binocular improvement exceeded full summation for low test contrast, but fell below full summation at higher test contrasts. In the absence of contours in one eye, background luminances are not expected to sum, yet binocular detection is an average of 41.5% better than monocular detection. This indicates a difference in the functional organization of the fused binocular channel and a monocular channel.

Monocular sensitivity during binocular viewing

Monocular detection of local luminance increments was studied psychophysically, during fusion, rivalry, and nonrivalry, Visibility of contrast flashes occurring less than 0.5 deg away from the center of the fovea, was affected by ipsilateral masking contours in such a way as to suggest that the contrast probe enhanced detectability in the tested eye. Visibility of luminance increments, whether flashed or continuously given in one monocular field, was best when a few contours were present in the tested eye but absent (or suppressed) in the partner eye. On the average, detection was poorest during rivalry. In fusion, detection was intermediate between rivalry and monocular dominance. It is proposed that background luminance summation is the specific mechanism in fusion.

Qualitative observations in visual science: “The farnsworth shelf”: Fusion at the site of the “ghosts”

Abstract The Keplerian projection theory of stereopsis predicts hidden intersections (the “ghosts”) in the binocular visual space. Experiments performed in real, as well as in stereoscopic space show that the fusional vergence may rest at the site of the “ghosts”; this normally happens when stereograms are perfectly fused. It is suggested that fusion always acts to minimize global disparity differences, unless the fixation reflex is elicited to oppose global fusion. The hidden (from awareness) structure of the binocular projection field represents a valuable “matrix” containing ready programs for changes in vergence, as well as for perception of spatial positions. The problem of the “ghosts” is interpreted as a normal instance of the still unknown relationship between stimulation and perception.

Anatomy of a Flash. 1. Two-Peak Masking and a Temporal Filling-In

A modified paradigm of Crawford masking was used to link masking to brightness fluctuation, as distinct from flash brightness. Thresholds were measured for a 10 ms incremental pulse (the ‘probe’) presented before, during, or after a 500 ms pulse (the ‘flash’). Both pulses were spatially coextensive with the background field, thus the criterion for probe detection was purely temporal. The flash occurred either in the tested eye, the opposite eye, or in both eyes. In all conditions, masking was strongly bimodal: thresholds peaked near flash onset and flash offset. The flash was perceived as a unitary event. Bimodal masking is attributed to cortical on-and off-effects, as (i) dichoptic masking was strong and (ii) the same incremental probe was masked by either incremental or decremental flashes. Strikingly, monocular probe thresholds were about equally elevated by binocular as by monocular flashes, although the binocular flashes were brighter. Therefore, some monocular features can be preserved in the larger net binocular response. A general conclusion is that masking depends on the same transient neural responses that bring about a brightness fluctuation, whereas the appearance of the flash as a single event, a unitary change of brightness, depends on a different mechanism, perhaps a sustained response that performs a temporal filling-in.

Binocular fusion regarded as simple summation of monocular photopic luminances

Binocular (B) and monocular (BM) increment intensity at threshold was measured on binocularly fused backgrounds of luminance I (30 cd/m2), or 2I (60 cd/m2). Simple summation (S) predicts S = BM(I)/B(2I) = 1. Mean results yield S = 0.99. The mean was similar in the 4 observers, and it did not change with absolute intensity at threshold, which decreased tenfold, as duration of the increment was increased from 3 and 240 msec. Simple summation of monocular luminances occurring during binocular fusion is the most likely explanation of the results.

Vision Comes to Mind

An overview is presented in which visual awareness is regarded as the epitome of the general mind—body problem. An experimental solution of the problem is considered first, followed by a philosophical outlook. It is argued that the scientific approach may eventually discover the neural correlate of visual awareness, but visual perceptions, even simple qualia like ‘brightness’ or ‘color’, occur in the mind of a conscious observer and are not reducible to observable activity of a specific set of neurons in the brain.