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A I Cogan

Publications and source records attributed to A I Cogan.

15 recordsLinked to original sources
PubMed1995

Binocular disparity processing with opposite-contrast stimuli.

Stereoscopic perception of relative depth with reversed-contrast half images differs in several important respects from stereopsis with matched-contrast half images. Thus, reversed-contrast images show no correlated shift in visual direction, indicating that the sensory-fusion mechanism ignores opposite-sign edges; one experiment addressed this aspect of the problem. Mainly, this was a quantitative study of opposite-contrast stereopsis, in which stereoacuity was measured as a function of bar width by means of narrow-band stimuli. Acuity was about an order of magnitude worse for reversed-contrast than for matched stimuli, but the ability to see valid (disparity-dependent) depth was not altogether lost even with wide (1 cycle deg-1) reversed-contrast bars. It is generally believed that depth with opposite-contrast stimuli is mediated by interaction between binocular stimuli components that have the same sign of contrast. Perceived depth was measured as a function of disparity and thus one of the predictions of that 'same-sign hypothesis' was tested experimentally; then, the magnitude of same-sign components was manipulated within the reversed-contrast stimuli, and thus the general prediction of the same-sign hypothesis was tested. The results show conclusively that the same-sign hypothesis cannot account for opposite-contrast stereopsis; its mechanism remains unknown.

Contrast Sensitivity↗
PubMed1992

Anatomy of a flash. 2. The 'width' of a temporal edge.

In a previous study it has been found by using the Crawford masking paradigm with spatially coextensive stimuli that the end points of even a weak flash have a strong masking effect on the probe pulse, but no threshold elevation was observed when the probe was 100 ms or longer from flash onset and offset. The relationship between the strength of a temporal edge (eg the amplitude of a sudden increase in field luminance) and the duration of masking has not been systematically investigated. In the present experiments the duration of masking produced by a step of luminance as a function of step size was measured. With steps whose amplitude was between ten and five hundred times detection threshold, the duration of masking was about constant, ie 70 and 50 ms for on-responses and off-responses, respectively. This refractory period defined the 'width' of a temporal edge. Unless two suprathreshold unidirectional luminance changes are separated by an interval exceeding the width of a temporal edge, they are perceived as a single change in brightness. A linear systems analysis led to predictions for ramp stimuli that were confirmed by experiment.

Attention↗
PubMed1990

Two-pulse monocular and binocular interactions at the differential luminance threshold.

Interaction between two pulses at the differential luminance threshold was studied for stimuli pairs presented to the same eye or to opposite eyes with an interocular delay. With monocular stimuli, the results replicated the earlier observations by Ikeda (1965) and Rashbass (1970) indicating linear interaction followed by rectification occurring at about 50-60 msec into the integration epoch. Binocular results were different, in accord with observations made in the contrast domain by Green and Blake (1981). Binocular stimuli of opposite polarity showed no cancellation. Binocular facilitation at threshold was found when either the stimuli of the same sign (+ + or - -) occurred with little interocular delay (stimulus onset asynchrony, SOA less than 15 msec), or the stimuli of the opposite sign (+ - or - +) were presented with an interocular delay between 15 and 100 msec SOA; the latter effect was at maximum with flashes 50 msec in duration presented with 50 msec interocular SOA. These results imply that binocular interaction takes place between rectified internal effects of luminance pulses. From the two-channel binocular model of Cogan (1987), binocular facilitation is attributed to the "fused" response derived from multiplicative excitation between same-sign (half-wave rectified), internal pulse responses. The absence of cancellation between simultaneous opposite-sign dichoptic stimuli is attributed to the "either-eye" binocular process dealing with full-wave rectified internal pulse responses to transient stimuli.

Humans↗
PubMed1989

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.

Attention↗
PubMed1989

Do background luminances interact during binocular fusion?

The question investigated in the experiments reported here was whether monocular background luminances sum during binocular fusion. Fusion was made explicit by using a random-dot stereogram (RDS) as a background stimulus. In the presence of the RDS, differential luminance thresholds were somewhat higher than in the uniform field: a full-field, binocular dot array acted as a mask for a full-field luminance change, but global depth had no effect at threshold. The amount of the binocular advantage at threshold was compared to the basic "threshold response," that is, the change in threshold resulting from raising the background luminance by a factor of 2. It was found that the amount of the binocular advantage was equivalent, on the average, to some 75% of the threshold response--significantly less than the 100% predicted by "simple summation." The amount of the binocular advantage varied substantially among observers and eyes, whereas the threshold response obeyed Weber's law in all cases: the variability was eye-, rather than threshold-dependent. Monocular thresholds did not decrease when taken with the nontest eye occluded rather than viewing a fused background. The proposition that the adaptation state of the visual system is increased during binocular fusion (Cogan, 1982) was not supported. Yet occluding the nontest eye, rather than presenting the test stimulus monocularly against a fused background, did change monocular thresholds in some eyes and observers. These findings are interpreted as evidence for a complex binocular background interaction involving both summation and inhibition.

Attention↗
PubMed1987

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 eye's 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, Ocular↗
PubMed1985

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.

Adaptation, Ocular↗
PubMed1984

Binocular summation on fused annular backgrounds.

A paradigm has been proposed to test the hypothesis that binocular fusion results in simple summation, S, of monocular photopic luminances: S = delta LBM(L)/delta LB(2L) = 1, where delta LBM and delta LB are threshold luminances for a monocular and a binocular increment, appearing on a fused background of luminance L or 2L, respectively. This prediction was tested psychophysically, with background size as parameter. Thresholds were measured for a brief (20 ms) foveal flash centered on a luminous disk encircled by a massive black annulus serving as a fusion lock. The flash was presented binocularly, or monocularly, at 30 and 60 cd m-2 background luminances. The diameter of the disk was varied from 3 min to 40 min visual angle. The size of the flash was constant (6 min diameter), except for the smallest disk (3 min diameter). All thresholds varied as a function of background size (the known 'sensitization effect'). The summation index S did not vary with background size. A mean value S = 0.9 was found; this is consistent with the notion that summation of the order of 90% occurs during binocular fusion. We surmise that even a weak monocular test flash makes fusion less than perfect.

Adult↗
PubMed1983

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.

Depth Perception↗
PubMed1982

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.

Computers↗
PubMed1980

Fluctuations of visibility during dichoptic viewing: Preliminary report.

Detection rates were determined for multiple flashes (20-msec duration, 7.5 arc min dia) presented foveally to one eye while the other eye had either a uniform target (the "monocular" condition), a congruent grating (fusion), or an orthogonal grating (rivalry). Average detection rates were highest in the monocular condition, and they were about the same in fusion and rivalry. In all conditions, the typical error (some 70% of all errors) was one stimulus missed out of several simultaneously presented. It was concluded that sensitivity fluctuates over the fovea, even in the absence of contours in the other eye. In two observers, the ratio of detection rates for the two eyes was about the same for all conditions; in two other observers, this ratio decreased (eye dominance increased) in fusion, and especially in rivalry, relative to the monocular condition. The question is raised whether a common link exists between spontaneous fluctuations of sensitivity and those induced by dichoptic contours.

Form Perception↗