Binocular luminance detection: availability of more than one central interaction

An anti-Hebbian model for binocular visual plasticity and its attentional modulation

Monocular deprivation during the critical period impairs the cortical structure and visual function of the deprived eye. Conversely, transient occlusion of one eye in adults enhances the predominance of that eye. This counter-intuitive effect of short-term monocular deprivation is a form of homeostatic plasticity. However, whether this sensory plasticity requires attention, and the underlying neural mechanisms remain unclear. Here, through a psychophysical experiment, we demonstrate that the deprivation effect is dramatically attenuated in the absence of attention. We develop a neural computational model incorporating the Hebbian learning rule in interocular inhibitory synapses (i.e., anti-Hebbian learning) to explain the deprivation effect. Our model predicts both the boosting of the deprived eye and its dependence on attention. Moreover, it accounts for other forms of binocular plasticity, including plasticity observed in prolonged binocular rivalry. We suggest that short-term binocular plasticity arises from the plasticity in inhibitory connections between the two monocular pathways.

Applying the efficient coding principle to understand encoding of multisensory and multimodality sensory signals

Sensory neurons often encode multisensory or multimodal signals. For example, many medial superior temporal (MST) neurons are tuned to heading direction of self-motion based on visual (optic flow) signals and vestibular signals. Middle temporal (MT) cortical neurons are tuned to object depth from signals of two visual modalities: motion parallax and binocular disparity. A MST neuron's preferred heading directions from different senses can be congruent (matched) or opposite from each other. Similarly, the preferred depths of a MT neuron from the two modalities are congruent in some neurons and opposite in other neurons. While the congruent tuning appears natural for cue integration, the functions of the opposite tuning have been puzzling. This paper explains these tunings from the efficient coding principle that sensory encoding extracts as much sensory information as possible while minimizing neural cost. It extends the previous applications of this principle to understand neural receptive fields in retina and the primary visual cortex, particularly multimodal encoding of cone signals or binocular signals. Congruent and opposite sensory signals that excite the congruent and opposite neurons, respectively, are the decorrelated sensory components that provide a general purpose, efficient, representation of sensory inputs before task specific object segmentation and recognition. It can be extended to encoding signals from more than two sensory sources, e.g., from three cone types. This framework also predicts a wider tuning width for the opposite than congruent neurons, neurons that are neither congruent nor opposite, and how neural receptive fields adapt to statistical changes of sensory environments.

Surround masking reveals binocular adding and differencing channels

Recent studies suggest that binocular adding S+ and differencing S- channels play an important role in binocular vision. To test for such a role in the context of binocular contrast detection and binocular summation, we employed a surround masking paradigm consisting of a central target disk surrounded by a mask annulus. All stimuli were horizontally oriented 0.5c/d sinusoidal gratings. Correlated stimuli were identical in interocular spatial phase while anticorrelated stimuli were opposite in interocular spatial phase. There were four target conditions: monocular left eye, monocular right eye, binocular correlated and binocular anticorrelated, and three surround mask conditions: no surround, binocularly correlated and binocularly anticorrelated. We observed consistent elevation of detection thresholds for monocular and binocular targets across the two binocular surround mask conditions. In addition, we found an interaction between the type of surround and the type of binocular target: both detection and summation were relatively enhanced by surround masks and targets with opposite interocular phase relationships and reduced by surround masks and targets with the same interocular phase relationships. The data were reasonably well accounted for by a model of binocular combination termed MAX (S+S-), in which the decision variable is the probability summation of modeled S+ and S- channel responses, with a free parameter determining the relative gains of the two channels. Our results support the existence of two channels involved in binocular combination, S+ and S-, whose relative gains are adjustable by surround context.

Detection of vertical interocular phase disparities using luster as cue

Interocular disparities in contrast generate an impression of binocular luster, providing a cue for their detection. Disparities in the carrier spatial phase of horizontally oriented Gabor patches also generate an impression of luster, so the question arises as to whether it is the disparities in local contrast that accompany the phase disparities that give rise to the luster. We examined this idea by comparing the detection of interocular spatial phase disparities with that of interocular contrast disparities in Gabor patches, in the latter case that differed in overall contrast rather than phase between the eyes. When bandwidth was held constant and Gabor spatial frequency was varied, the detection of phase and contrast disparities followed a similar pattern. However, when spatial frequency was fixed and Gabor envelope standard deviation (and hence number of modulation cycles) was varied, thresholds for detecting phase disparities followed a U-shaped function of Gabor standard deviation, whereas thresholds for contrast disparities, following an initial decline, were more-or-less constant as a function of Gabor standard deviation. After reviewing a number of possible explanations for the U-shape found with phase disparities, we suggest that the likely cause is binocular sensory fusion, the strength of which increases with the number of modulation cycles. Binocular sensory fusion would operate to reduce phase but not contrast disparities, thus selectively elevating phase disparity thresholds.

Binocular summation and efficient coding

Two eyes are better than one at detecting a pattern, an advantage termed binocular summation. It is widely believed that binocular summation is mediated by neurons that sum the two eyes' inputs. Here we suggest an alternative model based on a model of binocular interactions proposed by Cohn, Leong & Lasley (Vision Research, 1981, 21, 1017-1023) and further motivated by the efficient coding framework proposed by Li & Atick (Network: Computation in Neural Systems, 1994, 5, 157-174). In the model, termed MAX(S+S-), binocular summation is mediated by channels that compute the sum, S+, and difference, S-, of the two eyes' monocular signals. The S+ and S- signals are assumed to be perturbed by independent noise, have independent gains and contribute independently to detection via the MAX rule. To test the model we measured binocular summation for horizontally-oriented Gabor patches at a range of spatial-frequencies and bandwidths, at both contrast detection threshold and for increment thresholds on binocular pedestals at contrasts set to 10x detection threshold. The model's performance was compared to that of two conventional models of binocular summation, one in which the two eyes' signals remain separate at the decision stage, termed MAX(LR), the other in which the two eye's signals are summed by a single channel, termed B+, with both models incorporating interocular inhibition. The MAX(S+S-) model gave as good a performance as the other two models. Together with the evidence for the involvement of separately gain controlled S+ and S- signals underpinning a wide range of binocular behaviors, we conclude that the MAX(S+S-) model can and should be considered as a viable model for binocular summation.

Interocular ND filter suppression: Eccentricity and luminance polarity effects

The depth and extent of interocular suppression were measured in binocularly normal observers who unilaterally adapted to neutral density (ND) filters (0, 1.5, 2, and 3 ND). Suppression was measured by dichoptically matching sectors of a ring presented to the adapted eye to a fixed contrast contiguous ring presented to the non-adapted eye. Other rings of alternating polarity were viewed binocularly. Rings were defined by luminance (L), luminance with added dynamic binary luminance noise (LM), and contrast modulating the same noise (CM). Interocular suppression depth increased with increasing ND, nearing significance (p = 0.058) for 1.5 ND. For L and LM stimuli, suppression depth across eccentricity (±12° visual field) differed for luminance increment (white) versus luminance decrement (black) stimuli, potentially confounding eccentricity results. Suppression for increment-only (white) luminance stimuli was steeper centrally and extended across the visual field, but was deeper for L than for LM stimuli. Suppression for decrement-only (black) luminance stimuli revealed only central suppression. Suppression was deeper with CM than LM stimuli, suggesting that CM stimuli are extracted in areas receiving predominantly binocular input which may be more sensitive to binocular disruption. Increment (white) luminance stimuli demonstrate deeper interocular suppression in the periphery than decrement (black) stimuli, so they are more sensitive to changes in peripheral suppression. Asymmetry of suppression in the periphery for opposite polarity luminance stimuli may be due to interocular receptive field size mismatch as a result of dark adaptation separately affecting ON and OFF pathways. Clinically, measurement of suppression with CM stimuli may provide the best information about post-combination binocularity.

Interocular difference thresholds are mediated by binocular differencing, not summing, channels

Patterns in the two eyes' views that are not identical in hue or contrast often elicit an impression of luster, providing a cue for discriminating them from perfectly matched patterns. Here we attempt to determine the mechanisms for detecting interocular differences in luminance contrast, in particular in relation to the possible contributions of binocular differencing and binocular summing channels. Test patterns were horizontally oriented multi-spatial-frequency luminance-grating patterns subject to variable amounts of interocular difference in grating phase, resulting in varying degrees of local interocular contrast difference. Two types of experiment were conducted. In the first, subjects discriminated between a pedestal with an interocular difference that ranged upward from zero (i.e., binocularly correlated) and a test pattern that contained a bigger interocular difference. In the second type of experiment, subjects discriminated between a pedestal with an interocular difference that ranged downward from a maximum (i.e., binocularly anticorrelated) and a test pattern that contained smaller interocular difference. The two types of task could be mediated by a binocular differencing and a binocular summing channel, respectively. However, we found that the results from both experiments were well described by a simpler model in which a single, linear binocular differencing channel is followed by a standard nonlinear transducer that is expansive for small signals but strongly compressive for large ones. Possible reasons for the lack of involvement of a binocular summing channel are discussed in the context of a model that incorporates the responses of both monocular and binocular channels.

Adaptation to interocular difference

Patterns in the two eyes' views that are not identical in hue or contrast often elicit an impression of luster, providing a cue for discriminating them from perfectly matched patterns. Here we ask whether the mechanism for detecting interocular differences (IDs) is adaptable. Our stimuli were horizontally oriented multispatial-frequency grating patterns that could be subject to varying degrees of ID through the introduction of interocular phase differences in the grating components. Subjects adapted to patterns that were either correlated, uncorrelated, monocular (one eye only), or anticorrelated. Following adaptation, thresholds for detecting IDs were measured using a staircase procedure. It was found that ID thresholds were elevated following adaptation to uncorrelated, monocular, and anticorrelated but not correlated patterns. Threshold elevation was found to be maximal when the orientations of the adaptor and test gratings were the same, and when their spatial frequencies were similar. The results support the existence of a specialized mechanism for detecting IDs, the most likely candidate being the binocular differencing channel proposed in previous studies.

Conflict-sensitive neurons gate interocular suppression in human visual cortex

Neural suppression plays an important role in cortical function, including sensory, memory, and motor systems. It remains, however, relatively poorly understood. A paradigmatic case arises when conflicting images are presented to the two eyes. These images can compete for awareness, and one is usually strongly suppressed. The mechanisms that resolve such interocular conflict remain unclear. Suppression could arise solely from “winner-take-all” competition between neurons responsive to each eye. Alternatively, suppression could also depend upon neurons detecting interocular conflict. Here, we provide physiological evidence in human visual cortex for the latter: suppression depends upon conflict-sensitive neurons. We recorded steady-state visual evoked potentials (SSVEP), and used the logic of selective adaptation. The amplitude of SSVEP responses at intermodulation frequencies strengthened as interocular conflict in the stimulus increased, suggesting the presence of neurons responsive to conflict. Critically, adaptation to conflict both reduced this SSVEP effect, and increased the amount of conflict needed to produce perceptual suppression. The simplest account of these results is that interocular-conflict-sensitive neurons exist in human cortex: adaptation likely reduced the responsiveness of these neurons which in turn raised the amount of conflict required to produce perceptual suppression. Similar mechanisms may be used to resolve other varieties of perceptual conflict.

Contrast and lustre: A model that accounts for eleven different forms of contrast discrimination in binocular vision

Our goal here is a more complete understanding of how information about luminance contrast is encoded and used by the binocular visual system. In two-interval forced-choice experiments we assessed observers' ability to discriminate changes in contrast that could be an increase or decrease of contrast in one or both eyes, or an increase in one eye coupled with a decrease in the other (termed IncDec). The base or pedestal contrasts were either in-phase or out-of-phase in the two eyes. The opposed changes in the IncDec condition did not cancel each other out, implying that along with binocular summation, information is also available from mechanisms that do not sum the two eyes' inputs. These might be monocular mechanisms. With a binocular pedestal, monocular increments of contrast were much easier to see than monocular decrements. These findings suggest that there are separate binocular (B) and monocular (L,R) channels, but only the largest of the three responses, max(L,B,R), is available to perception and decision. Results from contrast discrimination and contrast matching tasks were described very accurately by this model. Stimuli, data, and model responses can all be visualized in a common binocular contrast space, allowing a more direct comparison between models and data. Some results with out-of-phase pedestals were not accounted for by the max model of contrast coding, but were well explained by an extended model in which gratings of opposite polarity create the sensation of lustre. Observers can discriminate changes in lustre alongside changes in contrast.

Efficient Coding Theory Predicts a Tilt Aftereffect from Viewing Untilted Patterns

The brain is bombarded with a continuous stream of sensory information, but biological limitations on the data-transmission rate require this information to be encoded very efficiently [1]. Li and Atick [2] proposed that the two eyes' signals are coded efficiently in the brain using mutually decorrelated binocular summation and differencing channels; when a channel is strongly stimulated by the visual input, such that sensory noise is negligible, the channel should undergo temporary desensitization (known as adaptation). To date, the evidence for this theory has been limited [3, 4], and the binocular differencing channel is missing from many models of binocular integration [5-10]. Li and Atick's theory makes the remarkable prediction that perceived direction of tilt (clockwise or counterclockwise) of a test pattern can be controlled by pre-exposing observers to visual adaptation patterns that are untilted or even have no orientation signal. Here, we confirm this prediction. Each test pattern consisted of different images presented to the two eyes such that the binocular summation and difference signals were tilted in opposite directions, to give ambiguous information about tilt; by selectively desensitizing one or other of the binocular channels using untilted or non-oriented binocular adaptation patterns, we controlled the perceived tilt of the test pattern. Our results provide compelling evidence that the brain contains binocular summation and differencing channels that adapt to the prevailing binocular statistics.

A model of binocular rivalry and cross-orientation suppression

Binocular rivalry and cross-orientation suppression are well-studied forms of competition in visual cortex, but models of these two types of competition are in tension with one another. Binocular rivalry occurs during the presentation of dichoptic grating stimuli, where two orthogonal gratings presented separately to the two eyes evoke strong alternations in perceptual dominance. Cross-orientation suppression occurs during the presentation of plaid stimuli, where the responses to a component grating presented to both eyes is weakened by the presence of a superimposed orthogonal grating. Conventional models of rivalry that rely on strong competition between orientation-selective neurons incorrectly predict rivalry between the components of plaids. Lowering the inhibitory weights in such models reduces rivalry for plaids, but also reduces it for dichoptic gratings. Using an exhaustive grid search, we show that this problem cannot be solved simply by adjusting the parameters of the model. Instead, we propose a robust class of models that rely on ocular opponency neurons, previously proposed as a mechanism for efficient stereo coding, to yield rivalry only for dichoptic gratings, not for plaids. This class of models reconciles models of binocular rivalry with the divisive normalization framework that has been used to explain cross-orientation. Our model makes novel predictions that we confirmed with psychophysical tests.

Binocular spatial induction for the perception of depth does not cross the midline

Although horizontal binocular retinal disparity between images in the two eyes resulting from their different views of the world has long been the centerpiece for understanding the unique characteristics of stereovision, it does not suffice to explain many binocular phenomena. Binocular depth contrast (BDC), the induction of an appearance of visual pitch in a centrally located line by pitched-from-vertical flanking lines, has particularly been the subject of a good deal of attention in this regard. In the present article, we show that BDC does not cross the median plane but is restricted to the side of the visual field containing a unilateral inducer. These results cannot be explained by the use of retinal disparity alone or in combination with any additional factors or processes previously suggested to account for stereovision. We present a two-channel three-stage neuromathematical model that accounts quantitatively for present and previous BDC results and also accounts for a large number of the most prominent features of binocular pitch perception: Stage 1 of the differencing channel obtains the difference between the retinal orientations of the images in the two eyes separately for the inducer and the test line; stage 1 of the summing channel obtains the corresponding sums. Signals from inducer and test stimuli are combined linearly in each channel in stage 2, and in stage 3 the outputs from the two channels are combined along with a bias signal from the body-referenced mechanism to yield ', the model's prediction for the perception of pitch.

Interocular interactions reveal the opponent structure of motion mechanisms

Interactions between motion sensors tuned to the same and to opposite directions were probed by means of measuring summation indexes for sensitivities (d') to contrast increments and/or decrements applied to drifting gratings presented in binocular and in dichoptic vision. The data confirm a phenomenon described by Stromeyer, Kronauer, Madsen & Klein (1984, J. Opt. Soc. Am. A 1, 876-884), whereby opposite polarity contrast changes applied to binocular gratings drifting in opposite directions yield sensitivities significantly higher than same sign changes for which performance complies with probability summation (PS). The effect disappears in dichoptic vision where opposite sign contrast changes yield a performance close to, or below PS, whether they are applied to same or to opposite direction stimuli. Same sign changes in dichoptic drifting stimuli yield a performance higher than PS independently of their relative directions and close to the performances obtained when these same sign changes are applied to dichoptic, static +/- 45 degrees gratings. Opposite sign changes applied to such static gratings yield PS. The data support the view according to which: (i) motion direction is extracted at the monocular site; (ii) motion sensors exhibit mutual inhibition within each eye when tuned to opposite directions; and (iii) binocular summation when tuned to the same direction. The data also suggest that (iv) independently of their directional tuning, all motion sensors converge on a binocular, motion non-specific ('flicker') unit; and that (v) signals carried by ON and OFF pathways are slightly inhibitory to each other.

Neural dynamics of binocular brightness perception

How does the visual cortex combine information from both eyes to generate perceptual representations of object surfaces? Important clues about this process may be derived from data about the perceived brightness of surface regions under binocular viewing conditions, including data about binocular brightness summation in response to Ganzfelds, the U-shaped data of Fechner's paradox that violates binocular brightness summation, and the effects of different combinations of monocular and binocular contours and surface luminance differences on threshold sensitivity to monocular flashes of light. How to reconcile these apparently contradictory data properties has been a severe challenge to previous models, and none has explained them all. The present article quantitatively simulates them all by further developing the FACADE vision model. Key model processes discount the illuminant and compute image contrasts in each monocular channel using shunting on-center off-surround networks; binocularly fuse these discounted monocular signals using shunting on-center off-surround networks with nonlinear excitatory and inhibitory signals; and use these binocularly fused activities to trigger filling-in of a binocular surface representation that represents perceived surface brightness. Previous models that have suggested explanations of subsets of these data are discussed.

Discrimination of binocular color mixtures in dichromacy: evaluation of the Maxwell-Cornsweet conjecture

We tested the Maxwell-Cornsweet conjecture that differential spectral filtering of the two eyes can increase the dimensionality of a dichromat's color vision. Sex-linked dichromats wore filters that differentially passed long- and middle-wavelength regions of the spectrum to each eye. Monocularly, temporal modulation thresholds (1.5 Hz) for color mixtures from the Rayleigh region of the spectrum were accounted for by a single, univariant mechanism. Binocularly, univariance was rejected because, as in monocular viewing by trichromats, in no color direction could silent substitution of the color mixtures be obtained. Despite the filter-aided increase in dimension, estimated wavelength discrimination was quite poor in this spectral region, suggesting a limit to the effectiveness of this technique.

Models of binocular luminance interaction evaluated using visually evoked potential and psychophysical measures: a tribute to M. Russell Harter

We determined subjects' responses to sine-wave modulated lights employing visually evoked potentials (VEPs) and psychophysical thresholds in a series of experiments. The stimuli had the same temporal frequency and mean luminance in each eye but the phase difference between the two eyes was varied so that phase was either the same (dioptic) or different (dichoptic) in the two eyes. The data were fit by a model which had two binocular pathways, one which summed monocular nonlinear elements and a second which had a nonlinearity following the combination of monocular linear elements. In the second channel the outputs of the monocular linear elements were summed at low luminance while at higher luminance levels they were subtracted. Based on variations in the threshold data with temporal frequency, the pathway which summed nonlinear monocular elements was identified with the magnocellular (M) pathway, and the pathway which combined monocular linear elements prior to a binocular nonlinear element was identified with the parvocellular (P) pathway.

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.

Temporal arteritis associated with Borrelia infection. A case report

A 71-year-old man had sudden vision loss associated with headache. A temporal artery biopsy revealed a typical picture of giant cell arteritis. Subsequent steroid therapy failed to restore sight. A later blood culture contained spirochetes compatible with Borrelia species, and a silver stain of the temporal artery biopsy specimen demonstrated a similar spirochete. Treatment with i.v. ceftriaxone sodium led to some limited return of sight. To our knowledge, this is the first case report of a spirochete compatible with Borrelia found in a temporal artery biopsy specimen.

Binocular combination of contrast signals

We studied the detectability of dichoptically presented vertical grating patterns that varied in the ratio of the contrasts presented to the two eyes. The resulting threshold data fall on a binocular summation contour well described by a power summation equation with an exponent near 2. We studied the effect of adding one-dimensional visual noise, either correlated or uncorrelated between the eyes, to the grating patterns. The addition of uncorrelated noise elevated thresholds uniformly for all interocular ratios, while correlated noise elevated thresholds for stimuli whose ratios were near 1 more than thresholds for other stimuli. We also examined the effects of monocular adaptation to a high-contrast grating on the form of the summation contour. Such adaptation elevates threshold in a manner that varies continuously with the interocular contrast ratio of the test targets, and increases the amount of binocular summation. Each of several current models can explain some of our results, but no one of them seems capable of accounting for all three sets of data. We therefore develop a new multiple-channel model, the distribution model, which postulates a family of linear binocular channels that vary in their sensitivities to the two monocular inputs. This model can account for our data and those of others concerning binocular summation, masking, adaptation and interocular transfer. We conclude that there exists a system of ocular dominance channels in the human visual system.

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.

Fechner's paradox in binocular contrast sensitivity

It is generally accepted that binocular spatial contrast sensitivity in normal observers is higher than monocular sensitivity by some 42% across all spatial frequencies, an amount predictable on the basis of neural summation of the two monocular responses. Such summation predicts that a reduction of sensitivity in one eye would result in a fall in binocular sensitivity to a level approaching, but never lower than, that of the other eye. We present evidence that reduction in monocular sensitivity caused by reduced luminance can, in some subjects, lower binocular sensitivity to a level below that of the other eye, an analogue of Fechner's brightness paradox. In other subjects the expected summation occurs and binocular sensitivity always remains at or above the monocular level.