Evidence for implication of primate area V1 in neural 3-D spatial localization processing.

We investigated the neural mechanisms underlying visual localization in 3-D space in area V1 of behaving monkeys. Three different sources of information, retinal disparity, viewing distance and gaze direction, that participate in these neural mechanisms are being reviewed. The way they interact with each other is studied by combining retinal and extraretinal signals. Interactions between retinal disparity and viewing distance have been shown in foveal V1; we have observed a strong modulation of the spontaneous activity and of the visual response of most V1 cells that was highly correlated with the vergence angle. As a consequence of these gain effects, neural horizontal disparity coding is favoured or refined for particular distances of fixation. Changing the gaze direction in the fronto-parallel plane also produces strong gains in the visual response of half of the cells in foveal V1. Cells tested for horizontal disparity and orientation selectivities show gain effects that occur coherently for the same spatial coordinates of the eyes. Shifts in preferred disparity also occurred in several neurons. Cells tested in calcarine V1 at retinal eccentricities larger than 10 degrees , show that horizontal disparity is encoded at least up to 20 degrees around both the horizontal and vertical meridians. At these large retinal eccentricities we found that vertical disparity is also encoded with tuning profiles similar to those of horizontal disparity coding. Combinations of horizontal and vertical disparity signals show that most cells encode both properties. In fact the expression of horizontal disparity coding depends on the vertical disparity signals that produce strong gain effects and frequent changes in peak selectivities. We conclude that the vertical disparity signal and the eye position signal serve to disambiguate the horizontal disparity signal to provide information on 3-D spatial coordinates in terms of distance, gaze direction and retinal eccentricity. We suggest that the relative weight among these different signals is the determining factor involved in the neural processing that gives information on 3-D spatial localization.     

Linear and nonlinear transparencies in binocular vision.

When the product of a vertical square-wave grating (contrast envelope) and a horizontal sinusoidal grating (carrier) are viewed binocularly with different disparity cues they can be perceived transparently at different depths. We found, however, that the transparency was asymmetric; it only occurred when the envelope was perceived to be the overlaying surface. When the same two signals were added, the percept of transparency was symmetrical; either signal could be seen in front of or behind the other at different depths. Differences between these multiplicative and additive signal combinations were examined in two experiments. In one, we measured disparity thresholds for transparency as a function of the spatial frequency of the envelope. In the other, we measured disparity discrimination thresholds. In both experiments the thresholds for the multiplicative condition, unlike the additive condition, showed distinct minima at low envelope frequencies. The different sensitivity curves found for multiplicative and additive signal combinations suggest that different processes mediated the disparity signal. The data are consistent with a two-channel model of binocular matching, with multiple depth cues represented at single retinal locations.     

Does vertical disparity scale the perception of stereoscopic depth?

It has been suggested that a measure of the gradients of vertical disparity over a surface may scale the mapping between horizontal disparity and perceived depth. We have investigated this possibility by obtaining estimates of the depth within stereograms that simulated two apposed fronto-parallel planes placed at different distances from an observer. The gradients of vertical disparity in a stereogram were set to simulate those appropriate to a viewing distance of 12.5 cm, 25 cm, 50 cm or 100 cm, whereas the distance specified by vergence and accommodative cues was always fixed at 50 cm. Judgements of the perceived depth between the two planes were uninfluenced by changes in the gradients of vertical disparity. It thus seems that the human visual system does not employ vertical disparity as a scaling parameter in stereoscopic depth judgements.     

Similar Mechanisms Underlie the Detection of Horizontal and Vertical Disparity Corrugations

Our aim was to compare sensitivity for horizontal and vertical disparity corrugations and to resolve whether these stimuli are processed by similar or radically different underlying mechanisms. We measure global disparity sensitivity as a function of carrier spatial frequency for equi-detectable carriers and found a similar optimal carrier relationship for vertical and horizontal stimuli. Sensitivity as a function of corrugation spatial frequency for stimuli of comparable spatial summation and composed of optimal, equi-detectable narrowband carriers did not significantly differ for vertical and horizontal stimuli. A small anisotropy was revealed when fixed, high contrast broadband carriers were used. In a separate discrimination-at-threshold experiment, multiple mechanisms of similar tuning were revealed to underlie the detection of both vertical and horizontal disparity corrugations. We conclude that the processing of the horizontal and vertical disparity corrugations occurs along similar lines.     

Vertical Binocular Disparity is Encoded Implicitly within a Model Neuronal Population Tuned to Horizontal Disparity and Orientation

Primary visual cortex is often viewed as a “cyclopean retina”, performing the initial encoding of binocular disparities between left and right images. Because the eyes are set apart horizontally in the head, binocular disparities are predominantly horizontal. Yet, especially in the visual periphery, a range of non-zero vertical disparities do occur and can influence perception. It has therefore been assumed that primary visual cortex must contain neurons tuned to a range of vertical disparities. Here, I show that this is not necessarily the case. Many disparity-selective neurons are most sensitive to changes in disparity orthogonal to their preferred orientation. That is, the disparity tuning surfaces, mapping their response to different two-dimensional (2D) disparities, are elongated along the cell''s preferred orientation. Because of this, even if a neuron''s optimal 2D disparity has zero vertical component, the neuron will still respond best to a non-zero vertical disparity when probed with a sub-optimal horizontal disparity. This property can be used to decode 2D disparity, even allowing for realistic levels of neuronal noise. Even if all V1 neurons at a particular retinotopic location are tuned to the expected vertical disparity there (for example, zero at the fovea), the brain could still decode the magnitude and sign of departures from that expected value. This provides an intriguing counter-example to the common wisdom that, in order for a neuronal population to encode a quantity, its members must be tuned to a range of values of that quantity. It demonstrates that populations of disparity-selective neurons encode much richer information than previously appreciated. It suggests a possible strategy for the brain to extract rarely-occurring stimulus values, while concentrating neuronal resources on the most commonly-occurring situations.     

Learning to use an invisible visual signal for perception

How does the brain construct a percept from sensory signals? One approach to this fundamental question is to investigate perceptual learning as induced by exposure to statistical regularities in sensory signals [1-7]. Recent studies showed that exposure to novel correlations between sensory signals can cause a signal to have new perceptual effects [2, 3]. In those studies, however, the signals were clearly visible. The automaticity of the learning was therefore difficult to determine. Here we investigate whether learning of this sort, which causes new effects on appearance, can be low level and automatic by employing a visual signal whose perceptual consequences were made invisible-a vertical disparity gradient masked by other depth cues. This approach excluded high-level influences such as attention or consciousness. Our stimulus for probing perceptual appearance was a rotating cylinder. During exposure, we introduced a new contingency between the invisible signal and the rotation direction of the cylinder. When subsequently presenting an ambiguously rotating version of the cylinder, we found that the invisible signal influenced the perceived rotation direction. This demonstrates that perception can rapidly undergo "structure learning" by automatically picking up novel contingencies between sensory signals, thus automatically recruiting signals for novel uses during the construction of a percept.     

Vertical disparity, egocentric distance and stereoscopic depth constancy: a new interpretation

There has long been a problem concerning the presence in the visual cortex of binocularly activated cells that are selective for vertical stimulus disparities because it is generally believed that only horizontal disparities contribute to stereoscopic depth perception. The accepted view is that stereoscopic depth estimates are only relative to the fixation point and that independent information from an extraretinal source is needed to scale for absolute or egocentric distance. Recently, however, theoretical computations have shown that egocentric distance can be estimated directly from vertical disparities without recourse to extraretinal sources. There has been little impetus to follow up these computations with experimental observations, because the vertical disparities that normally occur between the images in the two eyes have always been regarded as being too small to be of significance for visual perception and because experiments have consistently shown that our conscious appreciation of egocentric distance is rather crude and unreliable. Nevertheless, the veridicality of stereoscopic depth constancy indicates that accurate distance information is available to the visual system and that the information about egocentric distance and horizontal disparity are processed together so as to continually recalibrate the horizontal disparity values for different absolute distances. Computations show that the recalibration can be based directly on vertical disparities without the need for any intervening estimates of absolute distance. This may partly explain the relative crudity of our conscious appreciation of egocentric distance. From published data it has been possible to calculate the magnitude of the vertical disparities that the human visual system must be able to discriminate in order for depth constancy to have the observed level of veridicality. From published data on the induced effect it has also been possible to calculate the threshold values for the detection of vertical disparities by the visual system. These threshold values are smaller than those needed to provide for the recalibration of the horizontal disparities in the interests of veridical depth constancy. An outline is given of the known properties of the binocularly activated cells in the striate cortex that are able to discriminate and assess the vertical disparities. Experiments are proposed that should validate, or otherwise, the concepts put forward in this paper.     

Modeling of time disparity detection by the Hodgkin-Huxley equations

Phase-sensitive neurons in the electrosensory lateral line lobe in the electrosensory pathway of the wave-type electric fish, Gymnarchus niloticus, are specialized for sensing the time disparity between sensory inputs at different parts of the body surface that is necessary for an electrical behavior, jamming avoidance response. These neurons are sensitive to time disparity in the microsecond range between synaptic inputs that represent occurrence times of electrosensory signals at different areas on the body surface. We showed that an ideal Hodgkin-Huxley equation may serve as a time disparity detector that fits physiological precision, and the precision for the time disparity detection is largely regulated by the maximal g(K) conductance in the Hodgkin-Huxley equations.     

The precedence effect in the horizontal and vertical planes in experiments with a moving lagging signal

The precedence effect in the localization of a moving lagging sound source was studied in experiments on humans under the free field conditions in the presence of a stationary (lead) sound source. Broad-band noise (5–18 kHz) bursts 1 s in duration presented in the horizontal and vertical planes were used as signals. The lead-lag delays ranged from 1 to 40 ms. The results showed that, if the signals were presented in the horizontal plane, the probability of correct localization of the moving lagging signal was decreased for delays shorter than 25 ms; if the signals were presented in the vertical plane, it was decreased for delays shorter than 40 ms. If the delays were shorter than 8–10 ms, the subjects could not localize the moving lagging signal at all. In this interval of delays, the subjects could localize only the lead signal. The mean echo threshold for signals presented in the horizontal plane was smaller than for signals presented in the vertical plane (7.3 and 10.1 ms, respectively). However, comparison of these values across the sample of subject did not show significant differences [F(1, 5) = 5.52, p = 0.07]. The results of the study suggest that the precedence effect causes a tendency towards a stronger suppression of a moving lagging signal in the vertical plane than in the horizontal plane.     

Cue combination and the effect of horizontal disparity and perspective on stereoacuity

Relative depth judgments of vertical lines based on horizontal disparity deteriorate enormously when the lines form part of closed configurations (Westheimer, 1979). In studies showing this effect, perspective was not manipulated and thus produced inconsistency between horizontal disparity and perspective. We show that stereoacuity improves dramatically when perspective and horizontal disparity are made consistent. Observers appear to use unhelpful perspective cues in judging the relative depth of the vertical sides of rectangles in a way not incompatible with a form of cue weighting. However, 95% confidence intervals for the weights derived for cues usually exceed the a-priori [0-1] range.     

A computational model of stereoscopic prey capture in praying mantises

We present a simple model which can account for the stereoscopic sensitivity of praying mantis predatory strikes. The model consists of a single “disparity sensor”: a binocular neuron sensitive to stereoscopic disparity and thus to distance from the animal. The model is based closely on the known behavioural and neurophysiological properties of mantis stereopsis. The monocular inputs to the neuron reflect temporal change and are insensitive to contrast sign, making the sensor insensitive to interocular correlation. The monocular receptive fields have a excitatory centre and inhibitory surround, making them tuned to size. The disparity sensor combines inputs from the two eyes linearly, applies a threshold and then an exponent output nonlinearity. The activity of the sensor represents the model mantis’s instantaneous probability of striking. We integrate this over the stimulus duration to obtain the expected number of strikes in response to moving targets with different stereoscopic disparity, size and vertical disparity. We optimised the parameters of the model so as to bring its predictions into agreement with our empirical data on mean strike rate as a function of stimulus size and disparity. The model proves capable of reproducing the relatively broad tuning to size and narrow tuning to stereoscopic disparity seen in mantis striking behaviour. Although the model has only a single centre-surround receptive field in each eye, it displays qualitatively the same interaction between size and disparity as we observed in real mantids: the preferred size increases as simulated prey distance increases beyond the preferred distance. We show that this occurs because of a stereoscopic “false match” between the leading edge of the stimulus in one eye and its trailing edge in the other; further work will be required to find whether such false matches occur in real mantises. Importantly, the model also displays realistic responses to stimuli with vertical disparity and to pairs of identical stimuli offering a “ghost match”, despite not being fitted to these data. This is the first image-computable model of insect stereopsis, and reproduces key features of both neurophysiology and striking behaviour.     

The direction of retinal motion facilitates binocular stereopsis.

Visual information from binocular disparity and from relative motion provide information about three-dimensional structure and layout of the world. Although the mechanisms that process these cues have typically been studied independently, there is now a substantial body of evidence that suggests that they interact in the visual pathway. This paper investigates one advantage of such an interaction: whether retinal motion can be used as a matching constraint in the binocular correspondence process. Stimuli that contained identical disparity and motion signals but which differed in their fine-scale correlation were created to establish whether the direction, or the speed, of motion could enhance performance in a psychophysical task in which binocular matching is a limiting factor. The results of these experiments provide clear evidence that different directions of motion, but not different speeds, are processed separately in stereopsis. The results fit well with properties of neurons early in the cortical visual pathway which are thought to be involved in determining local matches between features in the two eyes'' images.     

Stereoscopic mechanisms: binocular responses of the striate cells of cats to moving light and dark bars

New knowledge concerning the internal structure and response properties of the receptive fields of striate cells calls for a fresh appraisal of their binocular interactions in the interest of a better understanding of the neural mechanisms underlying binocular depth discrimination. Binocular position-disparity response profiles were recorded from 71 simple and B-cells in response to moving light and dark bars. Predominantly excitatory (PE) cells (N = 48) had disparity response profiles that were spatially closely similar to their respective monocular responses. In addition, the centrally located excitatory subregions were flanked on one or both sides by non-specific inhibitory regions. PE cells with a preferred stimulus orientation within 30 degrees of the vertical (N = 17) showed binocular facilitations with maximal values that were always more than twice (mean 3.3) the sum of the two monocular responses to the same stimuli and generally greater than the facilitations shown by cells with orientations more than 30 degrees from the vertical (N = 29; mean 2.2 times the sum of the respective monocular responses). The strength of the binocular facilitation depended on the stimulus contrast, the facilitation decreasing with increasing contrast. The receptive-field disparity distribution of the 31 PE cells capable of making significant horizontal disparity discriminations has standard deviations of 0.37 degrees and 0.40 degrees, respectively. Predominantly inhibitory cells (PI) (N = 23) showed two basic types of disparity response profile: symmetric (N = 17) and asymmetric (N = 6). Uncertainty regarding the precise location of the binocular fixation point in the anaesthetized and paralysed preparation made it difficult to categorize PI cells adequately.     

Isotropic integration of binocular disparity and relative motion in the perception of three-dimensional shape

Richards (1985) showed that veridical three-dimensional shape may be recovered from the integration of binocular disparity and retinal motion information, but proposed that this integration may only occur for horizontal retinal motion. Psychophysical evidence supporting the combination of stereo and motion information is limited to the case of horizontal motion (Johnston et al., 1994), and has been criticised on the grounds of potential object boundary cues to shape present in the stimuli. We investigated whether veridical shape can be recovered under more general conditions. Observers viewed cylinders that were defined by binocular disparity, two-frame motion or a combination of disparity and motion, presented at simulated distances of 30 cm, 90 cm or 150 cm. Horizontally and vertically oriented cylinders were rotated about vertical and horizontal axes. When rotation was about the cylinder's own axis, no boundary cues to shape were introduced. Settings were biased for the disparity and two-frame motion stimuli, while more veridical shape judgements were made under all conditions for combined cue stimuli. These results demonstrate that the improved perception of three-dimensional shape in these stimuli is not a consequence of the presence of object boundary cues, and that the combination of disparity and motion is not restricted to horizontal image motion.     

Small-field, binocular neurons in the superficial layers of the frog optic tectum

Binocular neurons with receptive fields about 5 degrees across were recorded just beneath the pia. Most of them responded to dark stimuli in the lower half of their receptive field and to light stimuli above. There was almost no vertical disparity between the left and right fields and the modal value of the horizontal disparity of the population of cells was 1.7 degrees. Because frogs do not verge their eyes it is possible to calculate at what distance the receptive fields through the two eyes are superimposed. This calculation suggests that the neurons are tuned to detect features in the external world about 50 cm away. This is too far for the neurons to be involved in the frog's everyday distance vision. It is more likely that they are concerned with assessing the vertical position of a horizontal surface.     

Allometric space and allometric disparity: a developmental perspective in the macroevolutionary analysis of morphological disparity

Here, we advance novel uses of allometric spaces--multidimensional spaces specifically defined by allometric coefficients--with the goal of investigating the focal role of development in shaping the evolution of morphological disparity. From their examination, operational measures of allometric disparity can be derived, complementing standard signals of morphological disparity through an intuitive and process-oriented refinement of established analytical protocols used in disparity studies. Allometric spaces thereby become a promising context to reveal different patterns of evolutionary developmental changes and to assess their relative prevalence and importance. Such spaces offer a novel domain of investigation of phenotypic variation and should help in detecting large-scale trends, thus placing various macroevolutionary phenomena in an explicitly developmental context. Ammonoidea (Cephalopoda) at the Lower-Middle Jurassic transition were chosen as a case study to illustrate this methodological approach. We constructed two phenotypic spaces: a static, adult one (adult morphospace) and a dynamic, developmental one (allometric space). Comparative disparity analyses show a strikingly stable occupation in both spaces, despite extensive change in taxonomic composition. In contrast, disparity analyses of subclades reveal clearly distinct morphological and allometric disparity dynamics. Allometric approaches allow developmental insights into morphological diversification otherwise intractable from the analysis of adult morphospace alone.     

Vertiquant: A Device for Automatically Recording Vertical Migration of Plankton

Vertical migration is a key subject in understanding zooplankton ecology and its influence on aquatic ecosystems. This paper introduces a device for automatically recording vertical plankton migrations to study proximate factors regulating the stimulus, timing and amplitude of these movements under controlled laboratory conditions. The instrument records the light scattered by organisms at their respective depths and processes the signals in real time to a graphic representation of the organisms vertical distribution. Organisms of different taxa from a size of <40 μ, to > 10 000 μm were used for these experiments. Daphnia migrations in response to UV light are used to demonstrate the basic functions of the instrument.     

Audio-Visual Perception of 3D Cinematography: An fMRI Study Using Condition-Based and Computation-Based Analyses

The use of naturalistic stimuli to probe sensory functions in the human brain is gaining increasing interest. Previous imaging studies examined brain activity associated with the processing of cinematographic material using both standard “condition-based” designs, as well as “computational” methods based on the extraction of time-varying features of the stimuli (e.g. motion). Here, we exploited both approaches to investigate the neural correlates of complex visual and auditory spatial signals in cinematography. In the first experiment, the participants watched a piece of a commercial movie presented in four blocked conditions: 3D vision with surround sounds (3D-Surround), 3D with monaural sound (3D-Mono), 2D-Surround, and 2D-Mono. In the second experiment, they watched two different segments of the movie both presented continuously in 3D-Surround. The blocked presentation served for standard condition-based analyses, while all datasets were submitted to computation-based analyses. The latter assessed where activity co-varied with visual disparity signals and the complexity of auditory multi-sources signals. The blocked analyses associated 3D viewing with the activation of the dorsal and lateral occipital cortex and superior parietal lobule, while the surround sounds activated the superior and middle temporal gyri (S/MTG). The computation-based analyses revealed the effects of absolute disparity in dorsal occipital and posterior parietal cortices and of disparity gradients in the posterior middle temporal gyrus plus the inferior frontal gyrus. The complexity of the surround sounds was associated with activity in specific sub-regions of S/MTG, even after accounting for changes of sound intensity. These results demonstrate that the processing of naturalistic audio-visual signals entails an extensive set of visual and auditory areas, and that computation-based analyses can track the contribution of complex spatial aspects characterizing such life-like stimuli.     

Fine discrimination training alters the causal contribution of macaque area MT to depth perception

When a new perceptual task is learned, plasticity occurs in the brain to mediate improvements in performance with training. How do these changes affect the neural substrates of previously learned tasks? We addressed this question by examining the effect of fine discrimination training on the causal contribution of area MT to coarse depth discrimination. When monkeys are trained to discriminate between two coarse absolute disparities (near versus far) embedded in noise, reversible inactivation of area MT devastates performance. In contrast, after animals are trained to discriminate fine differences in relative disparity, MT inactivation no longer impairs coarse depth discrimination. This effect does not result from changes in the disparity tuning of MT neurons, suggesting plasticity in the flow of disparity signals to decision circuitry. These findings show that the contribution of particular brain area to task performance can change dramatically as a result of learning new tasks.     

Binocular vision: an orientation to disparity coding

A new study has shown that neurons in the visual cortex are specialized to encode the larger range of horizontal - relative to vertical - disparities that occurs in central vision. These results challenge the established 'energy' model of disparity processing.