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The relative importance of contrast and assimilation for determining the perceived brightness was estimated. Assimilation decreased when a test spot had such a binocular disparity that the spot and its background appeared on different depth planes respectively. However, contrast was not affected by the binocular depth cue. These results indicate that the cortex takes an important role in assimilation process.
A global shape judgement task was used to investigate the combination of stereopsis and kinetic depth. With both cues present, there were no distortions of shape perception, even under conditions where either cue alone did show such distortions. We suggest that the addition of motion information overcomes the stereo distance scaling problem. However, when incongruent combinations of disparity and motion were used, the results did not match predictions of a number of combination theories. These data could be described by a model which used weighted linear combination after correctly scaling disparities for viewing distance. When the motion cue was weakened by presenting only two frames of each motion sequence, stereo was weighted more heavily.
The perception of dynamic random-dot stereograms (RDS) depends on the physiological fusion of horizontally disparate binocular visual input. Thus, the use of RDS offers the possibility to study selectively cortical processing of visual information in man. We investigated the influence of horizontal disparity on the scalp topography of RDS evoked brain activity in 33 healthy subjects. Stereoscopic checkerboard patterns were presented in the center or lateralized in the left or right visual field with horizontal disparities changing at temporal frequencies of six or eight depth reversals/s using different disparity values ranging from 3.5 to 28 min of arc. In 11 subjects evoked potential fields were recorded from 16 electrodes, and 21 subjects participated in 30-channel recordings with electrodes located over the parietal and occipital brain areas. Stimulation frequency-related brain activity was obtained with all disparity values; however, with large or small disparities the potential field strength decreased significantly while largest responses were obtained with intermediate disparities. Significant differences were observed in RDS evoked brain activity when central and lateralized stimulus locations were compared. With lateral stimuli (extending from the fovea to 17.1-deg eccentricity) maximal amplitudes were obtained at larger disparities than with central stimuli. In addition there were pronounced differences between brain activity evoked with stimuli presented in the left or right visual field; however, there were very similar evoked potential signals recorded from electrodes located over the left and right hemispheres. Our findings indicate that the processing of disparity information with lateralized stimuli is different from the processing in the center of the visual field. In addition, lateralized stimulation yields a significant disparity tuning mainly with stereoscopic targets occurring to the right from the fixation point (but not with stimuli to the left) suggesting a functional difference between the visual half-fields.
An automated 2-rod stereoacuity test was modified to permit lateral head movement during testing so that depth perception by means of motion parallax could be assessed. The patient, a 48-year-old truck driver with one blind eye, found the test easy to perform. In 24 presentations of a 40 sec arc disparity he was 96% correct. With the same number of presentations at 20 sec arc he was 83% correct. In the latter case, the probability of success by chance alone was p = 0.0004.
The middle temporal (MT) visual area is widely accepted to play important roles in motion processing. It is unclear, however, whether MT contributes to visual perception during the viewing of static scenes, when there is little retinal image motion during the interval between saccades. Some previous studies suggest that MT neurons give little or no response to stationary stimuli that are flashed onto the receptive field, but no previous study has directly examined the fidelity with which MT neurons code visual information in moving versus stationary stimuli. In this study, we compare the ability of MT neurons to signal binocular disparity in moving versus stationary random-dot stereograms. Although responses to moving stimuli are typically stronger, many MT neurons give robust responses to stationary stereograms, and some MT neurons actually prefer stationary patterns to those moving at any tested speed. These responses to stationary stimuli are not caused by monitor refresh or microsaccades. Disparity tuning curves for moving and stationary stimuli are nearly identical in shape for most neurons. Although the disparity discriminability of MT neurons is generally higher for moving stereograms when responses are averaged over the entire 1.5 sec trial epoch, discriminability is comparable for moving and stationary stimuli during the first 200-300 msec of the response. Thus, in a normal time interval between saccades, MT neurons signal the binocular disparity of stationary stimuli with high fidelity. These findings show that MT can be a reliable source of visual information during the viewing of static scenes.
Stereoscopic depth perception relies on binocular disparities, or small geometric differences between the retinal images of each eye. The most reliable binocular depth judgments are those that are based on relative disparities between two simultaneously visible features in a scene. Many cortical areas contain neurons that are sensitive to disparity, but it is unclear whether any areas show a specific sensitivity to relative disparity. We recorded from neurons in the early cortical visual area V2 of the awake macaque during presentation of random-dot patterns. The depth of a central region ('center'), and that of an annular surrounding region ('surround'), were manipulated independently in these stimuli. Some cells were fully selective for the resulting relative disparities. Most showed partial selectivity, which nonetheless indicated a sensitivity for the depth relationship between center and surround. Both types of neural response could support psychophysical judgments of relative depth.
Tuning for the orientation of elongated, linear image elements (edges, bars, gratings), first discovered by Hubel and Wiesel, is considered a key feature of visual processing in the brain. It has been studied extensively in two dimensions (2D) using frontoparallel stimuli, but in real life most lines, edges and contours are slanted with respect to the viewer. Here we report that neurons in macaque area V4, an intermediate stage in the ventral (object-related) pathway of visual cortex, were tuned for 3D orientation--that is,for specific slants as well as for 2D orientation. The tuning for 3D orientation was consistent across depth position (binocular disparity) and position within the 2D classical receptive field. The existence of 3D orientation signals in the ventral pathway suggests that the brain may use such information to interpret 3D shape.
The size and depth constancies considered here operate only at near distances (< about 2 m) in a static stimulus situation with vergence as the only cue to distance. The innervation of the extraocular muscles, as evidenced by the corollary discharge, provides information about the vergence of the eyes and hence about the egocentric distance both for symmetrical and asymmetrical vergences. Size and depth constancies are regarded as the first and second stages of a linked two-stage process. In the lateral geniculate nuclei compensatory adjustments are separately applied to each retinal image as they are received from the two eyes. The modified ocular images, with their associated vertical and horizontal disparities, now provide synaptic inputs to binocularly activated cells in the visual cortex. Then, by a process akin to the induced effect, cortical cells with geniculate afferents with vertical disparities will have their outputs expressed in terms of horizontal disparities. The horizontal disparity outputs of these cortical cells are then further multiplied by the outputs from cortical cells with geniculate afferents with horizontal disparities. It is this second multiplicative process that brings about the quadratic relationship between horizontal retinal disparity and egocentric distance. The proposed mechanisms involve the known ability of the visual system to detect and respond to vertical as well as horizontal disparities and provide a definite role for the induced effect in the perceptual process. The above neural model is based on fairly simple equations that give a remarkably adequate description of the operation of the two constancies.
PURPOSE: To evaluate stereoscopic visual evoked potentials (S-VEP) in normal controls and in patients with glaucomatous optic nerve damage. METHODS: Computer-generated dynamic random-dot stereograms were used to elicit cortical visual evoked potentials using wireless electric liquid crystal shutter glasses. Normal subjects (n=22) and patients with glaucoma (n=22) were investigated using five different disparities from 9 to 40 arc min. Statistical dependency of measurements with different stimulus at identical patients was adjusted for. RESULTS: Peak times of onset and offset response of S-VEP can be significantly delayed in glaucomas. A general linear regression model confirmed that differences between patients and normals depend on disparity. S-VEP onset shows no significant difference between controls and glaucomas at 9 arc min disparity. At high disparities, however, peak time of the onset response was significantly (p<0.01) delayed in glaucomas when compared with normals (normals: 125.8+/-13 ms, glaucomas: 148.2+/-25.6 ms at 40 arc min). CONCLUSIONS: Visual evoked potential elicited by the onset of a random-dot stereogram can be used for objective measurement of stereoacuity in a clinical setting. Differences between controls and glaucomas in high and low disparities could indicate a stereo-specific deficit in glaucoma.
Spatial frequency selectivity has been incorporated into various theories of stereo matching, along with spatial scale interactions operating from coarse-to-fine spatial scales. We concentrate here on the role of fine scale information in the stereo matching process and show that fine scale information is capable of disambiguating matches made at coarser scales. An ambiguous coarse scale stimulus was created by presenting a low frequency (2 c/deg) sine wave in anti-phase to the two eyes, whose endpoints betrayed no information about which way the sine waves should be matched. It could be seen with crossed or uncrossed disparity equally validly and at chance from trial to trial. To this was added a fine scale (8 c/deg) filtered random dot stimulus specifying unambiguously a certain disparity. Observers judged the apparent depth of the two stimuli as the disparity of the fine scale stimulus was varied. The sine wave was usually perceived to have the same sign disparity as the fine scale stimulus. Depth matching with the two superimposed stimuli confirmed that the coarse scale stimulus was actually disambiguated, and seen with disparities equal to half its spatial period. The results suggest the operation of a cross-spatial scale matching disambiguation process, which can operate in a fine-to-coarse fashion.
Transparent motion stimuli allow us to investigate how visual motion is processed in the presence of multiple sources of information. We used stereo random-dot kinematograms to determine how motion processing is affected by the difference in direction and depth of two overlapping motion components. Observers judged whether a noise dot display contained one or two directions of motion. For all disparity differences, performance did not change among angles greater than 60 degrees, but the ability to detect transparent motion fell dramatically as the direction difference decreased below 60 degrees. When a disparity difference was added between the two motion components, detection became easier. We compared these results to an ideal-observer model limited by stimulus uncertainty and low-level sources of internal noise. The resulting measure of efficiency--the ratio of human to model performance--reflects changes in how motion stimuli are being processed. A decrease of both the direction and disparity differences had the effect of decreasing efficiency. These results suggest that the mechanism processing transparent motion may implement a smoothness constraint that tends to combine similar motions into a single percept.
The visual system must reconstruct the three-dimensional structure of an object from two-dimensional retinal images. Previous research has shown that macaque inferior temporal (IT) neurons, although belonging to the ventral visual stream, code for depth defined by binocular disparity gradients. Here, we demonstrate that macaque IT neurons also code for depth defined by texture gradients, a monocular depth cue. Single IT neurons were selective for the tilt of texture-defined surfaces, and the tilt preferences of individual neurons remained the same, whether surfaces were defined by texture or disparity cues. Furthermore, the tilt preference was invariant over different types of textures and slants, suggesting an abstract representation of surface tilt in ventral visual cortex.
Stereoacuity experiments tested definitions of binocularly disparate spatial positions by perturbing the binocular correspondence of the two half-images. Dichoptic translations perturbed zero-order retinal positions; expansions perturbed first-order horizontal separations; rotations perturbed first-order orientations; and anisotropic expansions deformed first-order two-dimensional (2D) structure. Each transformation perturbed relative positions in the two half-images by more than 100 arcsec, but stereoacuity thresholds remained about 10 arcsec. Binocular disparity involves second-order 2D differential structure of the monocular half-images, specifying local surface shape. Stereoacuity is much better than nonstereo acuity, suggesting that monocular spatial signals are binocularly correlated.
The human visual evoked potentials (VEPs) elicited by retinal disparity stimulation of the nasotemporal overlap area of the fundus, where ipsilaterally and contralaterally projecting retinal ganglion cells intermingle, were recorded bilaterally at the cortical hemispheres, and the contribution of the nasotemporal overlap to the mechanism of stereopsis was studied. The stimulus was a slit of a stereogram or correlogram applied to the center of the fovea of the right eye, or 3 degrees or 6 degrees nasal or temporal to the fovea. The negative waves of the VEPs evoked by the stereogram were located at O1 and O2. The VEP amplitudes evoked by the stereogram markedly exceeded those by the correlogram. In comparing the VEP amplitude of O1 with that of O2, when the slit of the stereogram hit the nasal side of the nasotemporal overlap O1 exceeded O2, and on the temporal side, this was reversed. The difference between the amplitudes at O1 and O2 was significant at 3 degrees from the fovea. These results showed electrophysiologically that the width of the nasotemporal overlap in the human retina is more or less 4 degrees and that the activity from the overlap area converges to disparity selective neurons in the visual cortex, suggesting that the overlap area is the region where cues to depth perception are obtained in the front-posterior plane of the fixation point.
A learning algorithm for a model binocular cell was derived according to an information maximization principle and by using a low signal-to-noise-ratio approximation. The algorithm updates cell's synaptic weights so that the information obtained from the cell's output is increased. According to the algorithm, model binocular cells were trained by using computer-generated stereo images as training data. As a result, cells tuned to various disparities were generated. Also, generated synaptic weight patterns of the cells were similar to Gabor-wavelets and receptive fields of simple cells in the visual cortex. Thus, they were orientation and spatial frequency selective as well as disparity selective. Gabor functions were used to fit the generated weight patterns. The fitting results indicated that the generated cells encode disparities in terms of phase disparity and/or position disparity. This result agrees with experimental findings by Anzai et al. [J Neurophys 82 (1999) 874] and is consistent with ICA-based theoretical results obtained [Network: Comput Neural Syst 11 (2000) 191].
We analyzed the functional organization of speed tuned neurons in extrastriate visual area MT. We sought to determine whether neurons tuned for particular speeds are clustered spatially and whether such spatial clusters are elongated normal to the cortical surface so as to form speed columns. Our data showed that MT neurons are indeed clustered according to preferred speed. Multiunit recordings were speed tuned, and the speed tuning of these signals was well correlated with the speed tuning of single neurons recorded simultaneously. To determine whether speed columns exist in MT, we compared the rates at which preferred speed changed in electrode tracks that traversed MT obliquely and normally to the cortical surface. If speed columns exist, the preferred speed should change at a faster rate during oblique electrode tracks. We found, however, that preferred speed changed at similar rates for either type of penetration. In the same data set, the rate of change of preferred direction and preferred disparity differed substantially in normal and oblique penetrations as expected from the known columnar organization of MT. Thus our results suggest that a columnar organization for speed tuned neurons does not exist in MT.
Changing size (looming) produces changes in accommodation and vergence. Dynamic responses of vergence and accommodation to sinusoidal looming of a Maltese cross were recorded with an SRI dual-Purkinje-image eyetracker and optometer. The ratio of these two motor responses was compared with the response accommodative convergence/accommodation (AC/A) ratio and convergence accommodation/convergence (CA/A) ratio determined from sinusoidal variations of blur and disparity respectively. The response to changing size was found to be more similar to the AC/A ratio than the CA/C ratio. In addition, when a changing disparity or changing blur stimulus was combined with a changing size stimulus, the response phase lags of accommodation and vergence were decreased. In addition, the CA/C ratio was increased when changing size was added to changing disparity, but the AC/A ratio was unaltered when changing size was added to changing blur. These results indicate that changing size is stimulating accommodation directly and vergence secondarily through an AC/A crosslink.
The experiments reported in this paper were designed to investigate how depth information from binocular disparity and motion parallax cues is integrated in the human visual system. Observers viewed simulated 3-D corrugated surfaces that translated to and fro across their line of sight. The depth of the corrugations was specified by either motion parallax, or binocular disparities, or some combination of the two. The amount of perceived depth in the corrugations was measured using a matching technique. A monocularly viewed surface specified by parallax alone was seen as a rigid, corrugated surface translating along a fronto-parallel path. The perceived depth of the corrugations increased monotonically with the amount of parallax motion, just as if observers were viewing an equivalent real surface that produced the same parallax transformation. With binocular viewing and zero disparities between the images seen by the two eyes, the perceived depth was only about half of that predicted by the monocular cue. In addition, this binocularly viewed surface appeared to rotate about a vertical axis as it translated to and fro. With other combinations of motion parallax and binocular disparity, parallax only affected the perceived depth when the disparity gradients of the corrugations were shallow. The discrepancy between the parallax and disparity signals was typically resolved by an apparent rotation of the surface as it translated to and fro. The results are consistent with the idea that the visual system attempts to minimize the discrepancies between (1) the depth signalled by disparity and that required by a particular interpretation of the parallax transformation and (2) the amount of rotation required by that interpretation and the amount of rotation signalled by other cues in the display.