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PubMed2003

Oculomotor consequences of feeble image size inequality at near reading distance.

Reading and working with a computer screen are activities of everyday life that take place at near vision. This study examines whether at such a near distance, normal subjects are capable of modifying the natural conjugacy of their saccades when exposed to a feeble image size inequality of 2%; similar inequality exists for persons who wear spectacles of slightly different power for the two eyes. Subjects were seated at 40 cm in front of a screen where a random dot pattern was projected. They were asked to make saccades of 7.5 degrees and 15 degrees along the horizontal and vertical principal meridians and horizontal saccades between secondary and tertiary positions in the upper and lower field. Five subjects performed the experiment with a 2% overall reduction lens inserted over their dominant eye; three subjects participated in a second experiment with an overall magnification lens of 2% also inserted in front of the dominant eye. The results showed a persistent decrease or increase in the size of the saccade of the dominant eye that was subject dependent for horizontal saccades, but consistent over subjects for vertical saccades. Persistent disconjugacy, at least for vertical saccades, is interpreted as evidence of learning.

Analysis of Variance↗
PubMed2004

The importance of perceived relative motion in the control of posture.

Two experiments investigated the role of optic flow in controlling posture. Both experiments measured postural sway in two virtual environments with different 3-D structure but the same optic flow. Observers attempted to maintain balance on one foot while viewing an object that appeared either rigid with respect to the environment or that appeared to move concomitantly with head movements. The apparent object motion concomitant with head motion was achieved by changing the perceived, but not physical, depth of the object. For both objects, the optic flow information was the same and only depth information was varied. Observers showed a decrease in stability (as measured by head sway) when viewing the object that appeared to move, suggesting that perceived relative motion, not optic flow, signals self-motion to the postural control system.

Adult↗
PubMed2000

Effects of stimulus size and eccentricity on horizontal and vertical vergence.

We measured the gain and phase of horizontal and vertical vergences of five subjects as a function of stimulus area and position. Vergence eye movements were recorded by the scleral search coil method as subjects observed dichoptic displays oscillating in antiphase either from side to side or up and down with a peak-to-peak magnitude of 0.5 degree at either 0.1 Hz or 1.0 Hz. The stimulus was a central textured disc with diameter ranging from 0.75 degree to 65 degrees, or a peripheral annulus with outer diameter 65 degrees and inner diameter ranging from 5 degrees to 45 degrees. The remaining field was black. For horizontal vergence at both stimulus frequencies, gain and the phase lag were about the same for a 0.75 degree stimulus as for a 65 degrees central stimulus. For vertical vergence, mean gain increased and mean phase lag decreased with increasing diameter of the central stimulus up to approximately 20 degrees. Thus, the stimulus integration area is much smaller for horizontal vergence than for vertical vergence. The integration area for vertical vergence is similar to that for cyclovergence, as revealed in a previous study. For both types of vergence, response gains were higher and phase lags smaller at 0.1 Hz than at 1.0 Hz. Also, gain decreased and phase lag increased with increasing occlusion of the central region of the stimulus. Vergence gain was significantly higher for a 45 degrees central disc than for a peripheral annulus with the same area. Thus, the central retina has more power to evoke horizontal or vertical vergence than the same area in the periphery. We compare the results with similar data for cyclovergence and discuss their ecological implications.

Adult↗
PubMed2003

[Development of dynamic stereopsis after eye muscle operations for binocular vision impairment].

BACKGROUND: Strabism usually impairs central stereopsis irreversibly after a short time duration,however, the retinal periphery is more resistant to deterioration by binocular impairment. METHODS: In this prospective study 46 strabismic patients (9-56 years old) were tested for dynamic stereopsis in the peripheral visual field up to 20 degrees eccentricity. The effect of realignment of the visual axes by surgery on dynamic stereopsis was tested before and after surgery in 40 out of these patients. Patients were tested qualitatively for dynamically stereoactive fields of vision and quantitatively for the threshold value needed to create a three-dimensional impression. RESULT: We found residual dynamic stereopsis in 30% of patients without central static stereopsis: 56% of the patients improved after surgery, either through a significant (p<0.01) gain of stereoactive fields or through a decrease of threshold values. CONCLUSION: Strabismus surgery has benefits for the patients outside classical tests and seems to be independent of the static depth perception.

Adolescent↗
PubMedDate not supplied

Assessment of cyclodisparity-induced slant perception with a synoptophore.

PURPOSE: To evaluate with a synoptophore slant perception induced by binocular cyclodisparities in normal subjects and to argue for the possibility of abnormal slant perception in patients with cyclo-vertical strabismus. METHODS: A vertical line with cyclodisparities that ranged from 0 degrees to +/- 10 degrees was presented to 17 normal subjects (mean +/- SD age, 28.4 +/- 5.6 years; 11 men and 6 women) with a synoptophore, and the perceived slant of the line in the pitch plane was measured by a matching method. Cyclodisparity thresholds for top-away and top-forward slants were also evaluated by the method of limits in a separate experiment. RESULTS: An incyclodisparity induced top-forward, while an excyclodisparity induced top-away, slant perception. The maximum slant angle was 37 degrees on average at a cyclodisparity of 10 degrees , and the mean slant gain (perceived angle/geometrically calculated angle) was 64 +/- 13%. The mean cyclodisparity thresholds for top-away and top-forward slants were -1.1 degrees and 0.6 degrees , respectively. CONCLUSION: The slant perception induced by cyclodisparities was reasonably assessed with the synoptophore. The cyclodisparity thresholds obtained in this experiment were much lower than the cyclodeviation range of the patients, indicating that a considerable number of patients may have abnormal slant perception once they achieve sensory fusion.

Adult↗
PubMed2006

Fourier-analysed steady-state VEPs in pre-school children with and without normal binocularity.

Pre-school children aged 4-5 were examined with steady-state VEP in response to a sinusoidal grating pattern with a spatial frequency of 4 c/deg, reversing at rates 5, 10, and 15 Hz. Normal children (n = 10) were compared with subjects lacking stereo perception (n = 6) and with subjects showing significant unilateral amblyopia with visual acuity in the worse eye <0.5 (n = 7). Fast Fourier Transform was used for analysis of the crude steady-state VEP responses. Compared to normals, the subjects lacking stereo perception showed a significantly lower power of the second harmonic in the response evoked by binocular stimulation with gratings reversed at 15 Hz. The amblyopic group showed a significant difference between the dominant and the non-dominant eye regarding the first harmonic power in the responses evoked by gratings reversed at 5 and 10 Hz. These findings are discussed in relation to the magnocellular and parvocellular visual pathways and suggested models for linear and non-linear processing of visual signals.

Amblyopia↗
PubMed1990

Real world occlusion constraints and binocular rivalry.

A surface occluding a more distant surface gives rise to interocularly unpaired regions to its immediate left and right. The unpaired region on the left side is visible only to the left eye, whereas that on the right side is visible only to the right eye. Thus for real world scenes there are opto-geometrical constraints which determine whether particular combinations of relative depth and right-eye-only or left-eye-only stimuli are ecologically valid or invalid. We report a demonstration and experiments to show that opto-geometrically "valid" unpaired regions are seen as continuous with the rear plane and escape interocular suppression, whereas "invalid" unpaired regions are perceived as closer and are suppressed vigorously. An additional experiment indicates that the results cannot be understood in terms of correspondence solving, but require neural mechanisms that embody real-world occlusion constraints. These results suggest a rather close interaction between stereopsis and rivalry "modules". Since explicit eye-of-origin information is lost relatively early in the hierarchical organization of cortical visual processing, we argue that occlusion-related constraints must be embodied at such early levels.

Chi-Square Distribution↗
PubMed1990

da Vinci stereopsis: depth and subjective occluding contours from unpaired image points.

Distant surfaces are occluded by nearer surfaces to different extents in the two eyes, leading to the existence of unpaired image points visible in one eye and not the other. An ecological analysis of the real world situation that could have given rise to such unpaired points indicates the presence of a depth constraint zone, defined by visibility lines between which possible real world points must lie. The leading edge of this zone starts at the edge of a fused binocular occluding surface and recedes linearly with increases in horizontal distance to the unpaired point. Psychophysical evidence indicates that the human visual system makes use of this unpaired information in a remarkably adaptive manner, showing an increase in perceived depth for increasing horizontal separations between the unpaired target and fused edge, at least over a significant angular range (approx. 25-40 min arc). We also show that unpaired points in binocular images can lead to the formation of subjective occluding contours and surface having the qualitatively appropriate sign of depth. Furthermore, we show that the visual system could not recover depth of unpaired points camouflaged from the other eye against silhouettes. Our findings indicate that the visual system makes use of occlusive relations in the real world to recover depth, contour, and surface from unpaired points. The fact that such processes must utilize eye-of-origin information implies that they share this essential characteristic with classical or Wheatstone stereopsis. The necessity of eye-of-origin information also suggests that the processing may begin relatively early in cortical visual processing, possibly as early as V1. Finally, the novel emergence of subjective occluding contours from unpaired monocular stimuli raises the possibility that this process is mediated by visual experience, built up by the association of unpaired points and occluding contours.

Depth Perception↗
PubMed1991

On the coexistence of stereopsis and binocular rivalry.

Dichoptically viewed complex texture stereograms with correlated spatial frequency information can yield stable depth perception, implying cooperative interaction between the two eyes. Dichoptically viewed dissimilar texture pairs may yield competition in the form of binocular rivalry. To study whether stereopsis and rivalry can spatially coexist when stimulus conditions for both are present, we had observers dichoptically view spatial frequency filtered random-dot patterns. The left eye viewed one half-image of an RDS; the right eye viewed the superimposition of the other RDS half-image (which when paired alone with the left-eye RDS yielded stereoscopic depth) and a noise target (which on its own engaged in rivalry with the right eye target). Observers judged the quality of depth and the rate of rivalry for these stereo-pairs. When the contrast of the noise component was low, observers experienced stereopsis and stable single vision that included the noise. At intermediate noise contrasts, local regions were seen either in rivalry or in stereoscopic depth, but rivalry and depth were not experienced at the same spatial location simultaneously. At high noise contrasts, the right eye target dominated almost exclusively, with little hint of stereopsis. Essentially the same pattern of results was obtained in forced-choice experiments in which observers judged the direction of stereoscopic tilt from vertical cosine gratings differing slightly in spatial frequency. Considered together, these results are inconsistent with theories positing that rivalry and stereopsis coexist at the same spatial location because they occur within independent, parallel pathways.

Depth Perception↗
PubMed1992

Orientation dependence in the recognition of familiar and novel views of three-dimensional objects.

We report four experiments that investigated the representation of novel three-dimensional (3D) objects by the human visual system. In the first experiment, canonical views were demonstrated for novel objects seen equally often from all test viewpoints. The next two experiments showed that the canonical views persisted under repeated testing, and in the presence of a variety of depth cues, including binocular stereo. The fourth experiment probed the ability of subjects to generalize recognition to unfamiliar views of objects previously seen at a limited range of attitudes. Both mono and stereo conditions yielded the same increase in the error rate with misorientation relative to the training attitude. Taken together, these results support the notion that 3D objects are represented by multiple specific views, possibly augmented by partial viewer-centered 3D information.

Computer Simulation↗
PubMed1993

Errors in direction-of-motion discrimination with dichoptically viewed stimuli.

At durations shorter than about 150 msec, a complex grating comprising a static 1-c/deg grating and a moving 3-c/deg grating is perceived as moving in the direction opposite that of the physical direction of motion. Here the phenomenon is further examined by measuring the perceived direction of motion of the fused images of a 1-c/deg grating presented to one eye and a moving 3-c/deg grating presented to the other. The strength of the illusion is almost unaffected by dichoptic presentation. This observation is consistent with the hypothesis that perceived motion is a consequence of the way the visual system integrates signals arising from different detectors tuned to the two component gratings.

Humans↗
PubMed1995

Interocular suppression in the primary visual cortex: a possible neural basis of binocular rivalry.

In an attempt to demonstrate a physiological basis for the alternating suppression of perception when the two eyes view very different contours (binocular rivalry), we studied the responses of neurons in the lateral geniculate nucleus (LGN) and area 17 of cats for drifting gratings of different orientation, spatial frequency and contrast in the two eyes. Almost half of the LGN neurons studied exhibited modest inhibitory interocular interaction, but independent of interocular differences in orientation. Monocularly driven units in layer 4 of area 17 behaved similarly. However, for the majority of binocular cortical cells, the response to a grating of optimal orientation in one eye was suppressed by a grating of very different orientation shown to the other eye, over a wide range of spatial frequency and independent of relative spatial phase. This interocular suppression exhibits a remarkable non-linearity: a grating of non-preferred orientation in one eye causes significant interocular suppression only if the neuron is already responding to an appropriate stimulus in the other eye [Sengpiel and Blakemore (1994) Nature, 368, 847-850]. We propose that the switches in perceptual dominance during binocular rivalry depend on interocular interactions at the level of binocular neurons of the primary visual cortex, which might involve intracortical inhibition between adjacent ocular dominance columns. The spontaneous alternations in perceptual suppression that occur during prolonged viewing of rivalrous patterns remain to be explained, although significant variation in the strength of neuronal suppression in such conditions was occasionally seen.

Action Potentials↗
PubMed1995

Depth discrimination of a crowded line is better when it is more luminant than the lines crowding it.

Observers usually cannot discriminate the relative depth of a crowded feature with respect to crowding features about 2 arc min distant if all the features have the same luminance. However, stereo thresholds significantly less than 20 arc sec are obtained when the crowded feature is about twice as luminant as the features crowding it. The thresholds depend only upon the ratio of the luminance of the target feature to the luminance of the crowding features and are independent of the absolute luminance of the features. With further increase in the relative luminance of the target feature, the performance eventually deteriorates and this deterioration is not due to difficulty in seeing the features which were individually clearly visible for all the luminances tested. The closest spacing of local crowded features that still allows good stereo discrimination is about the same as the spatial resolution attainable for many luminance-based non-stereo tasks.

Depth Perception↗
PubMed1994

Active vision in honeybees: task-oriented suppression of an innate behaviour.

In a pattern discrimination task, bees tend to fly along the contours contained in the patterns, as revealed by an earlier study. As opposed to this, in a task involving the detection of an edge between two striped surfaces placed at two different ranges, the bees avoid contour-following, as revealed by the present study. The study shows that, in the latter task, the bees learn to suppress the otherwise innate contour-following behaviour and adopt a flight strategy that provides them with the motion parallax cues necessary to cope with this task. Thus, the animal's active behaviour determines the type of visual information to be extracted from the environment.

Animals↗
PubMed1994

Visual suppression and its effect upon color and luminance sensitivity.

Psychophysical increment thresholds were compared for periods of phenomenological dominance or suppression produced by different stimulation of the two eyes. Three experimental procedures were used; binocular rivalry, permanent suppression and flash suppression. The amount of suppression produced by each procedure was evaluated under conditions intended to accentuate color or luminance system contribution to the detection of a spectral flash. All three procedures resulted in a different pattern of color and luminance suppression. Binocular rivalry suppressed color sensitivity more than luminance and within color, blue (439 nm) sensitivity was more suppressed than red (613 nm). Permanent suppression resulted in a similar pattern of suppression but only blue color sensitivity was reliably more suppressed than luminance sensitivity. Flash suppression produced distinctly different results such that blue color sensitivity was reliably less suppressed than luminance or red color sensitivity, which were not different from each other. Taken together these results provide clues as to where and when the physiological processes mediating visual suppression may be found in the nervous system.

Color Perception↗
PubMed1996

The binocular computation of visual direction.

How is a single visual direction assigned to a binocular feature for which the left and right eyes are signaling different directions? According to geometrical principles, binocular visual direction is the average of the visual directions measured from the left and right eyes. Contrary to this prediction, we have found that the relative visual direction between two Gabor targets presented at different stereoscopic depths could be manipulated by varying the contrast ratio between the left and right images. This finding is consistent with a new model in which the relative alignment of depth features is determined from a maximum-likelihood combination of the direction signals from the left and right eyes. In a second experiment we provide support for this model, showing that the magnitude of the contrast-dependent bias in visual direction is predicted by the uncertainty for spatial localization in the left and right images. Lastly we show that visual direction and stereopsis have different dependencies on interocular contrast differences, suggesting that the computation of stereo depth and visual direction are mediated via different mechanisms.

Contrast Sensitivity↗
PubMed1996

Binocular rivalry with isoluminant stimuli visible only via short-wavelength-sensitive cones.

To test whether the binocular contour rivalry mechanism is tritanopic, we presented isoluminant, rival stimuli visible only via the short-wavelength-sensitive (S) cones. We stimulated only the S cones with violet gratings superimposed on a bright yellow field that adapted the responses of the middle- and long-wavelength-sensitive (M and L) cones. We found that an S-cone grating presented to one eye rivalled with an orthogonal grating presented to the other. Rivalry persisted over a range of luminances and contrasts of the S-cone stimuli, and was greater than could be accounted for by nonrival fading. The spatial spread of rivalry from S-cone stimuli is similar to that for the same stimuli when visible also to the M and L cones (luminance stimuli). We found that an S-cone stimulus would rival with a luminance stimulus, and exploited this to determine the equivalent luminance contrast of S-cone stimuli by putting them in a rivalry competition with luminance stimuli. For rivalry, the equivalent luminance contrast of isoluminant, S-cone stimuli is much less than their S-cone contrast. The existence of rivalry with isoluminant stimuli, along with earlier evidence that such stimuli can support stereopsis, challenges the view that an achromatic channel alone drives certain higher level functions such as depth perception.

Contrast Sensitivity↗
PubMed1996

Binocular alignment in different depth planes.

A generally accepted notion in binocular vision is that we see the world as if viewed by a single eye, the cyclopean eye. A consequence of seeing the world from a single point in space is that the outlines of occluding and occluded surfaces have the same shape. We designed stereograms in which subjects aligned binocularly visible lines to each other. The lines were lying in different depth planes. In the vicinity of occluded areas, binocular alignment was achieved by alignment of the lines in the eye that viewed the monocularly visible details. Stereograms in which shapes of surfaces lying in different depth planes were compared to each other show that occluding and occluded surfaces do not have the same shape: a square surface occludes rectangular surfaces in other depth planes of which the horizontal widths are smaller than the vertical widths. This difference is perceived shape is not possible if the centre of binocular direction has a fixed position in the head.

Depth Perception↗