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At least 1,153 records · Page 64Linked to original sources
PubMed1989

A neural theory of binocular rivalry.

When the two eyes view discrepant monocular stimuli, stable single vision gives way to alternating periods of monocular dominance; this is the well-known but little understood phenomenon of binocular rivalry. This article develops a neural theory of binocular rivalry that treats the phenomenon as the default outcome when binocular correspondence cannot be established. The theory posits the existence of monocular and binocular neurons arrayed within a functional processing module, with monocular neurons playing a crucial role in signaling the stimulus conditions instigating rivalry and generating inhibitory signals to implement suppression. Suppression is conceived as a local process happening in parallel over the entire cortical representation of the binocular visual field. The strength of inhibition causing suppression is related to the size of the pool of monocular neurons innervated by the suppressed eye, and the duration of a suppression phase is attributed to the strength of excitation generated by the suppressed stimulus. The theory is compared with three other contemporary theories of binocular rivalry. The article closes with a discussion of some of the unresolved problems related to the theory.

Attention↗
PubMed2005

Attending to eye movements and retinal eccentricity: evidence for the activity distribution model of attention reconsidered.

When testing between spotlight and activity distribution models of visual attention, D. LaBerge, R. L. Carlson, J. K. Williams, and B. G. Bunney (1997) used an experimental paradigm in which targets are embedded in 3 brief displays. This paradigm, however, may be confounded by retinal eccentricity effects and saccadic eye movements. When the retinal eccentricities of the targets are equated and eye position is monitored, the pattern of results reported by LaBerge et al., which supported the activity distribution model, is not found. This result underscores the importance of considering targets' eccentricity and people's inclination to make saccadic eye movements in certain types of visual cognition tasks. ((c) 2005 APA, all rights reserved).

Attention↗
PubMed2004

Aging and the perception of depth and 3-D shape from motion parallax.

The ability of younger and older observers to perceive 3-D shape and depth from motion parallax was investigated. In Experiment 1, the observers discriminated among differently curved 3-dimensional (3-D) surfaces in the presence of noise. In Experiment 2, the surfaces' shape was held constant and the amount of front-to-back depth was varied; the observers estimated the amount of depth they perceived. The effects of age were strongly task dependent. The younger observers' performance in Experiment 1 was almost 60% higher than that of the older observers. In contrast, no age effect was obtained in Experiment 2. Older observers can effectively perceive variations in depth from patterns of motion parallax, but their ability to discriminate 3-D shape is significantly compromised.

Adult↗
PubMed1991

Motion parallax judgements of depth as a function of the direction and type of head movement.

We compared the relative effectiveness of rotating or translating the head, either horizontally or vertically, on the perception of depth resulting from motion parallax. Using Rogers and Graham's (1979) paradigm, we yoked the movement of random dots on a screen to movements of the head, simulating a corrugated surface. In two experiments, subjects nulled the apparent depth or motion seen in the display. Horizontal head movements yielded the most precise depth judgements, irrespective of whether the head translated or rotated. Motion thresholds were higher than those for depth and were independent of direction of head movement. In a third experiment, suprathreshold stimuli that simulated differing amounts of depth were used, and the subjects' perception of depth was virtually the same for all types and directions of head movement. In our stimulus situation, rotating or translating the head either vertically or horizontally produced motion parallax cues for depth that were equally effective. Our results also showed that, within a range, retinal image motion from head movement is converted into a depth signal and that above that range location constancy breaks down and motion is seen.

Adult↗
PubMed2001

Joint-encoding of motion and depth by visual cortical neurons: neural basis of the Pulfrich effect.

Motion and stereoscopic depth are fundamental parameters of the structural analysis of visual scenes. Because they are defined by a difference in object position, either over time or across the eyes, a common neural machinery may be used for encoding these attributes. To examine this idea, we analyzed responses of binocular complex cells in the cat striate cortex to stimuli of various intra- and interocular spatial and temporal shifts. We found that most neurons exhibit space-time-oriented response profiles in both monocular and binocular domains. This indicates that these neurons encode motion and depth jointly, and it explains phenomena such as the Pulfrich effect. We also found that the relationship between neuronal tuning of motion and depth conforms to that predicted by the use of motion parallax as a depth cue. These results demonstrate a joint-encoding of motion and depth at an early cortical stage.

Animals↗
PubMed1990

The influence of dopamine on spatial vision.

Contrast thresholds for, and contrast matches between, stationary gratings of three spatial frequencies (0.5, 2, and 8 c/deg) were measured on eight subjects with a history of schizophrenia, just before, and again two to three days after, a therapeutic injection of depot neuroleptic. The drug enhanced sensitivity at the low, and reduced it at the medium and high spatial frequency. After injection, subjects required more contrast to match the apparent contrast of the high, and less contrast to match that of the low, to that of the medium spatial frequency. Pupillary measurements suggested that these effects were not due to drug-induced changes in pupil size. The results are discussed in terms of the functional role of dopamine in the retina, and a possible application in therapy for amblyopia.

Adult↗
PubMed2005

Cortical responses to invisible objects in the human dorsal and ventral pathways.

The primate visual system is believed to comprise two main pathways: a ventral pathway for conscious perception and a dorsal pathway that can process visual information and guide action without accompanying conscious knowledge. Evidence for this theory has come primarily from studies of neurological patients and animals. Using fMRI, we show here that even though observers are completely unaware of test object images owing to interocular suppression, their dorsal cortical areas demonstrate substantial activity for different types of visual objects, with stronger responses to images of tools than of human faces. This result also suggests that in binocular rivalry, substantial information in the suppressed eye can escape the interocular suppression and reach dorsal cortex.

Adult↗
PubMed2003

[Electro-mechanic steering device for head-lamp mounted miniature video cameras].

BACKGROUND: Endoscopic or microscopic video recordings set a widely established standard for medico-legal documentation of operative procedures. In addition, they are an essential part of undergraduate as well as postgraduate medical education. Macroscopic operations in the head and neck can be recorded by miniaturised video cameras attached to the surgeon's head lamp. METHODS: The authors present an electro-mechanic steering device which has been designed to overcome the parallax error created with a head-mounted video camera, especially as the distance of the camera to the operative field varies. RESULTS: The device can be operated by the theatre staff, while the sterility of the operative field is maintained and the surgeon's physical working range remains unrestricted. As the video image is reliably centred to the operative field throughout the procedure, a better orientation and understanding for spectators who are unfamiliar with the surgical steps is obtained. CONCLUSIONS: While other adverse factors to macroscopic head-mounted video recordings, such as involuntary head movements of the surgeon, remain unchanged, the device adds to a higher quality of video documentation as it relieves the surgeon from adjusting the image field to the regions of interest. Additional benefit could be derived from an auto-focus feature or from image stabilising devices.

Endoscopy↗
PubMed1992

[Congenital nystagmus and intermittent exotropia. Suppression of nystagmus by fusional convergence].

We report on a patient with a combination of congenital nystagmus and intermittent exotropia. When the exotropia became manifest, a gross nystagmus appeared. When, however, the exotropia was overcome by fusinal convergence, the nystagmus was largely suppressed. The angle of squint was surgically reduced by such an amount that the exodeviation was just compensated, and the fusional convergence required to overcome the remaining angle sufficed to suppress the nystagmus.

Convergence, Ocular↗
PubMed1991

Reversals of visual depth caused by motion parallax.

The role of the velocity and direction of retinal movement in the determination of apparent depth from motion parallax was examined. Motion parallax was produced either by linking the movement of random-dots to head movement or by making this motion independent of the head movement. The results show that apparent depth was largely estimated from the velocity difference between the stimuli. The direction of retinal movement in the absence of head movement did not determine whether the pattern appeared to protrude or recede. Information about direction linked to head movement was able to stabilize protrusion/recession by providing a cue for the location of the fixation point. Depth reversal occurred less frequently in the presence than in the absence of head movement. When the fixation point shifted from the apparently protruding pattern to the apparently receding pattern, in both the presence and absence of head movement, depth reversal was readily observed.

Adolescent↗
PubMed1992

Colour inputs to random-dot stereopsis.

Recently it has been claimed by Livingstone and Hubel that, of three anatomically and functionally distinct visual channels (the magnocellular, parvocellular interblob, and blob channels), only the magnocellular channel is involved in the processing of stereoscopic depth. Since the magnocellular system shows little overt colour opponency, the reported loss of the ability to resolve random-dot stereograms defined only by colour contrast seems consistent with this view. However, Julesz observed that reversed-contrast stereograms could be fused if correlated colour information was added. In the present study, 'noise' (non-corresponding) pixels were injected into random-dot stereograms in order to increase fusion time. All six subjects tested were able to achieve stereopsis in less than three minutes when there was only correspondence in colour and not in luminance, and three when luminance contrast was completely reversed. This ability depends on information about the direction of colour contrast, not just the presence of chromatic borders. When luminance and chromatic contrast are defined in terms of signal-to-noise ratios at the photoreceptor mosaic, chromatic information plays at least as important a role in stereopsis as does luminance information, suggesting that the magnocellular channel is not uniquely involved.

Color Perception↗
PubMed1992

Resolving ambiguities in orientation, motion, and depth domains.

Three different perceptual systems--orientation, motion, and depth--can recover a global perceptual organization from spatially correlated random multielement patterns. In all three cases the global structure composed of random elements is evaluated by mechanisms performing measurements in the energy domain within appropriately defined local space-time areas. The selective increase in energy of one fraction of the elements may dramatically change the whole perceptual organization of the stimulus. In specially devised patterns one and the same element can belong to two or more separate perceptual organizations, the perceptual salience of one of which can be reinforced by a luminance increment of the elements comprising it. If a stimulus provides two different perceptual organizations to which each element could potentially belong, one of four possible solutions of the existing ambiguity will occur: suppression, rivalry, mixture, or parity. Two superimposed global orientation patterns either suppress or dominate over each other but cannot be seen simultaneously or in a mixed form. Characteristic of the depth system is that it allows multiple binocular matchings and parity of possible perceptual solutions. Finally, if a stimulus provides two or more paths along which each element may appear to move, the perceived global motion direction is determined by a mixture of directions of these competing motion paths. Dissimilarities in these ways of resolving ambiguities may be based on different principles defining regularity and coherence of an object in the orientation, motion, and depth domains.

Decision Making↗
PubMed1993

Perceiving surface orientation: pictorial information based on rectangularity can be overriden during observer motion.

Although the observer's motion can elicit perception of relative depth, it is less successful in doing so when competing pictorial information is available. However, the evidence for this may be affected by limited extents of motion and by equidistance tendencies. Results obtained when monocular observers judged the orientation-in-depth of trapezoidal and of rectangular surfaces, during lateral head motion of extents 0 cm to 30 cm, are described. When the motion extent was less than 30 cm, trapezoidal surfaces were misperceived because they were interpreted as rectangular; this pictorial information was overriden only when the motion extent was 30 cm. The results may reflect the sequential nature of motion information and the redundancy of information in normal viewing: pictorial information may take precedence when motion is limited, but motion information can be indefinitely augmented. Comments are directed to: (i) the use of Ames 'distorted rooms' in this area of research, and (ii) the 'ecological' interpretation of pictorial information.

Adult↗
PubMed1993

Head movement changes apparent depth order in a motion-parallax display.

The hypothesis that the apparent visual depth is determined by the proximal velocity relative to the position of the head was examined in three experiments. Apparent protrusion/recession changed when subjects observed a moving random-dot pattern with their heads tilted sideways or rotated in the horizontal plane. This is ascribed to lateral head movement, which increases the proximal velocity when the dots and the subjects' heads are moving in opposite directions, and decreases the proximal velocity when both are moving in the same direction. Changes in the direction of movement of the stimulus caused a reversal of the apparent protrusion/recession. The resultant proximal velocity of the stimulus determined the order of depth of surfaces when the movement of the stimulus was linked to the subject's head movement.

Adolescent↗
PubMed1993

The effects of visual depth and eccentricity on manual bias, induced motion, and vection.

The relationship between the effects of visual-surround roll motion on compensatory manual tracking of a central display and the perceptual phenomena of induced motion and vection were investigated. To determine if manual-control biases generated in the direction of surround rotation compensate primarily for the perceived counterrotation of the central display ('induced motion') or the perceived counterrotation of the entire body ('vection'), the depth and eccentricity of the visual surround were varied. In the first experiment, twelve subjects attempted to keep an unstable central display level while viewing rotating visual surrounds in three depth planes: near (approximately 20 cm in front of the central display), coplanar, and far (approximately 21 cm behind the central display). In the second experiment, twelve additional subjects viewed a rotating surround that was presented either in the full visual field (0-110 deg) or in central and peripheral regions of similar width. Manual-control biases and induced motion were shown to be closely related to one another and strongly influenced both by central and by peripheral surround motion at or beyond the plane of fixation. Vection, on the other hand, was shown to be much more dependent on peripheral visual inputs.

Adolescent↗