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PubMed2004

Perception of scene layout from optical contact, shadows, and motion.

Kersten et al (1997 Perception 26 171-192) found that the perceived motion of an object in a 3-D scene was determined by the motion of a shadow. In the present study, we compared the effect of a shadow to that of a second object on the ground in determining the perceived position in depth of a floating object in both dynamic and stationary scenes. Changing the second (lower) object from textured to dark increased the influence of the second object on the judged position of the first object. Giving the second object zero thickness had this effect only if it was also dark. Variations in the height of the floating object were important with a second object but not with a shadow, in motion scenes. With alternative shadows present, the position of the floating object was determined primarily by matching speeds, with matching sizes as a secondary factor. These results show some similarities but important differences between the effect of a second object and that of a shadow.

Depth Perception↗

PubMed2000

Parietal neurons represent surface orientation from the gradient of binocular disparity.

In order to elucidate the neural mechanisms involved in the perception of the three-dimensional (3D) orientation of a surface, we trained monkeys to discriminate the 3D orientation of a surface from binocular disparity cues using a Go/No-go type delayed-matching-to-sample (DMTS) task and examined the properties of the surface-orientation-selective (SOS) neurons. We recorded 57 SOS neurons from the caudal part of the lateral bank of the intraparietal sulcus (area CIP) of three hemispheres of two Japanese monkeys (Macaca fuscata). We tested 29 of 57 SOS neurons using the square plate of a solid figure stereogram (SFS) and random-dot stereogram (RDS) without perspective cues; almost all of the tested neurons (28/29) showed surface orientation selectivity for the SFS and/or the RDS without perspective cues. Eight of these 28 neurons (28.6%) showed selectivity for both the RDS and SFS, 7 (25.0%) were dominantly selective for the RDS, and 13 (46.4%) were dominantly selective for the SFS. These results suggest that neurons that show surface orientation tuning for the RDS without perspective cues compute surface orientation from the gradient of the binocular disparity given by the random-dot across the surface. On the other hand, neurons that show surface orientation tuning for the SFS without perspective cues may represent surface orientation primarily from the gradient of the binocular disparity along the contours. In conclusion, the SOS neurons in the area CIP are likely to operate higher order processing of disparity signals for surface perception by integrating the input signals from many disparity-sensitive neurons with different disparity tuning.

Animals↗

PubMed1995

The perception of 3-D structure from contradictory optical patterns.

Two experiments investigated observers' perception of 3-D structure when optical sources of information were contradictory. When motion and stereoscopic disparities specified different surfaces, the perceptual outcome dependent strongly on the direction of curvature present within each modality. Previous research has shown that the perception of surface slant and curvature is anisotropic for both motion and stereo and that it depends on the direction in which it takes place. In the present experiments, the modality with the "effective" direction of curvature tended to dominate or suppress the perception of surfaces in the other modality with less effective curvatures. The results have implications for models which attempt to combine 3-D data from different optical sources.

Depth Perception↗

PubMed1985

Automatic and attentional components in perception of shape-at-a-slant.

In perceiving shape-at-a-slant it is assumed that a sequence of operations is executed. The aim of these experiments was to determine the extent to which execution of these operations requires allocation of attention. Three hypotheses were considered: zero automaticity--that all of the operations require attention; partial automaticity--that the operations culminating in a representation of projective shape and slant-in-depth are automatic while the combinatorial operations culminating in a distally correlated shape require attention; full automaticity--that the entire sequence of operations is automatic, proceeding without allocation of attention. To decide among these hypotheses, subjects performed forced-choice shape recognition tests under two conditions: In the shape-directed condition subjects were motivated to process shape. In the numerosity-directed condition subjects were motivated to direct attention to discrimination of numerosity, thereby causing attention to be diverted from processing of shape. Examination of the pattern of choices on the recognition test showed results that conformed best to the hypothesis of partial automaticity.

Attention↗

PubMed1991

Depth-coded motion signals in plaid perception and optokinetic nystagmus.

When two sine-wave gratings drift in different directions at the same speed behind a circular window, a single coherent plaid is seen rather than one grating sliding over the other. We find that as the stereo depth separation of the two component gratings increases, the probability of seeing a plaid declines. The gain of the slow phase of vertical optokinetic nystagmus (OKN) also falls as the separation of the components increases. When the two grating components are in the same depth plane, the vertical eye velocity is greater than that of either component. This shows that the OKN is being driven by the plaid, whose vertical speed is roughly twice as fast as the components. We conclude that both perception and OKN are fed by the same motion signal, which arises after binocular combination and after plaid synthesis.

Depth Perception↗

PubMed1994

Perceiving textures: beyond filtering.

Classical texture segregation theory and current visual filtering models suggest that rapid texture segregation is determined by the properties of early cortical filtering. To examine this assumption, we manipulated the binocular disparity of texture elements (an L vs a bar) relative to adjoining squares. Such a manipulation has little effect on the early cortical filtered representations of the stimuli but can have a strong influence on surface representation. In particular, it can determine whether an image patch is seen as a distinct surface, in front, or part of a larger occluded surface, in back. When seen as part of an occluded surface, the distinctive shape of each texture element is reduced. Accordingly, we found that texture discrimination performance was impaired when the texture elements had uncrossed disparities ("back" case). Additional experiments indicated that this result was not due to the disparity manipulation alone, but occurred only when each texture element was perceived as part of an occluded surface. We conclude that in rapid texture discrimination, the visual system cannot ignore information regarding surface layout. This suggests that for purposes of texture perception, either: (a) the visual system has no access to the filtered image or (b) one cannot entertain the concept of a filtered image uninfluenced by higher order surface processing.

Depth Perception↗

PubMed1987

The representation of uniform motion in vision.

For veridical detection of object motion any moving detecting system must allocate motion appropriately between itself and objects in space. A model for such allocation is developed for simplified situations (points of light in uniform motion in a frontoparallel plane). It is proposed that motion of objects is registered and represented successively at four levels within frames of reference that are defined by the detectors themselves or by their movements. The four levels are referred to as retinocentric, orbitocentric, egocentric, and geocentric. Thus the retinocentric signal is combined with that for eye rotation to give an orbitocentric signal, and the left and right orbitocentric signals are combined to give an egocentric representation. Up to the egocentric level, motion representation is angular rather than three-dimensional. The egocentric signal is combined with signals for head and body movement and for egocentric distance to give a geocentric representation. It is argued that although motion perception is always geocentric, relevant registrations also occur at the three earlier levels. The model is applied to various veridical and nonveridical motion phenomena.

Concept Formation↗

PubMed2003

Reversible-figure perception: mechanisms of intentional control.

Observers can exert a degree of intentional control over the perception of reversible figures. Also, the portion of the stimulus that is selected for primary or enhanced processing (focal-feature processing) influences how observers perceive a reversible figure. Two experiments investigated whether voluntary control over perception of a Necker cube could be explained in terms of intentionally selecting appropriate focal features within the stimulus for primary processing. In Experiment 1, varying observers' intentions and the focus of primary processing produced additive effects on the percentage of time that one alternative was perceived. In Experiment 2, the effect of varying the focus of primary processing was eliminated by the use of a small cube, but the effect of intention was unaltered. The results indicate that intentional control over perception can be exerted independently of focal-feature processing, perhaps by top-down activation or priming of perceptual representations. The results also reveal the limits of intentional control.

Attention↗

PubMed1986

The effects of quantity and depth of processing on children's time perception.

Two experiments were conducted to investigate the effects of quantity and depth of processing on children's time perception. These experiments tested the appropriateness of two adult time-perception models (attentional and storage size) for younger ages. Children were given stimulus sets of equal time which varied by level of processing (deep/shallow) and quantity (list length). In the first experiment, 28 children in Grade 6 reproduced presentation times of various quantities of pictures under deep (living/nonliving categorization) or shallow (repeating label) conditions. Students also compared pairs of durations. In the second experiment, 128 children in Grades K, 2, 4, and 6 reproduced presentation times under similar conditions with three or six pictures and with deep or shallow processing requirements. Deep processing led to decreased estimation of time. Higher quantity led to increased estimation of time. Comparative judgments were influenced by quantity. The interaction between age and depth of processing was significant. Older children were more affected by depth differences than were younger children. Results were interpreted as supporting different aspects of each adult model as explanations of children's time perception. The processing effect supported the attentional model and the quantity effect supported the storage size model.

Age Factors↗

PubMed1994

Properties of the stereoscopic (cyclopean) motion aftereffect.

Across four experiments, this study investigated properties of the stereoscopic motion aftereffect (adaptation from moving retinal disparity information). The results showed that stereoscopic motion can induce an adaptation aftereffect across a wide range of conditions and observers, provided that the duration of adaptation is sufficiently long and a perceptually salient test pattern is viewed. Motion adaptation was found to transfer between the stereoscopic and luminance domains [replicating a previous report by Fox, Patterson and Lehmkuhle (1982) Investigative Ophthalmology and Visual Science (Suppl.), 22, 144], suggesting that motion perception from stereoscopic (second-order) and luminance (first-order) attributes is mediated by a common neural substrate.

Adaptation, Ocular↗

PubMed2004

Spatial integration in structure from motion.

In three experiments we investigated whether the perception of 3D structure from the optic-flow involves a process of spatial integration. The observer's task was to judge the 3D orientation of local velocity field patches. In two conditions, the patches were presented either in isolation, or as part of a global optic-flow. In Experiment 1, the global optic-flow was a linear velocity field. In Experiment 2, the patches were embedded in a randomly perturbed linear velocity field. In Experiment 3, the local patches belonged to a smoothly curved surface. The results of these three experiments lead to two main conclusions: (1) a process linking spatially separated patches into global entities does affect the perception of local surface orientation induced by the optic-flow, and (2) linearity or smoothness of the global velocity field are not necessary conditions for spatial integration.

Adult↗

PubMed1997

Cortical dynamics of three-dimensional figure-ground perception of two-dimensional pictures.

This article develops the FACADE theory of 3-dimensional (3-D) vision and figure-ground separation to explain data concerning how 2-dimensional pictures give rise to 3-D percepts of occluding and occluded objects. The model describes how geometrical and contrastive properties of a picture can either cooperate or compete when forming the boundaries and surface representation that subserve conscious percepts. Spatially long-range cooperation and spatially short-range competition work together to separate the boundaries of occluding figures from their occluded neighbors. This boundary ownership process is sensitive to image T junctions at which occluded figures contact occluding figures. These boundaries control the filling-in of color within multiple depth-sensitive surface representations. Feedback between surface and boundary representations strengthens consistent boundaries while inhibiting inconsistent ones. Both the boundary and the surface representations of occluded objects may be amodally completed, while the surface representations of unoccluded objects become visible through modal completion. Functional roles for conscious modal and amodal representations in object recognition, spatial attention, and reaching behaviors are discussed. Model interactions are interpreted in terms of visual, temporal, and parietal cortices.

Attention↗

PubMed1998

Stereoscopic head-mounted display incorporated into microsurgical procedures: technical note.

OBJECTIVE: We have previously evaluated video technologies that have allowed for the use of electronic imaging during microneurosurgical and endoscopic procedures. This stereoscopic camera set and monitor/recording system allows for the use of stereoscopic images during surgery by the primary surgical assistant and ancillary personnel. It also allows for stereoscopic recording and playback using a wide video home system format. We describe a novel prototype of a head-mounted display (HMD) that allows the surgical team to simultaneously visualize the surgical field stereoscopically and includes picture-in-picture, voice control, and stereoscopic recording capabilities. INSTRUMENTATION: A stereoscopic HMD with 640 x 480 (video graphics array) pixel resolution and the ability to display 24-bit images has been designed. This device weighs 900 g. It is interfaceable with common video display formats. RESULTS: This stereoscopic HMD is being evaluated in a prospective multicenter trial of open microsurgical and endoscopic minimally invasive procedures. In our experience to date, there have been no equipment failures or complications attributable to the use of the display system. The equipment was well accepted by users who reported significant benefits in visualization. CONCLUSION: The use of a stereoscopic HMD may result in improved efficiency and safety in both endoscopic and open microsurgical procedures. We have verified that the HMD is comfortable during the course of a surgical procedure, is reliable, and allows for accessibility to the operative field with an excellent field of view and three-dimensional perception. Positioning and dexterity within the operative field are also enhanced. Additional uses relate to surgical training, multimodal information display, and operative rehearsals.

Depth Perception↗

PubMed2000

How does the brain sustain a visual percept?

Perception involves the processing of sensory stimuli and their translation into conscious experience. A novel percept can, once synthesized, be maintained or discarded from awareness. We used event-related functional magnetic resonance imaging to separate the neural responses associated with the maintenance of a percept, produced by single-image, random-dot stereograms, from the response evoked at the onset of the percept. The latter was associated with distributed bilateral activation in the posterior thalamus and regions in the occipito-temporal, parietal and frontal cortices. In contrast, sustained perception was associated with activation of the pre-frontal cortex and hippocampus. This observation suggests that sustaining a visual percept involves neuroanatomical systems which are implicated in memory function and which are distinct from those engaged during perceptual synthesis.

Brain↗

PubMed1992

Experiencing and perceiving visual surfaces.

A theoretical framework is proposed to understand binocular visual surface perception based on the idea of a mobile observer sampling images from random vantage points in space. Application of the generic sampling principle indicates that the visual system acts as if it were viewing surface layouts from generic not accidental vantage points. Through the observer's experience of optical sampling, which can be characterized geometrically, the visual system makes associative connections between images and surfaces, passively internalizing the conditional probabilities of image sampling from surfaces. This in turn enables the visual system to determine which surface a given image most strongly indicates. Thus, visual surface perception can be considered as inverse ecological optics based on learning through ecological optics. As such, it is formally equivalent to a degenerate form of Bayesian inference where prior probabilities are neglected.

Depth Perception↗

PubMed2006

A single system explains human speed perception.

Motion is fully described by a direction and a speed. The processing of direction information by the visual system has been extensively studied; much less is known, however, about the processing of speed. Although it is generally accepted that the direction of motion is processed by a single motion system, no such consensus exists for speed. Psychophysical data from humans suggest two separate systems processing luminance-based fast and slow speeds, whereas neurophysiological recordings in monkeys generally show continuous speed representation, hinting at a single system. Although the neurophysiological findings hint at a single system, they remain inconclusive as only a limited amount of cells can be measured per study and, possibly, the putative different motion systems are anatomically separate. In three psychophysical motion adaptation experiments, we show that predictions on the basis of the two-motion system hypothesis are not met. Instead, concurrent modeling showed that both here-presented and previous data are consistent with a single system subserving human speed perception. These findings have important implications for computational models of motion processing and the low-level organization of the process.

Adaptation, Physiological↗

PubMed1982

Sigma-movement and optokinetic nystagmus elicited by stroboscopically illuminated stereopatterns.

Sigma-movement is an apparent movement seen when a stationary periodic visual pattern of the period PS is illuminated stroboscopically at the flash frequency fS and smooth gaze pursuit eye movements are performed across the pattern at an angular velocity Ve = PS X fS deg X S-1. Sigma-movement leads to an optokinetic nystagmus (Sigma OKN) which in turn sustains Sigma-movement perception. (1) Sigma-movement was also seen in an apparent three-dimensional periodic stripe pattern generated by two periodic monocular stimulus patterns with a certain degree of horizontal binocular disparity. (2) Sigma-movement perception and Sigma-OKN were also elicited by a stroboscopically illuminated, stationary, random dot stereostripe pattern. The periodicity PS of this pattern is generated on the cyclopean retina (Julesz 1971). The equation described above was also valid. When the time delay delta t between left eye and right eye flashes was varied, the apparent depth of the random dot stereostripe pattern decreased with increasing delta t, but the Sigma-effects were not affected. (3) Sigma-movement illusion and Sigma-pursuit movements can also be induced when real three-dimensional objects composed of periodic components are stroboscopically illuminated and adequate gaze or eye pursuit movements are induced. Sigma-movement is related to gaze movement and is therefore elicitable by eye, head or body movements. (4) Sigma-movement is presumably caused by the interaction of efference copy signals (generated in a cortical gaze pursuit system) and afferent visual signals. The present data indicate that neuronal mechanisms for this interaction are located--at least in part--at or beyond the level of binocular fusion and stereopsis.

Adult↗

PubMed1992

Perception of optical flow in cortical blindness: a case report.

Motion perception was studied in a subject with bilateral lesion of the visual cortex, involving severe damage to cortical areas V1 and V4, but with no apparent damage to visual associative areas situated in occipito-parietal and lateral occipito-temporal (presumably V5) zones. He was able to perceive optical flow motions simulating motion in depth in "blind" parts of his visual field, provided that the stimulus-onset was temporally dissociated from its motion. Moreover, he was able to discriminate between different velocities and directions of motion. The results suggest that perimetrically "blind" parts of the visual field in this patient have true capacities to process visual motion. They are discussed in reference to the subject's ability to move freely in his environment and in reference to the role of extrastriate visual pathways in visual motion processing.

Acceleration↗