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At least 1,315 records · Page 73Linked to original sources
PubMed2003

Perception can influence the vergence responses associated with open-loop gaze shifts in 3D.

We sought to determine if perceived depth can elicit vergence eye movements independent of binocular disparity. A flat surface in the frontal plane appears slanted about a vertical axis when the image in one eye is vertically compressed relative to the image in the other eye: the induced size effect (Ogle, 1938). We show that vergence eye movements accompany horizontal gaze shifts across such surfaces, consistent with the direction of the perceived slant, despite the absence of a horizontal disparity gradient. All images were extinguished during the gaze shifts so that eye movements were executed open-loop. We also used vertical compression of one eye's image to null the perceived slant resulting from prior horizontal compression of that image, and show that this reduces the vergence accompanying horizontal gaze shifts across the surface, even though the horizontal disparity is unchanged. When this last experiment was repeated using vertical expansions in place of the vertical compressions, the perceived slant was increased and so too was the vergence accompanying horizontal gaze shifts, although the horizontal disparity again remained unchanged. We estimate that the perceived depth accounted, on average, for 15-41% of the vergence in our experiments depending on the conditions.

Convergence, Ocular↗
PubMed1999

The size-distance paradox is a cognitive phenomenon.

The perceived size of a fixated object is known to be a function of the perceived fixation distance. The size-distance paradox has been posited as evidence that the perceived distance of a fixated object is, in turn, influenced by the object's perceived size. If this is correct then it challenges a widely accepted account (modified weak fusion) of how the nervous system combines multiple sources of information. We hypothesised that the influence of perceived size on the perception of distance is likely to be restricted to conscious perceptual judgements. If our hypothesis is correct then the size-distance paradox should not be observed when observers make action-based distance judgements. In line with this expectation we observed the size-distance paradox when participants made verbal reports on target distance but found no paradoxical judgements in a group who were asked to point at the target. We therefore suggest that the size-distance paradox should not be taken as evidence that perceived size feeds back into distance perception.

Adult↗
PubMed1996

The time course of 2-D shape discrimination in random dot stereograms.

Predictions of a coarse-to-fine and co-operative stereo matching algorithm were compared using a 2-D shape discrimination task for disparity-defined targets in 50% random dot stereograms. Uniform disparity targets, square wave modulated targets with a different mean disparity to the background, or uncorrelated dots could be seen at much briefer exposures (down to 33 msec) than square wave modulated targets with the same mean disparity as the background. In the latter case, performance at brief exposures was improved by using coarser disparity corrugations. The results are readily explained by a coarse-to-fine matching scheme such as that proposed by Marr & Poggio (1979) [Proceedings of the Royal Society of London (B), 204, 301-328] and suggest that the correspondence problem is not the limiting step in the perception of simple cyclopean forms.

Adult↗
PubMed2006

Facing mortality: a qualitative in-depth interview study on illness perception, lay theories and coping strategies of adult patients with acute leukemia 1 week after diagnosis.

In patients with acute leukemia we investigated the illness perceptions, lay theories and coping strategies 1 week after diagnosis. Semi-structured in-depth interviews were conducted with 12 patients. The transcribed interviews were analyzed by methods of qualitative research. Dramatic narrations of overwhelming threat in younger patients contrast to rationalization in elderly patients. Feelings of helplessness and efforts of normalization become apparent. Intense descriptions of physical injury due to invasive procedures allow verbalizing the fear of the disease. While coping strategies are complex, the overall importance of trust is recognized. Mortal fears are indirectly indicated. The results have consequences for psycho-oncological training and patient education.

Adaptation, Psychological↗
PubMed2001

Mental models of line drawings.

Mental models are internal representations of world structure, used to accomplish cognitive tasks. I postulate specific representations (of objects and images) and associated context (of world and view) for mental models of line drawings. I then analyze the representations and context to predict specific perceptual modes, including the relative strengths of these modes. The predicted modes are supported by a well-known example [from Rock, 1983 The Logic of Perception (Cambridge, MA: MIT Press)] where object perception changes with image orientation.

Bayes Theorem↗
PubMed1978

Neurones in cat parastriate cortex sensitive to the direction of motion in three-dimensional space.

1. On psychophysical grounds, Beverley & Regan suggested that in man different neural mechanisms mediate the binocular perception of movement in depth and the binocular perception of positional (static) depth. They proposed that the human visual pathway contains several neural mechanisms, each sensitive to a different direction of motion in space. These mechanisms compute the direction of motion from the relative speeds and directions of movement of the left and right retinal images.2. We have recorded from 101 units in area 18 of cat visual cortex, searching for neurones tuned to the direction of motion in three dimensions, with properties that could account for the proposed directionally tuned binocular motion detectors in man. The cat's left eye viewed one bar, while its right eye simultaneously viewed a second bar. Single units were stimulated by independently oscillating the bars from side to side. The apparent direction of movement in three dimensions was altered by varying the relative speeds of the bars and their relative directions of motion. The mean (positional) disparity of the bars could also be varied.3. For one class of neurone (twenty cells), binocular stimulation inhibited firing for trajectories parallel to the frontoparallel plane over a large volume of space. Strong firing was produced by oppositely directed bar movements. Some of these neurones were especially narrowly tuned to the direction of movement in depth, responding only to a range of 2-3 degrees , i.e. to moving bodies that would hit or only narrowly miss the cat. These cells emphasized the direction of movement at the expense of positional information.3. These units occurred in clusters. On the perpendicular penetrations in which they were found, they comprised a substantial majority of all cells encountered.5. For a second class of neurone (nine cells), binocular facilitation produced selective responses to objects moving along trajectories that missed the head.6. The two classes of neurone provide a basis for four proposed directionally tuned binocular motion detectors.7. A third class of neurone (seventeen cells) was selectively sensitive to movements parallel to the frontoparallel plane. There was strong binocular facilitation when the bars moved at the same speeds in the same directions: oppositely directed movements might be more than 100 times less effective. These neurones may signal positional disparity.8. These three classes of neurone cut across established categories. Only when both eyes were stimulated simultaneously with targets moving in different speeds and directions was it possible to demonstrate the binocular interactions described here.

Action Potentials↗
PubMed1983

Anisotropies in the perception of three-dimensional surfaces.

The appearance of certain three-dimensional surfaces was found to depend on the orientation of the depth contours forming the surface. This was true both when the depth was specified by motion parallax and when it was specified by binocular disparities. Slowly changing depth surfaces that generated a pattern of relative motions or disparities characterized by a one-dimensional expansion-compression were perceived differently from those that produced a shear transformation.

Depth Perception↗
PubMed2006

Integration of force and position cues for shape perception through active touch.

This article systematically explores cue integration within active touch. Our research builds upon a recently made distinction between position and force cues for haptic shape perception: when sliding a finger across a bumpy surface, the finger follows the surface geometry (position cue). At the same time, the finger is exposed to forces related to the slope of the surface (force cue). Experiment 1 independently varied force and position cues to the curvature of 3D arches. Perceived curvature could be well described as a weighted average of the two cues. Experiment 2 found more weight of the position cue for more convex high arches and higher weight of the force cue for less convex shallow arches--probably mediated through a change in relative cue reliability. Both findings are in good agreement with the maximum-likelihood estimation (MLE) model for cue integration and, thus, carry this model over to the domain of active haptic perception.

Adult↗
PubMed2006

Bayesian inference of form and shape.

The ability to visually perceive two-dimensional (2D) form and three-dimensional (3D) shape is one of our most fundamental faculties. This ability relies on considerable prior knowledge about the way edge elements in an image are likely to be connected together into a contour as well as the way these 2D contours relate to 3D shapes. The interaction of prior knowledge with image information is well modeled within a Bayesian framework. We review here the experimental evidence of shape perception seen as a Bayesian inference problem.

Bayes Theorem↗
PubMed2003

At least at the level of inferior temporal cortex, the stereo correspondence problem is solved.

Stereoscopic vision requires the correspondence problem to be solved, i.e., discarding "false" matches between images of the two eyes, while keeping correct ones. To advance our understanding of the underlying neuronal mechanisms, we compared single neuron responses to correlated and anticorrelated random dot stereograms (RDSs). Inferior temporal neurons, which respond selectively to disparity-defined three-dimensional shapes, showed robust selectivity for correlated RDSs portraying concave or convex surfaces, but unlike neurons in areas V1, MT/V5, and MST, were not selective for anticorrelated RDSs. These results show that the correspondence problem is solved at least in far extrastriate cortex, as it is in the monkey's perception.

Animals↗
PubMed1988

Perception of shape from shading.

The human visual system can rapidly and accurately derive the three-dimensional orientation of surfaces by using variations in image intensity alone. This ability to perceive shape from shading is one of the most important yet poorly understood aspects of human vision. Here we present several findings which may help reveal computational mechanisms underlying this ability. First, we find that perception of shape from shading is a global operation which assumes that there is only one light source illuminating the entire visual image. This implies that if two identical objects are viewed simultaneously and illuminated from different angles, then we would be able to perceive three-dimensional shape accurately in only one of them at a time. Second, three-dimensional shapes that are defined exclusively by shading can provide tokens for the perception of apparent motion, suggesting that the motion mechanism is remarkably versatile in the kinds of inputs it can use. Lastly, the occluding edges which delineate an object from its background can also powerfully influence the perception of three-dimensional shape from shading.

Depth Perception↗
PubMed2004

Are switches in perception of the Necker cube related to eye position?

The issue of the relation of eye position to perceptual reversals of the ambiguous figure of the 'Necker cube' dates back to Necker's original article [L.A. Necker (1832) The London & Edinburgh Philosophical Magazine and Journal of Science, 1, 329-337]. Despite the investigations of many distinguished psychophysicists since then, the question of whether perceptual switching is a cause or a consequence of associated changes in eye position has remained a matter of debate. In the present study we overcame methodological problems that have bedevilled many previous studies. We avoided any instruction that could interfere with the human subjects' free viewing of the Necker cube, tracked the eye position precisely and used biased versions of the cube that produced unambiguous percepts to determine how each subject actually looked at the cube. We show that, under these free-viewing conditions, there is a close link between the perception of the Necker cube and eye position. The average eye position of most subjects is at an extreme value at about the time when the subject's perception switches. From the biased cube trials we can infer that the polarity of the extreme corresponds to the percept which the subject had before the switch. These data indicate a bidirectional coupling between eye position and perceptual switching so that, after a subject's perceptual state changes, their eye position shifts to view the newly established percept. When the eye position approaches the corresponding extreme, the percept, in turn, becomes more and more likely to switch. This result suggests that the changed eye position itself might provide a negative feedback signal that suppresses the percept.

Adult↗
PubMed2006

The visual perception of plane tilt from motion in small field and large field: psychophysics and theory.

Subjects indicated the tilt of dotted planes rotating in depth, in monocular viewing, under perspective projection. The responses depended on the FOV (field of view) and on the angle W between the tilt and frontal translation (orthogonal to the rotation axis). Response accuracy increased with the FOV, and decreased with W. Our results support the processing of the second-order optic flow in all cases, but indicate that this flow is quantitatively small in small-field, leading to tilt ambiguities. We examine computational models based on the affine components of the optic flow to interpret our results.

Adult↗