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At least 289 records · Page 16Linked to original sources
PubMed1995

Words interact with colors in a globally aphasic patient: evidence from a Stroop-like task.

Stroop-like manual reaction time tasks were administered to 16 controls and one globally aphasic patient who was unable to match written color words with the corresponding color. Although the patient could not explicitly access the meaning of written color words, he showed a similar pattern of performance in the reaction time tasks as the controls: written words were processed automatically and they interacted with the processing of the color of the letters. Thus, the results of this study suggested that it is possible to process unrecognized words implicitly to a cross-domain semantic level where the processing of words interacts with the processing of colors. The results from this and other studies on implicit language processing indicate that linguistic stimuli can be analysed to relatively high levels without any awareness.

Aphasia↗
PubMed2000

The viewpoint-dependency of veridicality: psychophysics and modelling.

Human observers were shown projected angles, embedded in solid cross-like figures and were asked whether these projected angles could be the projection of an orthogonal angle in 3-D space (i.e. whether the two legs of the cross were orthogonal to each other). We found that performance depended on the viewpoint at which the angle was viewed: Both slant (i.e. the angle between the normal of the target angle relative to the plane of projection) and roll (i.e. the rotation around the normal of the target angle) had a systematic effect on the proportion of errors when observers were shown non-orthogonal angles. With orthogonal angles, however, this effect was absent (i.e. very low error rate with no systematic effect of slant and roll). Instead of assuming a viewpoint-dependent bias towards orthogonality, a computational analysis of the task, using a Bayesian approach, and a computer simulation showed that the viewpoint-dependency can be modelled by a fixed set of biases in order to constrain the set of possible scenes that could give rise to the projection.

Computer Graphics↗
PubMed2000

Electrophysiological correlates of visual impairments after traumatic brain injury.

Our aims were to investigate: (i) the VEP correlates of functional visual impairments following traumatic brain injury (TBI), in particular of the reduced spatial form perception; and (ii) the VEP correlates of visual sustained arousal in TBI patients. We used two approaches: (i) the analysis of latency and amplitude of the peaks; and (ii) the study of the correlations among the latencies of the peaks as a label of temporal synchronization. Thirty-five severe TBI outcome inpatients and 35 matching controls were studied. Pattern-reversal VEPs were recorded at Oz-Fz and Cz-A1, first without counting, then with counting of the reversals. Seven peaks of the waveform at Oz and eight peaks at Cz were measured. We found several differences in amplitude and latency between patients and controls, and between nocount/count. The temporal binding of the peaks within each channel and between the two channels was calculated by correlation matrices, and tested by factor analysis. Results indicated that the synchronization of the peaks within each channel did not differ between patients and controls. The temporal covariation between peaks occurring at Oz and Cz, however, was highly significantly altered in patients. This suggests that visual impairments in TBI patients may be due to a deranged synchronization of the activity of different brain regions.

Adolescent↗
PubMed2001

The lateral occipital complex and its role in object recognition.

Here we review recent findings that reveal the functional properties of extra-striate regions in the human visual cortex that are involved in the representation and perception of objects. We characterize both the invariant and non-invariant properties of these regions and we discuss the correlation between activation of these regions and recognition. Overall, these results indicate that the lateral occipital complex plays an important role in human object recognition.

Brain Mapping↗
PubMed2001

Interaction of visual prior constraints.

The visual system relies on two types of information to interpret a visual scene: the cues that can be extracted from the retinal images and prior constraints that are used to disambiguate the scene. Many studies have looked at how multiple visual cues are combined. We examined the interaction of multiple prior constraints. The particular constraints studied here are assumptions the observer makes concerning the location of the light source (for the shading cue to depth) and the orientation of a surface (for depth based on image contours). The reliability of each of the two cues was manipulated by changing the contrast of different parts of the stimuli. We developed a model based on elements of Bayesian decision theory that permitted us to track the weights applied to each of the prior constraints as a function of the cue reliabilities. The results provided evidence that prior constraints behave just like visual cues to depth: cues with more reliable information have higher weight attributed to their corresponding prior constraint.

Bayes Theorem↗
PubMed2001

3D after-effects are due to shape and not disparity adaptation.

There are a variety of stereoscopic after-effects in which exposure to a stimulus with a particular slant or curvature affects the perceived slant or curvature of a subsequently presented stimulus. These after-effects have been explained as a consequence of fatigue (a decrease in responsiveness) among neural mechanisms that are tuned to particular disparities or patterns of disparity. In fact, a given disparity pattern is consistent with numerous slants or curvatures; to determine slant or curvature, the visual system must take the viewing distance into account. We took advantage of this property to examine whether the mechanisms underlying the stereoscopic curvature after-effect are tuned to particular disparity patterns or to some other property such as surface curvature. The results clearly support the second hypothesis. Thus, 3D after-effects appear to be caused by adaptation among mechanisms specifying surface shape rather than among mechanisms signaling the disparity pattern.

Adaptation, Physiological↗
PubMed2001

Perception viewed as an inverse problem.

The modern study of perception began when Fechner published his 'Elements of Psychophysics' in 1860. This book has guided most perception research ever since. It has become increasingly clear that there are problems with Fechner's approach, which assumes that the percept is completely determined by the sensory input. Fechner's approach cannot explain the processes that allow our percepts to be veridical. Post-Fechnerian schools (Helmholtzian, Structural, Gestalt and Gibsonian) have tried to deal with this problem, but have not been successful. An alternative to the Fechnerian approach is required. This paper describes an alternative that has been developing over the last 20 years within the computer vision community. It treats perceptual interpretation as a solution of an inverse problem that depends critically on the operation of a priori constraints. Contemporary research, which adopted this approach, has concentrated on verifying the usefulness of Bayesian and standard regularization methods. This paper takes the next step; it discusses theoretical and empirical aspects of studying human perception as an inverse problem. It reviews the literature that illustrates the power of the inverse problem approach. This review leads to the suggestion that progress in the study of perception will benefit if the inverse approach were to be adopted by experimentalists, as well as by the computational modelers, who have been actively exploring its potential to date.

Bayes Theorem↗
PubMed2001

The stationarity hypothesis: an allocentric criterion in visual perception.

Having long considered that extraretinal information plays little or no role in spatial vision, the study of structure from motion (SfM) has confounded a moving observer perceiving a stationary object with a non-moving observer perceiving a rigid object undergoing equal and opposite motion. However, recently it has been shown that extraretinal information does play an important role in the extraction of structure from motion by enhancing motion cues for objects that are stationary in an allocentric, world-fixed reference frame (Nature 409 (2001) 85). Here, we test whether stationarity per se is a criterion in SfM by pitting it against rigidity. We have created stimuli that, for a moving observer, offer two interpretations: one that is rigid but non-stationary, another that is more stationary or less rigid. In two experiments, with subjects reporting either structure or motion, we show that stationary, non-rigid solutions are preferred over rigid, non-stationary solutions; and that when no perfectly stationary solutions is available, the visual system prefers the solution that is most stationary. These results demonstrate that allocentric criteria, derived from extra-retinal information, participate in reconstructing the visual scene.

Adult↗
PubMed2001

How vertical disparities assist judgements of distance.

The ratio of the vertical sizes of corresponding features in the two eyes' retinal images depends both on the associated object's distance and on its horizontal direction relative to the head (eccentricity). It is known that manipulations of vertical size ratio can affect perceived distance, size, depth and shape. We examined how observers use the vertical size ratio to determine the viewing distance. Do they use the horizontal gradient of vertical size ratio, or do they combine the vertical size ratio itself with the eccentricity at which it is found? Distance scaling (as measured by having subjects set an ellipsoid's size and shape to match a tennis ball) was no better when the judged object was 30 degrees to the right of the head (where vertical size ratios vary considerably with distance) than when it was located straight ahead. Distance scaling improved when vertical disparities were presented within larger visual fields, irrespective of where this was relative to the head. Our results support the proposal that subjects use the horizontal gradient of vertical size ratio to estimate the distance of an object that they are looking at.

Distance Perception↗
PubMed1997

Additive effect of luminance and color cues in generation of neon color spreading.

A series of experiments were designed to examine how luminance and color cues influence the occurrence of neon color spreading for the Ehrenstein pattern plus the cross pattern. The proportion of "see" responses for color spreading was obtained for different combinations of the luminance and color of the pattern components. The following were obtained. (1) Increase in the luminance of the inducing pattern and/or the color difference between the cross and the inducing pattern raised the proportion of "see" responses for color spreading. This implies that luminance and color signals additively contribute to the generation of the color spreading, but in different ways. (2) The "iso-spreading contours" for the generation of color spreading were determined on the two-dimensional isoluminant plane composed of the L-M and S-(L+M) axes. The contours were approximately described by rotated quadratic or ellipse functions. The additive interaction within the chromatic systems [the L-M system and the S-(L+M) system] was less significant than that across the luminance and chromatic systems. (3) The pattern of the experimental results could not be explained straightforwardly by existing models.

Color Perception↗
PubMed1997

Integrating contours within and through depth.

To better understand the role of disparity in contour integration we compared detection performance of "paths" composed of elements confined either to a single depth plane, or spanning multiple depth planes. In both cases paths defined by alignment of elements were embedded in a noise background-field made up of similar, but randomly positioned, elements covering the same depth range as the path elements. We show that a systematic disparity cue can enhance the detectability of paths which traverse depth, but that this detectability is weak compared to paths made up of elements of the same disparity. These results suggest that the outputs of disparity detectors tuned to different disparities can be linked to define contours.

Depth Perception↗
PubMed1997

Light source dependence in shape from shading.

We investigated shape constancy in human shape from shading under variations of illuminant direction using a local attitude probe in conjunction with a perturbation analysis. Stimuli were computer generated and depicted ellipsoids in a structured setting. Even with these simple shapes subjects settings were systematically biased in the illuminant direction and were consistent with a regression to image luminance gradients. These biases were reduced for high albedo scenes where interreflections make image illuminance more dependent on scene geometry. Adding texture to the surface reduced but did not eliminate this bias. These results suggest that we can expect little constancy in shape from shading under variations of illuminant direction without constraints from other cues.

Computer Simulation↗
PubMed1997

2D motion aliasing yielding 3D ambiguity. A study with variants of a Necker cube.

The 2D projection of a rotating Necker cube yields an ambiguous 3D interpretation based on both 2D shape and kinetic depth information. The present study shows that the alternation rate of the two 3D interpretations is constant with the rotation speed up to some critical value (around 25 turns/min for a cube whose sides subtend 2.5 deg) and increases monotonically thereafter. It is proposed that the additional perceptual reversals (PRs) observed at high rotation speeds are due to the increased frequency of the crossovers of the cube's edges. These crossovers yield 2D motion "aliasing" (or discontinuity) and "veridical" (or continuity) motion components. The motion aliasing (or crossover) hypothesis states that, in addition to the inherent ambiguity of the dynamic 2D projection of 3D objects, perceptual motion/perspective reversals will occur any time the discontinuity speed takes over the continuity speed. It is proposed that the relative strengths of the two components depend on the linear speed of the projected edges and that the discontinuity components take over the continuity one in the speed range where contrast sensitivity (or, above threshold, efficiency) is a decreasing function of speed. The motion aliasing hypothesis was tested and supported in a series of independent experiments showing that, for rotation speeds higher than 25 turns/min the PR rate increases with the crossover frequency at a constant speed, with linear speed at a constant crossover frequency and with the similarity of the crossing bars in terms of their orientation, polarity and spatial overlap. In addition, some of these experiments suggest that 2D shape and kinetic depth 3D-cues combine in such a way that the average PR rate they yield together is the same as the PR rate yielded by each of them independently. In the Discussion section we elaborate on issues related to the perceptual combination of ambiguous shape and kinetic depth, 3D cues.

Depth Perception↗
PubMed1997

Concentric orientation summation in human form vision.

Psychophysical data demonstrate that orientation information in concentric, random-dot Glass patterns is summed linearly to extract a global form percept. Surprisingly, no such global pooling was found for Glass patterns with parallel structure. A simple neural model explains these results and agrees with recent V4 single unit physiology. As V4 provides the major input to IT, global concentric units may play an important role in analyzing complex images such as faces. In support of this possibility, deficits in the perception of concentric Glass patterns have recently been linked to prosopagnosia.

Humans↗
PubMed1998

The viewpoint complexity of an object-recognition task.

There is an ongoing debate about the nature of perceptual representation in human object recognition. Resolution of this debate has been hampered by the lack of a metric for assessing the representational requirements of a recognition task. To recognize a member of a given set of 3-D objects, how much detail must the objects' representations contain in order to achieve a specific accuracy criterion? From the performance of an ideal observer, we derived a quantity called the view complexity (VX) to measure the required granularity of representation. VX is an intrinsic property of the object-recognition task, taking into account both the object ensemble and the type of decision required of an observer. It does not depend on the visual representation or processing used by the observer. VX can be interpreted as the number of randomly selected 2-D images needed to represent the decision boundaries in the image space of a 3-D object-recognition task. A low VX means the task is inherently more viewpoint invariant and a high VX means it is inherently more viewpoint dependent. By measuring the VX of recognition tasks with different object sets, we show that the current confusion about the nature of human perceptual representation is partly due to a failure in distinguishing between human visual processing and the properties of a task and its stimuli. We find general correspondence between the VX of a recognition task and the published human data on viewpoint dependence. Exceptions in this relationship motivated us to propose the view-rate hypothesis: human visual performance is limited by the equivalent number of 2-D image views that can be processed per unit time.

Algorithms↗
PubMed1998

Local and global factors affecting the coherent motion of gratings presented in multiple apertures.

Using stimuli composed of two independent gratings viewed through multiple apertures, we investigate a number of parameters affecting the integration of locally ambiguous motions into globally coherent motion. In four experiments, we varied local factors (grating spatial frequency, speed, contrast, duty cycle, orientation) and global factors (degree of similarity and common fate between the gratings, and symmetry in the configuration of the grating pattern) and examined their effects on global motion coherence. Our results, confirming accounts offered by previous investigators, indicate that local competition between motion signals generated by contours (ambiguous) and their line terminations (unambiguous) is important in determining global motion coherence in multiple-aperture stimuli. Our results also indicate that global factors can affect perceived coherence independently of local motion signals, suggesting the involvement of higher-level motion areas and a role for non-motion processes such as those involved in pattern and form perception. Comparing motion coherence with other two-dimensional (2-D) stimuli (plaids) shows that 2-D multiple-aperture stimuli are not analogous and that coherence models derived from plaid stimuli do not account for the data.

Humans↗
PubMed1998

Shape from shading: estimation of reflectance map.

The reflectance map used by the visual system for perception of shape from shading was estimated. In Experiment 1, an image of a cylinder or a sphere illuminated from the viewer's direction was presented, and subjects estimated the cross-section of perceived 3D-shape. The reflectance map was estimated from the relationship between the stimulus image intensities and the slants of the measured cross-section. The estimated reflectance maps were not the ones based on Lambertian reflectance properties. In Experiment 2, whether perceived shapes could be predicted based on the reflectance maps obtained in Experiment 1 was examined. Subjects performed the same shape estimation task with images of cylinders generated by the reflectance map obtained in Experiment 1. The perceived shapes coincided well with the shapes used for stimulus image generation. These results indicate that the visual system's estimation of shape from shading can be fully understood based on empirically obtained reflectance maps without mentioning its inaccurate nature which has been claimed by past studies.

Algorithms↗
PubMed1998

Large-scale tests of a keyed, appearance-based 3-D object recognition system.

We describe and analyze an appearance-based 3-D object recognition system that avoids some of the problems of previous appearance-based schemes. We describe various large-scale performance tests and report good performance for full-sphere/hemisphere recognition of up to 24 complex, curved objects, robustness against clutter and occlusion, and some intriguing generic recognition behavior. We also establish a protocol that permits performance in the presence of quantifiable amounts of clutter and occlusion to be predicted on the basis of simple score statistics derived from clean test images and pure clutter images.

Computer Simulation↗