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PubMed1995

The discrimination of dynamic orientation changes in gratings.

Thresholds were measured for discrimination of direction of a step angular rotation of gratings. The addition of simultaneous phase displacements (translation) had little effect on rotation thresholds for gratings over a considerable range; discrimination of rotation is unaffected by random directional translations an order of magnitude larger. Angular rotation discrimination thresholds increased with interstimulus interval (ISI). Thus discrimination is based at short ISIs (180 ms or less) on a percept of rotary motion, but at ISIs of several seconds by a spatial strategy (comparing static component orientations) relying on visual memory. Data points for the short-ISI region fell below the best-fitting straight line, and the slope of the short-ISI region of the curve was steeper than that of the long-ISI region. However, when either compound or simple gratings with uncorrelated spatial frequencies were used in the two stimulus frames, there was no evidence for a separate function at short ISIs. Orientation-change thresholds were measured for simple gratings as a function of contrast and spatial frequency. The contrast function showed saturation and the spatial frequency function was U-shaped. Rotation sensitivity for gratings is thus similar in its spatiotemporal properties to translation sensitivity. The findings support the proposal that rotation discrimination (at short ISIs) is achieved by a template mechanism combining signals from different directional detectors, rather than by congnitive comparison of the outputs of the directional mechanisms themselves.

Attention↗
PubMed2005

Height effects in real and virtual environments.

The study compared human perceptions of height, danger, and anxiety, as well as skin conductance and heart rate responses and postural instability effects, in real and virtual height environments. The 24 participants (12 men, 12 women), whose average age was 23.6 years, performed "lean-over-the-railing" and standing tasks on real and comparable virtual balconies, using a surround-screen virtual reality (SSVR) system. The results indicate that the virtual display of elevation provided realistic perceptual experience and induced some physiological responses and postural instability effects comparable to those found in a real environment. It appears that a simulation of elevated work environment in a SSVR system, although with reduced visual fidelity, is a valid tool for safety research. Potential applications of this study include the design of virtual environments that will help in safe evaluation of human performance at elevation, identification of risk factors leading to fall incidents, and assessment of new fall prevention strategies.

Accidental Falls↗
PubMed1999

An optimal estimation approach to visual perception and learning.

How does the visual system learn an internal model of the external environment? How is this internal model used during visual perception? How are occlusions and background clutter so effortlessly discounted for when recognizing a familiar object? How is a particular object of interest attended to and recognized in the presence of other objects in the field of view? In this paper, we attempt to address these questions from the perspective of Bayesian optimal estimation theory. Using the concept of generative models and the statistical theory of Kalman filtering, we show how static and dynamic events occurring in the visual environment may be learned and recognized given only the input images. We also describe an extension of the Kalman filter model that can handle multiple objects in the field of view. The resulting robust Kalman filter model demonstrates how certain forms of attention can be viewed as an emergent property of the interaction between top-down expectations and bottom-up signals. Experimental results are provided to help demonstrate the ability of such a model to perform robust segmentation and recognition of objects and image sequences in the presence of occlusions and clutter.

Attention↗
PubMed1995

The Pulfrich effect in optometric practice.

The Pulfrich effect can occur spontaneously, giving severe symptoms of changed visual perception in a variety of conditions in which an interocular latency difference has occurred. The symptoms principally involve misjudgements on the location of objects, especially when driving. Interocular latency differences can inadvertently be created in patients in practice and this study has investigated the Pulfrich effect in anisocoria, uniocular mydriasis, and with uniocular tint (X-chrom lens). In all cases a Pulfrich effect was consistently evident and compared to the size of the effect provoked by neutral density filters. The results show that with anisocoria, uniocular mydriasis and the X-chrom lens, the provoked Pulfrich effect is similar to that found reported previously in cases of trauma. These cases had severe visual symptoms so that care should be exercised in practice, where procedures likely to provoke the Pulfrich effect are contemplated. Specifically, patients who have received uniocular mydriasis in practice should be cautioned about the possible visual effects and advised not to drive until the effect of the mydriatic has finished and equal pupil sizes have been restored.

Anisocoria↗
PubMed2005

Selective biasing of stereo correspondence in an ambiguous stereogram.

In spite of numerous studies in stereoscopic perception, it is still not clear how the visual system matches features between the two eyes. One reason is that these previous studies used stimuli that presented little perceptual ambiguity, so the correspondence problem had only one solution. We present here a novel stimulus that presents a more complex correspondence problem. This stimulus is inspired by "wallpaper" stimuli and was specifically designed to put into conflict two possible constraints underlying stereo correspondence matching. These constraints are the nearest neighbour matching rule--that biases surfaces towards the horopter--and the nearest disparity rule--that biases surfaces to be smooth. By varying the contrast of adjacent image features in this stimulus, we were able to reveal and quantify a preference for nearest disparity matching. The magnitude of this preference is dependent upon the magnitude of possible disparities in the scene and is consistent with the idea that the visual system seeks to minimise local differences in disparity. We discuss these results with regard to the use of prior constraints in models of stereo matching.

Contrast Sensitivity↗
PubMed2006

[Associated heterophoria and the asymmetry of ocular prevalence for stereo images in front of or behind a reference plane].

BACKGROUND: An unequal weighting of the eyes in the directional perception of stereodisparate objects, is referred to as ocular prevalence of the right or left eye, respectively. Between 1962 and 1964 H.-J. Haase developed a valence test for the prismatic correction of heterophoria. He suggested that there would be less prevalence for stereo images presented in front of or behind a reference plane, and that this asymmetry of prevalence may be related to the direction of the associated heterophoria (eso- or exophoria). According to H.-J. Haase, the asymmetry may indicate a fixation disparity with a corresponding shift in retinal correspondence. Hence, the valence test could be an indicator for the prismatic correction of heterophoria. METHODS: Prevalence was tested in 37 subjects, using three methods: The subjects were asked to describe their perception (1), to make a paper drawing of their perception (2), and to align the position of the stereo images to the central fusion target with a computer-controlled device (3). METHODS 2 and 3 were used to reduce a possible suggestive influence on part of the investigator. The associated heterophoria was determined with the cross test by H.-J. Haase. RESULTS: Depending on whether the triangular stereo images were presented behind or in front of the reference plane, more or less prevalence was measured (mean values) in the group with exophoria than in the group with esophoria. These results were confirmed with all three methods. The asymmetry of prevalence was correlated with the direction of the associated heterophoria with r = 0.5. CONCLUSION: Statistically, these results confirm Haase's hypothesis of a relation between the asymmetry of ocular prevalence and the direction (eso- or exophoria) of the associated heterophoria. Since this relation holds true only for the group mean value, but not for each individual, the valence test cannot be generally recommended as an adjunct for the prismatic correction of heterophoria.

Adult↗
PubMed1997

Shading and stereo in early perception of shape and reflectance.

Recent experiments involving shaded 2-D stimuli have shown that early-vision mechanisms are capable of interpreting 3-D shape from shading. In particular, target discrimination tasks suggest that a target 'pops out' when background distractors, but not the target, can be interpreted as convex and lit from above or top-left. Since the problem of extracting 3-D shape from shading is intrinsically ill-defined, early vision may need to make these twin assumptions of convexity and top-left lighting in order to constrain the problem. Would these assumptions be recognized as unnecessary and consequently discarded when 3-D shape could be unambiguously defined by some other cue, like stereo disparity? A 2AFC stimulus onset asynchrony paradigm with masking was used in target discrimination experiments. The performance of five naive subjects on tasks where only shading cues were present was compared with that on tasks involving shading as well as stereo cues that define shape unambiguously. The results show that although stereo disparity information is incorporated by early-vision 3-D mechanisms, it is not used to overturn the default assumptions of lighting and shape. Stereo information is interpreted within the framework of top-left lighting, and a consistent preference for convexity is seen over concavity.

Contrast Sensitivity↗
PubMed1993

Tactile pattern recognition by graphic display: importance of 3-D information for haptic perception of familiar objects.

Haptic recognition of familiar objects by the early blind, the late blind, and the sighted was investigated with two-dimensional (2-D) and three-dimensional (3-D) stimuli produced by small tactor-pins. The 2-D stimulus was an outline of an object that was depicted by raising tactor-pins to 1.5 mm. The 3-D stimulus was a relief that was produced by raising the tactors up to 10 mm, corresponding to the height of the object. Mean recognition times for correct answers to the 3-D stimuli were faster than those for the 2-D stimuli, in all three subject groups. No statistically significant differences in percentage of correct responses between the 2-D and the 3-D stimuli were found for the late-blind and sighted groups, but the early-blind group demonstrated a significant difference. In addition, the haptic legibility for the quality of depiction of the object, without regard to whether or not the stimulus was understood, was measured. The haptic legibility of the 3-D stimuli was significantly higher than that of the 2-D stimuli for all the groups. These results suggest that 3-D presentation seems to promise a way to overcome the limitations of 2-D graphic display.

Adult↗
PubMed2003

Do humans optimally integrate stereo and texture information for judgments of surface slant?

An optimal linear system for integrating visual cues to 3D surface geometry weights cues in inverse proportion to their uncertainty. The problem of integrating texture and stereo information for judgments of planar surface slant provides a strong test of optimality in human perception. Since the accuracy of slant from texture judgments changes by an order of magnitude from low to high slants, optimality predicts corresponding changes in cue weights as a function of surface slant. Furthermore, since humans show significant individual differences in their abilities to use both texture and stereo information for judgments of 3D surface geometry, the problem admits the stronger test that individual differences in subjects' thresholds for discriminating slant from the individual cues should predict individual differences in cue weights. We tested both predictions by measuring slant discrimination thresholds and stereo/texture cue weights as a function of surface slant for multiple subjects. The results bear out both predictions of optimality, with the exception of an apparent slight under-weighting of texture information. This may be accounted for by factors specific to the stimuli used to isolate stereo information in the experiments. Taken together, the results are consistent with the hypothesis that humans optimally combine the two cues to surface slant, with cue weights proportional to the subjective reliability of the cues.

Cues↗
PubMed1991

Visual processing of rotary motion.

Local descriptions of velocity fields (e.g., rotation, divergence, and deformation) contain a wealth of information for form perception and ego motion. In spite of this, human psychophysical performance in estimating these entities has not yet been thoroughly examined. In this paper, we report on the visual discrimination of rotary motion. A sequence of image frames is used to elicit an apparent rotation of an annulus, composed of dots in the frontoparallel plane, around a fixation spot at the center of the annulus. Differential angular velocity thresholds are measured as a function of the angular velocity, the diameter of the annulus, the number of dots, the display time per frame, and the number of frames. The results show a U-shaped dependence of angular velocity discrimination on spatial scale, with minimal Weber fractions of 7%. Experiments with a scatter in the distance of the individual dots to the center of rotation demonstrate that angular velocity cannot be assessed directly; perceived angular velocity depends strongly on the distance of the dots relative to the center of rotation. We suggest that the estimation of rotary motion is mediated by local estimations of linear velocity.

Adult↗
PubMed1999

Local bias in autistic subjects as evidenced by graphic tasks: perceptual hierarchization or working memory deficit?

In the present study, copying tasks were used to assess hierarchical aspects of visual perception in a group of 10 nonsavant autistic individuals with normal intelligence. In Experiment 1, the hierarchical order of graphic construction and the constancy of this order were measured for the copying of objects and nonobjects. In comparison to control participants, autistic individuals produced more local features at the start of the copying. However, they did not differ from controls with respect to graphic constancy. Experiment 2 measured the effect of geometrical impossibility on the copying of figures. Results revealed that autistic individuals were less affected by figure impossibility than were controls. Therefore, these experiments seem to support the notion of a local bias for visual information processing in individuals with autism. Two interpretations are proposed to account for this effect. According to the hierarchical deficit hypothesis, individuals with autism do not manifest the normal global bias in perceiving scenes and objects. Alternatively, the executive function hypothesis suggests that autism brings about limitations in the complexity of information that can be manipulated in short-term visual memory during graphic planning.

Adolescent↗
PubMed1995

Motion and shape perception in cerebral akinetopsia.

Motion cues serve many purposes in primate vision. Consequently, akinetopsia, a defect of movement perception due to cerebral lesions, would be expected to comprise a range of motion-related defects. To address this issue we explored further the perceptual profiles in akinetopsic subject L.M. who has motion perception deficits due to extensive bilateral lesions of the dorsolateral visual association cortex that spare primary visual cortex, area V1. We used several different experimental procedures. Using random-dot motion stimuli, we showed that L.M. can still perceive global coherent motion and discriminate motion direction, yet these abilities fail even at moderate levels of background noise. L.M. also viewed a two-frame apparent (phi) motion stimulus known as a Ternus display. Her performance on this test suggests defective 'long-range' apparent motion mechanisms. These results are clearly in agreement with previous reports. Additional experiments showed that L.M. can still perceive 2-D shape and 3-D structure-from-motion (SFM). However, like motion direction discrimination, these abilities broke down at moderate levels of moving and stationary noise. Surprisingly, L.M. also had trouble perceiving 2-D shapes defined by non-motion signals including 'on' and 'off' transients, dynamic and static binocular disparity, and static texture cues. Our findings highlight the role of the visual association cortex in extracting salient information from noise.

Adult↗
PubMed1998

Veridical perception of global motion from disparate component motions.

Although it is in principle possible to determine the direction of motion of an object by combining the motion of its one-dimensional oriented contours (Fennema CL, Thompson WB. Comput. Graph. Image Processing 1979;9:301-315) there is still much debate on whether human observers can do so. The Intersection Of Constraint (IOC) rule proposed by Adelson and Movshon (Adelson EH, Movshon JA. Nature 1982;300:523-525), although compatible with the veridical object's motion, was challenged by recent psychophysical data obtained with type II plaids or lines moving behind apertures: perceived direction of motion is biased toward the vectorial average of the component motions, rather than in the direction predicted by the IOC rule. Since the velocity predicted by the vectorial rule is inconsistent with the physical velocity, its use leads to the puzzling prediction that the perceived position of a moving object becomes inconsistent with its actual position. In the present paper, the perceived path of a figure defined by its one-dimensional contours and moving behind apertures along a circular trajectory is compared with the discrepant predictions of the IOC and of the Vectorial model. The results show that the perceived path is close to veridical with these stimuli, therefore challenging the idea that the visual system uses a vector averaging rule.

Depth Perception↗
PubMed1995

Shape from shaded random surfaces.

The perception of surface relief from random shading patterns is measured by having observers adjust three-dimensional local probes, the projections of which are superimposed on the image. Three observers perform four settings of 91 probes on each of 14 images. These images are generated by calculating the Lambertian reflectance of a random superposition of elliptical Gaussian hills and valleys illuminated by a single distant light source as well as by ambient light. Neither the surface reflectance equation nor the light source direction is conveyed to our observers in any way. Mathematically, this "pure" shape-from-shading problem has highly non-unique solutions. Perception of a well-defined, stable shape therefore implies that the ambiguity is resolved, i.e. a gauge is fixed. We analyse the surface ambiguity or gauge freedom which is left unconstrained by pure shading information and we investigate possible ways of restricting it. Statistical analysis of the curl component of the field of probe settings reveals that the settings are significantly consistent with an underlying perceived surface. In spite of the large theoretical ambiguity in the stimuli, the settings are reproducible and show considerable inter-observer agreement. Even the correlation of the settings with the real surfaces is surprisingly large. If the settings are compared to the real surface normals, one finds a series of biases, the strongest of which is that the global surface slant is systematically underestimated, even in those cases where ending occluding contours or high-contrast luminance ridges, indicative of "almost" contours, are present in the image. Another bias then is that the corresponding rims on the surface are seen as roughly parallel to the picture plane.

Cues↗
PubMed1999

Human brain activity related to the perception of spatial features of objects.

The role of the parietal cortex in visuospatial analysis of object was investigated by cerebral blood flow measurements in seven objects using positron emission tomography. Data were acquired while subjects performed a matching task requiring the discrimination of simultaneously presented objects based on one of their spatial properties. Three properties were studied separately during three scanning conditions repeated twice:surface orientation, principal axis orientation, and size. Scans were also obtained during a sensorimotor control task (similar visual stimulation, same motor action, voluntary saccades toward each object) as well as during rest (no stimulation, eyes closed). Compared to rest, the three property matching tasks showed the same pattern of activation: the whole occipital lobe, the right intraparietal sulcus (IPS), and the right occipitotemporal (OT) junction. Compared to the control condition, only right IPS and OT junction were significantly activated during discrimination of the spatial properties. The IPS focus was located between the superior parietal lobule and the angular gyrus, and the OT activation overlapped the posterior part of the inferior temporal gyrus and the middle occipital gyrus. These results indicate that discrimination of spatial attributes requires the activation of both the parietal and the temporal cortices of the right hemisphere and provide further evidence that the IPS plays a critical role in visuospatial analysis of objects.

Adult↗
PubMed1986

A test of the minimum principle requires a perceptual coding system.

The minimum principle states that a perceiver will see the simplest possible interpretation of a pattern. Some theorists of human perception take this principle as a core explanatory concept. Others, especially Rock and Hochberg, hold the view that a perceptual minimum principle is untenable. Rock presents a great number of demonstrations which, in his opinion, rule out the minimum principle. Hochberg states that 'impossible' figures especially present a difficulty for this principle. It is argued here that, in order to test the minimum principle, a method is needed to describe interpretations of patterns in such a way that they can be ordered according to simplicity. To achieve this, Leeuwenberg's coding system was used. The analyses reported here of the patterns which Rock produces as evidence against the principle show that, contrary to Rock's claim, the way these patterns are preferentially perceived provides strong support for the minimum principle. Next, it is demonstrated that interpreting certain patterns as 'impossible' figures is not incompatible with the principle. Finally, it is argued that a test of the minimum principle is necessarily conflated with two other hypotheses, one concerning the metric of simplicity and one concerning the task conception of the experimental subjects.

Depth Perception↗
PubMed2002

Visual perception of collinearity.

In a metrical space, there exists an intimate relation between collinearity and parallelity. In particular, in a Riemannian space collinearity is just a special case of parallelity. Is this true for visual space as well? We investigated the visual perception of collinearity by having subjects align two bars in the horizontal plane at eye height. The distances of the bars from the subject and the angles at which they were placed were varied. We found deviations of up to 22 degrees. The deviations of the left and right bars could be split into two independent components: namely, the sum and the difference of the deviations of the left and right bars. We found that the former depended only on the ratio between the distances of each bar from the subject, whereas the latter was largely independent of the positions of the bars. The difference in deviations corresponded to the deviation from parallelity. Compared with the results in the parallelity task (Cuijpers, Kappers, & Koenderink, 2000b), the deviations from parallel were much smaller. As a consequence, the results of the two experiments cannot be described by the same Riemannian geometry. This indicates that the intrinsic geometry of visual space differs across tasks. This is conceivable if the intrinsic geometry of visual space is operationally defined.

Adult↗
PubMed1997

[Limits of perceptual robustness in perspective distortion].

Pictures often do not appear distorted even when viewed at oblique angles. Three hypotheses have been put forth to explain this robustness of virtual space toward affine transformations. First, array specificity holds that the perception of depicted space is fully specified by the information available at the point of observation. Second, the notion of a compensatory mechanism involves an unconscious recreation of the scene according to the original viewpoint. Third, the indiscrimination hypothesis denies the ability of the visual system to resolve or detect affine transformations up to a certain degree. Three experiments were conducted to investigate these claims. Using a double projection technique devised by Cutting (1987), Experiment I showed that observers are able to discriminate and compensate, to some degree, for affine transformations if information about the projection surface is available. However, observers relied on relative image velocities rather than reconstructing the object. In Experiment 2 additional observer motion was simulated. In single and double projection trials that required more difficult judgments of object rotation, compensation was poor and observers seemed to rely on local cues. Finally, real and simulated rotation of the projection surface revealed that observers are able to compensate for only one primary projection surface slant. The results reject the indiscrimination hypothesis and support the notion of array specificity.

Adult↗