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PubMed1999

Estimator reliability and distance scaling in stereoscopic slant perception.

When a horizontal or vertical magnifier is placed before one eye, a frontoparallel surface appears slanted. It appears slanted away from the eye with horizontal magnification (geometric effect) and toward the eye with vertical magnification (induced effect). According to current theory, the apparent slant in the geometric and induced effects should increase with viewing distance. The geometric effect does scale with distance, but there are conflicting reports as to whether the induced effect does. Ogle (1938 Archives of Ophthalmology 20 604-623) reported that settings in slant-nulling tasks increase systematically with viewing distance, but Gillam et al (1988 Perception & Psychophysics 44 473-483) and Rogers et al (1995 Perception 24 Supplement, 33) reported that settings in slant-estimation tasks do not. We re-examined this apparent contradiction. First, we conducted two experiments whose results are consistent with the literature and thus replicate the apparent contradiction. Next, we analyzed the signals available for stereoscopic slant perception and developed a general model of perceived slant. The model is based on the assumption that the visual system knows the reliability of various slant-estimation methods for the viewing situation under consideration. The model's behavior explains the contradiction in the literature. The model also predicts that, by manipulating eye position, apparent slant can be made to increase with distance for vertical, but not for horizontal, magnification. This prediction was confirmed experimentally.

Adult↗
PubMed2000

Getting the measure of vergence weight in nearness perception.

Combining multiple sources of information allows the human nervous system to construct an approximately Euclidean representation of near (personal) space. Within this space, binocular vergence is an important source of egocentric distance information. We investigated how the nervous system determines the significance (weight) accorded to vergence information when other (retinal) distance cues are present. We found that weight decreases with (1) increasing discrepancy between vergence information and other cues and (2) reduced vergence demand. The results also provided evidence that the nervous system represents vergence related distance information in units of nearness (the reciprocal of distance).

Adolescent↗
PubMed1997

Types of size disparity and the perception of surface slant.

We examined (i) perceived slant of a textured surface about a vertical axis as a function of disparity magnitude for horizontal-size disparity, vertical-size disparity, and overall-size disparity; and (ii) interactions between patterns with various types and magnitudes of size disparity and superimposed or adjacent zero-disparity stimuli. Horizontal-size disparity produced slant which increased with increasing disparity, was enhanced by superimposed zero-disparity stimuli, and induced contrasting slant in superimposed or adjacent zero-disparity stimuli. Vertical-size disparity produced opposite slant (induced effect) which was reduced to near zero by a superimposed zero-disparity pattern and both patterns appeared as one surface. Adjacent vertical-size-disparity and zero-disparity patterns appeared as separate surfaces with a wide curved boundary. Overall-size disparity produced slant which was enhanced by a superimposed zero-disparity pattern and less so by a zero-disparity line, and induced more slant in a zero-disparity line than in a zero-disparity pattern. The results are discussed in terms of depth underestimation of isolated surfaces, depth enhancement, depth contrast, and the processing of deformation disparity.

Adult↗
PubMed1979

A model for processing of movement in the visual system.

Processing of spatio-temporal information in the human visual system has been investigated thoroughly during the past decade, but is still far from being properly understood. Moreover, the theory of separation of information by means of sustained and transient channels already at the retinal level is not satisfactory, as experimental results indicate that these two types of channels span a continuum of temporal characteristics. It is however obvious, that the process of pattern recognition and velocity perception calls for their separation at some level of the hierarchy. In this communication, we extend our model of three-dimensional spatio-temporal frequency expansion in the visual system (Gafni and Zeevi, 1977) to show how velocity-information extraction channels, sensitive to direction and velocity exclusively, can be formed by simple summation of signals from well-defined sets of channels representing points in the frequency space. Correspondence of these channels to characteristics of the cortical neurons is discussed.

Depth Perception↗
PubMed1999

Using motor tasks to quantitatively judge 3-D surface curvatures.

The primary objective of this study was to quantitatively investigate the human perception of surface curvature by using virtual surfaces and motor tasks along with data analysis methods to estimate surface curvature from drawing movements. Three psychophysical experiments were conducted. In Experiment 1, we looked at subjects' sensitivity to the curvature of a curve lying on a surface and changes in the curvature as defined by Euler's formula, which relates maximum and minimum principal curvatures and their directions. Regardless of direction and surface shape (elliptic and hyperbolic), subjects could report the curvature of a curve lying on a surface through a drawing task. In addition, multiple curves drawn by subjects were used to reconstruct the surface. These reconstructed surfaces could be better accounted for by analysis that treated the drawing data as a set of curvatures rather than as a set of depths. A pointing task was utilized in Experiment 2, and subjects could report principal curvature directions of a surface rather precisely and consistently when the difference between principal curvatures was sufficiently large, but performance was poor for the direction of zero curvature (asymptotic direction) on a hyperbolic surface. In Experiment 3, it was discovered that sensitivity to the sign of curvature was different for perceptual judgments and motor responses, and there was also a difference for that of a curve itself and the same curve embedded in a surface. These findings suggest that humans are sensitive to relative changes in curvature and are able to comprehend quantitative surface curvature for some motor tasks.

Adult↗
PubMed1981

Use of foreground and background information in visually guided locomotion.

The effects of reducing the range of spatial perception on the accuracy of visually guided locomotion were studied in two experiments. Limiting the range of perception to only near objects produces changes in the flow of stimulus detail and reduces opportunities for the appearance of an aiming point and for motion parallax. Such conditions were found to produce inferior performance compared to full vision, or to minimal background information. A defined aiming point was also found to assist control when no other background was present. The results are discussed with reference to theories of locomotor control and the design of artificial spatial sensing aids for the blind.

Depth Perception↗
PubMed2006

Preservation of Emmert's law in a visual form agnosic.

Size constancy was investigated in DF, a patient with visual form agnosia, using a technique based on Emmert's law of visual after-images. DF was first given a task in which she was asked to indicate the distance of a vertical surface and a task where she had to estimate the width of a series of squares (widths ranging from 5 cm to 35 cm) placed at varying distances and having a constant visual angle. In the distance estimation task, DF greatly overestimated the distance of the vertical surface placed in front of her. DF also had great difficulty performing the size estimation task. DF then performed a task in which she stared at a bright 5 cm square for a brief period of time at a distance of 30 cm followed by the presentation of a vertical surface which varied in distance and was asked to indicate the width of the after-image either verbally or manually. DF's after-images conformed to the size-distance relationship predicted by Emmert's law--as the distance of the vertical surface increased her perception of the size of the after-images also increased. These data demonstrate that although DF is rather impaired in tasks that require explicit estimates of size and distance, at some level, DF must have relatively intact size constancy mechanisms given that her estimates of the width of the after-image conform to Emmert's law. Thus, the processes underlying explicit judgements of size and distance appear to differ from those underlying the size and distance scaling of after-images.

Adult↗
PubMed1995

The contribution of stimulus attributes of three-dimensional solid forms and level of discrimination to the visual and haptic percept of balance.

Design-trained subjects sorted fourteen solid forms under either visual or haptic conditions into groups on the basis of perceived similarity of balance or along a balance continuum. After these tasks of coarse and fine discrimination, each form was rated on ten bipolar stimulus attributes. Multidimensional scaling and property-fitting analyses were performed on the results of four experiments to determine the combined contribution of stimulus attributes and level of discrimination on the visual and haptic percept of balance. A balance dimension emerged for both modalities, but only when subjects were inclined to attend globally to the structure of a form by the coarse-discrimination task. Results indicate that visual balance is a holistic property of forms which derives from the synthesis of physical stimulus information. For touch, subjects appear to have equated balance with a symmetric distribution of weight/shape about the central axis of a form. Findings are related to theoretical notions of balance.

Adult↗
PubMed1992

Visual resolution of motion ambiguity with periodic luminance- and contrast-domain stimuli.

Visual motion processes were studied with luminance- and contrast-modulated gratings. A sine-wave luminance grating was displaced abruptly back and forth by 3/16 cycle. The display sequence is ambiguous in that each 3/16-cycle phase shift (short-path motion) could just as readily be seen as a 13/16-cycle shift (long-path motion) in the opposite direction. By varying the duration of the interval (IFI) between the two phase positions, the luminance of the IFI, or the spatial frequency of the grating, it was possible to bias the ambiguous percept in favor of short-path motions or long-path motions. A contrast-modulated grating displaced through 3/16 cycle always appeared t undergo short-path motion. Current motion models which incorporate Reichardt-type/energy mechanisms and certain types of auxiliary signal transformations which precede those mechanisms do not adequately explain the effects of IFI intensity on the perceived motion of a sinusoidal grating or the effect of IFI duration on the perceived motion of a contrast-modulated grating.

Contrast Sensitivity↗
PubMed2001

Perceived structure from optic flow: consistent versus variable mapping of 3-D Euclidean structure.

In an earlier study (Börjesson & Lind, 1996), the perception of Euclidean structure from polar projected two-frame apparent motion sequences was studied. The results showed that Euclidean structure is not perceived. However, at larger visual angles a certain consistency in the mapping between distal and perceived structure exists. The aim of the present study was to more precisely examine how this degree of consistency varies as a function of visual angle. In Experiments 1 and 2, slant judgments of simulated and real planes indicated that the degree of consistency is a positive function of visual angle. No definite sign of a Euclidean mapping could, however, be found even in the full view condition. Experiment 3 examined texture gradients and the response method used. The results showed that texture gradients did not influence the degree of consistency of the mapping between distal and judged depth and that the response method was both reliable and valid. However, texture gradients did influence the absolute values of the slant judgments. The role of Euclidean and affine mappings of distal structure is discussed and it is proposed that the perceptually important distinction is not between affine and Euclidean mapping, but rather between two types of affine mappings--consistent and variable.

Analysis of Variance↗
PubMed1994

Objective evaluation of human and computational stereoscopic visual systems.

Several computational models have been proposed to account for the abilities of human observers in solving the stereo correspondence problem. Such models are often evaluated by counting the number of correct matches that the model makes when presented with a test stimulus, but this measure is difficult to compare with conventional psychophysical measures of human stereo performance. This paper develops an absolute comparison of human and model stereo correspondence by using the concept of statistical efficiency, through which the performance of both humans and models is compared against that of an ideal observer. Some important assumptions need to be examined before the efficiencies of human and model can be compared because these assumptions affect the statistical performance of models. One assumption involves the use of an epipolar constraint to help constrain the matching problem but it is unclear what the resolution (or "height") of an epipolar line should be. Experimental estimates were obtained of the vertical region over which dot position may be varied without disruption of human stereoacuity. These estimates constrained the effective "height" of the zone above and below an epipolar line. Two models of stereopsis were then compared against human performance: the first (due to Pollard, Mayhew & Frisby, Perception, 14, 449-470, 1985) uses local constraints to solve the correspondence problem; the second is a simple area-based correlation model. For nearly all the stimuli tested, both models show strong similarities with human observers. For one particular stimulus, the efficiency of the correlation model is different from that of humans and of the PMF algorithm. This implies that a more complex process than simple correlation may be required to account for human stereo processing and indicates other limitations of correlation models.

Algorithms↗
PubMed1978

The Penrose triangle and a family of related figures.

Starting from a study of the Penrose triangle, the theory is advanced that our perception of the relative spatial positions of parts of a figure is mediated by a set of significant directions (or axes). Illustration and exploration of the theory in this paper revolves around examples drawn mostly from a family of figures related to the Penrose triangle. This family is defined partly in terms of the common nature of its members' anomalies, and partly by the feature that they are seen as configurations of 'beams'. This feature and the conditions for it are also examined by the use of examples.

Depth Perception↗
PubMed1982

A nonspecific learning effect in the perception of random-dot stereograms?

An experiment is reported the object of which was to check whether a small amount of nonspecific experience in perceiving random-dot stereograms could facilitate the perception of a previously unseen stereogram. The mean stereopsis perception time of a group of totally naive subjects was found to be significantly slower than that of a group who had previously been shown two different stereograms. Closer inspection of the data showed that this difference was primarily due to approximately one third of the naive group who were much slower than the 'experienced' group. It is therefore suggested that nonspecific experience provides most initial help for relatively slow perceivers, since many naive subjects can perform as well as those with prior experience of other stereograms.

Depth Perception↗
PubMed2004

Rapid processing of cast and attached shadows.

We used a visual-search method to investigate the role of shadows in the rapid discrimination of scene properties. Targets and distractors were light or dark 2-D crescents of identical shape and size, on a mid-grey background. From the dark stimuli, illusory 3-D shapes can be created by blurring one arc of the crescent. If the inner arc is blurred, the stimulus is perceived as a curved surface with attached shadow. If the outer arc is blurred, the stimulus is perceived as a flat surface casting a shadow. In a series of five experiments, we used this simple stimulus to map out the shadow properties that the human visual system can rapidly detect and discriminate. To subtract out 2-D image factors, we compared search performance for dark-shadow stimuli with performance for light-shadow stimuli which generally do not elicit strong 3-D percepts. We found that the human visual system is capable of rapid discrimination based upon a number of different shadow properties, including the type of the shadow (cast or attached), the direction of the shadow, and the displacement of the shadow. While it is clear that shadows are not simply discounted in rapid search, it is unclear at this stage whether rapid discrimination is acting upon shadows per se or upon representations of 3-D object shape and position elicited by perceived shadows.

Depth Perception↗
PubMed1996

Perception of local three-dimensional shape.

The authors present a series of 4 experiments designed to test the ability to perceive local shape information. Observers were presented with various smoothly varying 3-dimensional surfaces where they reported shape index and sign of Gaussian curvature at several probe locations. Results show that observers are poor at making judgments based on these local measures, especially when the region surrounding the local point is restricted or manipulated to make it noncoherent. Shape index judgments required at least 2 degrees of context surrounding the probe location, and performance on sign of Gaussian curvature judgments deteriorated as the contextual information was restricted as well.

Attention↗
PubMed1978

An additional dimension to grating perception.

Visual acuity (A) for a two-dimensional multiplicative sinusoidal contrast grid (sinusoidal chessboard) was measured as a function of spatial frequency (omega) of modulation in one of the dimensions. The reduction in acuity as frequency was increased was well described by the equation: A = -2pikomega + c, where k and c are constants. At suprathreshold contrasts, the frequency of modulation appeared to be doubled relative to the underlying modulation frequency. This doubling is not related to the frequency of the harmonic components of the chessboard, but suggests the existence of a perceptual mechanism sensitive to areas of sinusoidal modulation which are seen at twice the frequency of the variation in contrast.

Depth Perception↗
PubMed1988

A new visual problem: phenomenic folding.

It is possible to produce outline drawings that are perceived as representations of sheets or plates folded over themselves. However, only some of the many possible representations are immediately and necessarily perceived as such. Investigations were carried out to find out which elements must be included in a drawing if a subject is to perceive folding. Four necessary, though not individually sufficient, factors were detected. Other factors which are not necessary but which can intensify the perception of folding were also found. The four necessary factors are: (i) the existence of two phenomenically overlapping figures; (ii) at least one side of the upper figure must perfectly coincide with one side of the lower figure, this common side being defined as the folding line; (iii) the two phenomenically overlapping areas must be on the same side of the folding line; (iv) three segments must converge at the ends of the folding line. Some cognitive processes which appear to be involved in the phenomenon are also discussed.

Adolescent↗
PubMedDate not supplied

Sensory deviations subsequent to senile cataract.

PURPOSE: To evaluate the effect of extracapsular cataract extraction with posterior chamber intraocular lens (IOL) implantation in patients with sensory deviations subsequent to senile cataract. METHODS: Twenty patients with dense cataract and associated sensory deviations underwent follow-up between April 1996 and April 1998 after extracapsular cataract extraction with posterior chamber IOL implantation in the deviating eye. All patients underwent follow-up for a minimum period of 6 months. RESULTS: Preoperatively, all patients had a visual acuity of PL positive (perception of light present) and PR (projection of rays) accurate in the deviating eye. Two patients had an esodeviation, and 18 patients had an exodeviation. Postoperatively, all patients had a corrected visual acuity of 20/40 or better in the operated eye at 12 weeks. Nineteen patients had ocular alignment within +/-8 prism diopters of orthophoria at 12 weeks. CONCLUSION: Sensory deviations subsequent to senile cataract usually resolve spontaneously after cataract surgery if visual gains are 20/40 or better. The prognosis for binocular vision is good.

Cataract↗