Biomedical subjects

Benjamin T Backus

Publications and source records attributed to Benjamin T Backus.

8 recordsLinked to original sources

PubMed2006

The accuracy and reliability of perceived depth from linear perspective as a function of image size.

We investigated the ability to use linear perspective to perceive depth from monocular images. Specifically, we focused on the information provided by convergence of parallel lines in an image due to perspective projection. Our stimuli were trapezoid-shaped projected contours, which appear as rectangles slanted in depth. If converging edges of a contour are assumed to be parallel edges of a 3D object, then it is possible in principle to recover its 3D orientation and relative dimensions. This 3D interpretation depends on projected size; hence, if an image contour were scaled, accurate use of perspective predicts changes in perceived slant and shape. We tested this prediction and measured the accuracy and precision with which observers can judge depth from perspective alone. Observers viewed monocular images of slanted rectangles and judged whether the rectangles appeared longer versus wider than a square. The projected contours had varying widths (7, 14, or 21 deg) and side angles (7 or 25 deg), and heights were varied by a staircase procedure to compute a point of subjective equality and 75% threshold for each condition. Observers were able to reliably judge aspect ratios from the monocular images: Weber fractions were 6-9% for the largest rectangles, increasing to as high as 17% for small rectangles with high simulated slant. Overall, the contours judged to be squares were taller than the projections of actual squares, consistent with perceptual underestimation of depth. Judgments were modulated by image size in the direction expected from perspective geometry, but the effect of size was only about 20-30% of what was predicted. We simulated the performance of a Bayesian ideal observer that integrated perspective information with an a priori bias toward compression of depth and which was able to qualitatively model the pattern of results.

Cues↗

PubMed2006

Perception of surface slant from oriented textures.

When a surface covered with a regular texture is viewed in perspective, the projected texture provides a number of cues to 3D surface orientation. For oriented textures, one cue is perspective convergence: symmetry lines that are parallel along the surface project to lines that vary systematically in orientation. We investigated the contribution of perspective convergence to perception of 3D slant and tested whether slant from convergence depends on oriented spectral components. Subjects judged the sign of slant about a vertical axis of rotation. Textures were composed of filled circles in three spatial arrangements: a hex grid with symmetry lines at 0 and +/-60 deg relative to the tilt direction (aligned condition), a hex grid with symmetry lines at 90 and +/-30 deg (perpendicular condition), and random arrangements with similar average spacing (isotropic condition). The two hex grid textures differed in the amount of spectral energy present in the tilt direction (horizontal) but were otherwise closely matched. Slant discrimination thresholds for monocular stimuli were higher for isotropic textures than for either of the two hex grid textures and were higher for the perpendicular texture than for the aligned texture. In a second experiment, we measured the weight given to texture relative to binocular slant information for cue conflict stimuli (+/-5 deg). Weights were found to agree with individual subjects' monocular thresholds, in accordance with optimal estimation theory. We conclude that the visual system uses perspective convergence to perceive slant and that effective use of convergence requires the presence of spectral components aligned with the tilt direction.

Cues↗

PubMed2005

Demonstration of cue recruitment: change in visual appearance by means of Pavlovian conditioning.

Until half a century ago, associative learning played a fundamental role in theories of perceptual appearance [Berkeley, G. (1709) An Essay Towards a New Theory of Vision (Dublin), 1st Ed.]. But starting in 1955 [Gibson, J. J. & Gibson, E. J. (1955) Psychol. Rev. 62, 32-41], most studies of perceptual learning have not been concerned with association or appearance but rather with improvements in discrimination ability. Here we describe a "cue recruitment" experiment, which is a straightforward adaptation of Pavlov's classical conditioning experiment, that we used to measure changes in visual appearance caused by exposure to novel pairings of signals in visual stimuli. Trainees viewed movies of a rotating wire-frame (Necker) cube. This stimulus is perceptually bistable. On training trials, depth cues (stereo and occlusion) were added to force the perceived direction of rotation. Critically, an additional signal was also added, contingent on rotation direction. Stimuli on test trials contained the new signal but not the depth cues. Over 45 min, two of the three new signals that we tested acquired the ability to bias perceived rotation direction on their own. Results were consistent across the eight trainees in each experiment, and the new cue's effectiveness was long lasting. Whereas most adaptation aftereffects on appearance are opposite in direction to the training stimuli, these effects were positive. An individual new signal can be recruited by the visual system as a cue for the construction of visual appearance. Cue recruitment experiments may prove useful for reexamining of the role of experience in perception.

Conditioning, Classical↗

PubMed2005

Illusory motion from change over time in the response to contrast and luminance.

A striking illusion of motion is generated by static repeated asymmetric patterns (RAPs) such as Kitaoka's (2003) "Rotating Snakes" and Fraser and Wilcox's (1979) peripheral drift illusion. How do RAPs generate spurious motion signals, and what critical difference between RAPs and natural static scenes prevents the latter from appearing to move? Small involuntary eye movements during fixation have been suspected to play a critical role in these illusions, but here we give an account that does not depend on fixation jitter. We propose that these illusions result primarily from fast and slow changes over time in the neuronal representation of contrast ("contrast-driven RAPs") or luminance ("luminance-driven RAPs"). We show that temporal phase advance in the neural response at high contrast can account for the early, fast motion in contrast-driven RAPs (such as "Rotating Snakes") after each fixation change. An essential part of this explanation is that motion detectors fail to compensate for the dynamics of neuronal encoding. We argue that static natural patterns also generate local gain changes, but that these signals do not often trigger illusory motion because they are not usually aligned to drive global motion detectors. Movies in which real luminance changes over time, to mimic the proposed neuronal adaptations to contrast and luminance, evoke qualitatively similar percepts of motion. Experimental data are consistent with the explanation. Color and overall contrast both enhance the illusion.

Contrast Sensitivity↗

PubMed2005

Depth aftereffects mediated by vertical disparities: evidence for vertical disparity driven calibration of extraretinal signals during stereopsis.

Perceptual adaptation often results in a repulsive aftereffect: stimuli are seen as biased away from the adaptation stimulus (). Here we report the absence of a repulsive aftereffect for a vertical gradient of vertical disparity (or vertical size ratio, VSR). We exposed observers to a binocular stimulus consisting of horizontal lines. This stimulus contains vertical, but not horizontal disparities. The visual system was able to measure the VSR of this stimulus: although the lines themselves always appeared unslanted, the VSR carried by the lines had a dramatic effect on the apparent slant of a horizontal row of dots, as predicted by recent accounts of Ogle's (1938) induced effect (e.g., Backus, Banks, van Ee, & Crowell, 1999). Yet we observed no repulsive aftereffect for the VSR signal: after adaptation to horizontal lines that were vertically larger in one eye, we found an attractive aftereffect, the magnitude of which was largest in stimuli that did not contain a VSR signal. We interpret these results as a case of recalibration: disagreement between extra-retinal eye position signals (EP) and VSR causes a recalibration in the use of EP as used in the stereoscopic perception of slant.

Adaptation, Physiological↗

PubMed2004

Starry night: a texture devoid of depth cues.

From a modern Bayesian point of view, the classic Julesz random-dot stereogram is a cue-conflict stimulus: Texture cues specify an unbroken, unslanted surface, in conflict with any variation in depth specified by binocular disparity. We introduce a new visual texture-the starry night texture (SNT)--that is incapable of conveying slant, depth edges, or texture boundaries, in a single view. For SNT, changing density is equivalent to changing intensity, so an instance of the texture is characterized (up to the random locations of the texture elements) by what we call its densintensity. We consider deviations from the ideal that are needed to realize the texture in practice. In three experiments with computer-generated stimuli we examined human perception of SNT to show that (1) the deviations from the ideal that were needed to realize SNT do not affect the invariance of its appearance across changes in distance of several orders of magnitude; (2) as predicted, observers match SNT across changes in distance better than other textures; and (3) the use of SNT in a slant perception experiment did not reliably increase observers' reliance on stereoscopic slant cues, as compared with the sparse random-dot displays that have been commonly employed to study human perception of shape from binocular disparity and motion.

Journal Article↗

PubMed2003

The contribution of vergence change to the measurement of relative disparity.

The relative disparity between two objects in a scene can in principle be measured directly from the retinal images, without knowledge of eye position. But relative disparity increment thresholds are lowest when the relative disparity is small and the objects are not widely separated in the visual field: thus, some relative disparities are easier for the visual system to measure than others. We consider, after others, a second method by which the visual system could measure relative disparity, based on change in vergence ("delta vergence" or DV). The DV mechanism could be more reliable than the retinal mechanism when visual targets are widely separated in visual direction or depth. We used a cue-conflict paradigm to measure the extent to which perceived depth depends on DV. As target separation increased, so did reliance on DV. As intertarget disparity increased, reliance on DV increased for one observer but not for two others.

Convergence, Ocular↗

PubMed2002

Is vertical disparity used to determine azimuth?

The azimuth of a stimulus relative to the head can be determined from an extra-retinal, eye-position signal plus an estimate of the retinal eccentricity of the image. Alternatively, azimuth could be determined from retinal-image information alone. Specifically, stimulus azimuth could be estimated from two derivatives of vertical disparity: vertical size ratio (which varies with azimuth), and the horizontal gradient of vertical size ratio (a measure of distance). Here we examine the determinants of perceived azimuth in viewing conditions that, theoretically, should favor the use of vertical disparity. We find no evidence that vertical disparity is used. Perceived azimuth was determined completely by felt eye position and the retinal eccentricity of the image.

Depth Perception↗