Biomedical subjects

G H Silverman

Publications and source records attributed to G H Silverman.

10 recordsLinked to original sources

PubMed1989

Occlusion and the solution to the aperture problem for motion.

The "aperture problem" indicates that a local reading of the velocity of an oriented contour is inherently ambiguous, insufficient by itself to recover the velocity of image points. In Wallach's "barber pole" display consisting of moving diagonal lines within an elongated rectangular aperture, it has been suggested that the unambiguous motion of edge-terminators along the longer edges of the aperture propagates towards the motion-ambiguous center part of drifting stripes. This results in the perception of a surface moving in the direction of the longer axis of the aperture. By manipulating the stereoscopic disparity of a striped pattern relative to the aperture plane, we found that the disambiguating effects of terminators could be abolished if the striped pattern was in uncrossed disparity relative to the aperture plane. Also, the motion in 3 separate horizontally oriented, and vertically aligned apertures which would otherwise be seen as moving horizontally, was seen as "linked" together and moving vertically. This occurred only when the horizontally oriented segments separating these apertures were stereoscopically coded so that they appeared as occluders in front. These findings suggest that accidental or "extrinsic" terminators created by occluding edges are treated differently from real or "intrinsic" terminators, and that the real-world constraint of occlusion is thus implemented in the ambiguity-solving processes for motion.

Depth Perception↗

PubMed1989

Stereoscopic depth: its relation to image segmentation, grouping, and the recognition of occluded objects.

Image regions corresponding to partially hidden objects are enclosed by two types of bounding contour: those inherent to the object itself (intrinsic) and those defined by occlusion (extrinsic). Intrinsic contours provide useful information regarding object shape, whereas extrinsic contours vary arbitrarily depending on accidental spatial relationships in scenes. Because extrinsic contours can only degrade the process of surface description and object recognition, it is argued that they must be removed prior to a stage of template matching. This implies that the two types of contour must be distinguished relatively early in visual processing and we hypothesize that the encoding of depth is critical for this task. The common border is attached to and regarded as intrinsic to the closer region, and detached from and regarded as extrinsic to the farther region. We also suggest that intrinsic borders aid in the segmentation of image regions and thus prevent grouping, whereas extrinsic borders provide a linkage to other extrinsic borders and facilitate grouping. Support for these views is found in a series of demonstrations, and also in an experiment where the expected superiority of recognition was found when partially sampled faces were seen in a back rather than a front stereoscopic depth plane.

Attention↗

PubMed1988

An occlusion-related mechanism of depth perception based on motion and interocular sequence.

Objects occlude other objects in natural scenes, and this occlusive relationship increases the spatio-temporal complexity of sensory inputs to the two eyes, especially when objects are moving. We ask whether the visual system can employ clever strategies which make use of real-world constraints on inputs to the eyes to determine the depth of objects. Employing psychophysical methods, we found that occlusion-related geometric rules, which constrain the relationship between the direction of motion and the order and asynchrony of eyes, are implemented at early stages of cortical visual processing.

Depth Perception↗

PubMed1988

The aperture problem--I. Perception of nonrigidity and motion direction in translating sinusoidal lines.

To examine how local velocities from different regions of the visual field combine to form a coherent motion percept, we subjected a sinusoidal line stimulus to translational motion. Horizontal movement of a sinewave line along its axial direction is perceived as nonrigid if the angle at the zero crossing is smaller than a critical angle of about 15 deg. This angle is independent of spatial scale and the number of sinusoidal cycles. To extend the applicability of this concept of angle, we developed a mathematical model to predict an observer's sensitivity to small changes in motion direction based on two assumptions: (1) the computed velocity signal is obtained from the intersection of constraint lines defined by local velocity components, (2) local velocity components are contaminated by noise. Measurement of directional discrimination thresholds of moving targets confirmed our expectations. Thresholds varied as a function of the angle of the local contour independent of spatial scale and in quantitative accord with our assumptions.

Discrimination, Psychological↗

PubMed1988

The aperture problem--II. Spatial integration of velocity information along contours.

There exists a class of two-dimensional figures (including cumulative gaussian waveforms) whose contours have a limited range of orientations. These figures can appear as highly nonrigid if they undergo pure translation in the image plane. In the case of the cumulative gaussian waveform, it is the region around the inflection point that appears as nonrigid. Motivated by Hildreth's (1984) proposal, we made 5 predictions which were confirmed by the data: (0), rigidity of a figure can be dramatically increased if one attaches line terminators to the figure; (1), moving terminators "on" the figure increase rigidity far more than such terminators "off" the figure; (2) decreasing the velocity of the terminator decreases rigidity; (3) decreasing the distance between the terminator and the inflection point increases rigidity; (4) the effect of a moving terminator can be blocked by interposing a stationary terminator between it and a nonrigidly moving portion of the curve.

Humans↗

PubMed1986

Precise velocity discrimination despite random variations in temporal frequency and contrast.

Velocity discrimination is not affected by random changes in contrast or temporal frequency. Observers judged the relative velocity of a moving sinusoidal grating when target contrast was varied randomly from trial-to-trial over the range from 5 to 82%. The Weber fraction for the random mixture of interspersed contrast levels was about 0.06, comparable to velocity discrimination for targets presented at a fixed contrast. In a parallel experiment, the spatial frequency of the target was changed randomly from trial-to-trial, a procedure which produced concomitant random changes in the nominal temporal frequency. These variations had little effect on the velocity increment threshold; random changes in temporal frequency ranging from 2.25 to 8.25 Hz increased the Weber fraction from 0.05 to 0.07. Under identical experimental conditions, velocity discrimination was generally more precise than the discrimination of differences in temporal frequency, particularly when temporal frequency thresholds were measured with counterphase gratings. Our results indicate that velocity discrimination depends on velocity.

Discrimination, Psychological↗

PubMed1985

Sensitivity to shearing and compressive motion in random dots.

The sensitivity of the visual system to motion of differentially moving random dots was measured. Two kinds of one-dimensional motion were compared: standing-wave patterns where dot movement amplitude varied as a sinusoidal function of position along the axis of dot movement (longitudinal or compressional waves) and patterns of motion where dot movement amplitude varied as a sinusoidal function orthogonal to the axis of motion (transverse or shearing waves). Spatial frequency, temporal frequency, and orientation of the motion were varied. The major finding was a much larger threshold rise for shear than for compression when motion spatial frequency increased beyond 1 cycle deg-1. Control experiments ruled out the extraneous cues of local luminance or local dot density. No conspicuous low spatial-frequency rise in thresholds for any type of differential motion was seen at the lowest spatial frequencies tested, and no difference was seen between horizontal and vertical motion. The results suggest that at the motion threshold spatial integration is greatest in a direction orthogonal to the direction of motion, a view consistent with elongated receptive fields most sensitive to motion orthogonal to their major axis.

Humans↗

PubMed1985

Detection and discrimination of sinusoidal grating displacements.

Vertical sine-wave gratings of varying spatial frequency were stepped instantaneously to the right or to the left at differing phase angles (theta). Separate paradigms measured the contrast threshold for the detection of such a step and for the discrimination of the direction of the same step. By considering the grating before and after its displacement as a rotating phasor, we made the following predictions: (1) Contrast sensitivity for the detection of a displacement should rise as sin(theta). (2) Contrast sensitivity for the discrimination of the direction of the displacement should rise as sin(theta/2). Both predictions were confirmed using a range of spatial frequencies and phase angles. From the results of additional experiments, by measuring the discrimination of the direction thresholds as a function of contrast, we derived a nonlinear contrast response function for the motion system. This function appears to saturate fully at fairly low levels, in the neighborhood of 2 to 3% under the conditions examined. Our results suggest a direct connection among the contrast sensitivity, the contrast response function, and motion-hyperacuity thresholds.

Differential Threshold↗

PubMed1984

Temporal and spatial characteristics of the upper displacement limit for motion in random dots.

The upper displacement limit (Dmax) or Braddick limit was measured in random dots. We then interposed a variable duration pause at half the distance jumped. Of interest was to see the shortest time (delta t) which would yield a value of two times Dmax, thus indicating the time required to process one additional Braddick limit. A surprisingly short interval (as short as 10 msec under some conditions) was required. Furthermore for intermediate durations (20 to over 100 msec), the total limit was often more than doubled, indicating a nonlinear potentiation or sequential recruitment between successive displacements. Increasing field size had the following effects: (1) it increased Dmax, (2) it decreased delta t (the processing time), (3) it increased Vmax (the maximum velocity), (4) it increased the amount of sequential recruitment.

Humans↗

PubMedDate not supplied

Serial and parallel processing of visual feature conjunctions.

Treisman and others have reported that the visual search for a target distinguished along a single stimulus dimension (for example, colour or shape) is conducted in parallel, whereas the search for an item defined by the conjunction of two stimulus dimensions is conducted serially. For a single dimension the target 'pops out' and the search time is independent of the number of irrelevant items in the set. For conjunctions, the search time increases as the set becomes larger. Thus, it seems that the visual system is incapable of conducting a parallel search over two stimulus dimensions simultaneously. Here we extend this conclusion for the conjunction of motion and colour, showing that it requires a serial search. We also report two exceptions: if one of the dimensions in a conjunctive search is stereoscopic disparity, a second dimension of either colour or motion can be searched in parallel.

Color↗