Biomedical subjects

R F Hess

Publications and source records attributed to R F Hess.

At least 55 records · Page 3Linked to original sources

PubMed1999

Contour integration and scale combination processes in visual edge detection.

Contours in the natural visual environment consist mainly of edges which are spatially broad-band and whose (cosinusoidal) components have arrival phases close to +/-90 deg. Because early visual processing is thought to be based on a local Fourier description, the representation of edges requires two forms of filter combination: scale integration (filter combination across spatial frequency) and contour integration (filter combination across space). In order to determine how these two types of combination fit together, we determined spatial-frequency tuning for the detection of contours composed of broadband edge elements, alternating with narrow-band Gabor elements. A contour integration system operating independently at a number of spatial scales should be able to ignore the distracting influence of edge structure in such patterns. However, subjects cannot ignore edge structure indicating that local phase-alignment across spatial scale is coded prior to, or concurrent with, contour integration. Moreover, unlike contours composed of Gabors, the bandwidth of local elements is important for edge integration; the coding of element bandwidth seems to be dependent on the phase alignment of features across spatial frequency.

Computer Simulation↗

PubMed1999

Comparison of motion and stereopsis: linear and nonlinear performance.

To address the issue of whether the luminance-dependent (linear) and contrast-dependent (nonlinear) processes in stereo and motion have a common computational basis, we compare both carrier-dependent and envelope-dependent performance for these two modalities by using the same stimulus and task: two-flash apparent motion/depth for a wide range of displacements. We do this for different densities, bandwidths, contrasts, spatial frequencies, and exposure durations. The results suggest that there is concordance not only between the luminance-dependent (linear) processes of motion and stereo but also between the envelope-dependent (nonlinear) processes of both modalities. Only one exception was found, but we show this to be amenable to an explanation based on a different contrast dependence for the nonlinear mechanisms of stereo and motion. This suggests that the computational basis of linear and nonlinear processes may be similar for stereopsis and motion.

Depth Perception↗

PubMed1999

Temporal detection in human vision: dependence on spatial frequency.

In the study of perception of temporal changes in luminance, it is customary to model perceptual performance as based on one or more linear filters. The task is then to estimate the temporal impulse responses or the representation of the impulse response in the frequency domain. Previously, temporal masking data have been used to estimate the properties and numbers of these temporal mechanisms (filters) in central vision for 1-cycle-per-degree (cpd) targets [Vision Res. 38, 1023 (1998)]. The same methods have been used to explore how properties of the estimated filters change with stimulus contrast energy [J. Opt. Soc. Am. A 14, 2557 (1997)]. We present estimated properties for temporal mechanisms that detect low spatial-frequency patterns. The results indicate that two filters provide the best model for performance when mask contrast is significant. There are also differences between properties for mechanisms that detect signal spatial frequencies of 1 cpd and 1/3 cpd. The sensitivity of the low-pass mechanism relative to the bandpass mechanism is reduced at 1/3 cpd, consistent with previous findings.

Artifacts↗

PubMed1998

Is human motion detection subserved by a single or multiple channel mechanism?

Two recent versions of a single channel model of motion perception have had impressive success in explaining direction discrimination by human observers for spatially filtered noise images in two-flash apparent motion. It has been argued that the dramatic breakdown in motion perception which occurs when one image in the two-flash sequence is low-pass filtered can be explained only by a single channel model. We show that neither version of the single channel model which has been proposed can explain performance for noise images chosen to provide comparable stimulation in the spatial channels known to subserve human vision. A multi-channel model of motion perception has little difficulty in explaining these results.

Discrimination, Psychological↗

PubMed1998

Contour integration in anisometropic amblyopia.

Contour integration was measured in a group of anisometropic amblyopes to test the idea recently put forward that positional uncertainty sets a fundamental limit to contour integration in amblyopia. Anisometropic amblyopia, unlike strabismic amblyopia, has little or no positional uncertainty once the initial filtering loss has been taken into account. Therefore, according to the explanation put forward to explain strabismic amblyopia, anisometropes should exhibit normal contour integration. We show that this expectation is realized for five of our six anisometropic amblyopes.

Adult↗

PubMed1998

Orientation masks 3-Gabor alignment performance.

Several workers have concluded that Gabor alignment tasks are performed by using central tendencies of the micropatterns as a cue. One reason for this conclusion was that the 3-Gabor alignment task is performed equally well whether the orientations of the patches are collinear or orthogonal to the group orientation. We wished to find out if the orientation of the micropatterns has any effect on performance. We tested subjects in 3-micropattern alignment tasks using a variety of orientational conditions. If three vertically-aligned Gabor patches were vertical, horizontal or both, or if bullseye or Gaussian blobs were used, no difference in performance was found. If, however, the orientation of the patches was randomized, performance became much worse. Similarly, if the three patches were at 45 deg, thresholds were raised. The effect of orientation was maintained across different spatial frequencies. Control conditions involving randomization of the phase of the sinusoidal carrier, or jitter on the size of Gaussian blobs, confirmed that a central tendency of the micropatterns was indeed being used by subjects, indicating that the role of orientation in this task is that of a mask, rather than of a cue.

Cues↗

PubMed1998

Estimating multiple temporal mechanisms in human vision.

When studying human ability to perceive temporal changes in luminance it is customary to estimate either temporal impulse response shapes or temporal modulation transfer functions, the representation of the impulse response in the frequency domain. The advantages and limitations of previous methods are summarized. We then describe an approach based on use of an impulse response basis set that resolves some of those limitations. We next present psychophysical results for spatiotemporal signal detection in spatiotemporal noise, together with an economical model of performance. The model is based on accepted notions of psychophysical detection mechanisms and the filter basis set described in the first part of the paper. The best-fitting model requires only eight parameters, as opposed to the 198 parameters required to separately fit each psychometric function, and captures both qualitative and quantitative properties of the psychophysical data. Finally, the best-fitting model indicates that only two temporal filters are necessary to describe the performance of each of three subjects under the specific stimulus conditions employed here.

Humans↗

PubMed1998

Two mechanisms underlie processing of stochastic motion stimuli.

We have constructed "limited lifetime" stochastic motion stimuli using Gabor functions instead of dots, thereby controlling the local attributes of spatial frequency and orientation. Human psychophysical data for direction discrimination using these stimuli reveal two qualitatively distinct kinds of processing. For small displacements, direction discrimination performance as a function of displacement is scaled with spatial frequency in a manner consistent with a linear filtering motion mechanism. Motion perception for relatively large displacements is not directly related to the spatial frequency, and is consistent with a nonlinear process which signals motion of contrast envelopes.

Discrimination, Psychological↗

PubMed1998

The role of "contrast enhancement" in the detection and appearance of visual contours.

We test the proposition that the appearance and detection of visual contours is based on an increase in the perceived contrast of contour elements. First we show that detection of contours is quite possible in the presence of very high levels of variability in contrast. Second we show that inclusion in a contour does not induce Gabor patches to appear to be of higher contrast than patches outside of a contour. These results suggest that, contrary to a number of current models, contrast or its assumed physiological correlate (the mean firing rate of early cortical neurons) is not the determining information for identifying the contour.

Contrast Sensitivity↗

PubMed1998

Is the rod visual field temporally homogeneous?

Cone vision has been shown to be temporally inhomogeneous across the visual field. In the periphery, contrast sensitivity is lower for low temporal frequencies and higher for high temporal frequencies. Here we ask a similar question for rod vision at mesopic luminances. Isolation is obtained by testing a well documented rod monochromat. We show that the rod visual field exhibits only a modest degree of temporal inhomogeneity.

Color Vision Defects↗

PubMed1998

When stereopsis does not improve with increasing contrast.

It is well known that stereoacuity for conventional (1st-order) stimuli improves with increasing contrast with an approximate slope of -0.5 on log-log axes (Halpern DL, Blake RR. Perception 1988;17:483-495; Legge GE, Gu Y. Vis Res 1989;29:989-1004). In the experiments reported here a variety of stimuli were used (Gabor patches, amplitude modulated stimuli and 1D noise patches) and tasks (stereoacuity and Dmax) to determine if 2nd-order stereopsis shows a similar square root dependence. The results consistently demonstrate that the effect of contrast on stereopsis is quite different for the 2nd-order stimuli. Increases in stimulus contrast have little effect on performance; the resulting slopes are very shallow. The pattern of results is similar when the interocular contrast ratio is varied, demonstrating that 2nd-order processing is more resilient to stimulus differences in the two eyes than 1st-order.

Contrast Sensitivity↗

PubMed1998

Spatial-frequency tuning of visual contour integration.

We examine the mechanism that subserves visual contour detection and particularly its tuning for the spatial frequency of contour components. We measured the detection of contours composed of Gabor micropatterns within a field of randomly oriented distractor elements. Distractors were randomly assigned one of two spatial frequencies, and elements lying along the contour alternated between these values. We report that the degree of tolerable spatial-frequency difference between successive contour elements is inversely proportional to the orientation difference between them. Spatial-frequency tuning (half-width at half-height) for straight contours is approximately 1.3 octaves but, for contours with a 30 degrees difference between successive elements, drops to approximately 0.7 octaves. Integration of curved contours operates at a narrower bandwidth. Much orientation information in natural images arises from edges, and we propose that this narrowing of tuning is related to the reduction in interscale support that accompanies increasing edge curvature.

Humans↗

PubMed1998

Relationship between facilitation at threshold and suprathreshold contour integration.

We reevaluate the facilitation at threshold previously reported between aligned micropatterns and assess the role of such lateral spatial interactions in suprathreshold contour integration tasks. Contrary to previous claims, we show that these interactions are phase dependent. Furthermore, they are clearly evident only for foveal viewing, are not evident for curved alignments (> 20 degrees), and do not produce any suprathreshold consequence for contrast perception. Such findings question their usefulness for contour integration of smoothly curved suprathreshold paths.

Form Perception↗

PubMed1997

Absence of contour linking in peripheral vision.

Human foveal vision is subserved initially by groups of spatial, temporal and orientational 'filters', the outputs of which are combined to define perceptual objects. Although a great deal is known about the filtering properties of individual cortical cells, relatively little is known about the nature of this 'linking' process. One recent approach has shown that the process can be thought of in terms of an association field whose strength is determined conjointly by the orientation and distance of the object. Here we describe a fundamental difference in this feature-linking process in central and peripheral parts of the visual field, which provides insight into the ways that foveal and peripheral visual perception differ. In the fovea, performance can be explained only by intercellular linking operations whereas in the periphery intracellular filtering will suffice. This difference represents a substantial economy in cortical neuronal processing of peripheral visual information and may allow a recent theory of intercellular binding to be tested.

Form 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

A reduced motion aftereffect in strabismic amblyopia.

The motion aftereffect was measured using both static and dynamic test stimuli in a group of normal observers and a group of strabismic amblyopes. Amblyopes exhibited a reduced direct aftereffect for both static and dynamic stimuli and only two of the eight amblyopes exhibited any measurable interocular transfer for either test stimulus. It is hard to explain these results in terms of either the known spatial (contrast sensitivity and positional sensitivity) or motion deficits previously reported in amblyopia. These results suggest a primary motion deficit in amblyopia affecting both the static and dynamic motion aftereffects.

Adaptation, Ocular↗

PubMed1997

Contour integration in strabismic amblyopia: the sufficiency of an explanation based on positional uncertainty.

Contour integration was measured in a group of strabismic amblyopes to determine if an explanation based solely on positional uncertainty was sufficient to explain performance. The task involved the detection of paths composed of micropatterns with correlated carrier orientations embedded in a field of similar micropatterns of random position and orientation (Field et al. Contour integration by the human visual system; Evidence for a local "association field". Vision Research, 33, 173-193, 1993). The intrinsic positional uncertainty for each amblyopic eye was measured with the same stimulus and it was found that in 10 out of our 11 amblyopic subjects, the reduced performance of the amblyopic eye could be modelled by the normal eye with an equivalent amount of positional uncertainty added to the stimulus. We conclude that the rules by which cellular outputs are combined, at least as reflected by this task, are normal in amblyopia.

Amblyopia↗

PubMed1997

The spatial mechanisms mediating symmetry perception.

This paper examines the role of spatial frequency and orientation tuned channels in the perception of visual symmetry. Subjects discriminated between band-pass filtered, white noise textures that either did or did not contain vertical bilateral symmetry (VBS, i.e., around a vertical midline) as a function of the spatial phase disruption imposed on the images. Resistance to phase noise is largely scale-invariant for isotropically filtered images, but horizontally filtered images are consistently more noise-resistant than vertical. However, when stimuli are rotated through 90 deg (horizontal bilateral symmetry, HBS) performance is better with vertically filtered images suggesting a general advantage for orientations orthogonal to the axis of symmetry. At these orientations symmetry may be signaled directly by clusters of features along the axis. Our data further suggest that the established disadvantage for HBS may be attributable to an over-reliance on the output of horizontal filters. We compare models which exploit feature clustering around the axis by measuring the co-alignment in the output of oriented filters. Models using filters oriented orthogonal to the axis of symmetry predict the psychophysical performance for isotropic patterns and for patterns filtered orthogonal to the axis. For patterns filtered parallel to the axis, our data suggest that visual attention may play a role.

Humans↗