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R F Hess

Publications and source records attributed to R F Hess.

At least 73 records · Page 4Linked to original sources
PubMed1997

Scale selection for second-order (non-linear) stereopsis.

In addition to the conventional luminance spatial frequency-dependent, disparity processing mode, there is a second-order luminance spatial frequency-independent type of processing available to the stereoscopic system. Here we use gaussian-enveloped, amplitude-modulated grating patches to determine how the stereoscopic system responds to the presence of two sources of second-order disparity information at different scales when there is no disparity information available via the conventional luminance-based system. In the first experiment we show that the stereoscopic system uses the disparity signal provided by the stimulus envelope, even though it is at a coarser scale than that provided by the amplitude modulation (AM). We then demonstrate that if the stimulus envelope is degraded via blurring, or if it is fixed at zero disparity, then performance depends on the finer-scale AM disparity signal. To show that the stereoscopic system uses the disparity signal provided by the AM we extend the carrier grating outside the borders of the AM stimulus, thereby making the boundary of the patch less discernible. Results obtained using this stimulus suggest that when two sources of second-order disparity information are present within the same stimulus (i.e., with no reliable luminance-based disparity signal available), the disparity signal provided by the coarser-scale contrast envelope vetos the finer-scale disparity signal. The coarse-scale disparity information dominates as long at it provides an adequate disparity signal. When it is degraded, however, the finer-scale signal takes precedence.

Depth Perception↗
PubMed1997

Temporal detection in human vision: dependence on stimulus energy.

We have previously proposed and evaluated an economical model of human performance in tasks requiring spatiotemporal signal detection in spatiotemporal noise [Vision Research (to be published)]. The model was successful in describing human psychophysical performance and provides a means for comparing temporal filters (mechanisms) employed under different stimulus conditions. We present investigations into how estimates of temporal mechanisms depend on the contrast energy of the stimulus. Temporal-sensitivity changes result in covariation of the cutoff and peak frequencies of the low-pass and bandpass mechanisms, respectively, with stimulus energy. The results indicate that sensitivity to high temporal frequencies increases as stimulus energy increases, commensurate with extent physiological evidence in cat and primate.

Humans↗
PubMed1997

Second-order motion perception in peripheral vision: limits of early filtering.

Spatial and temporal analysis of contrast-modulated sine-wave gratings reveals that the second-order motion stimulus contains two sidebands, with equal energy but moving in opposite directions, flanking a stationary carrier. Any early linear spatial filtering process in the visual system that attenuates one sideband more than the other will be detrimental to the balance between the two sidebands, so that the perceived direction of the carrier might be opposite to that of the envelope motion. We tested this hypothesis by using contrast-modulated gratings presented centrally or at 20 deg in the horizontal nasal field with a two-alternative forced-choice staircase paradigm. We found that when the envelope frequency was close to that of the carrier, a second-order stimulus whose envelope motion direction was correctly identified in the fovea appeared to drift in the opposite direction in the periphery. Further increasing the envelope spatial frequency resulted in a reversed motion percept in both central and peripheral viewing conditions. For subjects to identify correctly the direction of motion of the envelope, the spatial frequency ratio of the carrier to the envelope had to be more than 2 in the fovea and more than 6 in the periphery. These phenomena in second-order motion perception can be explained by a linear model of motion detection with an early spatial filtering process. Further experiments and computer simulation show that undersampling of the carrier has little effect on second-order motion perception in the periphery, as long as the carrier is detectable.

Adult↗
PubMed1997

Depth perception during diplopia is direct.

Although depth is experienced with targets at large disparities when they are seen as double or diplopic, whether that depth is as direct as with fused targets has been a matter of considerable uncertainty. Researchers have often claimed that judgments of the depth of diplopic targets during simple near/far tasks rely upon indirect associations with eye-muscle proprioception or a copy of the vergence drive signal. We designed a four-alternative task that could not be performed without a direct appreciation of depth. Observers judged the depths of each of two Gabor stereo pairs presented simultaneously. Disparities were always above each observer's measured diplopia threshold. The signs of the disparities were varied independently and observers reported the perceived depth near and far for each target. Our results demonstrate conclusively that depth during diplopia requires neither proprioception nor an efferent copy but is direct.

Depth Perception↗
PubMed1996

Is the site of non-linear filtering in stereopsis before or after binocular combination?

There is recent evidence that both linear and non-linear filtering operations subserve stereoscopic localization. For example, for spatially band-pass stimuli, the overall Gaussian envelope, which is not explicitly represented by the output of linear filters, can provide coarse disparity information. Here we ask three questions about the nature of this non-linear processing in stereopsis. First, is the site of the non-linearity before or after binocular combination? Second, is the stimulus envelope extracted by orientation or non-orientation selective spatial filters? Finally, we ask whether the envelope-based 3-D localization performance is similar to that for monocular 2-D localization as would be the case if the localization of the monocular contrast envelope was common to both operations. Our results suggest that envelope extraction occurs before binocular combination and that the filters involved are orientation selective. Finally, we provide preliminary evidence that is compatible with the proposal that 3-D and 2-D localization use the same envelope extraction operations.

Contrast Sensitivity↗
PubMed1996

Color and luminance vision in human amblyopia: shifts in isoluminance, contrast sensitivity losses, and positional deficits.

The deficits for contrast detection and positional accuracy were compared for chromatic and luminance mechanisms within a group of strabismic and anisometropic amblyopes. We found that the isoluminant point was shifted towards red in the amblyopic compared to the fellow normal eye. This was not accounted for by eccentric fixation by the amblyopic eye. Contrast sensitivity deficits were similar for luminance and color stimuli in normal and amblyopic visual systems. In the majority of our amblyopic subjects, however, the deficits in positional acuity were greater for the chromatic than the luminance stimuli.

Adolescent↗
PubMed1996

Effect of exposure duration on spatial uncertainty in normal and amblyopic eyes.

Previous studies have found that the spatial uncertainty of amblyopes is critically dependent on temporal factors. These studies claim that the spatial uncertainty is much greater at short exposure durations. We have reassessed the effect of exposure duration on the spatial uncertainty of normal and amblyopic eyes using a task in which we can compensate for the loss in contrast sensitivity which inevitably occurs as exposure duration is shortened. Our task involved a three-element alignment task, where each of the elements were spatial Gabors at two different separations. We ensured that our stimuli were always displayed at a fixed ratio above contrast detection thresholds at each exposure duration. Our results show that for normal subjects, for well separated equivisible stimuli, there is only a weak effect of exposure duration. A similar dependence is found for the dominant and amblyopic eyes of a group of strabismic amblyopes. Dominant eyes of strabismic amblyopes show increased spatial uncertainty compared with normal subjects. Amblyopic eyes of strabismic amblyopes show increased spatial uncertainty compared with their dominant fellow eye which is invariant with exposure duration. Some subjects show a larger positional deficit at short durations when the stimuli are almost abutting.

Adolescent↗
PubMed1996

Localization of element clusters: multiple cues.

The visual system commonly has to estimate the relative location of a textured region but the stimulus features used to perform that task are yet to be determined. The use of centroid, midpoint and peak activity cues would all be reasonable. In the current experiment an attempt was made to assess the relative efficacy of these three cues. The observers were required to indicate whether a cloud of either 3, 10 or 100 elements was located to the left or right of an imaginary line formed between two reference elements. Performance was compared to that expected from the use of the three cues. It was concluded that the cue used varied as the characteristics of the cloud changed and therefore that the visual system is not restricted to the use of a single cue type when localizing object clusters.

Contrast Sensitivity↗
PubMed1996

Uncalibrated distortions vs undersampling.

In a recent paper of ours [Hess & Field (1993). Vision Research, 33, 2663-2670], we claim that there was a predictable relationship between position errors and contrast errors for an undersampled system. In this paper we re-state our main points. We feel that the response to that paper by Levi and Klein in the accompanying article does not require us to produce changes in our original position. We believe that the data support the notion that the principal causes of the positional errors in the normal periphery and the in the amblyopic visual system are due to uncalibrated distortions in the local signs of visual neurons. We believe that undersampling plays a major role in producing positional errors only in the far periphery at, or very near, the acuity limit. We maintain that our initial studies provide strong evidence that undersampling is insufficient as an explanation for the positional errors in the periphery of normals (Hess & Field, 1993) or the central field of amblyopes [Hess & Field (1994). Vision Research, 34, 3397-3406.

Amblyopia↗
PubMed1996

Positional loss in strabismic amblyopia: inter-relationship of alignment threshold, bias, spatial scale and eccentricity.

In order to understand the spatial loss in strabismic amblyopia and its relationship to the contrast sensitivity deficit, we measured alignment performance for a three element vertical alignment task in which the elements were equi-visible, spatial Gabors. We derived the threshold and bias and compared these for stimuli of different spatial scale and eccentricity. Our results suggest that: (1) the deficits for alignment thresholds and bias are uncorrelated; (2) in the majority of strabismic amblyopes, both deficits are scale invariant; (3) the form of the regional distribution depends on the spatial measure used and the scale at which it is measured; and (4) there is a poor correlation between the deficit for either spatial measure and the contrast sensitivity loss.

Amblyopia↗
PubMed1996

Primitives used in the spatial localization of nonabutting stimuli: peaks or centroids.

In order to determine whether simple luminance profiles are located by their peaks or centroids we performed a three element alignment task where the central element's degree of luminance asymmetry was randomly chosen from a flat distribution (skew noise). The central element with its randomly chosen skew was either positioned using the peak or centroid of its distribution. Accuracy is invariant with the magnitude of the skew noise for the centroid but not the peak condition. We conclude that the human visual system assigns position tags using centroids not peaks of luminance distributions for gabors. However this is not the case for Gaussian blobs, where a measure closer to the midpoint is used for our stimulus arrangement.

Humans↗
PubMed1996

Rod temporal channels.

Mechanisms underlying rod temporal contrast sensitivity have been considered in terms of a fast retinal signal predominating at mesopic levels and a slower retinal signal predominating at scotopic levels. Here we use a small signal masking method, which has previously been used to delineate the cone-mediated cortical temporal channels, to investigate their rod-mediated cortical counterparts. The results suggest that there are three different rod-mediated cortical temporal channels, one which is lowpass and two which are bandpass. These mechanisms co-exist at all light levels and their relative sensitivity depend on the stimulus spatio-temporal frequency.

Color Vision Defects↗
PubMed1996

Is amblyopia an impediment to binocular function?

While there is good agreement that the strabismic and/or anisometropic deficit in early life plays a key role in disrupting developing binocular connections, it is still not clear what impediment amblyopia is to binocular function in the adult. The answer to this depends upon our combined understanding of the normal stereo mechanism and the amblyopic deficit.

Amblyopia↗
PubMed1995

The processing of temporal modulation at different levels of retinal illuminance.

How does our temporal vision change as the mean illuminance reduces? We have examined the processing of near-threshold temporal information for a range of illuminance values (2850--0.15 phot td). At high illuminance, the modulation transfer function can be shown to be mediated via three underlying temporal filters that vary in sensitivity with spatial frequency. As the mean illuminance decreases these channels appear to change their sensitivity. Even at the lowest (scotopic) illuminance levels we were able to find evidence for at least two channels mediating detection threshold. There are also changes in the tuning properties of these channels such that the processing of high temporal frequencies is differentially compromised, resulting in a reduction in the flicker fusion limit of each channel, and a shift in the peak of the band-pass channel. The slope of the fall-off in sensitivity at high temporal frequencies is unaffected by test spatial frequency at each illuminance level, suggesting its limiting factor is one that is insensitive to spatial frequency. We propose that the changes in the tuning of the temporal filters occur because of an early (e.g. photoreceptor) change in the response dynamics, or by interactions between photoreceptors, rather than changes at or beyond the level of the channel response.

Adult↗
PubMed1995

Dmax for stereopsis depends on size, not spatial frequency content.

Stereoacuity depends not only on the carrier frequency of Gabor stimuli, but also upon their size. To determine if this is also the case at large disparities, we have measured the upper limit for stereopsis, "Dmax", and assessed its dependence on carrier frequency and overall envelope size. The results differ markedly from the stereoacuity data. Dmax for stereopsis is primarily dependent on the size of the envelope of the Gabor patch, and is relatively independent of its carrier frequency. These results support the proposition that stereopsis is achieved at large disparities by way of non-linear processing (envelope extraction).

Contrast Sensitivity↗
PubMed1995

Contour integration across depth.

In order to investigate the extent of the local connections subserving contour integration across depth, we measured performance for detecting the continuity of a path of Gabor elements distributed in depth and embedded in a three-dimensional field of random background elements. The results show that performance cannot be explained in terms of monocular performance and that contour information is not limited to single disparity planes. Path detection does indeed involve the integration of information across different, very disparate depth planes. The rules which emerge are in general similar to that already described in the two-dimensional case in as far as orientation and disparity are important. Unlike the two-dimensional case, three-dimensional integration operates over relatively large three-dimensional distances.

Depth Perception↗
PubMed1995

Adaptation to spatial offsets.

After prolonged viewing of a three-element target in which the middle element is spatially offset, subsequent viewing of the same three elements in alignment results in the middle element appearing to be offset in the opposite direction. This adaptational aftereffect to a spatial offset was investigated with elements which were spatial-frequency narrowband and equidetectable to ascertain (a) the properties of the mechanisms involved and (b) the nature of the underlying computation. Evidence is presented in favour of an orientational-grouping, rather than a purely positional computation, underlying this aftereffect. A dual site of adaptation is proposed: one which receives input from the orientation extracted from the output of linear filters, and another which receives input from the orientation derived from grouping processes working on the contrast-energy representation. These may correspond to the mechanisms which are thought to underlie the processing of real and subjective contours.

Adaptation, Ocular↗