Biomedical subjects

R F Hess

Publications and source records attributed to R F Hess.

At least 91 records · Page 5Linked to original sources

PubMed1995

Metric for separation discrimination by the human visual system.

It is now clear that models of positional coding for elements sufficiently separated to permit individual identification require not only a first stage of linear filtering but also a second stage of representation that is preceded by a rectifying type of nonlinearity. To address the issue of the metric of this second stage we measure separation discrimination for Gabor stimuli of different sizes and of different peak spatial frequencies and separation, with and without different types of lateral distractors. Our results show that there is only a weak dependence of separation discrimination on the spatial frequency of equidetectable, spatially narrow-band stimuli; however, carrier spatial frequency can affect the influence that lateral distractors have on separation judgments. We conclude that (1) the second-stage representation is a space-size one consistent with the fact that there are scaled distributions of energy detectors of different sizes and (2) the influence of distractor elements suggests a spatial-frequency influence either at the second-stage representation or at a site beyond this second stage.

Contrast Sensitivity↗

PubMed1995

Amblyopia: a tale of two approaches.

Our understanding of the physiological basis for amblyopia in humans has undergone dramatic changes over the past two decades. The first such change occurred when we were first able to record from single neurones in different parts of the visual pathway and develop neural models of the anomaly. The second came from psychophysical investigations and evaluations of these animal models. Our understanding has progressed from one focused on the properties of single cortical cells to an appreciation of the anomalous behaviour of cellular networks.

Amblyopia↗

PubMed1994

The coding of spatial position by the human visual system: effects of spatial scale and retinal eccentricity.

In this study we investigate the nature of the computations that underlie the encoding of spatial position by the human visual system. Specifically, we explore the relationship between alignment accuracy and retinal eccentricity for stimuli where local luminance, local contrast, and orientation cues do not underlie performance. Spatial scale is especially important for such a comparison because of the well documented spatial inhomogeneity of the human visual field. The results suggest that the relationship between spatial localization and eccentricity is invariant with spatial scale if accuracy and eccentricity are expressed in terms of the stimulus envelope size. We show that the photoreceptor disarray does not determine the limit to performance for this task, the limit is post-receptoral and can be modelled in terms of a positional uncertainty within the early filters located before the response envelope has been extracted. This uncertainty varies with eccentricity in a similar way within each spatial array.

Contrast Sensitivity↗

PubMed1994

Is the spatial deficit in strabismic amblyopia due to loss of cells or an uncalibrated disarray of cells?

We examine two competing explanations for the spatial localization deficit in human strabismic amblyopia, namely neural undersampling and uncalibrated neural disarray. An undersampling hypothesis would predict an associated deficit for contrast discrimination for which we find no evidence in strabismic amblyopia. A neural disarray hypothesis would predict an associated deficit in the degree to which stimuli appear spatially distorted. We find evidence for such a deficit in strabismic amblyopia. We propose that the spatial deficit in strabismic amblyopia is due to a filter-based distortion which is unable to be re-calibrated by higher visual centres.

Amblyopia↗

PubMed1994

Linear and non-linear filtering in stereopsis.

To better understand the spatial filtering operations underlying stereopsis, and their relationship to those underlying monocular localization of the same stimuli, we examined the dependence of stereoacuity on carrier and envelope size of Gabor patches. For stimuli of broad spatial bandwidth, stereoacuity depends on the carrier spatial frequency whereas for stimuli of narrow bandwidth, stereoacuity depends on the modulation frequency. The dependence of stereoacuity on the separation of the reference elements differs for stimuli of broad and narrow spatial frequency bandwidths. These relationships suggests that stereopsis has access to two different types of information from the early filters which we term, linear and non-linear. This distinction is important not only for understanding the relationship between monocular and stereoscopic localization, but also for understanding the different filter operations underlying stereopsis.

Contrast Sensitivity↗

PubMed1994

Localization of element clusters by the human visual system.

A spatial localization task was used to determine the accuracy of localization of random element clusters about their centroid. The random element clusters were generated within a circular region and the observers' task was to decide whether the cluster lay to the right or left of a reference line, defined by two vertically separated reference elements. Both the reference elements and the cluster elements themselves were comprised of spatially narrowband stimuli (gabor patches), and were presented at a constant suprathreshold level. The thresholds for localization of the element cluster were measured with varying element number, and for a number of different sizes of circular region and element. The relationship between localization threshold and element number was not monotonic. Thresholds were found to rise with increasing element number up to about six elements within a region, and to then fall with further increase in element number. Asymptotic thresholds at high element number were indistinguishable from those obtained for filled circles of the same size. The results do not conform to any one of a number of models based on a centroid analysis alone. By manipulating the spatial and orientational properties of the elements comprising the cluster to be localized it was determined that the more central mechanism underlying localization receive input from different spatial and orientation filters.

Contrast Sensitivity↗

PubMed1994

Pupillary function in human amblyopia.

Quantitative measurements of pupillary function (response amplitude and latency) were made for normal eyes and for normal and fellow amblyopic eyes of groups of strabismic and anisometropic amblyopes. Stimuli consisted of luminance modulation of a large, evenly lit area (pupil light reflex) as well as contrast modulation of sinusoidal gratings (pupil grating response) of fixed, space-averaged luminance. Measurements were made of the direct and the consensual reflex under monocular stimulation. A comparison of the amplitude of the pupil light reflex as a function of luminance modulation showed no significant differences between normal and fellow amblyopic eyes for both the strabismic and anisometropic groups of amblyopes studied. A similar comparison of the associated response latencies showed significant difference between normal and fellow amblyopic eyes for both groups. In general, reductions in response amplitude and latency of the pupil grating response were found in individuals from each group when comparing the good and the affected eyes, although the observed group differences were only significant in the strabismic group. Interestingly, statistically significant reductions in both amplitude and latency for both the pupil light reflex and the pupil grating response were found between the eyes of normal observers and the so-called normal eyes of amblyopes in both groups studied. These results suggest that the type of pupillary deficit in amblyopia is a complicated one, depending not only on the type of amblyopia (strabismic or anisometropic) and the type of stimulus employed (light or pattern), but also on the parameter assessed (amplitude or latency) and whether the amblyopic result is referenced to its fellow normal eye or to the normal eye of a non-amblyopic observer. Since the pupil response to light flux changes is not mediated exclusively via the retinal projection to the midbrain and may also involve the activity of central visual pathways, the results obtained in this study cannot be used to provide definitive evidence for the site of abnormality in amblyopia.

Adolescent↗

PubMed1994

Direction identification thresholds for second-order motion in central and peripheral vision.

Evidence bearing on the question of whether first-order and second-order motion are detected by use of the same or different principles has been sought. This question was approached by measuring thresholds for correctly identifying the direction of motion of various second-order motion patterns. The patterns used were contrast-modulated noise patterns in which the contrast of a carrier was modulated sinusoidally in one dimension, and the modulating waveform drifted smoothly while the carrier itself remained stationary. The carrier used was in most cases static two-dimensional noise; other carriers gave similar results. Thresholds were measured in terms of amplitude of contrast modulation (modulation depth) for each of a range of envelope drift speeds and spatial frequencies in the fovea and at several viewing eccentricities. Along with direction-identification thresholds, thresholds for either simple detection of the modulation or for correctly identifying the orientation of the modulation were simultaneously measured. Thresholds for direction identification were generally somewhat higher than those for simple detection. However, they were in most cases very similar to thresholds for identification of orientation, as found for conventional luminance gratings. Contrary to some reports, sensitivity to contrast-modulated patterns declines with eccentricity at a similar rate to that found with luminance gratings. The results suggest that first-order and second-order motion are either detected by a common motion-detection mechanism or are detected by different mechanisms that use a common principle of motion detection.

Humans↗

PubMed1994

Suprathreshold temporal-frequency discrimination in the fovea and the periphery.

To address the question of whether temporal-frequency information in the fovea and the periphery is processed in fundamentally different ways we measured temporal-frequency-discrimination thresholds for spatiotemporally narrow-band stimuli presented at suprathreshold contrast. Temporal-frequency-discrimination thresholds are similar (within a factor of 2) at the fovea and at 30 degrees in the periphery. We use a line-element approach and three spatiotemporally separable temporal mechanisms to model foveal and peripheral data with the same degree of fidelity. These findings suggest that not only are the front-end temporal mechanisms in the fovea and periphery likely to be similar but also the way in which their outputs are combined at more central sites is the same.

Contrast Sensitivity↗

PubMed1993

Neural recruitment explains "Weber's law" of spatial position.

We ask whether the well known Weber's law between spatial localization and element separation for high contrast, spectrally broad-band stimuli is a consequence of the organization of the early visual filters, or a fundamental constraint on the computation of spatial position by more central mechanisms. We address this question by identifying the individual contributions of mechanisms tuned to different ranges of spatial frequencies and contrast. We measure spatial-alignment and bisection error as a function of element separation at each of a number of spatial scales, using spectrally narrow-band stimuli of fixed supra-threshold contrast. We show that stimuli which minimize the extent of neural recruitment across different spatial channels before the site of extraction of the local contrast energy (and to a lesser extent across different contrast channels) do not exhibit Weber's law for either alignment or bisection. We present evidence that Weber's law for localization with increasing separation, found for stimuli of high contrast and broad-band spatial frequency content, is a consequence of the successive disengagement of unitary neural mechanisms, each of which has different spatial and contrast properties, and none of which individually exhibits Weber's law for spatial position.

Contrast Sensitivity↗

PubMed1993

Motion sensitivity and spatial undersampling in amblyopia.

The nature of the visual deficit in human amblyopia has been keenly sought over the last decade. Some confusion has arisen as to whether the motion-sensitive mechanisms known to exist in normal vision are selectively affected in humans with amblyopia. To address this issue we compare contrast thresholds for detection and direction discrimination of drifting sine-wave gratings in a group of humans with amblyopia. The results suggest that over the vast majority of the spatio-temporal range, direction of motion can be discriminated at detection threshold. Over a narrow part of the visible range there is a differential loss of sensitivity for direction discrimination over that of simple detection. However such an effect also occurs for normal vision under spatially scaled conditions and it seems likely that it is mediated by non-motion sensitive mechanisms. We show that one possible cause of this loss of direction discrimination, namely spatial undersampling within the central region of the amblyopic visual field, is not a viable explanation.

Amblyopia↗

PubMed1993

Contour integration by the human visual system: evidence for a local "association field".

The Gestalt law of "good continuation" has been used to describe a variety of phenomena demonstrating the importance of continuity in human perception. In this study, we consider how continuity may be represented by a visual system that filters spatial data using arrays of cells selective for orientation and spatial frequency. Many structures (e.g. fractal contours) show a form of redundancy which is well represented by the continuity of features as they vary across space and frequency. We suggest that it is possible to take advantage of the redundancy in continuous, but non-aligned features by associating the outputs of filters with similar tuning. Five experiments were performed, to determine the rules that govern the perception of continuity. Observers were presented with arrays of oriented, band-pass elements (Gabor patches) in which a subset of the elements was aligned along a "jagged" path. Using a forced-choice procedure, observers were found to be capable of identifying the path within a field of randomly-oriented elements even when the spacing between the elements was considerably larger than the size of any of the individual elements. Furthermore, when the elements were oriented at angles up to +/- 60 deg relative to one another, the path was reliably identified. Alignment of the elements along the path was found to play a large role in the ability to detect the path. Small variations in the alignment or aligning the elements orthogonally (i.e. "side-to-side" as opposed to "end-to-end") significantly reduced the observer's ability to detect the presence of a path. The results are discussed in terms of an "association field" which integrates information across neighboring filters tuned to similar orientations. We suggest that some of the processes involved in texture segregation may have a similar explanation.

Form Perception↗

PubMed1993

Is the increased spatial uncertainty in the normal periphery due to spatial undersampling or uncalibrated disarray?

There are two possible causes for the elevated positional uncertainty of the peripheral field; undersampling by post receptoral arrays or uncalibrated disarray within post receptoral arrays. In order to assess the relative influences of these two factors to peripheral spatial uncertainty, we apply an approach which has enjoyed much success in the field of colour vision to examine the trade-off between position and contrast errors for spatial vision. We show that, if our positional uncertainty in the periphery is due to spatial undersampling, a correlated contrast inaccuracy should be present. We find no evidence for this expected linkage in the peripheral field at any spatial scale. We conclude that uncalibrated neural disarray rather than undersampling is the cause of peripheral positional uncertainty.

Awareness↗

PubMed1992

Is the visual field temporally homogeneous?

We asked the question "is the visual field temporally homogeneous when the well known spatial inhomogeneity is taken into account?" Our results show that the visual field exhibits inhomogeneity for stimuli of low spatial frequency for which the spatial inhomogeneity is minimal. This inhomogeneity takes the form of a reduction in sensitivity for stimuli of low temporal frequency in the periphery and an enhancement of sensitivity for stimuli of high temporal frequency in the periphery. The low temporal frequency loss in the periphery is post-receptoral and must involve a selective loss of sensitivity of the low pass filter. The enhanced high frequency sensitivity may be post-receptoral or purely receptoral. The consequence of such an inhomogeneity for subsequent stages of visual processing is discussed.

Adult↗

PubMed1992

The coding of spatial position by the human visual system: effects of spatial scale and contrast.

In this study we investigate the nature of the computations that underlie the encoding of spatial position by the human visual system. Specifically, we explore the relationship between alignment accuracy and spatial scale on the one hand, and between alignment accuracy and contrast on the other. We do this for stimuli where local luminance, local contrast, and orientation cues do not underlie performance. The results suggest that spatial localisation is independent of spatial scale and weakly dependent on contrast. We present subsequent models based on the properties of some classes of visual cortical neurones, namely multiplicative noise and contrast energy detection of complex cells, which describe the form of these relationships.

Contrast Sensitivity↗

PubMed1992

Binocular integration of contrast information in amblyopia.

We asked whether suppression in amblyopia could be accounted for by dichoptic masking as described in normals, operating in the presence of a contrast threshold difference between the two eyes. A dichoptic masking paradigm was employed to investigate binocular interaction in a mixed group of amblyopic subjects. Normal dichoptic masking was not seen after threshold differences between the two eyes were accounted for in the majority of subjects studied. We found that the binocular dysfunction did not merely follow as a consequence of the known monocular loss and that it depended upon the aetiology of the amblyopia and the spatial frequency of the stimulus.

Adolescent↗

PubMed1992

Temporal properties of human visual filters: number, shapes and spatial covariation.

The temporal properties of the foveal visual filters were revealed using a method which is a variant on previously used noise masking paradigms. This enables the temporal properties of the mechanisms underlying threshold detection of a spatio-temporal probe to be measured. In accord with recent suggestions these results support the existence of three temporal mechanisms. The evidence for the third, higher temporal mechanism is only persuasive at low spatial frequencies. Furthermore, the results suggest that although there is some degree of spatio-temporal covariation in the filtering properties either of individual filters or across the filter population, the well known spatio-temporal covariation in human detection sensitivity is adequately explained by a sensitivity scaling of individual temporal filters with approximately invariant temporal properties.

Contrast Sensitivity↗

PubMed1992

Temporal frequency filters in the human peripheral visual field.

The temporal filtering properties of the human peripheral field were investigated by means of measuring: (1) modulation transfer functions for a range of spatial frequencies at four visual field locations (0, 10, 30 and 50 degrees), (2) the contrast of a masking stimulus required to extinguish the visibility of just suprathreshold probes. Results suggest that the number of temporal filters governing detection threshold is dependent upon both eccentricity and spatial frequency. For near-foveal viewing three temporal filters were found (one low-pass and two band-pass), whereas at far eccentricities only one was found (band-pass). A similar result was obtained by modeling the modulation transfer function by simply scaling the sensitivities of three independently derived filters. Our data suggest that (1) changes in the modulation transfer function with respect to spatial frequency and eccentricity can be adequately explained by the changes in sensitivity of a small number of spatio-temporal separable filters; (2) the peripheral field is not merely a coarser version of the fovea but has qualitative differences which may be thought to emphasize the transient properties of the stimulus.

Contrast Sensitivity↗