Biomedical subjects

Robert F Hess

Publications and source records attributed to Robert F Hess.

At least 37 records · Page 2Linked to original sources

PubMed2005

A residual deficit for global motion processing after acuity recovery in deprivation amblyopia.

PURPOSE: In a case of bilateral deprivation amblyopia due to congenital cataracts, the global motion sensitivity for stimuli that were well within the passband of the amblyopic visual system were assessed. METHODS: A stochastic global motion stimulus was used, comprising spatially narrow elements with varied spatial frequency, density, contrast, and area distribution. To determine threshold, a two-alternate, forced-choice direction discrimination task was used. RESULTS: There was a selective deficit for global motion processing that was not due to the visibility of the stimuli and was nonselective for spatial scale. The eye with the more complete recovery (acuity 20/20) from pattern deprivation in childhood exhibited the more severe global motion deficit. discussion. The results suggest a primary extrastriate deficit in the dorsal pathway, possibly involving the middle temporal (MT) and the medial superior temporal (MST) cortical areas, that is unrelated to the acuity deficit thought to be in area V1. A similar deficit has recently been shown in strabismic amblyopia.

Amblyopia↗

PubMed2005

Integration, segregation, and binocular combination.

The human visual system can accurately judge the mean of a distribution of different orientation samples. We ask whether the site of this integration is before or after the sites of binocular combination and disparity processing. Furthermore, we are interested in whether the efficiency with which local orientation information is integrated depends on the eye of origin. Our results suggest that orientation integration occurs after binocular integration but before disparity coding. We show that the effectiveness of added orientation noise is not only less than expected on signal or noise grounds but also that it depends on the dominance of the eye to which it is presented, suggesting an interocular opponent interaction in which the dominant eye input has higher gain.

Dominance, Ocular↗

PubMed2005

Foveal contour interaction: detection and discrimination.

Contour interaction, the detrimental effect of flanking features on the discrimination of optotypes, has been studied mainly close to the visual acuity limit. We were interested to know how these results compare with those for the detection of targets. According to the simplest model of contour interaction, comparable detection effects would be expected. The case for low-level masking would be further strengthened if the form and nature of the dependence on flank separation and flank polarity followed that typically found in studies of lateral spatial masking [Vision Res. 33, 993 (1993)]. Landolt Cs subtending a visual angle of 0.25 degrees, 0.5 degrees, and 1.0 degrees were presented and contrast thresholds for detecting the presence of the Landolt C and discriminating its orientation were measured in five normal subjects as a function of flank separation and flank polarity. The results obtained for the relationship between detection and discrimination depend on the size of the target used. For small letters, discrimination but not detection was significantly affected by flanking bars. For large letters, detection and discrimination were affected to the same extent. However, in this case the effectiveness of opposite-polarity flanks and the finding that facilitation occurred at close, not far, flank separations suggests that the simplest explanation in terms of masking may not be applicable.

Differential Threshold↗

PubMed2004

The spatial localization deficit in visually deprived kittens.

We measured the spatial localization abilities (alignment accuracy) of visually deprived kittens by use of similar spatially bandpass stimuli (Gaussian blobs) to those employed for the assessment of human amblyopes. The tests of vision were conducted on kittens reared with either strabismus or following different periods of monocular deprivation. As with amblyopic humans, the deficits in alignment accuracy were scaled in proportion to blob size and were not only considerably larger than those of grating acuity but also were not correlated with either the acuity or contrast sensitivity losses. Tests with stimuli of various contrast revealed that the deficits could not be explained in terms of the contrast sensitivity loss in this eye. The positional deficits that arise from anomalous visual development are independent of the contrast sensitivity loss and profound.

Amblyopia↗

PubMed2004

Low spatial frequencies are suppressively masked across spatial scale, orientation, field position, and eye of origin.

Masking is said to occur when a mask stimulus interferes with the visibility of a target (test) stimulus. One widely held view of this process supposes interactions between mask and test mechanisms (cross-channel masking), and explicit models (e.g., J. M. Foley, 1994) have proposed that the interactions are inhibitory. Unlike a within-channel model, where masking involves the combination of mask and test stimulus within a single mechanism, this cross-channel inhibitory model predicts that the mask should attenuate the perceived contrast of a test stimulus. Another possibility is that masking is due to an increase in noise, in which case, perception of contrast should be unaffected once the signal exceeds detection threshold. We use circular patches and annuli of sine-wave grating in contrast detection and contrast matching experiments to test these hypotheses and investigate interactions across spatial frequency, orientation, field position, and eye of origin. In both types of experiments we found substantial effects of masking that can occur over a factor of 3 in spatial frequency, 45 degrees in orientation, across different field positions and between different eyes. We found the effects to be greatest at the lowest test spatial frequency we used (0.46 c/deg), and when the mask and test differed in all four dimensions simultaneously. This is surprising in light of previous work where it was concluded that suppression from the surround was strictly monocular (C. Chubb, G. Sperling, & J. A. Solomon, 1989). The results confirm that above detection threshold, cross-channel masking involves contrast suppression and not (purely) mask-induced noise. We conclude that cross-channel masking can be a powerful phenomenon, particularly at low test spatial frequencies and when mask and test are presented to different eyes.

Contrast Sensitivity↗

PubMed2004

Investigating local network interactions underlying first- and second-order processing.

We compared the spatial lateral interactions for first-order cues to those for second-order cues, and investigated spatial interactions between these two types of cues. We measured the apparent modulation depth of a target Gabor at fixation, in the presence and the absence of horizontally flanking Gabors. The Gabors' gratings were either added to (first-order) or multiplied with (second-order) binary 2-D noise. Apparent "contrast" or modulation depth (i.e., the perceived difference between the high and low luminance regions for the first-order stimulus, or between the high and low contrast regions for the second-order stimulus) was measured with a modulation depth-matching paradigm. For each observer, the first- and second-order Gabors were equated for apparent modulation depth without the flankers. Our results indicate that at the smallest inter-element spacing, the perceived reduction in modulation depth is significantly smaller for the second-order than for the first-order stimuli. Further, lateral interactions operate over shorter distances and the spatial frequency and orientation tuning of the suppression effect are broader for second- than first-order stimuli. Finally, first- and second-order information interact in an asymmetrical fashion; second-order flankers do not reduce the apparent modulation depth of the first-order target, whilst first-order flankers reduce the apparent modulation depth of the second-order target.

Contrast Sensitivity↗

PubMed2004

Poor encoding of position by contrast-defined motion.

Second-order (contrast-defined) motion stimuli lead to poor performance on a number of tasks, including discriminating form from motion and visual search. To investigate this deficiency, we tested the ability of human observers to monitor multiple regions for motion, to code the relative positions of shapes defined by motion, and to simultaneously encode motion direction and location. Performance with shapes from contrast-defined motion was compared with that obtained from luminance-defined (first-order) stimuli. When the position of coherent motion was uncertain, direction-discrimination thresholds were elevated similarly for both luminance-defined and contrast-defined motion, compared to when the stimulus location was known. The motion of both luminance- and contrast-defined structure can be monitored in multiple visual field locations. Only under conditions that greatly advantaged contrast-defined motion, were observers able to discriminate the positional offset of shapes defined by either type of motion. When shapes from contrast-defined and luminance-defined motion were presented under comparable conditions, the positional accuracy of contrast-defined motion was found to be poorer than its luminance-defined counterpart. These results may explain some, but possibly not all, of the deficits found previously with second-order motion.

Cognition↗

PubMed2004

Integration of orientation information in amblyopia.

A recent report suggests that amblyopes are deficient in processing local orientation at supra-threshold contrasts. To determine whether amblyopes are also poor at integrating local orientation signals, we assessed performance for an orientation integration task in which the orientations of static signals are integrated across space. Our results show that amblyopic visual systems can integrate local static oriented signals with the same level of efficiency as normal visual systems. Although internal noise was slightly elevated, there was no indication that fewer samples were used to achieve optimal performance. This finding suggests normal integration of local orientation signals in amblyopia.

Adult↗

PubMed2003

Sensitivity for global shape detection.

In order to understand the nature of the mechanisms responsible for global shape detection, we measured coherence thresholds in a 2IFC task where subjects judged which of two arrays of Gabors contained global circular structure. The stimulus was composed of an array of oriented Gabor patches positioned on a polar grid. Thresholds were obtained for different array parameters (e.g. different area, density, number and positions of elements) as well as for different element parameters (e.g. different carrier spatial frequencies, contrasts, polarities and orientations). Global structure was detected when ~10% of the elements were coherently oriented. Neither the properties of the array (density, area, number or position of elements), nor those of the individual elements (carrier spatial frequency, contrast, polarity) altered coherence thresholds. Varying contrast or carrier spatial frequency within individual arrays also did not alter performance. Sensitivity was invariant to positional perturbations of the array grid. Only jittering the local orientation of elements decreased sensitivity. The underlying mechanisms are broadly tuned for contrast, spatial frequency and the spatial positioning of image samples. Detecting circular structure is a robust process and, in this case, a purely global one. Sensitivity was highest for circular as opposed to radial or spiral shapes.

Contrast Sensitivity↗

PubMed2003

The detection of direction-defined and speed-defined spatial contours: one mechanism or two?

It is now accepted that the visual system integrates local orientation information across space to define spatial contours [Vision Research 33 (1993) 173]. More recently, it has been shown that similar integration occurs for the direction of local motion signals, in different parts of the visual field, if they are aligned along the axis of a spatial contour [Vision Research 42 (2002) 653]. Here we ask whether similar spatial-linking rules hold for contours comprised of local elements that share only a common speed (but not direction), in the presence of background elements which collectively have the same mean speed as the contour but considerable random variation in the speeds of the individual elements. Furthermore we investigate the detection of spatial contours that are defined by a common speed that is different (both locally and globally) from that of the background elements. The results show that there is a significant, albeit relatively weak, speed-association field with preferential linking between spatially proximal elements that have similar speeds. Although a salient speed difference between the contour and the background elements enhances detection performance for motion-defined contours, it does so primarily via a different route to that of direction linking. We suggest that for motion-defined contours the Gestalt notions of "common fate" and "good continuity", that describe the parsing of local velocity information into objects, boundaries and contours, are mediated via separate underlying perceptual mechanisms.

Humans↗

PubMed2003

Deficits to global motion processing in human amblyopia.

We investigated global motion processing in a group of adult amblyopes using a method that allows us to factor out any influence of the known contrast sensitivity deficit. We show that there are independent global motion processing deficits in human amblyopia that are unrelated to the contrast sensitivity deficit, and that are more extensive for contrast-defined than for luminance-defined stimuli. We speculate that the site of these deficits must include the extra-striate cortex and in particular the dorsal pathway.

Adult↗

PubMed2003

Border distinctness in amblyopia.

On the basis of the contrast sensitivity loss in amblyopia which mainly affects higher spatial frequencies, one would expect amblyopes to perceive sharp edges as blurred. We show that they perceive sharp edges as sharp and have veridical edge blur perception. Contrary to the currently accepted view, this suggests that the amblyopic visual system is not characterized by a blurred visual representation.

Amblyopia↗

PubMed2003

Differential binocular input and local stereopsis.

Using fractal noise images, we measured the dependence of Dmin and Dmax for stereo on the interocular differences of spatial frequency and contrast. Dmin exhibits a strong dependence on the highest spatial frequency contained in the image, while Dmax exhibits a weaker dependence on the lowest spatial frequency contained within the image. Neither relationship was found to be different when the filtering was restricted to only one eye's image, although the effect of differential lowpass filtering for Dmin was greater than that of binocular lowpass filtering. Contrast is thought to affect stereo performance particularly when it is reduced in only one eye's image. We show that, at least for broadband fractal images representative of everyday natural images, interocular contrast differences are no more disruptive than binocular ones. These results bear upon the nature of the matching process in stereopsis. The fact that these interocular spatial frequency and contrast manipulations do not selectively degrade stereopsis beyond that expected from a consideration of purely monocular effects is consistent with matching occurring within multiple spatial channels prior to their combination.

Contrast Sensitivity↗

PubMed2003

Automatic volumetric segmentation of human visual retinotopic cortex.

Previous identification of early visual cortical areas in humans with phase-encoded retinotopic mapping techniques have relied on an accurate cortical surface reconstruction. Here a 3D phase-encoded retinotopic mapping technique that does not require a reconstruction of the cortical surface is demonstrated. The visual field sign identification is completely automatic and the method directly supplies volumes for a region-of-interest analysis, facilitating the application of cortical mapping to a wider population. A validation of the method is provided by simulations and comparison to cortical surface-based methodology.

Adult↗

PubMed2003

Cortical specialization for processing first- and second-order motion.

Distinct mechanisms underlying the visual perception of luminance- (first-order) and contrast-defined (second-order) motion have been proposed from electrophysiological, human psychophysical and neurological studies; however a cortical specialization for these mechanisms has proven elusive. Here human brain imaging combined with psychophysical methods was used to assess cortical specializations for processing these two kinds of motion. A common stimulus construction was employed, controlling for differences in spatial and temporal properties, psychophysical performance and attention. Distinct cortical regions have been found preferentially processing either first- or second-order motion, both in occipital and parietal lobes, producing the first physiological evidence in humans to support evidence from psychophysical studies, brain lesion sites and computational models. These results provide evidence for the idea that first-order motion is computed in V1 and second-order motion in later occipital visual areas, and additionally suggest a functional dissociation between these two kinds of motion beyond the occipital lobe.

Adult↗

PubMed2003

Perceived blur in amblyopia.

PURPOSE: The well-documented fact that visual acuity and contrast sensitivity in amblyopia are attenuated at high spatial frequencies predicts that amblyopes should perceive objects as blurred, because they do not have the high spatial frequency information necessary to represent sharp edges adequately. In the current study, the representation of blur in amblyopia with blur-discrimination and blur-matching tasks was explored in a series of experiments. METHODS: Monocular blur-discrimination thresholds were measured in a spatial two-alternative forced-choice procedure. Observers were required to discriminate which edge (right or left) appeared to be the lesser blurred. Observers also interocularly matched edges that were identical with those used in the blur-discrimination tasks, with the exception that they were viewed dichoptically at all times. RESULTS: Blur-discrimination thresholds were elevated in both the amblyopic and fellow fixing eyes but were within the normal range for interocular matching thresholds. CONCLUSIONS: The results suggest that blur is veridically represented in the amblyopic visual system. The surprising result is that all amblyopes, even those with the most severe visual loss, veridically matched all blurred edges, including the sharpest ones. This implies that amblyopes are able to represent levels of blur that are defined by spatial structure beyond their resolution limit.

Adult↗

PubMed2003

Threshold vision in amblyopia: orientation and phase.

PURPOSE: The visual deficit in amblyopia involves both elevated contrast thresholds and distorted suprathreshold percepts at high spatial frequencies. It is currently unclear whether these two anomalies are part of the same neural disturbance or whether they reflect different neural dysfunction. METHODS: The quality of the spatial percepts in amblyopia was assessed at detection threshold. The ability of amblyopes to discriminate the orientation and local spatial phase of well-localized spatial stimuli was measured at the detection threshold. Measurements were made as a function of spatial frequency. RESULTS: Performance seemed normal for orientation discrimination, even at high spatial frequencies, but, in some cases, it was disturbed in phase discrimination. CONCLUSIONS: A different explanation and neural basis is needed to encompass both threshold and suprathreshold spatial deficits in amblyopia.

Adult↗

PubMed2003

Integration of first- and second-order orientation.

The problem of how visual information such as orientation is combined across space bears on key visual abiities, such as texture perception. Orientation signals can be derived from both luminance and contrast, but it is not well understood how such information is pooled or how these different orientation signals interact in the integration process. We measured orientation discrimination thresholds for arrays of equivisible first-order and second-order Gabors. Thresholds were measured as the orientation variability in the arrays increased, and we estimated the number of samples (or efficiency) and internal noise of the mechanism being used. Observers were able to judge the mean orientation of arrays of either first- or second-order Gabors. For arrays of first-order and arrays of second-order Gabors, estimates of the number of samples used increased as the number of Gabors increased. When judging the orientation of arrays of either order, observers were able to ignore randomly oriented Gabors of the opposite order. If observers did not know which Gabor type carried the more useful orientation information, they tended to use the information from first-order Gabors (even when this was poorer information). Observers were unable to combine information from first- and second-order Gabors, though this would have improved their performance. The visual system appears to have separate integrators for combining local orientation across space for luminance- and contrast-defined features.

Discrimination, Psychological↗