Biomedical subjects

Robert F Hess

Publications and source records attributed to Robert F Hess.

At least 19 recordsLinked to original sources

PubMed2006

The global processing deficit in amblyopia involves noise segregation.

Some studies have reported deficits in amblyopia for global form and motion integration, whereas other studies have shown global integration of form and motion information to be normal in amblyopia. Here, we attempt to resolve this discrepancy by showing that amblyopes only exhibit selective performance deficits on global tasks that contain noise as well as signal. We hypothesized that signal integration is normal, but noise segregation is not. We used comparable global orientation and motion direction discrimination tasks to measure integration performance in the presence of controlled amounts of pedestal noise (i.e., elements whose orientations or directions were randomly selected). We modelled the performance using an equivalent noise model, which has the parameters of internal noise and number of samples. Our results show that amblyopic eyes can integrate form (i.e., orientation) and motion information (i.e., motion direction) similarly to normals when all the information is signal (i.e., no pedestal noise). However, introducing pedestal noise perturbs the performance of the amblyopic eyes significantly more than that of the normal eyes.

Amblyopia↗

PubMed2006

Flank facilitation and contour integration: different sites.

Observers' ability to integrate features into extended contours, and to exploit the flanking structure to facilitate contrast detection (flank facilitation), exhibit a similar dependence on element spacing and orientation. Here, we investigate whether this reflects the operation of a common cortical mechanism by comparing performance for both tasks under monocular, binocular, dichoptic, and stereoscopic viewing conditions. Our results clearly implicate different cortical sites for flank-facilitated detection and contour integration; the former is a purely monocular phenomenon and must therefore occur at the earliest stages of cortical processing. In contrast, contour integration is a binocular process and occurs after the encoding of relative disparity, suggesting substantial extra-striate involvement. We conclude that the sites, and therefore the mechanisms, underlying these two seemingly related psychophysical phenomena are different.

Form Perception↗

PubMed2006

Discrimination of amplitude spectrum slope in the fovea and parafovea and the local amplitude distributions of natural scene imagery.

A number of studies have investigated whether human visual performance can be related to the general form of the amplitude spectra (i.e., 1/f(alpha)) of natural scenes. Here, it is argued that there are some discrepancies in the data between some of those studies and that one possible explanation for the discrepancies may be related to differences in methodology (e.g., stimuli presented to the fovea as opposed to the parafovea). We sought to resolve some of the discrepancies with two psychophysical paradigms involving alpha discrimination with visual noise and natural scene image patches presented to the fovea or parafovea. Fovea-parafovea threshold differences were apparent for stimuli possessing alpha values < 1.0, with the parafovea typically showing highest thresholds for reference alpha values in the 0.74-0.85 range. Both fovea and parafovea thresholds were lowest in the 1.2-1.4 range. In addition, we conducted a local amplitude distribution analysis (i.e., assessed local alpha) with a large set of high-resolution natural scene imagery and found that the results of that analysis provided a better account of the alpha discrimination thresholds for stimuli presented to the fovea as opposed to the parafovea.

Adult↗

PubMed2006

The role of spatial phase in texture segmentation and contour integration.

It has been recently argued that the visual system possesses just two phase "detector" mechanisms, namely, +cosine and -cosine (P. C. Huang, F. A. Kingdom, & R. F. Hess, 2006). This suggests rather limited access to the rich distribution of receptive field phase that neurophysiologists tell us are represented in the different response profiles of striate simple cells. Whereas that study has suggested that striate receptive field phase is not directly available to perception for the detection/discrimination of localized stimuli, here we investigate whether such information might be used in more integrative striate or extrastriate functions such as texture segregation or contour integration. Specifically, given that simple cells have different local absolute phase response profiles, we ask whether a network of simple cells with similar phase preferences interact in such a way as to extract textures, contours, or both based on phase alone. Two novel texture segmentation experiments and one contour integration experiment were carried out with the intention of providing an answer to the question of how useful is local absolute spatial phase for texture segmentation and contour integration. The results support the possibility of two phase mechanisms (+/-cosine) for global texture segmentation, as well as for contour integration, when the elements that make up a given contour are orthogonal to contour paths.

Form Perception↗

PubMed2006

What image properties regulate eye growth?

The growth of the eye, unlike other parts of the body, is not ballistic. It is guided by visual feedback with the eventual aim being optimal focus of the retinal image or emmetropization . It has been shown in animal models that interference with the quality of the retinal image leads to a disruption to the normal growth pattern, resulting in the development of refractive errors and defocused retinal images . While it is clear that retinal images rich in pattern information are needed to control eye growth, it is unclear what particular aspect of image structure is relevant. Retinal images comprise a range of spatial frequencies at different absolute and relative contrasts and in different degrees of spatial alignment. Here we show, by using synthetic images, that it is not the local edge structure produced by relative spatial frequency alignments within an image but rather the spatial frequency composition per se that is used to regulate the growth of the eye. Furthermore, it is the absolute energy at high spatial frequencies regardless of the spectral slope that is most effective. Neither result would be expected from currently accepted ideas of how human observers judge the degree of image "blur" in a scene where both phase alignments and the relative energy distribution across spatial frequency (i.e., spectral slope) are important.

Animals↗

PubMed2006

Second-order spatial frequency and orientation channels in human vision.

We compared the number of spatial frequency and orientation mechanisms underlying first- versus second-order processing by measuring discrimination at detection threshold for first- and second-order Gabors to determine the smallest difference in spatial frequency and orientation that permits accurate discrimination at threshold. For second-order gratings, the number of channels is the same as for first-order gratings for spatial frequencies up to about 2 cpd; however, there are fewer second-order channels at higher spatial frequencies. In contrast, the number of labeled channels for orientation is the same for first- and second-order gratings. In conclusion, our findings provide evidence for distinct spatial frequency and orientation labeled detectors in second-order visual processing. We also show that, relative to first-order, there are fewer second-order channels processing higher spatial frequencies. This is consistent with a filter-rectify-filter scheme for second-order in which the second stage of filtering is at lower spatial frequencies.

Contrast Sensitivity↗

PubMed2006

Modulation of V1 activity by shape: image-statistics or shape-based perception?

It is current dogma that neurons in primary visual cortex extract local edges from the scene from which later visual areas reconstruct more meaningful shapes. Recent neuroimaging studies, however, have shown V1 modulations by the degree of structure in the image (shape). These V1 modulations due to the level of shape coherence have been explained in one of two possible ways: due to changes in image statistics or shape-based perceptual influences from higher visual areas. Here we compare both hypotheses using stimuli composed of Gabor arrays constructed to form circular shapes that can be successively degraded by manipulating the orientations of individual Gabors while maintaining local and global statistics. In a first experiment, we confirm that V1 responses are inversely correlated with the degree of structure in the image. In a second experiment, stimulus predictions are compared based on the degree of circular shape or change in the image statistic varied (orientation variance) in the image. We find that these V1 modulations to shape change are correlated with low-level changes in orientation contrast rather than shape perception per se.

Adult↗

PubMed2006

Stereo dynamics are not scale-dependent.

The experiments reported here focus on the temporal dynamics of stereopsis in an effort to shed light on how low level mechanisms might contribute to the execution of coarse-to-fine processing in the human stereo system. Because previous studies have used a variety of stimuli and configurations, we assess the effect of exposure duration on stereo thresholds using band-limited Gabor patches for a range of stimulus configurations. In preliminary studies, we found that the best stereo sensitivity-spatial frequency relationship was obtained when using configurations in which the size and target-reference spacing were consistent with spatially scaled stimuli. Sub-optimal stereo sensitivity as a function of spatial frequency was observed when the size and separation were fixed. Further, we found that the temporal properties of stereopsis were consistently sustained in nature irrespective of the stimulus spatial frequency content. This latter finding suggests that if coarse-to-fine stereo processing does occur it does not follow as a consequence of the dynamics of low-level disparity transduction.

Contrast Sensitivity↗

PubMed2006

Integration of local motion is normal in amblyopia.

We investigate the global integration of local motion direction signals in amblyopia, in a task where performance is equated between normal and amblyopic eyes at the single element level. We use an equivalent noise model to derive the parameters of internal noise and number of samples, both of which we show are normal in amblyopia for this task. This result is in apparent conflict with a previous study in amblyopes showing that global motion processing is defective in global coherence tasks [Vision Res. 43, 729 (2003)]. A similar discrepancy between the normalcy of signal integration [Vision Res. 44, 2955 (2004)] and anomalous global coherence form processing has also been reported [Vision Res. 45, 449 (2005)]. We suggest that these discrepancies for form and motion processing in amblyopia point to a selective problem in separating signal from noise in the typical global coherence task.

Adult↗

PubMed2005

The accessibility of spatial channels for stereo and motion.

Using fractal noise images, we measured the dependence of D(min) on the spatial passband (spatial frequency and orientation) over which information was correlated either between the eyes for stereo or between sequential frames for motion. Without affecting the amplitude spectrum of the noise stimulus we used idealized filters to scramble the phase of components outside a pre-defined passband. Using a simple Gaussian model in which performance depends on the signal/noise within a restricted spatial region, we obtained estimates of the bandwidth of the narrowest underlying spatial frequency and orientation spectral region subserving these two comparable tasks. Spatial bandwidths varied with peak spatial frequency but were very broad approximating the spectrum of the stimulus itself. Orientation properties of the underlying mechanisms were isotropic. These results suggest that the independent activity of individual narrowband spatial channels is not perceptually accessible for these tasks.

Depth Perception↗

PubMed2005

Size matters, but not for everyone: individual differences for contrast discrimination.

It is very well known that contrast detection thresholds improve with the size of a grating-type stimulus, but it is thought that the benefit of size is abolished for contrast discriminations well above threshold [e.g., Legge, G. E., & Foley, J. M. (1980)]. Here we challenge the generality of this view. We performed contrast detection and contrast discrimination for circular patches of sine wave grating as a function of stimulus size. We confirm that sensitivity improves with approximately the fourth-root of stimulus area at detection threshold (a log-log slope of -0.25) but find individual differences (IDs) for the suprathreshold discrimination task. For several observers, performance was largely unaffected by area, but for others performance first improved (by as much as a log-log slope of -0.5) and then reached a plateau. We replicated these different results several times on the same observers. All of these results were described in the context of a recent gain control model of area summation [Meese, T. S. (2004)], extended to accommodate the multiple stimulus sizes used here. In this model, (i) excitation increased with the fourth-root of stimulus area for all observers, and (ii) IDs in the discrimination data were described by IDs in the relation between suppression and area. This means that empirical summation in the contrast discrimination task can be attributed to growth in suppression with stimulus size that does not keep pace with the growth in excitation.

Adult↗

PubMed2005

The spatial frequency and orientation selectivity of the mechanisms that extract motion-defined contours.

The human visual system can undertake a specialized form of motion integration, one that enables the presence of extended spatial contours to be disambiguated from their backgrounds. We have shown previously that the visual system can selectively integrate local motion signals when their directions are along spatial contours and its efficiency is inversely related to the curvature of the contour involved (Ledgeway, T., & Hess, R. F. (2002). Vision Research, 42, 653-659). This integration primarily involves the direction, rather than the speed, of local motion signals. In the present study, we sought to investigate both the spatial frequency and orientation tuning of this specialized contour integration process, using a path detection paradigm. The results show that the tuning for spatial frequency is very broad, in line with previous studies that have examined this issue. In contrast, the orientation selectivity of the mechanism mediating contour extraction under these conditions is relatively narrowband. Thus, spatial frequency but not orientation pooling appears to take place prior to the extraction of motion-defined contours, a situation that is different from that previously shown for spatial contours composed of static, oriented elements.

Form Perception↗

PubMed2005

Detection, discrimination and integration of second-order orientation information in strabismic and anisometropic amblyopia.

To better understand the nature of the cortical deficit in amblyopia we undertook a systematic investigation of second-order processing in 8 amblyopic and 8 normal observers. We investigated local detection, discrimination and global integration. Our local stimulus consisted of a Gaussian patch of fractal noise multiplied by a 1-d sinusoidal modulator. Our global stimulus consisted of an array of such elements. We revealed second-order detection deficits for stimuli with equi-visible carriers. Orientation discrimination for an isolated second-order patch was comparable in normal and amblyopic eyes. We showed that pure integration of second-order patterns can be normal in amblyopia.

Adult↗

PubMed2005

Interocular suppression is gated by interocular feature matching.

We present a new form of contrast masking in which the target is a patch of low spatial frequency grating (0.46 c/deg) and the mask is a dark thin ring that surrounds the centre of the target patch. In matching and detection experiments we found little or no effect for binocular presentation of mask and test stimuli. But when mask and test were presented briefly (33 or 200 ms) to different eyes (dichoptic presentation), masking was substantial. In a 'half-binocular' condition the test stimulus was presented to one eye, but the mask stimulus was presented to both eyes with zero-disparity. This produced masking effects intermediate to those found in dichoptic and full-binocular conditions. We suggest that interocular feature matching can attenuate the potency of interocular suppression, but unlike in previous work (McKee, S. P., Bravo, M. J., Taylor, D. G., & Legge, G. E. (1994) Stereo matching precedes dichoptic masking. Vision Research, 34, 1047) we do not invoke a special role for depth perception.

Accommodation, Ocular↗

PubMed2005

The influences of visibility and anomalous integration processes on the perception of global spatial form versus motion in human amblyopia.

Do amblyopes demonstrate general irregularities in processes of global image integration? Or are these anomalies stimulus specific? To address these questions we employed directly analogous global-orientation and global-motion stimuli using a method that allows us to factor out any influence of the low-level visibility loss [Simmers, A. J., Ledgeway, T., Hess, R. F., & McGraw, P. V. (2003). Deficits to global motion processing in human amblyopia. Vision Research 43, pp. 729-738]. The combination of orientation and motion coherence thresholds reported here provides comparable psychophysical measures of global processing by spatial-sensitive and motion-sensitive mechanisms in the amblyopic visual system. The results show deficits in both global-orientation and global-motion processing in amblyopia, which appear independent of any low-level visibility loss, but with the most severe deficit affecting the extraction of global motion. This provides evidence for the existence of a dominant temporal processing deficit in amblyopia.

Adult↗

PubMed2005

Contributions of local orientation and position features to shape integration.

Contour integration plays an important role in linking local elements into global shape and the binding strength among local elements depends on both orientation and position features. The very high sensitivity reported for detecting the sinusoidal deformation of circular contours may result from the presence and concordance of both orientation and position cues to shape difference. In this study, position and orientation-defined micropatch-sampled radial frequency (MSRF) patterns were employed, which permit the independent assessment of the contributions of local orientation and position features to shape integration. It was demonstrated that, while both local orientation and position features can encode shape deformation, the human visual system is more sensitive to orientation-defined shape difference than to position-defined shape difference. Furthermore, integration of the local orientation feature into shape is more than two times stronger than that of local position, and may involve a global pooling mechanism. Nevertheless, optimal shape discrimination performance requires the analysis of both local orientation and position features.

Adult↗

PubMed2005

Grouping local orientation and direction signals to extract spatial contours: empirical tests of "association field" models of contour integration.

Over the last decade or so a great deal of psychophysical research has attempted to delineate the principles by which local orientations and motions are combined across space to facilitate the detection of simple spatial contours. This has led to the development of "association field" models of contour detection which suggest that the strength of linking between neighbouring elements in an image, is determined by the degree to which they aligned along smooth (first-order) curves. To test this assumption we used a path detection paradigm to compare the ability of observers to identify the presence of contours defined by either spatial orientation, motion direction or by specific combinations of both types of visual attribute. The relative alignment of the local orientations and/or directions with respect to the axis of the depicted contour was systematically varied. For orientation-defined contours detection was best when the elements were aligned along (parallel with) the contour axis, approached chance levels for obliquely oriented elements and then improved for elements that were orthogonal to the contour axis (i.e., performance was a U-shaped function of degree of orientation misalignment). This pattern of results was found for both straight and curved contours and is not readily explicable in terms of current association field theories. For motion-defined contours, however, performance simply deteriorated as the relative directions of the constituent path elements were progressively misaligned with respect to the contour. Thus the rules by which local orientations are linked to define spatial contours are qualitatively different from those used for linking local directions and each may be mediated by distinct visual mechanisms. When both orientation and motion cues were simultaneously available, contour detection performance was generally enhanced, in a manner that is consistent with probability summation. We suggest that association field models of orientation linking may need to be extended in light of the present findings.

Cues↗

PubMed2005

Haphazard neural connections underlie the visual deficits of cats with strabismic or deprivation amblyopia.

Identification of the neural basis of the visual deficits experienced by humans with amblyopia, particularly when associated with strabismus (strabismic amblyopia), has proved to be difficult in part because of the inability to observe directly the neural changes at various levels of the human visual pathway. Much of our knowledge has necessarily been obtained on the basis of sophisticated psychophysical studies as well as from electrophysiological explorations on the visual pathways in animal models of amblyopia. This study combines these two approaches to the problem by employing similar psychophysical probes of performance on animal models of two forms of amblyopia (deprivation and strabismic) to those employed earlier on human amblyopes (Hess & Field, 1994, Vis. Res., 34, 13397-13406). The tests explore two competing explanations for the visual deficits, namely an evenly distributed loss of neural connections (undersampling) with the amblyopic eye as opposed to disordered connections with this eye (neural disarray). Unexpectedly, the results in animal models of deprivation amblyopia were not in accord with expectations based upon an even distribution of lost connections with the amblyopic eye. However, the results were similar to those observed in a strabismic amblyopic animal and to strabismic amblyopic humans. We suggest that deprivation amblyopia may be accompanied by an uneven loss of connections that results in effective neural disarray. By contrast, amblyopia associated with strabismus might arise from neural disarray of a different origin such as an alteration of intrinsic cortical connections.

Amblyopia↗