Biomedical subjects

R F Hess

Publications and source records attributed to R F Hess.

At least 37 records · Page 2Linked to original sources

PubMed2000

Impoverished second-order input to global linking in human vision.

Recent evidence points to the importance of global operations across spatial regions larger than individual cortical receptive fields. Studies of contour integration and motion trajectory detection suggest that network operations between local detectors underlie the encoding of extended contours in space and extended trajectories in motion. Here we ask whether such network operations also occur between second-order-detectors known to exist in visual cortex. We compared performance for stimuli composed of either first-order or second-order elements equated for visibility, and we show that unlike the first-order case, there is little or no linking interaction between local second-order detectors. Near chance performance was found for elements defined by second-order attributes when observers had to identify either an elongated spatial contour or an extended motion trajectory embedded in noise elements. This implies that the network operations thought to underlie these two global tasks receive, at best, an impoverished input from local detectors that encode second-order image attributes.

Contrast Sensitivity↗

PubMed2000

The properties of the motion-detecting mechanisms mediating perceived direction in stochastic displays.

Previous studies [e.g. Baker & Hess, 1998. Vision Research, 38, 1211-1222] have shown that perceived direction in displays composed of multiple, limited-lifetime, Gabor micropatterns (G) is influenced by movement both at the fine spatial scale of the internal luminance modulation (first-order motion) and the coarse spatial scale of the Gaussian, contrast window (second-order motion). However it is presently indeterminate as to whether this pattern of results is indicative of the processes by which first-order and second-order motion signals interact within the visual system per se or those by which motion information, irrespective of how it is defined, is utilised across different spatial scales. To address this issue, and more generally the properties of the mechanisms that analyse motion in such displays, we employed stochastic motion sequences composed of either G, G added to a static carrier (G + C) or G multiplied with a carrier (G*C). Crucially G*C, unlike both G and G + C, micropatterns contain no net first-order motion and second-order motion only at the scale of the internal contrast modulation. For small displacements perceived direction in all cases showed a dependence on the internal sinusoidal spatial structure of the micropatterns and characteristic oscillations were typically observed, consistent with models in which first-order motion and second-order motion are encoded on the basis of similar low-level mechanisms. Importantly for larger displacements, and also when the internal spatial structure was randomised on successive exposures (so that motion at this spatial scale was unreliable), performance tended to be veridical for all types of micropattern, even though under these conditions displacements of the G*C micropatterns should have been invisible to current, low-level, motion-detecting schemes. This suggests that both low-level motion sensors and mechanisms utilising a different motion-detecting strategy such as high-level, attentive, feature-tracking may mediate perceptual judgements in stochastic displays.

Humans↗

PubMed2000

The effects of blur and size on monocular and stereoscopic localization.

Monocular localization of non-abutting stimuli and stereoscopic localization of the same second-order targets are performed with the same precision (Wilcox, L.M. & Hess, R.F. (1996) Is the site of non-linear filtering in stereopsis before or after binocular combination? Vision Research, 36, 391-399). Further, both tasks show a similar dependence on the scale of the stimulus. Since prior studies used Gaussian-enveloped stimuli, modifications of stimulus scale produced concurrent changes in edge blur. The experiments reported here assess the relative contributions of size and blur to the observed dependence on envelope scale for both monocular localization and stereoacuity. Stereoacuity for first-order targets was found to be an order of magnitude better than stereoacuity for second-order targets and monocular acuity for both first- and second-order targets. Further, while first-order stereopsis was found to depend solely on blur, second-order stereoacuity and monocular acuity were affected by both size and blur. These results suggest that while stereoacuity for first-order stimuli may be determined by a correlative process limited by early additive noise, stereoacuity for second-order stimuli and monocular acuity for non-abutting targets are more likely limited by stimulus-dependent spatial subsampling.

Depth Perception↗

PubMed2000

The foveal 'crowding' effect: physics or physiology?

It has been known for some time that both foveal and peripheral visual acuity is higher for single letters than for letters in a row. Early work showed that this was due to the destructive interaction of adjacent contours (termed 'crowding' or contour interaction). It has been assumed to have a neural basis and a number of competing explanations have been advanced which implicate either high-level or low-level stages of visual processing. Our results suggest a much simpler explanation, one primarily determined by the physics of the stimulus rather than the physiology of the visual system. We show that, under conditions of contour interaction or 'crowding', the most relevant physical spatial frequency band of the letter is displaced to higher spatial frequencies and that foveal vision tracks this change in spatial scale.

Form Perception↗

PubMed2000

Global factors that determine the maximum disparity for seeing cyclopean surface shape.

A disparity gradient limit explains why the maximum amplitude of sinusoidal disparity gratings increases with decreasing disparity spatial frequency. It also explains why the largest disparity for binocular fusion (diplopia threshold) varies directly with stimulus element separation. Does a disparity gradient limit also apply to the detection of cyclopean shape? A previous study addressed this question and concluded that it does not. We examined this question by measuring the largest disparity amplitude (dmax) at which observers could judge the shape of cyclopean disparity gratings. We used trapezoidal, triangular, sinusoidal, and square wave gratings in order to dissociate the effects of disparity gradient and disparity spatial frequency. Gabor micropatterns were used to minimize potential scale-dependent interactions with luminance processing. Our results support a disparity gradient limit for cyclopean shape perception, with additional factors being involved at high disparity spatial frequencies. Combining the gradient limit hypothesis with lowpass disparity filtering describes the pattern of dmax for both smooth and discontinuous surface shapes.

Depth Perception↗

PubMed2000

Local luminance factors that determine the maximum disparity for seeing cyclopean surface shape.

We measured the maximum disparity grating amplitude (d(max)) for seeing cyclopean surface shape, using stereograms made from dense arrays of micropatterns, whose luminance characteristics were manipulated. In Experiment 1, we used disparity gratings made from Gabor micropatterns. D(max) was found to vary inversely both with luminance spatial frequency and with Gabor size, but was constant for a constant bandwidth (frequency times size). To test whether this was due to changes in bandwidth per se or to changes in the number of local features, in Experiment 2 we manipulated the local feature content with a range of micropatterns that we termed 'edgels'. The results supported neither hypothesis. In Experiment 3 we varied the phases of the Fourier components of square wave edgels, thereby introducing more features, and we found that this did not change d(max). Taken together, our results show that d(max) decreases with an increase in the number of local luminance cycles at each luminance scale. D(max) is mainly limited by false target matching between similar components of the micropatterns. Stereopis, in terms of surface shape perception, is served only by first order mechanisms, and only by luminance filters that are broadband.

Depth Perception↗

PubMed2000

The roles of polarity and symmetry in the perceptual grouping of contour fragments.

We describe two experiments that investigate the roles of polarity and symmetry in the perceptual grouping of contour fragments. Observers viewed, for one second on each presentation, arrays of oriented, spatial-frequency band-pass, elements, in which a subset of the elements was aligned along a twisting curve. In each of five conditions we measured observers' ability to detect aligned combinations of even- and odd-symmetric elements, of the same and different polarities, against a background of 'noise' elements. As with previous experiments we found that the 'path' could be reliably detected, even when the elements of the path were oriented at angles of up to +/- 60 deg relative to each other. Detection of the path was still possible when the polarity of path elements alternated. However, the probability of detection of the path was raised significantly when the path elements were all of the same polarity. Perceptual grouping of even-symmetric elements was no different to perceptual grouping of odd-symmetric elements. The results provide evidence, that in achieving integration of contour fragments, the visual system uses a process that is to some degree phase selective. We use the results to describe how the visual system may resolve natural contours when they occur against backgrounds that vary over a wide range of intensities. The data presented here have been published in conference-abstract form (Hayes et al., 1993; Field et al., 1997).

Dark Adaptation↗

PubMed2000

Regional sensitivity for shape discrimination.

In a task where subjects had to detect smooth deviations from circularity, we assessed whether performance varied with eccentricity. Our stimuli were circular 4th derivatives of Gaussian contours (CD4s) whose radii were sinusoidally modulated. We used D4s of different peak spatial frequencies and overall diameters. Although performance declined with eccentricity for all radial frequencies tested, once scaling was taken into account, sensitivity was similar at all eccentricities. This was the basis of the scale-invariance also exhibited by this task. Thus, shape discrimination does not appear to be a specialized function limited to central vision.

Form Perception↗

PubMed2000

Contour interaction in fovea and periphery.

It has been known for some time that both foveal and peripheral visual acuity are higher for single letters than for letters in a row. Early work showed that this was due to the destructive interaction of adjacent contours (termed contour interaction). It has been assumed to have a neural basis, and a number of competing explanations have been advanced that implicate either high-level or low-level stages of visual processing. Our previous results for foveal vision suggested a much simpler explanation, one determined primarily by the physics of the stimulus rather than the physiology of the visual system. We show that, under conditions of contour interaction or crowding, the most relevant physical spatial-frequency band of the letter is displaced to higher spatial frequencies and that foveal vision tracks this change in spatial scale. In the periphery, however, beyond 5 degrees, the physical explanation is not sufficient. Here we show that there are genuine physiological lateral spatial interactions, which are due to changes in the spatial scale of analysis.

Adult↗

PubMed1999

On the relationship between the spatial channels for luminance and disparity processing.

To determine the relationship between the spatial channels for luminance and shape-from-stereo-disparity processing we measured disparity modulation sensitivity as a function of disparity spatial frequency for sinusoidal modulations of a field of Gabor micropatterns of differing luminance spatial frequency. We first examine the effects of contrast, spatial bandwidth and element density and show that it is only the last of these which is critical for the shape of the disparity modulation threshold function. We show that the shape of this function depends on the luminance spatial frequency of the surface that is modulated in depth. Specifically, low corrugation frequencies enjoy a greater scale support from the early luminance spatial filters than do high corrugation frequencies. The results are consistent with higher spatial frequency disparity channels receiving a greater input from higher spatial frequency luminance channels.

Contrast Sensitivity↗

PubMed1999

Contour integration in the peripheral field.

Contour integration was measured in the normal peripheral field to determine if an explanation based solely on the known peripheral 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, D. J., Hayes A., & Hess, R. F. (1993). Vision Research, 33, 173-193). The intrinsic positional uncertainty for each eccentric locus was measured with the same stimulus and it did not account for levels of peripheral performance. We show that peripheral performance on this task does not get worse with eccentricity beyond about 10 degrees and that these results can be modeled by simple filtering without any subsequent cellular linking interactions.

Discrimination, Psychological↗

PubMed1999

A deficit in strabismic amblyopia for global shape detection.

Using a task which relied upon the detection of sinusoidal deformations from circularity, we show that strabismic amblyopes exhibit deficits which are not critically dependent on either the scale of deformation or the spatial frequency characteristics of the stimulus (circular D4) itself. We show that this loss is not due to the restricted passband of the amblyopic eye. Furthermore, in a pedestal distortion experiment, we show that the suprathreshold form of this loss is consistent with an elevated level of 'intrinsic noise' rather than a loss in 'sampling efficiency'.

Amblyopia↗

PubMed1999

Stereoscopic depth but not shape perception from second-order stimuli.

Depth can be seen using either linear (first-order) or non-linear (second-order) stereo micropatterns when, in the latter, contrast envelopes contain the disparity information. We examined whether a second-order mechanism can contribute to the perception of 3-D surface shape. Using a variety of different stimulus types, we show that for each, shape is easy to see with linear stimuli. Over a wide range of parameters however, none of our observers perceived shape, however faintly, from the non-linear stimuli. To explore why these elements failed, we simplified our stimulus to a step-edge in depth and measured performance while varying the number of elements. We show how performance declined when more than two non-linear elements were used. We discuss reasons for the limitation found for non-matching elements, including a dissociation for stereopsis between seeing surface shape and depth.

Contrast Sensitivity↗

PubMed1999

The interaction of first- and second-order cues to orientation.

The visual system is sensitive to orientation information defined both by first-order (luminance) and by second-order (texture) cues. We consider how these orientation cues are computed and how they affect one another. We measured the perceived orientation of the first and second-order components of Gabor patches (the carrier and envelope, respectively) and report a dependence of the perceived orientation of each on the orientation of the other, and on the spatial frequency of the carrier. Fixing the carrier orientation near that of the envelope interferes with envelope orientation judgements. This interference is reduced by adding a small (subthreshold) rotation to the carrier indicating that the site of interference is early. When the gross relative orientation of carrier and envelope is varied, the carrier appears systematically tilted towards the envelope. However, provided envelope and carrier are separated by more than approximately 10 degrees, the perceived envelope orientation appears tilted away from the carrier. The size of these effects increases with decreasing carrier spatial frequency, and with increasing exposure duration. When the envelope and carrier are both non parallel and non-perpendicular Fourier energy is distributed asymmetrically across orientation. We demonstrate that, for a channel-based orientation code, this asymmetry induces a shift in mean orientation that is sufficient to explain illusory tilting of carriers. The illusory tilting of the envelope, as a function of carrier orientation and spatial frequency, demonstrates that human ability to demodulate contrast information is far from ideal and cannot be explained by existing two-stage filter-rectify-filter models. We propose that illusory tilting of the envelope is due to selective connectivity between first- and second-stage filters whose purpose is to dissociate the type of image structure producing each class of cue.

Cues↗

PubMed1999

Discriminating local continuity in curved figures.

We assessed whether the visual system's ability to discriminate subtle perturbations from smoothness in curved shapes was based on 1st-order properties or 2nd-order properties. We investigated which of the two would determine performance in a task where the observer had to detect spatial jitter on aligned, unaligned or unoriented Gabor patches forming either an open or enclosed path. Surprisingly, performance was no better in the conditions employing aligned micropatterns, implicating the use of 2nd-order properties. Varying the peak spatial frequency or the size, (standard deviation of the Gaussian envelope), produced little change in the jitter threshold. By contrast, increasing the spacing between the Gabor patches had a large detrimental effect. Randomizing the orientation of the Gabors also hampered performance. These results indicate that orientation linking may only aid psychophysical performance in detection tasks. If variance was imposed on the size of the blobs (a 2nd-order property), performance was degraded. Variance on the carrier spatial frequency (a 1st-order property) resulted in a smaller worsening of performance. Overall, our results imply that shape discrimination is performed by mechanisms sensitive to 2nd-order micropattern properties, although some dependence on 1st-order properties exists.

Discrimination, Psychological↗

PubMed1999

The neural deficit in strabismic amblyopia: sampling considerations.

In order to understand the nature of the neural loss in strabismic amblyopia, we have applied a technique which has been used in the normal periphery to psychophysically probe the sampling properties of the neuronal population. We ask whether there is a 'sampling' deficit and if so whether it is based on either an absolute loss of neurons (i.e. spatial undersampling) or an irregular arrangement of a normal number of neurons (i.e. irregular sampling). Our results suggest that neural pooling restricts the spatial frequency region where sampling considerations are important to a very small part of the visible high spatial frequency range. Within this limited region, irregular sampling rather than spatial undersampling is the greater contributor to the strabismic amblyope deficit.

Amblyopia↗

PubMed1999

The orientation discrimination deficit in strabismic amblyopia depends upon stimulus bandwidth.

We show that the previously reported orientation deficit in amblyopia (Skottun, B. C., Bradley, A., & Freeman, R. D. (1986). Orientation discrimination in amblyopia. Investigative Ophthalmology and Visual Science, 30, 532-537) also occurs for arrays of randomly positioned Gabor micropatterns for which explanations based on either neural disarray or local neural interactions would not hold. Furthermore, when using Gabors, we show that the deficit varies with the spatial frequency and orientational bandwidth of the stimuli used to measure it. We discuss two competing explanations for this, one based on a broader underlying detector bandwidth in amblyopia (both orientation and spatial frequency) and the other based on a selective deficit of first-order, as opposed to second-order orientation processing in strabismic amblyopia. Our results favour the latter interpretation.

Amblyopia↗

PubMed1999

Are judgements of circularity local or global?

We assessed, in a task where subjects had to detect smooth deviations from circularity, whether the underlying mechanisms were localised in space to the size of the individual perturbations or whether they computed global shape. By manipulating the phase, the number of cycles of modulation and the spatial arrangement of the perturbations we argue that although either aspect can be detected, performance is ultimately limited by a global shape detecting mechanism. We show that this global mechanism receives input from spatially coarse, crossed orientationally tuned filters whose peak position in orientation depends on the overall shape to be detected.

Discrimination, Psychological↗