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R F Hess

Publications and source records attributed to R F Hess.

At least 19 recordsLinked to original sources
PubMed2006

The extent of the dorsal extra-striate deficit in amblyopia.

Previously, we have shown that humans with amblyopia exhibit deficits for global motion discrimination that cannot be simply ascribed to a reduction in visibility or contrast sensitivity. Deficits exist in the processing of global motion in the fronto-parallel plane that suggest reduced extra-striate function (i.e., MT) in amblyopia. Here, we ask whether such a deficit also exists for rotation and radial components of optic flow that are first processed at higher sites along the dorsal pathway (i.e., MSTd). We show that similar motion processing deficits occur in our amblyopic group as a whole for translation, rotation, and radial components of optic flow and that none of these can be solely accounted for by the reduced visibility of the stimuli. Furthermore, on a subject-by-subject basis there is no significant correlation between the motion deficits for radial and rotational motion and those for translation, consistent with independent deficits in dorsal pathway function up to and including MSTd.

Adult↗
PubMed2006

Only two phase mechanisms, +/-cosine, in human vision.

We evaluated the proposal that there exist detectors of the following four cardinal phases in human vision: +cosine, -cosine, +sine, and -sine. First, we assessed whether there was evidence that these cardinal phases were processed by independent 'labeled lines,' using a discrimination at detection threshold paradigm. Second, we assessed whether suprathreshold phase discrimination was best at phases intermediate between these cardinal values. Third, we tried to replicate previous evidence showing that an absence of facilitation occurs only between cosine pedestals and sine tests (or vice-versa). In all three experimental approaches we found no compelling evidence for four cardinal phase groupings. We did however find evidence for independent detectors for pure increments and decrements (+/-cosine). We suggest that phase discrimination, whether at threshold or suprathreshold, is mediated by mechanisms that encode the relative positions and contrasts of local increments and decrements within the stimulus.

Discrimination, Psychological↗
PubMed2003

How many positions can we perceptually encode, one or many?

Here we show that our sensitivity for discriminating relative position across the visual field is limited. In experiment 1 we show that we are much worse at detecting a texture defined by the relative position of elements within an array than would be expected if we had access to multiple estimates of relative position across the visual field. In experiment 2 we show that human performance is impaired for positional judgments when there is uncertainty as to which of a number of possible elements is misaligned. This impairment is greater than one would expect from an ideal observer model and greater than that found for a comparable task involving orientation. It is consistent with positional thresholds being determined by only one estimate of relative position. In experiment 3 we estimate the number of suprathreshold positional signals that can be pooled at the same time across the visual field using a standard summation variance paradigm. The results suggest that the human visual system is limited to one estimate of position, but additional estimates can be built up serially over time; however, this process is slow and probably cognitive in nature. These experiments taken as a whole suggest that only one estimate of relative position (i.e. relative to a predefined reference) at a time is accessible at the perceptual level.

Attention↗
PubMed2002

Orientation sensitivity in human visual motion processing.

Orientation tuning of receptive fields is well documented in the spatial domain, but considerable variability exists amongst published estimates of orientation sensitivity of motion receptive fields. We used a two-frame motion sequence, in which one frame was binary noise and the other was a horizontally displaced and filtered version of the same noise field, to examine the orientation sensitivity of human motion mechanisms. Initially, orientations orthogonal to the direction of motion were removed from each filtered frame. Observers indicated perceived direction of motion in a single interval, binary choice task. D(max) was determined for different amounts of removed orientations, and found to remain constant across the removal of energy up to approximately +/-60 deg from vertical. In a second experiment, the orientations removed were now parallel to the direction of motion of the stimulus. D(max) fell as a cosine function with increasing removal of orientation information, in agreement with off-orientation looking or matched filtering predictions. The two experiments show the presence of mechanisms both broadly tuned and more narrowly tuned for orientation. A control experiment introduced an interstimulus interval between the two frames of our motion sequence. Performance on the direction discrimination task was severely degraded, indicating that the original results are not explicable in terms of a feature-tracking or long-range motion process. The presence of both broadly and narrowly tuned mechanisms implies multiple possible solutions to the processing of coherent plaid motion.

Humans↗
PubMed2002

Temporal detection in human vision: dependence on eccentricity.

Studies of human perception of time-varying luminance often aim to estimate either temporal impulse response shapes or temporal modulation transfer functions (MTFs) of putative temporal processing mechanisms. Previously, temporal masking data have been used to estimate the properties and numbers of these temporal mechanisms in central vision for 1 cycle per degree (cpd) targets [Fredericksen and Hess (1998)]. The same methods have been used to explore how these properties change with stimulus energy [Fredericksen and Hess (1997)] and spatial frequency [Fredericksen and Hess (1999)]. We present here analyses of the properties of temporal mechanisms that detect temporal variations of luminance in peripheral vision. The results indicate that a two-filter model provides the best model for our masking data, but that no multiple filter model provides an acceptable fit across the range of parameters varied in this study. Single-filter modelling shows differences between processing mechanisms at 1 cpd in central vision and those that operate eccentrically. We find evidence that this change is because of differences in relative sensitivities of the mechanisms, and to differences in fundamental mechanism impulse responses.

Electronics↗
PubMed2001

Synthesis and characterization of six new quaternary actinide thiophosphate compounds: Cs(8)U(5)(P(3)S(10))(2)(PS(4))(6)(,) K(10)Th(3)(P(2)S(7))(4)(PS(4))(2), and A(5)An(PS(4))(3), (A = K, Rb, Cs; An = U, Th).

Six new actinide metal thiophosphates have been synthesized by the reactive flux method and characterized by single-crystal X-ray diffraction: Cs(8)U(5)(P(3)S(10))(2)(PS(4))(6) (I), K(10)Th(3)(P(2)S(7))(4)(PS(4))(2) (II), K(5)U(PS(4))(3) (III), K(5)Th(PS(4))(3) (IV), Rb(5)Th(PS(4))(3) (V), and Cs(5)Th(PS(4))(3) (VI). Compound I crystallizes in the monoclinic space group P2(1)/c with a = 33.2897(1) A, b = 14.9295(1) A, c = 17.3528(2) A, beta = 115.478(1) degrees, Z = 8. Compound II crystallizes in the monoclinic space group C2/c with a = 32.8085(6) A, b = 9.0482(2) A, c = 27.2972(3) A, beta = 125.720(1) degrees, Z = 8. Compound III crystallizes in the monoclinic space group P2(1)/c with a = 14.6132(1) A, b = 17.0884(2) A, c = 9.7082(2) A, beta = 108.63(1) degrees, Z = 4. Compound IV crystallizes in the monoclinic space group P2(1)/n with a = 9.7436(1) A, b = 11.3894(2) A, c = 20.0163(3) A, beta = 90.041(1) degrees, Z = 4, as a pseudo-merohedrally twinned cell. Compound V crystallizes in the monoclinic space group P2(1)/c with a = 13.197(4) A, b = 9.997(4) A, c = 18.189(7) A, beta = 100.77(1) degrees, Z = 4. Compound VI crystallizes in the monoclinic space group P2(1)/c with a = 13.5624(1) A, b = 10.3007(1) A, c = 18.6738(1) A, beta = 100.670(1) degrees, Z = 4. Optical band-gap measurements by diffuse reflectance show that compounds I and III contain tetravalent uranium as part of an extended electronic system. Thorium-containing compounds are large-gap materials. Raman spectroscopy on single crystals displays the vibrational characteristics expected for [PS(4)](3)(-), [P(2)S(7)](4-), and the new [P(3)S(10)](5)(-) building blocks. This new thiophosphate building block has not been observed except in the structure of the uranium-containing compound Cs(8)U(5)(P(3)S(10))(2)(PS(4))(6).

Journal Article↗
PubMed2001

The cortical deficit in humans with strabismic amblyopia.

To further our understanding of the cortical deficit in strabismic amblyopia, we measured, compared and mapped functional magnetic resonance imaging (fMRI) activation between the fixing and fellow amblyopic eyes of ten strabismic amblyopes. Of specific concern was whether the function of any visual area was spared in strabismic amblyopia, as recently suggested by both positron emission tomography (PET) and fMRI studies, and whether there was a close relationship between the fMRI response and known psychophysical deficits. To answer these questions we measured the psychophysical deficit in each subject and used stimuli whose relationship to the psychophysical deficit was known. We observed that stimuli that were well within the amblyopic passband did produce reduced fMRI activation, even in visual area V1. This suggests that V1 is anomalous in amblyopia. A similar level of reduction was observed in V2. In two subjects, we found that stimuli outside the amblyopic passband produced activation in visual area V3A. We did not find a close relationship between the fMRI response reduction in amblyopia and either of the known psychophysical deficits even though the fMRI response reduction in amblyopia did covary with stimulus spatial frequency.

Adolescent↗
PubMed2001

Dynamics of contour integration.

To determine the dynamics of contour integration the temporal properties of the individual contour elements were varied as well as those of the contour they form. A temporal version of a contour integration paradigm (Field, D. J., Hayes, A., & Hess, R. F. (1993) Vision Research, 33, 173-193) was used to assess these two temporal dynamics as a function of the contrast of individual elements and the curvature of the contour. The results show that the dynamics of contour integration are good when the contrast of the individual elements is modulated in time (10-30 Hz), but are poor when contour linking per se is temporally modulated (1-12 Hz). The dynamics of contour linking is not dependent on the absolute contrast of the linking elements, so long as they are visible, but does vary with the curvature of the contour. For straight contours, temporal resolution is around 6-12 Hz but falls to around 1-2 Hz for curved contours.

Analysis of Variance↗
PubMed2001

Contour interaction in amblyopia: scale selection.

It has been known for some time that visual acuity in amblyopia is higher for single letters than for letters in a row (termed crowding). Early work showed that this could not be accounted for on the basis of the destructive interaction of adjacent contours (termed contour interaction), which was shown to be, in resolution units, normal in amblyopia. We have re-examined this issue using a letter stimulus that is modulated about a mean light level. This allows an examination of the effects of contrast polarity and spatial filtering within the contour interaction paradigm. We show that the majority of strabismic amblyopes that we investigated exhibit an anomalous contour interaction that, in some cases, was dependent on the contrast polarity of the flanking stimuli. Furthermore, we show that while amblyopes do select the optimum scale of analysis for unflanked stimuli, they do not select the optimum scale of analysis for flanked stimuli. For reasons that may have to do with their poorer shape discrimination, they select a non-optimal scale to process flanked stimuli.

Adolescent↗
PubMed2001

Spatial coherence does not affect contrast discrimination for multiple Gabor stimuli.

Gestalt grouping rules imply a process or mechanism for grouping together local features of an object into a perceptual whole. Several psychophysical experiments have been interpreted as evidence for constrained interactions between nearby spatial filter elements and this has led to the hypothesis that element linking might be mediated by these interactions. A common tacit assumption is that these interactions result in response modulation which disturbs a local contrast code. We addressed this possibility by performing contrast discrimination experiments using two-dimensional arrays of multiple Gabor patches arranged either (i) vertically, (ii) in circles (coherent conditions), or (iii) randomly (incoherent condition), as well as for a single Gabor patch. In each condition, contrast increments were applied to either the entire test stimulus (experiment 1) or a single patch whose position was cued (experiment 2). In experiment 3, the texture stimuli were reduced to a single contour by displaying only the central vertical strip. Performance was better for the multiple-patch conditions than for the single-patch condition, but whether the multiple-patch stimulus was coherent or not had no systematic effect on the results in any of the experiments. We conclude that constrained local interactions do not interfere with a local contrast code for our suprathreshold stimuli, suggesting that, in general, this is not the way in which element linking is achieved. The possibility that interactions are involved in enhancing the detectability of contour elements at threshold remains unchallenged by our experiments.

Contrast Sensitivity↗
PubMed2001

What does the Ternus display tell us about motion processing in human vision?

The Ternus display is a moving visual stimulus which elicits two very different percepts, according to the length of the interstimulus interval (ISI) between each frame of the motion sequence. These two percepts, referred to as element motion and group motion, have previously been analysed in terms of the operation of a low-level, dedicated short-range motion process (in the case of element motion), and of a higher-level, attentional long-range motion process (in the case of group motion). We used a novel Ternus configuration to show that both element and group motion are, in fact, mediated solely by a process sensitive to changes in the spatial appearance of the Ternus elements. In light of this, it appears that Ternus displays tell us nothing about low-level motion processing, implying that previous studies using Ternus displays, for instance those dealing with dyslexia, require reinterpretation. Further manipulations of the Ternus display revealed that the orientation and spatial-frequency discrimination of the process underlying the analysis of Ternus displays is far worse than thresholds for spatial vision. We conclude that Ternus displays are analysed via a long-range motion, or feature-tracking, process, and that this process is distinct from spatial vision.

Discrimination, Psychological↗
PubMed2001

Luminance spatial scale facilitates stereoscopic depth segmentation.

Are differences in luminance spatial frequency between surfaces that overlap in depth useful for surface segmentation? We examined this question, using a novel stimulus termed a dual-surface disparity grating. The dual-surface grating was made from Gabor micropatterns and consisted of two superimposed sinusoidal disparity gratings of identical disparity-modulation spatial frequency and orientation but of opposite spatial phase. Corrugation amplitude thresholds for discrimination of the orientation of the dual-surface grating were obtained as a function of the difference in Gabor (luminance) spatial frequency between the two surfaces. When the Gabor micropatterns on the two surfaces were identical in spatial frequency, thresholds were very high and in some instances impossible to obtain. However, with as little as a 1-octave difference in spatial frequency between the surfaces, thresholds fell sharply to near-asymptotic levels. The fall in thresholds paralleled a change in the appearance of the stimulus from one of irregular depth to stereo transparency. The most parsimonious explanation for this finding is that the introduction of a between-surface luminance spatial-frequency difference reduces the number of spurious cross-surface binocular matches, thus helping to reveal the three-dimensional structure of the stimulus.

Depth Perception↗
PubMed2001

Centrifugal bias for second-order but not first-order motion.

Limited-lifetime Gabor stimuli were used to assess both first- and second-order motion in peripheral vision. Both first- and second-order motion mechanisms were present at a 20-deg eccentricity. Second-order motion, unlike first-order, exhibits a bias for centrifugal motion, suggesting a role for the second-order mechanism in optic flow processing.

Anisotropy↗
PubMed2001

Detection of constrast-defined shape.

We assessed the accuracy of contrast-defined shape detection of stimuli of constant aspect ratio, namely, circular bandpass stimuli whose radii were sinusoidally varied about a mean radius. Performance for these contrast-defined shapes, which we show is determined by the global rather than the local attributes of the stimulus, is 2-8 times worse than that for their luminance-defined counterparts, suggesting separate processing limitations. By spatially and orientationally filtering the two-dimensional fractal-noise carriers of which these stimuli were composed, we determined whether there are specific rules concerning the spatial and orientational input to shape detectors from mechanisms sensitive to the carrier structure. The results suggest that second-order circularity detectors receive mixed input from spatial-frequency-tuned and orientationally tuned cells.

Contrast Sensitivity↗
PubMed2001

Contour interaction for an easily resolvable stimulus.

It is well known that adjacent contours can reduce the visual acuity of single letters. Although this has traditionally been considered only in terms of a neural-based interaction, it has recently been suggested that the information content in the stimulus may account for the interaction. Here we ask the question, "Do similar interference effects occur for the discrimination of low-contrast letters whose size is larger than that corresponding to the resolution limit?" If so, previous acuity-based interaction results may be of more general importance. We show that while there are interference effects of nearby contours, they are of a form different from that observed at the resolution limit. In particular, the contrast polarity of the nearby contour is unimportant, which in turn suggests that a physical explanation is inappropriate. Also, the discrimination of an easily resolvable, unflanked Landolt C target requires information over a much wider spatial-frequency range than its counterpart at the resolution limit.

Discrimination, Psychological↗
PubMed2000

Stereodeficient subjects demonstrate non-linear stereopsis.

There appear to be two modes of stereoscopic processing: a conventional linear operation that is dependent on correspondence between local luminance components in the two eyes' views, and a non-linear or second-order processing mode. This second mode may use disparity information provided by particular 'non-Fourier' features of the stimulus such as the contrast envelope. Preliminary results suggest that people who fail standard clinical stereotests are able to extract non-linear disparity information from Gabor stimuli [McColl & Mitchell, 1998. Vision Research, 38, 1889-1900]. Here we evaluate the status of the non-linear mechanism in such individuals by using two types of contrast enveloped stimuli, namely random line and Gabor micropatterns, in a task that requires near/far depth judgements [Ziegler & Hess, 1999. Vision Research, 39, 1491-1507]. Although our sample was small, three of our four subjects who had performed poorly on at least one standard clinical test of stereopsis could perform the task, as well as one 'stereoblind' subject who had failed all four standard clinical tests. The overall results suggest that individuals with stereoanomalies show a diversity of deficits, but some nevertheless can see depth using 'non-linear' mechanisms.

Amblyopia↗
PubMed2000

What limits the contribution of second-order motion to the perception of surface shape?

Both motion and stereopsis can be derived from contrast as well as luminance defined stimuli. It is currently assumed that these two different sources of information about objects feed into one common stage. Thus it would not be expected that their role in visual perception would be different. Here we show that although motion can be carried by contrast-defined elements, such motion is not used to define three-dimensional (3D) surfaces. A similar effect has been reported in stereopsis; although such contrast-defined elements can give signed disparity signals they nevertheless do not contribute to the percept of shape. We show that the reason for this lies in the inability of the second order signals to cohere or bind across space/spatial scales rather than a characteristic of the elementary motion signals per se.

Contrast Sensitivity↗