Robert F Hess

McGill University, Montréal, Quebec, Canada

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Publications (326)991.7 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: To assess the effects of spatial frequency and phase alignment of mask components in pattern masking, target threshold vs. mask contrast (TvC) functions for a sine-wave grating (S) target were measured for five types of mask: a sine-wave grating (S), a square-wave grating (Q), a missing fundamental square-wave grating (M), harmonic complexes consisting of phase-scrambled harmonics of a square wave (Qp), and harmonic complexes consisting of phase-scrambled harmonics of a missing fundamental square wave (Mp). Target and masks had the same fundamental frequency (0.46 cpd) and the target was added in phase with the fundamental frequency component of the mask. Under monocular viewing conditions, the strength of masking depends on phase relationships among mask spatial frequencies far removed from that of the target, at least 3 times the target frequency, only when there are common target and mask spatial frequencies. Under dichoptic viewing conditions, S and Q masks produced similar masking to each other and the phase-scrambled masks (Qp and Mp) produced less masking. The results suggest that pattern masking is spatial frequency broadband in nature and sensitive to the phase alignments of spatial components.
    Journal of Vision 02/2012; 12(2):14. DOI:10.1167/12.2.14 · 2.73 Impact Factor
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    ABSTRACT: Converging evidence from human psychophysics and animal neurophysiology indicates that amblyopia is associated with abnormal function of area MT, a motion sensitive region of the extrastriate visual cortex. In this context, the recent finding that amblyopic eyes mediate normal perception of dynamic plaid stimuli was surprising, as neural processing and perception of plaids has been closely linked to MT function. One intriguing potential explanation for this discrepancy is that the amblyopic eye recruits alternative visual brain areas to support plaid perception. This is the hypothesis that we tested. We used functional magnetic resonance imaging (fMRI) to measure the response of the amblyopic visual cortex and thalamus to incoherent and coherent motion of plaid stimuli that were perceived normally by the amblyopic eye. We found a different pattern of responses within the visual cortex when plaids were viewed by amblyopic as opposed to non-amblyopic eyes. The non-amblyopic eyes of amblyopes and control eyes differentially activated the hMT+ complex when viewing incoherent vs. coherent plaid motion, consistent with the notion that this region is centrally involved in plaid perception. However, for amblyopic eye viewing, hMT+ activation did not vary reliably with motion type. In a sub-set of our participants with amblyopia we were able to localize MT and MST within the larger hMT+ complex and found a lack of plaid motion selectivity in both sub-regions. The response of the pulvinar and ventral V3 to plaid stimuli also differed under amblyopic vs. non-amblyopic eye viewing conditions, however the response of these areas did vary according to motion type. These results indicate that while the perception of the plaid stimuli was constant for both amblyopic and non-amblyopic viewing, the network of neural areas that supported this perception was different.
    NeuroImage 01/2012; 60(2):1307-15. DOI:10.1016/j.neuroimage.2012.01.078 · 6.36 Impact Factor
  • Perception 01/2012; 41(12):1512-1513. · 1.11 Impact Factor
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    ABSTRACT: Factors such as strabismus or anisometropia during infancy can disrupt normal visual development and result in amblyopia, characterized by reduced visual function in an otherwise healthy eye and often associated with persistent suppression of inputs from the amblyopic eye by those from the dominant eye. It has become evident from fMRI studies that the cortical response to stimulation of the amblyopic eye is also affected. We were interested to compare the hemodynamic response function (HRF) of early visual cortex to amblyopic vs. dominant eye stimulation. In the first experiment, we found that stimulation of the amblyopic eye resulted in a signal that was both attenuated and delayed in its time to peak. We postulated that this delay may be due to suppressive effects of the dominant eye and, in our second experiment, measured the cortical response of amblyopic eye stimulation under two conditions--where the dominant eye was open and seeing a static pattern (high suppression) or where the dominant eye was patched and closed (low suppression). We found that the HRF in response to amblyopic eye stimulation depended on whether the dominant eye was open. This effect was manifested as both a delayed HRF under the suppressed condition and an amplitude reduction.
    Journal of Vision 12/2011; 11(14). DOI:10.1167/11.14.16 · 2.73 Impact Factor
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    ABSTRACT: It is known that information from an amblyopic eye can be strongly suppressed when both eyes are open. The authors investigated the way in which suppression influences the relative perception of suprathreshold contrast and luminance between a person's eyes under dichoptic viewing conditions. Stimuli consisted of four patches of luminance or four patches containing gratings. Two patches were presented to each eye. Ten amblyopes with mild suppression (six strabismic, three anisometropic and strabismic, and one deprivation; mean age, 34.5 years) and three control observers with normal vision (mean age, 33.0 years) matched the appearance of the stimuli presented to each eye. The match involved manipulation of either luminance or contrast. Amblyopes with mild suppression decreased stimulus luminance in the fellow fixing eye or increased luminance in the amblyopic eye to achieve a match (mean matching luminance, 21.1 and 39.6 cd/m(2) for the fellow fixing eye and the amblyopic eye, respectively; standard luminance, 30 cd/m(2)). This interocular mismatch was also observed when luminance was variable and contrast was kept constant (mean matching luminance, 22.8 cd/m(2) for the fellow fixing eye). On the other hand, the amblyopic eye showed no loss of perceived contrast. There was little or no mismatch between the two eyes of control participants with normal binocular vision. Amblyopes have monocular deficits in contrast perception but dichoptic deficits in luminance perception, suggesting that suppression in its mild form involves luminance processing.
    Investigative ophthalmology & visual science 11/2011; 52(12):9011-7. DOI:10.1167/iovs.11-7748 · 3.66 Impact Factor
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    ABSTRACT: An investigation of long timescale (5 minutes) fMRI neuronal adaptation effects, based on retinotopic mapping and spatial frequency stimuli, is presented in this paper. A hierarchical linear model was developed to quantify the adaptation effects in the visual cortex. The analysis of data involved studying the retinotopic mapping and spatial frequency adaptation effects in the amblyopic cortex. Our results suggest that, firstly, there are many cortical regions, including V1, where neuronal adaptation effects are reduced in the cortex in response to amblyopic eye stimulation. Secondly, our results show the regional contribution is different, and it seems to start from V1 and spread to the extracortex regions. Thirdly, our results show that there is greater adaptation to broadband retinotopic mapping as opposed to narrowband spatial frequency stimulation of the amblyopic eye, and we find significant correlation between fMRI response and the magnitude of the adaptation effect, suggesting that the reduced adaptation may be a consequence of the reduced response to different stimuli reported for amblyopic eyes.
    PLoS ONE 10/2011; 6(10):e26562. DOI:10.1371/journal.pone.0026562 · 3.53 Impact Factor
  • Journal of Vision 09/2011; 11(11):747-747. DOI:10.1167/11.11.747 · 2.73 Impact Factor
  • R F Hess, B Mansouri, B Thompson
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    ABSTRACT: To develop a treatment for amblyopia based on re-establishing binocular vision. A novel procedure is outlined for measuring and reducing the extent to which the fixing eye suppresses the fellow amblyopic eye in adults with amblyopia. We hypothesize that suppression renders a structurally binocular system, functionally monocular. We demonstrate that strabismic amblyopes can combine information normally between their eyes under viewing conditions where suppression is reduced by presenting stimuli of different contrast to each eye. Furthermore we show that prolonged periods of binocular combination leads to a strengthening of binocular vision in strabismic amblyopes and eventual combination of binocular information under natural viewing conditions (stimuli of the same contrast in each eye). Concomitant improvement in monocular acuity of the amblyopic eye occurs with this reduction in suppression and strengthening of binocular fusion. Additionally, stereoscopic function was established in the majority of patients tested. We have implemented this approach on a headmounted device as well as on a handheld iPod. This provides the basis for a new treatment of amblyopia, one that is purely binocular and aimed at reducing suppression as a first step.
    Strabismus 09/2011; 19(3):110-8. DOI:10.3109/09273972.2011.600418
  • Andrew Isaac Meso, Robert F Hess
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    ABSTRACT: The visual system exploits a cortical hierarchy to process complex inputs such as those defined by modulations of motion and/or texture. One class of visual stimuli, composed of alternate stripes of opposing motion requires at least 2 separate stages of computation within this cortical hierarchy, thought to involve cortical area V1 and extra-striate regions like global motion area MT respectively. Using a psychophysical task, we characterise sensitivity to such stimuli containing periodic spatial modulations of motion gradients as a function of the ratio of the spatial parameters at the two processing levels by manipulating the spatial properties of the carrier and modulator. We find band-passed functions for foveal stimulus presentations showing an optimum sensitivity at ratios in the range of r≤10, informative of the coupling relationship between frequency channels at the carrier and modulator levels. An annulus stimulus (excluding the fovea) with a radius of 15.5° exhibited optima of sensitivity at r>15. This difference in the optimal coupling between filtering stages reflects a processing architecture that changes with eccentricity, consistent with the previously observed smaller differences between mean receptive field sizes in striate and extra-striate filtering stages in the fovea compared to the periphery. This is also important for visual psychophysics when comparing sensitivity for first and second order stimuli across retinal eccentricity.
    Neuroscience Letters 08/2011; 503(2):77-82. DOI:10.1016/j.neulet.2011.07.052 · 2.06 Impact Factor
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    ABSTRACT: Individuals with alternating fixation due to strabismus have often been considered prime examples of monocular visual function. A growing body of evidence suggests, however, that, at least in the case of a fixed-angle strabismus, excitatory binocular function is possible in the strabismic visual cortex if interocular suppression is taken into account. We investigated whether excitatory binocular function might also be possible for patients with alternating strabismus. Suprathreshold binocular interaction was tested in two individuals with alternating fixation and no amblyopia using a dichoptic motion coherence paradigm that can measure and account for interocular suppression. Both participants exhibited strong interocular suppression when stimuli of the same contrast were presented to each eye, whereas no such suppressive interactions were present for controls; however, in significantly reducing the contrast of the stimuli presented to the fixing eye, excitatory binocular interactions were demonstrated in both participants similar to those measured in controls without the contrast imbalance. The cortical mechanisms necessary for combining information from the two eyes seem to have been present but suppressed in our 2 participants with alternating fixation, just as they have been shown to be present in patients with fixed-angle strabismus.
    Journal of AAPOS: the official publication of the American Association for Pediatric Ophthalmology and Strabismus / American Association for Pediatric Ophthalmology and Strabismus 08/2011; 15(4):345-9. DOI:10.1016/j.jaapos.2011.03.017 · 1.14 Impact Factor
  • Andrew Isaac Meso, Robert F Hess
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    ABSTRACT: We investigated sensitivity to orientation modulation using visual stimuli with bandpass filtered noise carriers. We characterized the relationship between the spatial parameters of the modulator and the carrier using a 2-AFC detection task. The relationship between these two parameters is potentially informative of the underlying coupling between first- and second-stage filtering mechanisms, which, in turn, may bear on the interrelationship between striate and extrastriate cortical processing. Our previous experiments on analogous motion stimuli found an optimum sensitivity when the ratio of the carrier and modulator spatial frequency parameters (r) was approximately ten. The current results do not exhibit an optimum sensitivity at a given value of the ratio r. Previous experiments involving second-order modulation sensitivity show an inconsistent range of estimates of optimum sensitivity at values of r between 5 and 50. Our results, using a complementary approach, confirm these discrepancies, demonstrating that the coupling between carrier and modulator frequency parameters depends on a number of stimulus-specific factors, such as contrast sensitivity, stimulus eccentricity, and absolute values of the carrier and modulator spatial frequency parameters. We show that these observations are true for a stimulus limited in eccentricity and that this orientation-modulated stimulus does not exhibit scale invariance. Such processing can not be modeled by a generic filter-rectify-filter model.
    Journal of the Optical Society of America A 08/2011; 28(8):1721-31. DOI:10.1364/JOSAA.28.001721 · 1.45 Impact Factor
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    ABSTRACT: Radial Frequency (RF) patterns can be used to study the processing of familiar shapes, e.g. triangles and squares. Opinion is divided over whether the mechanisms that detect these shapes integrate local orientation and position information directly, or whether local orientations and positions are first combined to represent extended features, such as curves, and that it is local curvatures that the shape mechanism integrates. The latter view incorporates an intermediate processing stage, the former does not. To differentiate between these hypotheses we studied the processing of micro-patch sampled RF patterns as a function of the luminance polarity of successive elements on the contour path. Our first study measures shape after effects involving suprathreshold amplitude RF shapes and shows that alternating the luminance polarity of successive micro-patch elements disrupts adaptation of the global shape. Our second study shows that polarity alternations also disrupt sensitivity to threshold-amplitude RF patterns. These results suggest that neighbouring points of the contour shape are integrated into extended features by a polarity selective mechanism, prior to global shape processing, consistent with the view that for both threshold amplitude and suprathreshold amplitude patterns, global processing of RF shapes involves an intermediate stage of processing.
    Vision research 06/2011; 51(15):1760-6. DOI:10.1016/j.visres.2011.06.003 · 2.38 Impact Factor
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    ABSTRACT: To better understand the neural basis of sensory dominance in the normal population, we applied a recently established test designed to quantify the degree of suppression in amblyopia to participants with normal binocular vision. This test quantifies the degree of dichoptic imbalance in coherent motion sensitivity by manipulating the contrast of stimuli seen by the two eyes. The contrast at which balanced dichoptic motion sensitivity occurs is referred to as the "balance point" and is an estimate of the degree of suppression. We apply the same logic to the measurement of sensory dominance by measuring the distribution of "balance points" within the normal population. We show that although most subjects are balanced or only weakly imbalanced, a minority is strongly imbalanced. To ascertain the site of sensory dominance, we assessed the degree to which normal sensory balance can be modulated by changing the interocular mean luminance. We found that mismatches in mean luminance between the two eyes had a pronounced effect on the balance point determination. Because cells in the lateral geniculate nucleus exhibit a strong modulation to sustained changes in the mean light level, this may suggests that the inhibitory circuits underlying sensory eye dominance are located at a precortical site.
    Optometry and vision science: official publication of the American Academy of Optometry 06/2011; 88(9):1072-9. DOI:10.1097/OPX.0b013e3182217295 · 2.04 Impact Factor
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    Robert F Hess, Goro Maehara
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    ABSTRACT: To determine whether conscious perception has access to brief temporal event, we asked subjects in an odd-man out paradigm to determine which of the four Gaussian blobs was flickering asynchronously in time. We measure synchrony thresholds as a function of the base temporal frequency for spatially scaled stimuli in foveal and peripheral vision. The results are consistent with a time delay of around 67 milliseconds (ms) for foveal vision and 91 ms for peripheral vision. We conclude that conscious perception has access to only relatively long (∼67 ms) time events.
    05/2011; 2(2):142-9. DOI:10.1068/i0418
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    ABSTRACT: This study had three main goals: to assess the degree of suppression in patients with strabismic, anisometropic, and mixed amblyopia; to establish the relationship between suppression and the degree of amblyopia; and to compare the degree of suppression across the clinical subgroups within the sample. Using both standard measures of suppression (Bagolini lenses and neutral density [ND] filters, Worth 4-Dot test) and a new approach involving the measurement of dichoptic motion thresholds under conditions of variable interocular contrast, the degree of suppression in 43 amblyopic patients with strabismus, anisometropia, or a combination of both was quantified. There was good agreement between the quantitative measures of suppression made with the new dichoptic motion threshold technique and measurements made with standard clinical techniques (Bagolini lenses and ND filters, Worth 4-Dot test). The degree of suppression was found to correlate directly with the degree of amblyopia within our clinical sample, whereby stronger suppression was associated with a greater difference in interocular acuity and poorer stereoacuity. Suppression was not related to the type or angle of strabismus when this was present or the previous treatment history. These results suggest that suppression may have a primary role in the amblyopia syndrome and therefore have implications for the treatment of amblyopia.
    Investigative ophthalmology & visual science 03/2011; 52(7):4169-76. DOI:10.1167/iovs.11-7233 · 3.66 Impact Factor
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    ABSTRACT: We describe a compact and convenient clinical apparatus for the measurement of suppression based on a previously reported laboratory-based approach. In addition, we report and validate a novel, rapid psychophysical method for measuring suppression using this apparatus, which makes the technique more applicable to clinical practice. By using a Z800 dual pro head-mounted display driven by a MAC laptop, we provide dichoptic stimulation. Global motion stimuli composed of arrays of moving dots are presented to each eye. One set of dots move in a coherent direction (termed signal) whereas another set of dots move in a random direction (termed noise). To quantify performance, we measure the signal/noise ratio corresponding to a direction-discrimination threshold. Suppression is quantified by assessing the extent to which it matters which eye sees the signal and which eye sees the noise. A space-saving, head-mounted display using current video technology offers an ideal solution for clinical practice. In addition, our optimized psychophysical method provided results that were in agreement with those produced using the original technique. We made measures of suppression on a group of nine adult amblyopic participants using this apparatus with both the original and new psychophysical paradigms. All participants had measurable suppression ranging from mild to severe. The two different psychophysical methods gave a strong correlation for the strength of suppression (rho = -0.83, p = 0.006). Combining the new apparatus and new psychophysical method creates a convenient and rapid technique for parametric measurement of interocular suppression. In addition, this apparatus constitutes the ideal platform for suppressors to combine information between their eyes in a similar way to binocularly normal people. This provides a convenient way for clinicians to implement the newly proposed binocular treatment of amblyopia that is based on antisuppression training.
    Optometry and vision science: official publication of the American Academy of Optometry 02/2011; 88(2):E334-43. DOI:10.1097/OPX.0b013e318205a162 · 2.04 Impact Factor
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    ABSTRACT: Amblyopia is characterised by visual deficits in both spatial vision and motion perception. While the spatial deficits are thought to result from deficient processing at both low and higher level stages of visual processing, the deficits in motion perception appear to result primarily from deficits involving higher level processing. Specifically, it has been argued that the motion deficit in amblyopia occurs when local motion information is pooled spatially and that this process is abnormally susceptible to the presence of noise elements in the stimulus. Here we investigated motion direction discrimination for abruptly presented two-frame Gabor stimuli in a group of five strabismic amblyopes and five control observers. Motion direction discrimination for this stimulus is inherently noisy and relies on the signal/noise processing of motion detectors. We varied viewing condition (monocular vs. binocular), stimulus size (5.3-18.5°) and stimulus contrast (high vs. low) in order to assess the effects of binocular summation, spatial summation and contrast on task performance. No differences were found for the high contrast stimuli; however the low contrast stimuli revealed differences between the control and amblyopic groups and between fellow fixing and amblyopic eyes. Control participants exhibited pronounced binocular summation for this task (on average a factor of 3.7), whereas amblyopes showed no such effect. In addition, the spatial summation that occurred for control eyes and the fellow eye of amblyopes was significantly attenuated for the amblyopic eyes relative to fellow eyes. Our results support the hypothesis that pooling of local motion information from amblyopic eyes is abnormal and highly sensitive to noise.
    Vision research 02/2011; 51(6):577-84. DOI:10.1016/j.visres.2011.02.001 · 2.38 Impact Factor
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    ABSTRACT: We have developed a prototype device for take-home use that can be used in the treatment of amblyopia. The therapeutic scenario we envision involves patients first visiting a clinic, where their vision parameters are assessed and suitable parameters are determined for therapy. Patients then proceed with the actual therapeutic treatment on their own, using our device, which consists of an Apple iPod Touch running a specially modified game application. Our rationale for choosing to develop the prototype around a game stems from multiple requirements that such an application satisfies. First, system operation must be sufficiently straightforward that ease-of-use is not an obstacle. Second, the application itself should be compelling and motivate use more so than a traditional therapeutic task if it is to be used regularly outside of the clinic. This is particularly relevant for children, as compliance is a major issue for current treatments of childhood amblyopia. However, despite the traditional opinion that treatment of amblyopia is only effective in children, our initial results add to the growing body of evidence that improvements in visual function can be achieved in adults with amblyopia.
    IEEE transactions on neural systems and rehabilitation engineering: a publication of the IEEE Engineering in Medicine and Biology Society 02/2011; 19(3):280-9. DOI:10.1109/TNSRE.2011.2115255 · 2.82 Impact Factor
  • Journal of American Association for Pediatric Ophthalmology and Strabismus 02/2011; 15(1). DOI:10.1016/j.jaapos.2011.01.151 · 1.14 Impact Factor
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    ABSTRACT: We investigate the effective connectivity in the lateral geniculate nucleus and visual cortex of humans with amblyopia. Six amblyopes participated in this study. Standard retinotopic mapping stimuli were used to define the boundaries of early visual cortical areas. We obtained fMRI time series from thalamic, striate and extrastriate cortical regions for the connectivity study. Thalamo-striate and striate-extrastriate networks were constructed based on known anatomical connections and the effective connectivities of these networks were assessed by means of a nonlinear system identification method. The effective connectivity of all networks studied was reduced when driven by the amblyopic eye, suggesting contrary to the current single-cell model of localized signal reduction, that a significant part of the amblyopic deficit is due to anomalous interactions between cells in disparate brain regions. The effective connectivity loss was unrelated to the fMRI loss but correlated with the degree of amblyopia (ipsilateral LGN to V1 connection), suggesting that it may be a more relevant measure. Feedforward and feedback connectivities were similarly affected. A hemispheric dependence was found for the thalamo-striate feedforward input that was not present for the feedback connection, suggesting that the reduced function of the LGN recently found in amblyopic humans may not be solely determined by the feedback influence from the cortex. Both ventral and dorsal connectivities were reduced.
    NeuroImage 01/2011; 54(1):505-16. DOI:10.1016/j.neuroimage.2010.07.053 · 6.36 Impact Factor

Publication Stats

7k Citations
991.70 Total Impact Points

Institutions

  • 1990–2015
    • McGill University
      • • Division of Ophthalmology
      • • Department of Psychology
      Montréal, Quebec, Canada
  • 2013
    • Sun Yat-Sen University
      • State Key Laboratory of Oncology
      Guangzhou, Guangdong Sheng, China
  • 2011
    • University of Waterloo
      • School of Optometry & Vision Science
      Waterloo, Ontario, Canada
    • University of Auckland
      • Department of Optometry and Vision Sciences
      Auckland, Auckland, New Zealand
  • 2009
    • Queensland University of Technology
      Brisbane, Queensland, Australia
  • 2004–2007
    • Aston University
      • • Department of Optometry
      • • School of Life and Health Sciences
      Birmingham, ENG, United Kingdom
  • 2003
    • University College London
      • Institute of Ophthalmology
      London, ENG, United Kingdom
  • 2001
    • University of Bristol
      Bristol, England, United Kingdom
  • 1996–2000
    • Cornell University
      • Department of Psychology
      Ithaca, NY, United States
    • Université du Québec à Montréal
      Montréal, Quebec, Canada
  • 1998
    • University of California, Irvine
      • Department of Cognitive Sciences
      Irvine, CA, United States
  • 1989–1992
    • University of Cambridge
      Cambridge, England, United Kingdom