Robert F Hess

McGill University, Montréal, Quebec, Canada

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Publications (306)936.82 Total impact

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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
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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 · 6.13 Impact Factor
  • P.-C. Huang, G. Maehara, R. F. Hess
    Journal of Vision 12/2010; 10(15):37-37. DOI:10.1167/10.15.37 · 2.73 Impact Factor
  • Journal of Vision 12/2010; 8(17):40-40. DOI:10.1167/8.17.40 · 2.73 Impact Factor
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    ABSTRACT: Although eye dominance assessment is used to assist clinical decision-making, current understanding is limited by inconsistencies across the range of available tests. A new psychophysical test of sensory eye dominance has been developed that objectively measures the relative contribution of each eye to a fused suprathreshold binocular percept. Six standard tests and the newly developed test were used to measure motor and sensory dominance in a group of 44 binocularly normal individuals (mean age, 29.5 ± 9.10 years). The new test required observers to perform a motion coherence task under dichoptic viewing conditions, wherein a population of moving, luminance-defined signal (coherently moving) and noise (randomly moving) dots were presented separately to each eye. The observers judged the motion direction of the signal dots. Motion coherence thresholds were measured by varying the ratio of signal-to-noise dots, in a staircase procedure. The new dichoptic motion coherence threshold test revealed a clear bimodal distribution of sensory eye dominance strength, wherein the majority of the participants (61%) showed weak dominance, but a significant minority (39%) showed strong dominance. Subsequent analysis revealed that the strong-dominance group showed greater consistency across the range of traditional eye dominance tests used. This new quantitative dichoptic motion coherence threshold technique suggests that there are two separate sensory eye dominance strength distributions among observers with normal binocular vision: weak and strong eye dominance. This finding may provide a basis for clinical decision-making by indicating whether eye dominance is likely to be an important consideration in a particular patient.
    Investigative ophthalmology & visual science 12/2010; 51(12):6875-81. DOI:10.1167/iovs.10-5549 · 3.66 Impact Factor
  • G. Maehara, P. C. Huang, R. F. Hess
    Journal of Vision 12/2010; 8(17):68-68. DOI:10.1167/8.17.68 · 2.73 Impact Factor
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    ABSTRACT: In this study, we investigate how the responses of the human visual pathway to temporal frequency are modified as information transfers between the lateral geniculate nucleus (LGN) and primary visual cortex (V1) and to the extrastriate areas of the dorsal and ventral streams (V2, V3, VP, V3A, V4, and MT). We use high-field fMRI (4 T) to record simultaneously the responses of these areas across temporal frequency for chromatic stimuli (L/M-cone opponent and S-cone opponent) and stimuli of high and low achromatic contrasts. We find that: (1) the LGN has relatively low-pass responses for temporal frequency at both high and low achromatic contrasts, indicating that LGN cell spiking activity is not well reflected in the BOLD response. In addition, M cell-like temporal responses were not found, even at low contrasts. (2) Responses in V1 and extrastriate areas V2, V3, VP, and V3A display a progressively low-pass dependence on temporal frequency for achromatic stimuli (2-16 Hz) and are flat for chromatic stimuli (2-8 Hz), showing little overall difference between chromatic and achromatic cortical temporal filtering. (3) Strongly differential effects are found between dorsal and ventral stream processing by the level of MT and V4. V4 shows a significant low-pass temporal dependence for all achromatic and chromatic stimuli, whereas MT has temporally high-pass or flat responses for achromatic and chromatic stimuli. MT was the only visual area that showed M cell-like responses. We conclude that the dorsal and ventral pathways of human vision progressively develop characteristic differences in temporal processing that affect both chromatic and achromatic stimuli.
    Journal of Vision 11/2010; 10(13):13. DOI:10.1167/10.13.13 · 2.73 Impact Factor
  • Robert F Hess, Anne Gabrielle Zaharia
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    ABSTRACT: We examine how global translational motion sensitivity varies with the mean luminance of the stimulus. Using DC-balanced, spatially narrowband micropatterns (radial Log Gabors) matched in terms of detectability (multiples above contrast threshold), we show that global translational motion sensitivity is invariant with the mean luminance. Contrast detection thresholds, however, show a characteristic spatial frequency dependence on mean luminance. Similar results were found for central and peripheral regions of the field and for a range of different micropattern velocities (2.1 deg/s to 84 deg/s). Thus, the sensitivity of global motion processing that occurs in extrastriate cortical areas, unlike the detectability of the stimuli themselves that occurs in lower visual areas, does not vary with mean luminance.
    Journal of Vision 11/2010; 10(13):22. DOI:10.1167/10.13.22 · 2.73 Impact Factor
  • R. F. Hess, T. Ledgeway
    Journal of Vision 11/2010; 2(7):121-121. DOI:10.1167/2.7.121 · 2.73 Impact Factor
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    ABSTRACT: Structure from motion (SFM) is the ability to perceive three-dimensional structure from stimuli containing only two-dimensional motion signals and this ability seems to be a result of high-level cortical processes. It has long been thought that local motion signals defined by second-order cues only weakly contribute to perception of SFM since performance on purely second-order SFM tasks is poor, relative to first-order stimuli. We hypothesized that the mechanisms responsible for deriving SFM were insensitive to low-level stimulus attributes such as the first- or second-order nature of the dots composing the stimulus, in other words: that they were "cue-invariant", but that large differences in sensitivity to local first- and second-order motions were responsible for previous findings. By manipulating the relative strength of first-order dots in an SFM stimulus that combines first- and second-order dots, we show that the two types of motion can separately support SFM and co-operatively interact to produce vivid three-dimensional percepts. This provides strong support that the mechanisms underlying SFM are cue-invariant.
    Journal of Vision 11/2010; 10(13):6. DOI:10.1167/10.13.6 · 2.73 Impact Factor
  • Journal of Vision 11/2010; 2(7):223-223. DOI:10.1167/2.7.223 · 2.73 Impact Factor
  • Journal of Vision 10/2010; 3(9):456-456. DOI:10.1167/3.9.456 · 2.73 Impact Factor

Publication Stats

7k Citations
936.82 Total Impact Points


  • 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
  • 2001–2002
    • 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