New paper in IOVS

Schmidtmann, G., Ruiz, T., Reynaud, A., Spiegel, D.P., Lague-Beauvais, M., Hess, R.F., Farivar, R. (2017), Sensitivity to binocular disparity is reduced by mild traumatic brain injury. Invest Ophthalmol Vis Sci. 2017; 58:2630–2635 [PDF

Purpose: The impairment of visual functions is one of the most common complaints following mild traumatic brain injury (mTBI). Traumatic brain injury–associated visual deficits include blurred vision, reading problems, and eye strain. In addition, previous studies have found evidence that TBI can diminish early cortical visual processing, particularly for second-order stimuli. We investigated whether cortical processing of binocular disparity is also affected by mTBI.

Methods: In order to investigate the influence of mTBI on global stereopsis, we measured the quick Disparity Sensitivity Function (qDSF) in 22 patients with mTBI. Patients with manifest strabismus and double vision were excluded. Compared with standard clinical tests, the qDSF is unique in that it offers a quick and accurate estimate of thresholds across the whole spatial frequency range.

Results: Results show that disparity sensitivity in the mTBI patients were significantly reduced compared with the normative dataset (n = 61). The peak spatial frequency was not affected.

Conclusions: Our results suggest that the reduced disparity sensitivity in patients with mTBI is more likely caused by cortical changes (e.g., axonal shearing, or reduced interhemispheric communication) rather than oculomotor dysfunction.

New paper in Journal of Vision

Akhavein, H., & Farivar, R. (2017). Gaze behavior during 3-D face identification is depth cue invariant. Journal of Vision, 17(2):9, 1–12.   PDF

Gaze behavior during scene and object recognition can highlight the relevant information for a task. For example, salience maps—highlighting regions that have heightened luminance, contrast, color, etc. in a scene—can be used to predict gaze targets. Certain tasks, such as face recognition, result in a typical pattern of fixations on high salience features. While local salience of a 2-D feature may contribute to gaze behavior and object recognition, we are perfectly capable of recognizing objects from 3-D depth cues devoid of meaningful 2-D features. Faces can be recognized from pure texture, binocular disparity, or structure-from-motion displays (Dehmoobadsharifabadi & Farivar, 2016; Farivar, Blanke, & Chaudhuri, 2009; Liu, Collin, Farivar, & Chaudhuri, 2005), and yet these displays are devoid of local salient 2-D features. We therefore sought to determine whether gaze behavior is driven by an underlying 3-D representation that is depth-cue invariant or depth-cue specific. By using a face identification task comprising morphs of 3-D facial surfaces, we were able to measure identification thresholds and thereby equate for task difficulty across different depth cues. We found that gaze behavior for faces defined by shading and texture cues was highly comparable, but we observed some deviations for faces defined by binocular disparity. Interestingly, we found no effect of task difficulty on gaze behavior. The results are discussed in the context of depth-cue invariant representations for facial surfaces, with gaze behavior being constrained by low-level limits of depth extraction from specific cues such as binocular disparity.

New Paper in Frontiers in System Neuroscience

Farivar R & Michaud-Landry D (2016) Construction and Operation of a High-Speed, High-Precision Eye Tracker for Tight Stimulus Synchronization and Real-Time Gaze Monitoring in Human and Animal Subjects. Front. Syst. Neurosci. 10:73. doi: 10.3389/fnsys.2016.00073

Measurements of the fast and precise movements of the eye—critical to many vision, oculomotor, and animal behavior studies—can be made non-invasively by video oculography. The protocol here describes the construction and operation of a research-grade video oculography system with ~0.1° precision over the full typical viewing range at over 450 Hz with tight synchronization with stimulus onset. The protocol consists of three stages: (1) system assembly, (2) calibration for both cooperative, and for minimally cooperative subjects (e.g., animals or infants), and (3) gaze monitoring and recording.

New Paper in The Journal of Physiology

Non-uniform phase sensitivity in spatial frequency maps of the human visual cortex

Complex natural scenes can be decomposed into their oriented spatial frequency (SF) and phase relationships, both of which are represented locally at the earliest stages of cortical visual processing. The SF preference map in the human cortex, obtained using synthetic stimuli, is orderly and correlates strongly with eccentricity. In addition, early visual areas show sensitivity to the phase information that describes the relationship between SFs and thereby dictates the structure of the image. Taken together, two possibilities arise for the joint representation of SF and phase: either the entirety of the cortical SF map is uniformly sensitive to phase, or a particular set of SFs is selectively phase sensitive—for example, greater phase sensitivity for higher SFs that define fine-scale edges in a complex scene. To test between these two possibilities, we constructed a novel continuous natural scene video whereby phase information was maintained in one SF band but scrambled elsewhere. By shifting the central frequency of the phase-aligned band in time, we mapped the phase-sensitive SF preference of the visual cortex. Using fMRI, we found that phase sensitivity in early visual areas is biased toward higher SFs. Compared to a SF map of the same scene obtained using linear filtered stimuli, a much larger area of V1 and V2 is sensitive to the phase alignment of higher SFs. The results of early areas cannot be explained by attention. Our results suggest non-uniform sensitivity to phase alignment in population-level SF representations, with phase alignment being particularly important for fine-scale edge representations of natural scenes.

Farivar, R., Clavagnier, S., Hansen, B. C., Thompson, B. and Hess, R. F. (2016), Non-uniform phase sensitivity in spatial frequency maps of the human visual cortex. J Physiol. Accepted Author Manuscript. doi:10.1113/JP273206

MIND THE BRAIN

Watch CTV News Press Coverage.

MIND THE BRAIN campaign is designed to educate and increase public awareness of traumatic brain injury (TBI) and the TBI Research program at RI-MUHC. Activity booths will provide many hands-on learning opportunities about the effects of TBI on the brain. Clinicians from the MUHC will guide you as you learn and experience what happens after a traumatic brain injury. Experience MRI's, obstacle courses, virtual reality, neurosurgery and more!
(10am to 4pm November 26th, 2016 Livingston Hall L6.500)
*Register Online for free! 

Society for Neuroscience Meeting 2016 - Posters

Monkeys See Snakes Like Humans Do? A comparative analysis of internal noise and efficiency of contour integration.

A. Zhang, P. Khayat, H. Akhavein, A. Baldwin, R. Hess, R. Farivar

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Accessing Cortical Inhibitory Processes Through the Delay of the fMRI BOLD Response

S. Proulx & R. Farivar

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Visual Contour Integration in mild Traumatic Brain Injury reveals increased internal noise

T. Ruiz, D. Spiegel, A. Baldwin, R. Hess, R. Farivar

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Representational similarity analysis reveals similarity between subjects in movie viewing using MEG

Y. Chen, A. Dehmoobadsharifabadi, E. Bock, S. Baillet, R. Farivar

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New paper in Vision Research

First- and second-order contrast sensitivity functions reveal disrupted visual processing following mild traumatic brain injury.

Spiegel, D. P., Reynaud, A., Ruiz, T., Laguë-Beauvais, M., Hess, R., & Farivar, R.

Vision is disrupted by traumatic brain injury (TBI), with vision-related complaints being amongst the most common in this population. Based on the neural responses of early visual cortical areas, injury to the visual cortex would be predicted to affect both 1(st) order and 2(nd) order contrast sensitivity functions (CSFs)-the height and/or the cut-off of the CSF are expected to be affected by TBI. Previous studies have reported disruptions only in 2(nd) order contrast sensitivity, but using a narrow range of parameters and divergent methodologies-no study has characterized the effect of TBI on the full CSF for both 1(st) and 2(nd) order stimuli. Such information is needed to properly understand the effect of TBI on contrast perception, which underlies all visual processing. Using a unified framework based on the quick contrast sensitivity function, we measured full CSFs for static and dynamic 1(st) and 2(nd) order stimuli. Our results provide a unique dataset showing alterations in sensitivity for both 1(st) and 2(nd) order visual stimuli. In particular, we show that TBI patients have increased sensitivity for 1(st) order motion stimuli and decreased sensitivity to orientation-defined and contrast-defined 2(nd) order stimuli. In addition, our data suggest that TBI patients' sensitivity for both 1(st) order stimuli and 2(nd) order contrast-defined stimuli is shifted towards higher spatial frequencies.

Spiegel, D. P., Reynaud, A., Ruiz, T., Laguë-Beauvais, M., Hess, R., & Farivar, R. (2016). First- and second-order contrast sensitivity functions reveal disrupted visual processing following mild traumatic brain injury. Vision Research, 122, 43–50. PDF

Averaged CSFs (top row) and model estimates based on the group pseudomedian estimates of qCSF parameters (bottom row). Averaged CSFs represent geometric mean with shading representing the standard deviation. Left panels TBI group; right panels normative dataset. cpd = cycles per degree. 1st Ori – 1st order orientation, 1st Mot – 1st order motion, 2nd Ori – 2nd order orientation modulation, 2nd Mot – 2nd order motion modulation, 2nd Cont – 2nd order contrast modulation. 

Averaged CSFs (top row) and model estimates based on the group pseudomedian estimates of qCSF parameters (bottom row). Averaged CSFs represent geometric mean with shading representing the standard deviation. Left panels TBI group; right panels normative dataset. cpd = cycles per degree. 1st Ori – 1st order orientation, 1st Mot – 1st order motion, 2nd Ori – 2nd order orientation modulation, 2nd Mot – 2nd order motion modulation, 2nd Cont – 2nd order contrast modulation.