[Show abstract][Hide abstract] ABSTRACT: Orientation-modulated stimuli are thought to be processed via a two-stage process, the first stage involving the detection of the carrier by mechanisms in striate cortex and the second stage involving the detection of the modulation by way of integration of carrier-based information by mechanisms in extra-striate cortex. Much is known about the spatial properties of the channels underlying carrier detection but less is known about the properties of the channels involved in modulation detection. Using a discrimination at detection paradigm, we show that the mechanisms underlying modulation detection are tuned for envelope spatial frequency and orientation. The tuning of these channels is not substantially different from that previously described for carrier mechanisms, namely 1-2 octaves for spatial frequency and 30° for orientation. For orientation, these stimuli exhibit an oblique effect that is dependent on absolute carrier orientation, suggesting facilitative interactions between first- and second-stage processes.
Experimental Brain Research 05/2012; 220(2):135-45. DOI:10.1007/s00221-012-3124-6 · 2.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The concept of a critical period for visual development early in life during which sensory experience is essential to normal neural development is now well established. However recent evidence suggests that a limited degree of plasticity remains after this period and well into adulthood. Here, we ask the question, "what limits the degree of plasticity in adulthood?" Although this limit has been assumed to be due to neural factors, we show that the optical quality of the retinal image ultimately limits the brain potential for change. We correct the high-order aberrations (HOAs) normally present in the eye's optics using adaptive optics, and reveal a greater degree of neuronal plasticity than previously appreciated.
[Show abstract][Hide abstract] ABSTRACT: Amblyopic observers exhibit a range of low- and high-level cortical deficits, and there is strong evidence that the extrastriate cortex is selectively affected for signal/noise tasks but not for simple integration tasks. We tested amblyopic and control observers on a structure-from-motion (SFM) task involving signal integration to gauge whether extrastriate processing is compromised at a level where dorsal and ventral information is combined. SFM tasks require integration of local elements to perceive the global structure using motion-defined depth cues. Observers were monocularly presented with a 2-IFC shape discrimination task and asked to indicate whether two consecutive SFM stimuli represented the same or different depth-defined shapes. Amblyopic observers had higher depth thresholds than control observers, even after controlling for low-level differences in contrast thresholds across eyes and observers. Combined with the presence of the deficit in both the amblyopic and fellow-fixing eyes, this suggests a high-level locus for the SFM deficit.
Journal of Vision 04/2012; 12(4). DOI:10.1167/12.4.4 · 2.39 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Noninvasive brain stimulation is a technique for inducing changes in the excitability of discrete neural populations in the human brain. A current model of the underlying pathological processes contributing to the loss of motor function after stroke has motivated a number of research groups to investigate the potential therapeutic application of brain stimulation to stroke rehabilitation. The loss of motor function is modeled as resulting from a combination of reduced excitability in the lesioned motor cortex and an increased inhibitory drive from the nonlesioned hemisphere over the lesioned hemisphere. This combination of impaired neural function and pathological suppression resonates with current views on the cause of the visual impairment in amblyopia. Here, we discuss how the rationale for using noninvasive brain stimulation in stroke rehabilitation can be applied to amblyopia, review a proof-of-principle study demonstrating that brain stimulation can temporarily improve amblyopic eye function, and propose future research avenues.
[Show abstract][Hide abstract] ABSTRACT: It has been argued that the human visual system is optimized for identification of broadband objects embedded in stimuli possessing orientation averaged power spectra fall-offs that obey the 1/f(β) relationship typically observed in natural scene imagery (i.e., β=2.0 on logarithmic axes). Here, we were interested in whether individual spatial channels leading to recognition are functionally optimized for narrowband targets when masked by noise possessing naturalistic image statistics (β=2.0). The current study therefore explores the impact of variable β noise masks on the identification of narrowband target stimuli ranging in spatial complexity, while simultaneously controlling for physical or perceived differences between the masks. The results show that β=2.0 noise masks produce the largest identification thresholds regardless of target complexity, and thus do not seem to yield functionally optimized channel processing. The differential masking effects are discussed in the context of contrast gain control.
Vision research 03/2012; 60:101-13. DOI:10.1016/j.visres.2012.03.017 · 1.82 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We describe the successful translation of computerized and space-consuming laboratory equipment for the treatment of suppression to a small handheld iPod device (Apple iPod; Apple Inc., Cupertino, California). A portable and easily obtainable Apple iPod display, using current video technology offers an ideal solution for the clinical treatment of suppression. The following is a description of the iPod device and illustrates how a video game has been adapted to provide the appropriate stimulation to implement our recent antisuppression treatment protocol. One to 2 hours per day of video game playing under controlled conditions for 1 to 3 weeks can improve acuity and restore binocular function, including stereopsis in adults, well beyond the age at which traditional patching is used. This handheld platform provides a convenient and effective platform for implementing the newly proposed binocular treatment of amblyopia in the clinic, home, or elsewhere.
[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 · 2.39 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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 · 2.39 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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 · 3.23 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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 · 1.00 Impact Factor
[Show abstract][Hide abstract] 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 · 1.56 Impact Factor
[Show abstract][Hide abstract] 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 · 1.82 Impact Factor
[Show abstract][Hide abstract] 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 · 1.60 Impact Factor
[Show abstract][Hide abstract] 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.