Binocular rivalry (BR) is a dynamic visual illusion that provides insight into the cortical mechanisms of visual awareness, stimulus selection, and object identification. When dissimilar binocular ...images cannot be fused, perception switches every few seconds between the left and right eye images. The speed at which individuals switch between alternatives is a stable, partially heritable trait. In order to isolate the monocular and binocular processes that determine the speed of rivalry, we presented stimuli tagged with a different flicker frequency in each eye and applied stimulus-phase locked MEG source imaging. We hypothesized that the strength of the evoked fundamental or intermodulation frequencies would vary when comparing Fast and Slow Switchers. Ten subjects reported perceptual alternations, with mean dominance durations between 1.2-4.0 sec. During BR, event-related monocular input in V1, and broadly in higher-tier ventral temporal cortex, waxed and waned with the periods of left or right eye dominance/suppression. In addition, we show that Slow Switchers produce greater evoked intermodulation frequency responses in a cortical network composed of V1, lateral occipital, posterior STS, retrosplenial & superior parietal cortices. Importantly, these dominance durations were not predictable from the brain responses to either of the fundamental tagging frequencies in isolation, nor from any responses to a pattern rivalry control condition, or a non-rivalrous control. The novel cortical network isolated, which overlaps with the default-mode network, may contain neurons that compute the level of endogenous monocular difference, and monitor accumulation of this conflict over extended periods of time. These findings are the first to relate the speed of rivalry across observers to the 'efficient coding' theory of computing binocular differences that may apply to binocular vision generally.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Summary Background Leber congenital amaurosis, caused by mutations in RPE65 and LRAT , is a severe form of inherited retinal degeneration leading to blindness. We aimed to assess replacement of the ...missing chromophore 11-cis retinal with oral QLT091001 (synthetic 9-cis-retinyl acetate) in these patients. Methods In our open-label, prospective, phase 1b trial, we enrolled patients (aged ≥6 years) with Leber congenital amaurosis and RPE65 or LRAT mutations at McGill University's Montreal Children's Hospital. Patients received 7 days of oral QLT091001 (10–40 mg/m2 per day). We assessed patients at baseline and days 7, 9, 14, and 30, and then 2 months and every 2 months thereafter for up to 2·2 years for safety outcomes and visual function endpoints including Goldmann visual fields (GVF), visual acuity, and functional MRI assessment. We regarded patients as having an improvement in vision if we noted at least a 20% improvement in retinal area on GVF compared with baseline or a visual acuity improvement of five or more letters compared with baseline in two consecutive study visits (or any improvement from no vision at baseline). This study is registered with ClinicalTrials.gov , number NCT01014052. Findings Between December, 2009, and June, 2011, we enrolled and treated 14 patients aged 6–38 years who were followed up until March, 2012. Ten (71%) of 14 patients had an improvement in GVF areas (mean increase in retinal area of 28–683%). Six (43%) patients had an improvement in visual acuity (mean increase of 2–30 letters). Self-reported or parent-reported improvements in activities of daily living supported these findings. After 2 years, 11 (79%) patients had returned to their baseline GVF retinal area and ten (71%) had returned to baseline visual acuity letter values. Thus, three (21%) patients had a sustained GVF response and four (30%) had a sustained visual acuity response. Four patients had functional MRI scans, which correlated with visual response or absence of response to treatment. No serious adverse events occurred, although we noted transient headaches (11 patients), photophobia (11 patients), reduction in serum HDL concentrations (four patients), and increases in serum triglycerides (eight patients) and aspartate aminotransferase concentrations (two patients). Interpretation Non-invasive oral QLT091001 therapy is well tolerated, and can rapidly improve visual function in some patients with Leber congenital amaurosis and RPE65 and LRAT mutations. Funding QLT, Foundation Fighting Blindness Canada, CIHR, FRSQ, Reseau Vision.
Binocular rivalry (BR) is a visual phenomenon in which perception alternates between two non-fusible images presented to each eye. Transition periods between dominant and suppressed images are marked ...by mixed percepts, where participants report fragments of each image being dynamically perceived. Interestingly, BR remains robust even when typical images are subdivided and presented in complementary patches to each eye, a phenomenon termed interocular grouping (IOG). The objective of the present study was to determine if increasing grouping demand in the context of BR changes the perceptual experience of rivalry. In 48 subjects with normal vision, mean dominant and mixed percept durations were recorded for classic BR and IOG conditions with increasing grouping demands from two, four, and six patches. We found that, as grouping demands increased, the duration of mixed periods increased. Indeed, durations of dominant and mixed percepts, as well as percentage of time spent in dominant or mixed state, differed significantly across conditions. However, durations of global dominant percepts remained relatively stable and saturated at about 1.5 seconds, despite the exponential increase in possible mixed combinations. Evidence shows that this saturation followed a nonlinear trend. The data also indicate that grouping across the vertical meridian is slightly more stable than for the horizontal meridian. Finally, individual differences in speed of alternation identified during BR were maintained in all interocular grouping conditions. These results provide new information about binocular visual spatial integration and will be useful for future studies of the underlying neural substrates and models of binocular vision.
Binocular rivalry is an example of bistable visual perception extensively examined in neuroimaging. Magnetoencephalography can track brain responses to phasic visual stimulations of predetermined ...frequency and phase to advance our understanding of perceptual dominance and suppression in binocular rivalry. We used left and right eye stimuli that flickered at two tagging frequencies to track their respective oscillatory cortical evoked responses. We computed time‐resolved measures of coherence to track brain responses phase locked with stimulus frequencies and with respect to the participants' indications of alternations of visual rivalry they experienced. We compared the brain maps obtained to those from a non‐rivalrous control replay condition that used physically changing stimuli to mimic rivalry. We found stronger coherence within a posterior cortical network of visual areas during rivalry dominance compared with rivalry suppression and replay control. This network extended beyond the primary visual cortex to several retinotopic visual areas. Moreover, network coherence with dominant percepts in primary visual cortex peaked at least 50 ms prior to the suppressed percept nadir, consistent with the escape theory of alternations. Individual alternation rates were correlated with the rate of change in dominant evoked peaks, but not for the slope of response to suppressed percepts. Effective connectivity measures revealed that dominant (respectively, suppressed) percepts were expressed in dorsal (respectively ventral) streams. We thus demonstrate that binocular rivalry dominance and suppression engage distinct mechanisms and brain networks. These findings advance neural models of rivalry and may relate to more general aspects of selection and suppression in natural vision.
Magnetoencephalography was used to track brain responses for perceptual dominance and suppression in binocular rivalry. Stronger coherence was found within a network of visual areas during rivalry dominance compared with rivalry suppression and replay control. Coherence peaks for dominance occurred early and rate of change predicted alternation rates. Effective connectivity revealed that dominant (respectively, suppressed) percepts were biased towards dorsal (respectively, ventral) streams.
Stereoanomalous (SA) subjects have normal visual acuity but reduced stereopsis and may have a prevalence of up to 30%. It has been suggested that, in SA subjects, an imbalance in interocular ...inhibition might underlie an asymmetry in sensory eye dominance (SED). Our study expands upon previous findings by examining binocular rivalry (BR) mean dominance durations, dichoptic masking (DM) thresholds and SED for a group of SA subjects compared to naïve controls. We examined BR dominance durations and DM thresholds for 15 stereonormal (SN) subjects and 10 SA subjects with normal or corrected-to-normal visual acuity. All subjects had visual acuity of 20/40 or better and less than or equal to two lines difference between eyes. Individuals who scored ≥6/9 on the Randot stereo test and <100 arcmin on the PacMan Stereo Acuity test were considered SN. We compared near-vertical and near-horizontal oriented sine-wave gratings for BR and DM in order to dissociate stereo-related mechanisms that rely on horizontal disparities from other eye-based integration mechanisms. Mean randot scores for SN subjects were 8.5/9 with a PacMan stereoacuity of 33 arcmin, and SA subjects scored 2.5/9 and 3,380 arcmin, respectively. The mean difference in SED was 0.19 for SN and 0.48 for SA when measured with a neutral density filter bar. The SA group showed a large interocular difference in BR durations that was significantly greater than normal (p = 0.004) and correlated with loss of stereoacuity. Moreover, the interocular difference for DM was similarly greater for SA subjects (p = 0.04) although a proportional difference in monocular sensitivity could partially account for this. We also found that both SN and SA subjects presented higher DM thresholds and, to some extent, sensitivity for vertical than horizontal orientations. SA subjects show an abnormal bias toward their dominant eye for both BR and DM. These data suggest that common mechanisms of monocular sensitivity and interocular inhibition may limit multiple binocular measures and provides a practical link to better understand the heterogeneity of stereopsis in amblyopia.
The neural correlates of binocular rivalry have been actively debated in recent years, and are of considerable interest as they may shed light on mechanisms of conscious awareness. In a related ...phenomenon, monocular rivalry, a composite image is shown to both eyes. The subject experiences perceptual alternations in which the two stimulus components alternate in clarity or salience. The experience is similar to perceptual alternations in binocular rivalry, although the reduction in visibility of the suppressed component is greater for binocular rivalry, especially at higher stimulus contrasts. We used fMRI at 3T to image activity in visual cortex while subjects perceived either monocular or binocular rivalry, or a matched non-rivalrous control condition. The stimulus patterns were left/right oblique gratings with the luminance contrast set at 9%, 18% or 36%. Compared to a blank screen, both binocular and monocular rivalry showed a U-shaped function of activation as a function of stimulus contrast, i.e. higher activity for most areas at 9% and 36%. The sites of cortical activation for monocular rivalry included occipital pole (V1, V2, V3), ventral temporal, and superior parietal cortex. The additional areas for binocular rivalry included lateral occipital regions, as well as inferior parietal cortex close to the temporoparietal junction (TPJ). In particular, higher-tier areas MT+ and V3A were more active for binocular than monocular rivalry for all contrasts. In comparison, activation in V2 and V3 was reduced for binocular compared to monocular rivalry at the higher contrasts that evoked stronger binocular perceptual suppression, indicating that the effects of suppression are not limited to interocular suppression in V1.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Diffusion tensor imaging (DTI) has been implemented in a breadth of scientific investigations of optic neuropathies, though it has yet to be fully adopted for diagnosis or prognosis. This is ...potentially due to a lack of standardization and weak replication of results. The aim of this investigation was to review DTI results from studies specific to three distinct optic neuropathies in order to probe its current clinical utility.
We reviewed the DTI literature specific to primary open-angle glaucoma (POAG), optic neuritis (ON), and traumatic optic neuropathy (TON) by systematically searching the PubMed database on March 1
, 2023. Four distinct DTI metrics are considered: fractional anisotropy (FA), along with mean diffusivity (MD, axial diffusivity (AD), and radial diffusivity (RD). Results from within-group, between-group, and correlational studies were thoroughly assessed.
POAG studies most consistently report a decrease in FA, especially in the optic radiations, followed in prevalence by an increase in RD and then MD, whilst AD yields conflicting results between studies. It is notable that there is not an equal distribution of investigated DTI metrics, with FA utilized the most, followed by MD, RD, and AD. Studies of ON are similar in that the most consistent findings are specific to FA, RD, and MD. These results are specific to the optic nerve and radiation since only one study measured the intermediary regions. More studies are needed to assess the effect that ON has on the tracts of the visual system. Finally, only three studies assessing DTI of TON have been performed to date, displaying low to moderate replicability of results. To improve the level of agreement between studies assessing each optic neuropathy, an increased level of standardization is recommended.
Both POAG and ON studies have yielded some prevalent DTI findings, both for contrast and correlation-based assessments. Although the clinical need is high for TON, considering the limitations of the current diagnostic tools, too few studies exist to make confident conclusions. Future use of standardized and longitudinal DTI, along with the foreseen methodological and technical improvements, is warranted to effectively study optic neuropathies.
The cortical mechanisms associated with conscious object recognition were studied using functional magnetic resonance imaging (fMRI). Participants were required to recognize pictures of masked ...objects that were presented very briefly, randomly and repeatedly. This design yielded a gradual accomplishment of successful recognition. Cortical activity in a ventrotemporal visual region was linearly correlated with perception of object identity. Therefore, although object recognition is rapid, awareness of an object's identity is not a discrete phenomenon but rather associated with gradually increasing cortical activity. Furthermore, the focus of the activity in the temporal cortex shifted anteriorly as subjects reported an increased knowledge regarding identity. The results presented here provide new insights into the processes underlying explicit object recognition, as well as the analysis that takes place immediately before and after recognition is possible.
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