Binocular vision adaptively suppresses delayed monocular signals

A neutral density filter placed before one eye will produce a dichoptic imbalance in luminance, which attenuates responses to visual stimuli and lags neural signals from retina to cortex in the filtered eye. When stimuli are presented to both the filtered and unfiltered eye (i.e., binocularly), neural responses show little attenuation and no lag compared with their baseline counterpart. This suggests that binocular visual mechanisms must suppress the attenuated and delayed input from the filtered eye; however, the mechanisms involved remain unclear. Here, we used a Steady-State Visual Evoked Potential (SSVEP) technique to measure neural responses to monocularly and binocularly presented stimuli while observers wore an ND filter in front of their dominant eye. These data were well-described by a binocular summation model, which received the sinusoidal contrast modulation of the stimulus as input. We incorporated the influence of the ND filter with an impulse response function, which adjusted the input magnitude and phase in a biophysically plausible manner. The model captured the increase in attenuation and lag of neural signals for stimuli presented to the filtered eye as a function of filter strength, while also generating the filter phase-invariant responses from binocular presentation for EEG and psychophysical data. These results clarify how binocular visual mechanisms—specifically interocular suppression—can suppress the delayed and attenuated signals from the filtered eye and maintain normal neural signals under imbalanced luminance conditions

Binocular summation revisited: beyond √2

Our ability to detect faint images is better with two eyes than with one, but how great is this improvement? A meta-analysis of 65 studies published across more than five decades shows definitively that psychophysical binocular summation (the ratio of binocular to monocular contrast sensitivity) is significantly greater than the canonical value of √2. Several methodological factors were also found to affect summation estimates. Binocular summation was significantly affected by both the spatial and temporal frequency of the stimulus, and stimulus speed (the ratio of temporal to spatial frequency) systematically predicts summation levels, with slow speeds (high spatial and low temporal frequencies) producing the strongest summation. We furthermore show that empirical summation estimates are affected by the ratio of monocular sensitivities, which varies across individuals, and is abnormal in visual disorders such as amblyopia. A simple modeling framework is presented to interpret the results of summation experiments. In combination with the empirical results, this model suggests that there is no single value for binocular summation, but instead that summation ratios depend on methodological factors that influence the strength of a nonlinearity occurring early in the visual pathway, before binocular combination of signals. Best practice methodological guidelines are proposed for obtaining accurate estimates of neural summation in future studies, including those involving patient groups with impaired binocular vision

Neuroimaging of binocular vision in human amblyopia

Amblyopia is a visual developmental condition that usually occurs when one eye receives abnormal input. For many years amblyopia was thought to be untreatable beyond 8 years old, after which the visual system would become functionally monocular. Recent research has shown that binocular mechanisms do remain intact in amblyopia and therefore investigating the nature of the deficit is crucial for understanding where neural problems arise and how they can be treated. Chapter 3 used population receptive field (pRF) modelling to further understand the cortical problems caused by amblyopia. Findings suggest that neurons responding to the amblyopic eye have reduced spatial resolution within striate and extrastriate areas. Chapters 4 and 5 aimed to test the predictions of different computational models of amblyopia using functional magnetic resonance imaging (fMRI) and electroencephalography (EEG), within the same group of participants. This is the first study to use a model driven approach to directly compare both neuroimaging methods within the same participants. The pattern of fMRI responses from the amblyopic eye showed evidence of a response gain effect and unbalanced interocular suppression, whereas EEG responses showed evidence of a contrast gain shift. Finally, Chapter 6 used EEG to objectively measure visual improvements, following treatment for amblyopia in children and adults. Measurable steady-state EEG responses were found for both groups; however, there was no convincing evidence of improvements in amblyopic eye responses throughout treatment. The studies undertaken in this thesis contribute to the wider understanding of the neural basis of amblyopia. Two different neuroimaging methods are compared, which has enabled insight into how current computational models of amblyopia could be improved. It is hoped that this research will further the development of treatments for amblyopia, by providing more insight into how binocular visual processes break down between the eyes

Change blindness: eradication of gestalt strategies

Arrays of eight, texture-defined rectangles were used as stimuli in a one-shot change blindness (CB) task where there was a 50% chance that one rectangle would change orientation between two successive presentations separated by an interval. CB was eliminated by cueing the target rectangle in the first stimulus, reduced by cueing in the interval and unaffected by cueing in the second presentation. This supports the idea that a representation was formed that persisted through the interval before being 'overwritten' by the second presentation (Landman et al, 2003 Vision Research 43149–164]. Another possibility is that participants used some kind of grouping or Gestalt strategy. To test this we changed the spatial position of the rectangles in the second presentation by shifting them along imaginary spokes (by ±1 degree) emanating from the central fixation point. There was no significant difference seen in performance between this and the standard task [F(1,4)=2.565, p=0.185]. This may suggest two things: (i) Gestalt grouping is not used as a strategy in these tasks, and (ii) it gives further weight to the argument that objects may be stored and retrieved from a pre-attentional store during this task

Artificial vision: feasibility of an episcleral retinal prosthesis & implications of neuroplasticity

Background. A visual prosthesis is a conceptual device designed to activate residual functional neurons in the visual pathway of blind individuals to produce artificial vision. Such device, when applied to stimulate the vitreous surface of the retina, has proven feasible in producing patterned light perception in blind individuals suffering from dystrophic diseases of the retina, such as aged-related macular degeneration (AMD). However the practicality of such approach has been challenged by the difficulty of surgical access and the risks of damaging the neuroretina. Positioning a visual implant over the scleral surface of the eye could present a safer alternative but this stimulation modality has not been tested in diseased retinas. Additionally, recent research has shown that the adult neocortex retains substantial plasticity following a disruption to its visual input and the potential deterioration in visual capabilities as a result of such experience modification may undermine the overall bionic rescue strategy. Methods. Two animal models mimicking the principal pathologies found in AMD, namely photoreceptor degeneration and reduced retinal ganglion cell mass, were used to evaluate the efficacy of trans-scleral stimulation of the retina by recording electrical evoked potentials in the visual cortex. The visual performance following the loss of pattern vision induced by bilateral eyelid suturing in adult mice was examined by analysing visual evoked potentials. Findings. Spatially differentiated cortical activations were obtained notwithstanding the underlying retinopathy in the experiment animals. The charge density thresholds were found to be similar to controls and below the bioelectric safety limit. After prolonged visual deprivation (weeks) in the mouse, the visual cortical responses evoked by either electrical or photic stimuli were both significantly reduced. An assessment of different visual capabilities using patterned stimuli demonstrated that whilst visual acuity and motion sensitivity were preserved, significant depression in luminance and contrast sensitivities was detected. Conclusion. Trans-scleral stimulation of the retina is a feasible approach for the development of a visual prosthesis. Following visual loss the adult brain exhibits significant experience-dependent modifications. These new insights may force a revision on the current bionic rescue strategy

VEP estimation of visual acuity: a systematic review

PURPOSE:Visual evoked potentials (VEPs) can be used to measure visual resolution via a spatial frequency (SF) limit as an objective estimate of visual acuity. The aim of this systematic review is to collate descriptions of the VEP SF limit in humans, healthy and disordered, and to assess how accurately and precisely VEP SF limits reflect visual acuity. METHODS:The protocol methodology followed the PRISMA statement. Multiple databases were searched using "VEP" and "acuity" and associated terms, plus hand search: titles, abstracts or full text were reviewed for eligibility. Data extracted included VEP SF limits, stimulus protocols, VEP recording and analysis techniques and correspondence with behavioural acuity for normally sighted healthy adults, typically developing infants and children, healthy adults with artificially degraded vision and patients with ophthalmic or neurological conditions. RESULTS:A total of 155 studies are included. Commonly used stimulus, recording and analysis techniques are summarised. Average healthy adult VEP SF limits vary from 15 to 40 cpd, depend on stimulus, recording and analysis techniques and are often, but not always, poorer than behavioural acuity measured either psychophysically with an identical stimulus or with a clinical acuity test. The difference between VEP SF limit and behavioural acuity is variable and strongly dependent on the VEP stimulus and choice of acuity test. VEP SF limits mature rapidly, from 1.5 to 9 cpd by the end of the first month of life to 12-20 cpd by 8-12 months, with slower improvement to 20-40 cpd by 3-5 years. VEP SF limits are much better than behavioural thresholds in the youngest, typically developing infants. This difference lessens with age and reaches equivalence between 1 and 2 years; from around 3-5 years, behavioural acuity is better than the VEP SF limit, as for adults. Healthy, artificially blurred adults had slightly better behavioural acuity than VEP SF limits across a wide range of acuities, while adults with heterogeneous ophthalmic or neurological pathologies causing reduced acuity showed a much wider and less consistent relationship. For refractive error, ocular media opacity or pathology primarily affecting the retina, VEP SF limits and behavioural acuity had a fairly consistent relationship across a wide range of acuity. This relationship was much less consistent or close for primarily macular, optic nerve or neurological conditions such as amblyopia. VEP SF limits were almost always normal in patients with non-organic visual acuity loss. CONCLUSIONS:The VEP SF limit has great utility as an objective acuity estimator, especially in pre-verbal children or patients of any age with motor or learning impairments which prevent reliable measurement of behavioural acuity. Its diagnostic power depends heavily on adequate, age-stratified, reference data, age-stratified empirical calibration with behavioural acuity, and interpretation in the light of other electrophysiological and clinical findings. Future developments could encompass faster, more objective and robust techniques such as real-time, adaptive control. REGISTRATION:International prospective register of systematic reviews PROSPERO (https://www.crd.york.ac.uk/PROSPERO/), registration number CRD42018085666

Seeing 3D surfaces: neural stimulation, learning and masking

In the present dissertation, I assessed the visual hierarchy stages that support the visual perception of three-dimensional (3D) surfaces. In the first experimental chapter, I used fMRI-guided rTMS to probe the cortical areas involved in the perception of slanted surfaces. Results hint at a functional contribution of the dorso-parietal visual stream (posterior parietal cortex; PPC) to slant estimation, however, further work is needed to fully understand the nature of its involvement. I then showed that fMRI-guided rTMS-induced disruption of the ventral stream (area LO) eliminates the facilitation observed, in 3D surface discrimination, when disparity and motion cues congruently inform depth. This finding indicates that LO encodes signals for the integration of depth cues. Then, I showed rTMS evidence that disparity and orientation signal-in-noise discriminations causally relate to PPC’s function. Interestingly, this relation diminishes after training on a visual feature other than the one employed during rTMS testing. This finding indicates that training, even across visual features, can influence neuronal organization. Finally, I used backward masking to show that brightness masking incorporates the 3D information of disparity-defined slant. This finding suggests that brightness estimation is mediated by mid-level neuronal mechanisms, at a cortical stage where binocular signals have been combined

Lateralised repetition priming for face recognition: priming occurs in the right hemisphere only

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Neural mechanisms of visual awareness and their modulation by social threat

The human brain can extract an enormous wealth of visual information from our surroundings. However, only a fraction of this immense data set ever becomes available to the observer’s awareness. How and why certain information is selected for awareness are questions under active investigation. Following two introductory chapters, this thesis contains six inter-related experimental chapters, through which I will explore two key outstanding questions in this field, using bistable perceptual paradigms to study conscious and non-conscious visual processing in healthy human volunteers. The first major theme focuses on adding new insight into the brain regions and networks that facilitate transfer between non-conscious and conscious modes of visual processing. In Chapters 3 and 4 I will use fMRI, both in task-related and resting-state conditions, to delineate areas, and their interactions (in terms of effective connectivity), which are relevant for transition between different conscious perceptual experiences. In Chapter 5 I will focus on one specific region in the proposed perceptual transition-related network (the frontal eye field) and explore its causal role in access to awareness using repetitive TMS. The second key question explored in this thesis is how social cues in the visual environment influence non-conscious visual processing as well as transition to conscious vision. In Chapter 6 I will study behavioural effects of non-conscious social cues from faces, and the relationship of these effects to focal brain anatomy. Based on finding slower perceptuomotor performance when non-conscious faces contain threatening cues in Chapter 6, I hypothesise that a defensive freezing response is engaged in such situations. The final two experimental chapters will explore the correlates of putative human freezing in the context of non-conscious social threat: using fMRI and psychophysiological measurements to study effects on perceptual transition in Chapter 7, and relating TMS-induced motor-evoked potentials and concurrent psychophysiological measurements to non-conscious perceptuomotor performance in Chapter 8. Taken together, the presented findings shed new light on the network of brain regions involved in transition between non-conscious and conscious modes of visual processing. In addition, they uncover novel mechanisms through which socially relevant visual cues shape our awareness of the visual world, with particular emphasis on the engagement of defensive responses by socially threatening stimuli. The concluding chapter discusses the implications of these findings and explores relevant avenues for future work