Electrophysiological Correlates of Visual Object Category Formation in a Prototype-Distortion Task

In perceptual learning studies, participants engage in extensive training in the discrimination of visual stimuli in order to modulate perceptual performance. Much of the literature in perceptual learning has looked at the induction of the reorganization of low-level representations in V1. However, much remains to be understood about the mechanisms behind how the adult brain (an expert in visual object categorization) extracts high-level visual objects from the environment and categorically represents them in the cortical visual hierarchy. Here, I used event-related potentials (ERPs) to investigate the neural mechanisms involved in object representation formation during a hybrid visual search and prototype distortion category learning task. EEG was continuously recorded while participants performed the hybrid task, in which a peripheral array of four dot patterns was briefly flashed on a computer screen. In half of the trials, one of the four dot patterns of the array contained the target, a distorted prototype pattern. The remaining trials contained only randomly generated patterns. After hundreds of trials, participants learned to discriminate the target pattern through corrective feedback. A multilevel modeling approach was used to examine the predictive relationship between behavioral performance over time and two ERP components, the N1 and the N250. The N1 is an early sensory component related to changes in visual attention and discrimination (Hopf et al., 2002; Vogel & Luck, 2000). The N250 is a component related to category learning and expertise (Krigolson et al., 2009; Scott et al., 2008; Tanaka et al., 2006). Results indicated that while N1 amplitudes did not change with improved performance, increasingly negative N250 amplitudes did develop over time and were predictive of improvements in pattern detection accuracy

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

Visual recognition memory: a view from V1

Although work in primates on higher-order visual areas has revealed how the individual and concerted activity of neurons correlates with behavioral reports of object recognition, very little is known about the underlying mechanisms for visual recognition memory. Low-level vision, even as early as primary visual cortex (V1) and even in subjects as unsophisticated as rodents, promises to fill this void. Although this latter approach sacrifices interrogation of many of the most astounding features of visual recognition, it does provide experimental constraint, proximity to sensory input, and a wide range of interventional approaches. The tractability of rodent visual cortex promises to reveal the molecular mechanisms and circuits that are essential for a fundamental form of memory.National Eye Institute (Grant RO1EY023037

Toluene inhalation exposure for 13 weeks causes persistent changes in electroretinograms of Long–Evans rats

Studies of humans chronically exposed to volatile organic solvents have reported impaired visual functions, including low contrast sensitivity and reduced color discrimination. These reports, however, lacked confirmation from controlled laboratory experiments. To address this question experimentally, we examined visual function by recording visual evoked potentials (VEP) and/or electroretinograms (ERG) from four sets of rats exposed repeatedly to toluene. In addition, eyes of the rats were examined with an ophthalmoscope and some of the retinal tissues were evaluated for rod and M-cone photoreceptor immunohistochemistry. The first study examined rats following exposure to 0, 10, 100 or 1000 ppm toluene by inhalation (6 hr/d, 5 d/wk) for 13 weeks. One week after the termination of exposure, the rats were implanted with chronically indwelling electrodes and the following week pattern-elicited VEPs were recorded. VEP amplitudes were not significantly changed by toluene exposure. Four to five weeks after completion of exposure, rats were dark-adapted overnight, anesthetized, and several sets of electroretinograms (ERG) were recorded. In dark-adapted ERGs recorded over a 5-log (cd-s/m2) range of flash luminance, b-wave amplitudes were significantly reduced at high stimulus luminance values in rats previously exposed to 1000 ppm toluene. A second set of rats, exposed concurrently with the first set, was tested approximately one year after the termination of 13 weeks of exposure to toluene. Again, dark-adapted ERG b-wave amplitudes were reduced at high stimulus luminance values in rats previously exposed to 1000 ppm toluene. A third set of rats was exposed to the same concentrations of toluene for only 4 weeks, and a fourth set of rats exposed to 0 or 1000 ppm toluene for 4 weeks were tested approximately 1 year after the completion of exposure. No statistically significant reductions of ERG b-wave amplitude were observed in either set of rats exposed for 4 weeks. No significant changes were observed in ERG a-wave amplitude or latency, b-wave latency, UV- or green-flicker ERGs, or in photopic flash ERGs. There were no changes in the density of rod or M-cone photoreceptors. The ERG b-wave reflects the firing patterns of on-bipolar cells. The reductions of b-wave amplitude after 13 weeks of exposure and persisting for 1 year suggest that alterations may have occurred in the inner nuclear layer of the retina, where the bipolar cells reside, or the outer or inner plexiform layers where the bipolar cells make synaptic connections. These data provide experimental evidence that repeated exposure to toluene may lead to subtle persistent changes in visual function. The fact that toluene affected ERGs, but not VEPs, suggests that elements in the rat retina may be more sensitive to organic solvent exposure than the rat visual cortex

Cue-dependent circuits for illusory contours in humans.

Objects' borders are readily perceived despite absent contrast gradients, e.g. due to poor lighting or occlusion. In humans, a visual evoked potential (VEP) correlate of illusory contour (IC) sensitivity, the "IC effect", has been identified with an onset at ~90ms and generators within bilateral lateral occipital cortices (LOC). The IC effect is observed across a wide range of stimulus parameters, though until now it always involved high-contrast achromatic stimuli. Whether IC perception and its brain mechanisms differ as a function of the type of stimulus cue remains unknown. Resolving such will provide insights on whether there is a unique or multiple solutions to how the brain binds together spatially fractionated information into a cohesive perception. Here, participants discriminated IC from no-contour (NC) control stimuli that were either comprised of low-contrast achromatic stimuli or instead isoluminant chromatic contrast stimuli (presumably biasing processing to the magnocellular and parvocellular pathways, respectively) on separate blocks of trials. Behavioural analyses revealed that ICs were readily perceived independently of the stimulus cue-i.e. when defined by either chromatic or luminance contrast. VEPs were analysed within an electrical neuroimaging framework and revealed a generally similar timing of IC effects across both stimulus contrasts (i.e. at ~90ms). Additionally, an overall phase shift of the VEP on the order of ~30ms was consistently observed in response to chromatic vs. luminance contrast independently of the presence/absence of ICs. Critically, topographic differences in the IC effect were observed over the ~110-160ms period; different configurations of intracranial sources contributed to IC sensitivity as a function of stimulus contrast. Distributed source estimations localized these differences to LOC as well as V1/V2. The present data expand current models by demonstrating the existence of multiple, cue-dependent circuits in the brain for generating perceptions of illusory contours

Early Visual Processing in Autism Spectrum Disorder as Assessed by Visual Evoked Potentials

Understanding early visual processing and the integrity of the visual pathways in Autism Spectrum Disorder (ASD) could help to develop a potential neuromarker. If these early stages of visual perception are compromised it could be impacting higher cognitive abilities that are necessary for social perception. For example, atypical visual behaviors such as poor eye gaze, difficulty with facial expression, and difficulty processing motion have been highly documented in social and nonsocial domains in ASD. These symptoms have been linked to abnormal sensory processing suggesting possible impairments in the magnocellular visual pathway (M-pathway). To assess early visual processing and the integrity of the visual pathways we used achromatic pattern-reversal along with a motion-onset and offset stimuli in children and adolescents with and without a diagnosis of ASD. Visual-evoked potentials (VEPs) were used to investigate early visual processing in adolescents with ASD compared to neurotypicals (NTs). For pattern-reversal, we used a black-and-white checkerboard with two different sizes (1° and 0.25°) and four different contrast levels (0.025 contrast, 0.05 contrast, 0.1 contrast, and 0.98 contrast). To study motion-onset and offset we used an expanding and contracting ‘dartboard.’ These stimuli were displayed to a total of seven male ASD and eight male NT subjects, ranging in age from 10-15 years old. VEPs were recorded on the scalp midline over the occipital (Oz) and parietal (Pz) cortices. For pattern-reversal, we examined the negative component N75, and the positive component P100. For motion-onset and motion-offset, we explored the positive component P100 and the negative component N135. VEPs responses were analyzed using measures of peak latency, peak amplitude, mean amplitude, and fractional area latency. Our results point to a disruption of the M-pathway where the ASD subjects often showed hyper-responsiveness to lower contrast stimuli presented at the largest check size. Individual waveforms in ASD subjects were variable, and may not be useful as a reliable early neuromarker. Some measures of the VEP seem to be related to symptom severity as assessed by the GARS-2, although these results never reached significance. For motion-onset, the ASD group presented larger amplitudes for the components P100 and N135 at electrode size Oz. Alterations to early visual processing in the ASD group suggest specific difficulties in the magnocellular system which could be causing a cascade of symptoms that impairs social communication. Although individual waveform variability limits the use of VEPs as a neuromarker, there is some potential relationship to symptom severity that deserves further study

Psychophysical performance, contingent negative variations, visually evoked cortical potentials, and selective attention : a behavioral and neurophysiological assessment of learning disabilities in children

The present experiment was designed to assess whether any attentional, perceptual, or neurophysiological differences exist between children classified as reading disabled and normal. A visual discrimination task was employed, which required attentional and perceptual capabilities; wherein the children were required to selectively attend and respond to one stimulus of a pair and to ignore the other stimulus. Four pairs of stimuli (colors, line orientations, letters, and words) of different levels of complexity were discriminated in order to provide clues as to the possible level of neural processing accounting for the reading disability. The children's ability to attend and to discriminate each stimulus in a pair was measured both behaviorally by psychophysical measures of response accuracy (d’) and reaction time, and also electrophysiologically by visually evoked cortical potentials (VEPs) and contingent negative variations (CNVs). A secondary purpose of the study was to examine whether the learning disability was restricted to one sensory modality. Children who were diagnosed as having either a visual or an auditory disability participated in the experiment so as to determine whether only the visual learning disability children would have difficulty with the visual discrimination task. Therefore, three groups of subjects, matched for age, sex, and IQ, were employed: normal controls (NC), visual learning disabled (VLD), and auditory learning disabled (ALD)

Visual Impairment in the absence of ON-pathway signal

Congenital retinal diseases are a major cause of childhood and lifelong visual im- pairment. Such conditions can manifest a variable array of severe and subtle ef- fects on vision. Assessment of visual function in children can be challenging; yet, knowledge about phenotype, genotype and impact of these disorders is crucial for providing appropriate support, tailored diagnostics and for developing treatments. ON-and OFF-pathways are separately transmitting information on brightness and darkness from the retina to the cortex, where their interplay is crucial in visual perception. This project investigated the effects of retinal ON-pathway dysfunction on vision. A cohort of 109 patients with ON-pathway dysfunction was examined from four subgroups of visual electrophysiological phenotypes (incomplete and complete Congenital Stationary Night Blindness - CSNB, Duchenne Muscular Dystrophy - DMD, and congenital disorders of N-glycosylation - PMM2-CDG). Using spe- cialised visual evoked potential stimuli, designed to distinguish the ON-and OFF- pathway signal arrival at the striate cortex, marked ON system delays were revealed in patients with subtypes of CSNB, DMD mutations post exon 30 and PMM2-CDG. A child-friendly psychophysical software called LumiTrack was developed to assess motion and contrast perception, two important qualities conveyed by ON-and OFF-pathways. Patients with subtypes of CSNB and PMM2-CDG showed abnormalities in motion perception and subnormal contrast sensitivity, while patients with DMD performed at the level of healthy volunteers. These impairments may occur due to a delay of signal transmission through the retina, resulting in an ON/OFF signal asymmetry within the visual system. A genotype-phenotype comparison suggested a trend of increasing ON/OFF asymmetry associated with genetic defects affecting proteins placed later within the photoreceptor / ON bipolar cell signalling cascade. This systematic study of cortical and behavioural visual function in patients with ON-pathway dysfunction highlights the impairments encountered by patients in visual qualities important for everyday life

Integration of Assistive Technologies into 3D Simulations: Exploratory Studies

Virtual worlds and environments have many purposes, ranging from games to scientific research. However, universal accessibility features in such virtual environments are limited. As the impairment prevalence rate increases yearly, so does the research interests in the field of assistive technologies. This work introduces research in assistive technologies and presents three software developments that explore the integration of assistive technologies within virtual environments, with a strong focus on Brain-Computer Interfaces. An accessible gaming system, a hands-free navigation software system, and a Brain-Computer Interaction plugin have been developed to study the capabilities of accessibility features within virtual 3D environments. Details of the specification, design, and implementation of these software applications are presented in the thesis. Observations and preliminary results as well as directions of future work are also included

History of Reading Struggles Linked to Enhanced Learning in Low Spatial Frequency Scenes

People with dyslexia, who face lifelong struggles with reading, exhibit numerous associated low-level sensory deficits including deficits in focal attention. Countering this, studies have shown that struggling readers outperform typical readers in some visual tasks that integrate distributed information across an expanse. Though such abilities would be expected to facilitate scene memory, prior investigations using the contextual cueing paradigm failed to find corresponding advantages in dyslexia. We suggest that these studies were confounded by task-dependent effects exaggerating known focal attention deficits in dyslexia, and that, if natural scenes were used as the context, advantages would emerge. Here, we investigate this hypothesis by comparing college students with histories of severe lifelong reading difficulties (SR) and typical readers (TR) in contexts that vary attention load. We find no differences in contextual-cueing when spatial contexts are letter-like objects, or when contexts are natural scenes. However, the SR group significantly outperforms the TR group when contexts are low-pass filtered natural scenes [F(3, 39) = 3.15, p<.05]. These findings suggest that perception or memory for low spatial frequency components in scenes is enhanced in dyslexia. These findings are important because they suggest strengths for spatial learning in a population otherwise impaired, carrying implications for the education and support of students who face challenges in school