Biology subjects

Greenwood, J. A.

Publications and source records attributed to Greenwood, J. A..

14 recordsLinked to original sources

bioRxiv2026

Incomplete letter recognition is limited by cortical and not optical factors: Simulating the visual deficits of dementia in healthy adults

Incomplete letter recognition tasks are frequently used to detect visual deficits arising from neurodegenerative syndromes, including Posterior Cortical Atrophy (PCA; visual-variant Alzheimers disease). A recent development of this approach is the Graded Incomplete Letters Test (GILT), which measures recognition thresholds for letters degraded by removing pixelated sections (decreasing completeness). Although GILT thresholds are strongly elevated in PCA relative to typical adults, the precise cortical visual impairments underlying these deficits are unclear, as is the potential contribution from age-related optical limitations. We compared candidate cortical factors (crowding and global integration) with optical limitations (blur and low contrast) by simulating these factors in typical adults (n=6) viewing incomplete letter stimuli. Participants identified foveally presented letters (12 alternatives), with completeness varied using QUEST. At baseline, thresholds averaged [~]5% completeness. Optical factors were simulated by separately applying blur and lowered contrast. These factors had minimal effect on thresholds, except where blur/contrast levels approached visibility limits, where thresholds rose modestly but remained far below clinical levels in PCA. Cortical factors were simulated by increasing crowding (disruptions from clutter) through peripheral presentation, with global-integration impairments simulated by varying pixel size to alter the distribution of degradation (limiting spatial integration) or degrading letters dynamically with limited-lifetime pixels (limiting temporal integration). These manipulations substantially elevated thresholds, with combined crowding and global-integration impairments increasing thresholds to levels comparable with PCA. We conclude that impaired incomplete letter recognition is driven primarily by cortical rather than optical factors, and that neurodegenerative deficits may reflect the combined impact of multiple cortical limitations.

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bioRxiv2026

Visual crowding in albinism: Evidence for a cortical sensory deficit with oculomotor influences

PurposeFoveal vision in individuals with albinism is impaired not only by reduced visual acuity but also by elevated crowding - the disruption of object recognition in clutter. Because albinism is characterised by both retinal underdevelopment and nystagmus (uncontrolled eye movements), it is unclear whether crowding is elevated primarily from image motion due to eye movements or an additional sensory deficit. To disentangle these factors, we examined the spatial and featural selectivity of foveal crowding in albinism, comparing performance with controls and prior data from individuals with idiopathic infantile nystagmus syndrome (IINS), where nystagmus occurs without retinal underdevelopment. MethodsAdults with albinism (n=8) and age-matched controls (n=8; 19-49 years) identified the orientation of foveal Landolt-C targets. In Experiment 1, targets were presented alone or flanked horizontally or vertically to assess spatial selectivity. In Experiment 2, flankers were of the same or opposite contrast polarity to assess featural selectivity. Stimulus size was adaptively scaled using QUEST to estimate gap-size thresholds. ResultsCrowding was substantially elevated in albinism, relative to both controls and IINS. Experiment 1 revealed stronger crowding for horizontally than vertically positioned flankers in albinism, mirroring the predominant direction of nystagmic eye movements. In Experiment 2, opposite-polarity flankers did not reduce crowding, indicating an absence of selectivity for target-flanker similarity. ConclusionsFoveal crowding in albinism is markedly elevated, with a nystagmus-related spatial anisotropy and a lack of featural selectivity. These characteristics suggest that these elevations reflect both retinal image motion and a substantial sensory deficit arising from abnormal visual development.

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bioRxiv2025

Temporal confounds emulate multivariate fMRI measures of perceptual learning

Human perception inherently involves learning. We can experience the same stimulus completely differently depending on our prior knowledge. Understanding the neural basis of perception therefore requires measurements that capture these temporal dynamics. Multi-voxel pattern analysis (MVPA) approaches are widely used to characterise changes in neural representations over time. A popular example is the perceptual reorganisation paradigm, which investigates the neural correlates of enhanced recognition of distorted images after cueing with the undistorted version. Studies typically report increased representational similarity of these two images in early visual cortex. However, as these paradigms include an inherent ordering of stimuli that precludes trial randomisation or counterbalancing, they are vulnerable to temporal confounds common to fMRI. Here, we investigate how these confounds could influence current understanding of perceptual reorganisation. We tested different perceptual reorganisation paradigm designs derived from published fMRI studies, and found substantial design-driven order effects at the single-subject level for all paradigms. For certain designs, these effects artificially amplified neural indices of perceptual reorganisation at both the single-subject and group levels, emulating widespread signatures of perceptual reorganisation across the brain. To disentangle perceptual learning processes from measurement artefacts, we recommend (i) selecting designs that minimise the effect of stimulus order confounds on contrasts of interest, (ii) correcting for these confounds, and (iii) confirming results are perceptually driven via negative controls, e.g., stimuli or brain areas not expected to produce perceptual effects. Our work demonstrates how current understanding of perceptual learning mechanisms based on multivariate neuroimaging approaches could be influenced by non-obvious design confounds that misdirect interpretations towards distributed neural processing, and offers practical solutions to address this.

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bioRxiv2025

Reduced susceptibility to experimentally-induced complex visual hallucinations with age

Visual hallucinations occur across many clinical conditions, but can also be induced experimentally in healthy individuals, using high-frequency flicker (Ganzflicker) and sensory deprivation (Ganzfeld). It is unclear how hallucinatory proneness changes across the lifespan, with prior questionnaire-based studies showing mixed results. As factors such as multi-sensory acuity loss and relatively increased reliance on prior knowledge may increase as we age, and these are considered risk factors for hallucination proneness, we hypothesised that reported decreases in hallucinations might reflect stigma-related underreporting. We therefore sought to measure hallucination proneness in 44 healthy adults spanning the adult lifespan (younger age group; n=22; age 19-39 years, mean 27.2 {+/-} 5.5; older age group n=22; age 59-79 years, mean 68.0 {+/-} 5.8), quantifying the tendency to experience complex and simple hallucinations in an experimental environment designed to elicit hallucinations. We find that reports of complex hallucinations (those representing objects, scenes, etc) are lower in older adults than in younger adults, both in real time and retrospectively. None of our measured cognitive or sensory measures (visual acuity, contrast sensitivity, perceptual reorganisation, imagery vividness, memory inhibition, and suggestibility) mediate this relationship. We conclude that reduced complex hallucination proneness appears normative in ageing, and that experiencing hallucinations in older individuals may signal underlying pathology.

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bioRxiv2025

The development of visual acuity and crowding: Finding the balance between fine detail and 'gist' processing

The adult visual system is characterised by high-resolution foveal vision and a peripheral field limited by crowding, the disruption to object recognition in clutter that gives a summary gist over fine detail. In children, crowding is elevated foveally, with the estimated age where foveal crowding drops to adult-like levels varying widely from 5 to 12+ years. As crowding restricts key processes like reading, characterisation of this developmental trajectory is critical. Using methods optimised to measure crowding in children, adults and typically-developing children (n=119; 3-13 years) judged the orientation of a foveal VacMan target either in isolation or surrounded by ghost flankers. For isolated elements, acuity (measured as gap-size thresholds) dropped rapidly to adult-like levels at 5-6 years. Thresholds rose when flanked/crowded, with elevations highest at 3-4 years, persisting at 5-6 years, and dropping to adult-like levels at 7-8 years. A meta-analysis of our results and 13 prior studies reveals a consistent developmental trajectory, despite wide methodological variations. We further demonstrate that developmental crowding shows the same selectivity for target-flanker similarity as peripheral crowding, consistent with common mechanisms. This prolonged development reveals a shifting balance in the visual system between the processing of fine detail vs. the gist of the scene.

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bioRxiv2024

Shared spatial selectivity in early visual cortex and face-selective brain regions

Face recognition relies on dedicated brain regions that are widely considered to show unique selectivity, including a disproportionate vulnerability to face inversion and a relative invariance to stimulus location. This proposed spatial invariance contrasts with accounts of common visuospatial coding whereby high-level category-selective areas inherit spatial properties from earlier regions. Critically, early regions (V1-V3) show characteristic retinotopic variations, with greater cortical sampling along the horizontal than vertical meridian and in the lower than upper field, mirroring established behavioural advantages for face recognition at these locations. We examined whether face-selective regions (OFA, pFus, mFus) share these spatial anisotropies, and whether these properties could drive the observed variations in face recognition. Using wide-field retinotopic mapping with upright and inverted faces ({+/-}21{degrees} eccentricity), we estimated population receptive fields (pRFs) and visual-field coverage. Though pRFs were substantially larger in face-selective regions than in V1-V3, pRF sizes did not vary in line with behavioural anisotropies. In contrast, both early and face-selective regions showed higher pRF numbers and greater visual-field coverage along the horizontal meridian and in the lower field. These sampling differences provide a plausible neural substrate for behavioural anisotropies in face recognition. We also show that pRF numbers in mFus were greater for upright than inverted faces, likely contributing to the perceptual advantage for upright faces. Together, our findings indicate that variations in visual-field sampling within face-selective cortex parallel those of early visual areas, supporting a hierarchical model in which the spatial selectivity of category-selective areas is built on that of earlier regions. Significance statementHigh-level face-selective cortex is often treated as functionally and spatially distinct from early visual areas. We show instead that these regions all share systematic patterns of spatial biases. Using population receptive field (pRF) analyses, both early and face-selective regions showed greater sampling (higher pRF numbers) and increased visual-field coverage along the horizontal than vertical meridian, and in the lower than upper visual field, matching the pattern of anisotropies in face-recognition performance. This relationship was absent for pRF sizes, indicating that behavioural anisotropies are more closely linked to sampling density and coverage. This common pattern of spatial sampling embeds specialised face-processing systems within a hierarchical network in which high-level regions retain fundamental aspects of visuospatial organisation from early visual cortex.

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bioRxiv2023

A common cortical basis for variations in visual crowding

Peripheral vision is limited by crowding, the disruptive effect of clutter on object recognition. Crowding varies markedly around the periphery, with e.g. stronger performance decrements with increasing eccentricity and in the upper vs. lower visual field. Although a number of neural substrates have been proposed for crowding, none to date can explain the full pattern of these variations. Here we examine the effects of crowding on object appearance. These effects are central to many models of crowding, and also vary markedly, causing target objects to appear more similar to flanker objects (assimilation) in some instances and dissimilar (repulsion) in others. We took 3 manipulations known to vary crowded performance (flankers in the same vs. different hemifield, the upper-lower visual field anisotropy, and the radial-tangential flanker anisotropy) and examined whether the effects on appearance vary similarly. In all cases, manipulations that increased performance impairments also increased assimilative errors, e.g. flankers on the radial axis around fixation gave high threshold elevation and assimilation, with reduced elevation and repulsion errors for tangential flankers. These linked variations in performance and appearance are well described by a population-coding model of crowding that varies the weighted combination of target vs. flanker population responses. We further demonstrate that this pattern is inconsistent with crowding being driven by either the cortical distance between elements or receptive-field size variations on their own. Instead, using a series of models we show that crowding could be driven by receptive field overlap - the intermixing of the spatial distribution of target/flanker population responses. Crowding is strong (with high performance decrements and assimilative biases) when the degree of spatial overlap in population responses is high and reduced (with low threshold elevation reduced assimilation or repulsion) when these responses are separable.

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bioRxiv2023

The resolution of face perception varies systematically across the visual field

Visual abilities tend to vary predictably across the visual field - for simple low-level stimuli, visibility is better along the horizontal vs. vertical meridian and in the lower vs. upper visual field. In contrast, face perception abilities have been reported to show either distinct or entirely idiosyncratic patterns of variation in peripheral vision, suggesting a dissociation between the spatial properties of low- and higher-level vision. To assess this link more clearly, we extended methods used in low-level vision to develop an acuity test for face perception, measuring the smallest size at which facial gender can be reliably judged in peripheral vision. In 3 experiments, we show the characteristic inversion effect, with better acuity for upright faces than inverted, demonstrating the engagement of high-level face-selective processes in peripheral vision. We also observe a clear advantage for gender acuity on the horizontal vs. vertical meridian and a smaller-but-consistent lower- vs. upper-field advantage. These visual field variations match those of low-level vision, indicating that higher-level face processing abilities either inherit or actively maintain the characteristic patterns of spatial selectivity found in early vision. The commonality of these spatial variations throughout the visual hierarchy means that the location of faces in our visual field systematically influences our perception of them.

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bioRxiv2023

Flicker and Ganzfeld induced visual hallucinations differ in frequency and content

Hallucinatory experiences, defined as perception in the absence of external stimuli, can occur in both pathological and non-pathological states and can be broadly phenomenologically divided into those of a simple and a complex nature. Non-pathological visual hallucinations can be induced experimentally using a variety of stimulation conditions. To assess whether these techniques drive a shared underlying hallucinatory mechanism, despite these differences, we compared two methods: flicker and perceptual deprivation (Ganzfeld). Specifically, we measured the frequency and complexity of the hallucinations produced by these techniques. We utilised button press, retrospective drawing, interviews, and questionnaires to quantify hallucinatory experience in 20 participants. With both experimental techniques, we found that simple hallucinations were more common than complex hallucinations. We also found that on average, flicker was more effective than Ganzfeld at eliciting a higher number of hallucinations, though Ganzfeld hallucinations were longer than flicker hallucinations. There was no interaction between experimental condition and hallucination complexity, suggesting that the increased bottom-up visual input in flicker increased both simple and complex hallucinations similarly. A correlation was observed between the total proportional time spent hallucinating in flicker and Ganzfeld, which was replicated in a retrospective questionnaire measure of experienced intensity, suggesting a shared hallucinatory mechanism between the two methodologies. We attribute these findings to a shared low-level core hallucinatory mechanism, such as excitability of visual cortex, which is amplified in flicker compared to Ganzfeld due to heightened bottom-up input.

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bioRxiv2021

Crowding changes appearance systematically in peripheral, amblyopic, and developing vision

Visual crowding is the disruptive effect of clutter on object recognition. Although most prominent in adult peripheral vision, crowding also disrupts foveal vision in typically-developing children and those with strabismic amblyopia. Do these crowding effects share the same mechanism? Here we exploit observations that crowded errors in peripheral vision are not random: target objects appear either averaged with the flankers (assimilation), or replaced by them (substitution). If amblyopic and developmental crowding share the same mechanism then their errors should be similarly systematic. We tested foveal vision in children aged 3-8 years with typical vision or strabismic amblyopia, and peripheral vision in typical adults. The perceptual effects of crowding were measured by requiring observers to adjust a reference stimulus to match the perceived orientation of a target Vac-Man element. When the target was surrounded by flankers that differed by {+/-}30{degrees}, all three groups (adults and children with typical or amblyopic vision) reported orientations between the target and flankers (assimilation). Errors were reduced with {+/-}90{degrees} differences, but primarily matched the flanker orientation (substitution) when they did occur. A population pooling model of crowding successfully simulated this pattern of errors in all three groups. We conclude that the perceptual effects of amblyopic and developing crowding are systematic and resemble the near periphery in adults, suggesting a common underlying mechanism. PrecisCrowding strongly disrupts peripheral vision, as well as the foveal vision of children with typical vision and amblyopia. We show that typically developing and amblyopic children make the same crowded errors as adults in the visual periphery, consistent with a common mechanism in all three cases.

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bioRxiv2021

Eye movements elevate crowding in congenital idiopathic nystagmus

Idiopathic infantile nystagmus syndrome is a disorder characterised by involuntary eye movements, which leads to decreased acuity and visual function. One such function is visual crowding; a process whereby objects that are easily recognised in isolation become impaired by nearby flankers. Crowding typically occurs in the peripheral visual field, though elevations in foveal vision have been reported in congenital nystagmus, similar to those found with amblyopia. Here we examine whether the elevated foveal crowding with nystagmus is driven by similar mechanisms to those documented in amblyopia - long-term neural changes associated with a sensory deficit - or by the momentary displacement of the stimulus through nystagmus eye movements. We used a Landolt-C orientation identification task to measure threshold gap sizes with and without either horizontally or vertically placed pairs of flanking Landolt-Cs. Because nystagmus is predominantly horizontal, crowding should be stronger with horizontal flankers if eye movements cause the interference, whereas a sensory deficit would more likely be equivalent for the two dimensions. Consistent with an origin in eye movements, we observe elevations in nystagmic crowding that are above that of typical vision, and stronger with horizontal than vertical flankers and not found in amblyopic or typical vision. We further demonstrate the same pattern of performance can be obtained in typical vision with stimulus movement that simulates nystagmus. Consequently, we propose that the origin of nystagmic crowding lies in the eye movements, either through image smear of the target and flanker elements or through relocation of the stimulus into peripheral retina.

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bioRxiv2020

Heritability of visual cortex architecture and perception

How much of the functional organization of our visual system is inherited? Here we tested the heritability of retinotopic maps in human visual cortex using functional magnetic resonance imaging. We demonstrate that retinotopic organization shows a closer correspondence in monozygotic (MZ) compared to dizygotic (DZ) twin pairs, suggesting a partial genetic determination. Using population receptive field (pRF) analysis to examine the preferred spatial location and selectivity of these neuronal populations, we estimate a heritability around 30% for polar angle preferences and spatial selectivity, as quantified by pRF size, in extrastriate areas V2 and V3. Our findings are consistent with heritability in both the macroscopic arrangement of visual regions and stimulus tuning properties of visual cortex. This could constitute a neural substrate for variations in a range of perceptual effects, which themselves have been found to be at least partially genetically determined. These findings also add convergent evidence for the hypothesis that functional map topology is linked with cortical morphology. HighlightsO_LIWe analyzed retinotopic maps from monozygotic and dizygotic twin pairs C_LIO_LIVisual field maps in V1-V3 are more similar in monozygotic twins C_LIO_LIHeritability is greater in V1 and V3 for polar angle and population receptive field sizes C_LIO_LIEccentricity maps show lesser degree of heritability C_LIO_LIFurther evidence for link between cortical morphology and topology of retinotopic maps C_LI Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=182 HEIGHT=200 SRC="FIGDIR/small/077545v4_ufig1.gif" ALT="Figure 1"> View larger version (77K): org.highwire.dtl.DTLVardef@ceacd6org.highwire.dtl.DTLVardef@b51604org.highwire.dtl.DTLVardef@b860f8org.highwire.dtl.DTLVardef@e67398_HPS_FORMAT_FIGEXP M_FIG C_FIG

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bioRxiv2019

Dissociable effects of visual crowding on the perception of colour and motion

Our ability to recognise objects in peripheral vision is fundamentally limited by crowding, the deleterious effect of clutter that disrupts the recognition of features ranging from orientation and colour to motion and depth. Prior research is equivocal on whether this reflects a singular process that disrupts all features simultaneously or multiple processes that affect each independently. We examined crowding for motion and colour, two features that allow a strong test of feature independence. Cowhide stimuli were presented 15 degrees in peripheral vision, either in isolation or surrounded by flankers to give crowding. Observers reported either the target direction (clockwise/counterclockwise from upwards) or its hue (blue/purple). We first established that both features show systematic crowded errors (predominantly biased towards the flanker identities) and selectivity for target-flanker similarity (with reduced crowding for dissimilar target/flanker elements). The multiplicity of crowding was then tested with observers identifying both features: a singular object-selective mechanism predicts that when crowding is weak for one feature and strong for the other that crowding should be all-or-none for both. In contrast, when crowding was weak for colour and strong for motion, errors were reduced for colour but remained for motion, and vice versa with weak motion and strong colour crowding. This double dissociation reveals that crowding disrupts certain combinations of visual features in a feature-specific manner, ruling out a singular object-selective mechanism. The ability to recognise one aspect of a cluttered scene, like colour, thus offers no guarantees for the correct recognition of other aspects, like motion. Significance statementOur peripheral vision is primarily limited by crowding, the disruption to object recognition that arises in clutter. Crowding is widely assumed to be a singular process, affecting all of the features (orientation, motion, colour, etc.) within an object simultaneously. In contrast, we observe a double dissociation whereby observers make errors regarding the colour of a crowded object whilst correctly judging its direction, and vice versa. This dissociation can be reproduced by a population-coding model where the direction and hue of target/flanker elements are pooled independently. The selective disruption of some object features independently of others rules out a singular crowding mechanism, posing problems for high-level crowding theories, and suggesting that the underlying mechanisms may be distributed throughout the visual system.

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bioRxiv2018

Population receptive field estimates for motion-defined stimuli

The processing of motion changes throughout the visual hierarchy, from spatially restricted local motion in early visual cortex to more complex large-field global motion at later stages. Here we used functional magnetic resonance imaging (fMRI) to examine spatially selective responses in these areas related to the processing of random-dot stimuli defined by differences in motion. We used population receptive field (pRF) analyses to map retinotopic cortex using bar stimuli comprising coherently moving dots. In the first experiment, we used three separate background conditions: no background dots (dot-defined bar-only), dots moving coherently in the opposite direction to the bar (kinetic boundary) and dots moving incoherently in random directions (global motion). Clear retinotopic maps were obtained for the bar-only and kinetic-boundary conditions across visual areas V1-V3 and in higher dorsal areas. For the global-motion condition, retinotopic maps were much weaker in early areas and became clear only in higher areas, consistent with the emergence of global-motion processing throughout the visual hierarchy. However, in a second experiment we demonstrate that this pattern is not specific to motion-defined stimuli, with very similar results for a transparent-motion stimulus and a bar defined by a static low-level property (dot size) that should have driven responses particularly in V1. We further exclude explanations based on stimulus visibility by demonstrating that the observed differences in pRF properties do not follow the ability of observers to localise or attend to these bar elements. Rather, our findings indicate that dorsal extrastriate retinotopic maps may primarily be determined by the visibility of the neural responses to the bar relative to the background response (i.e. neural signal-to-noise ratios) and suggests that claims about stimulus selectivity from pRF experiments must be interpreted with caution.

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