Biology subjects

Dekker, T. M.

Publications and source records attributed to Dekker, T. M..

13 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

Protective effect of prior knowledge in ageing

Visual perception depends on both sensory input and prior knowledge, and the quality of these information sources changes across the lifespan. As people age, accumulated knowledge, and its influence on perception, tend to increase, while declines in low-level visual functions reduce input quality and bias perception toward prior information. Yet, the combined effects of these changes on higher-level visual perception remain unclear, partly because concurrent cognitive decline may complicate the use of stored knowledge. In this study, we examined how prior knowledge shapes visual perception in younger (<40 years) and older (>60 years) adults using ambiguous two-tone images, whose recognition depends on prior knowledge. Providing participants with the original photographs used to generate the two-tones triggered perceptual reorganisation, improving recognition in both groups. Although older adults recognised fewer two-tones overall - a pattern linked to reduced visual acuity - they showed a comparable benefit from prior knowledge when cued. These findings suggest that, despite age-related sensory decline, the ability to draw on prior knowledge remains robust in older adulthood and may help preserve perceptual function.

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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

An fMRI-based Approach for Measuring Contrast Sensitivity Across the Visual Field in Visual Cortex

Peripheral vision is crucial for daily activities and quality of life, yet traditional measures of visual function like visual acuity primarily assess central vision. Visual field tests can evaluate peripheral vision but require extended focus combined with precise fixation, often very challenging for patients with severe sight loss. Functional MRI (fMRI) with population receptive field (pRF) mapping offers a non-invasive way to map scotomas but is limited by its reliance on single contrast levels and the necessity of accurate fixation. We developed an fMRI-based approach to measure contrast sensitivity across the visual field without the need for precise fixation. By combining large-field stimulation with varying spatial frequencies and contrast levels with either pRF mapping or a retinotopic atlas based on anatomical landmarks, we modeled contrast sensitivity in the primary visual cortex (V1) over a large (40 deg) expanse of the visual field. In seven normal-sighted participants, we characterized differences in V1 cortical sensitivity across eccentricities and visual quadrants, finding reliable and reproducible patterns of sensitivity differences at individual and session levels. To assess the methods tolerance to fixation variability, we further investigated how different levels of eye movement affect cortical sensitivity patterns in two participants. We found that cortical sensitivity patterns were largely preserved across eye movement, particularly at low spatial frequencies. This suggests that our approach can accommodate several degrees of fixation instability, making it suitable for populations with unstable or biased fixation for whom visual field maps are harder to acquire behaviorally (e.g., patients with dense central scotoma or strabismus). Additionally, our method effectively visualized cases of simulated and disease-linked sensitivity loss at the cortical level. Crucially, we demonstrated that these results could be largely recovered using a structure-based retinotopic atlas, eliminating the need for pRF mapping and precise fixation - although such an approach reduced sensitivity. This approach, integrating large-field stimulation with a retinotopic atlas, offers a promising tool for monitoring vision loss and recovery in patients with various visual impairments, addressing a significant challenge in current clinical assessments.

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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

Where the present gets remembered: Sensory regions communicate with the brain over the longest timescales.

In natural contexts, the brain simultaneously processes sensory information over diverse timescales. Here we reveal how this is reflected in the organisation of asynchronous connectivity in the brain. In 86 individuals watching feature-length movies during functional neuroimaging, we calculated the delay at peak connectivity between brain regions. We found the longest delays in received whole-brain functional connectivity within sensory regions (:S 18 seconds). Two complementary dimensionality reduction approaches were used to probe the spatial organisation of connection delays and weights. First, clustering of received delays separated sensory, and transmodal/associative outputting regions, suggesting that putatively localised functions are associated with asynchronous local-to-whole-brain connectivity patterns. Next, we organised delayed connectivity maps by likeness, unveiling five orthogonal gradients of variation, each demonstrating associations between early-sensory and transmodal/associative regions. Together, these findings challenge contemporary conceptualisations of the brains temporal hierarchy by emphasising the role of sensory regions as sites of integration across timescales.

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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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bioRxiv2022

Emergence of perceptual reorganisation from prior knowledge in human development and Convolutional Neural Networks

Perception in the mature human visual system relies heavily on prior knowledge. Here we show for the first time that prior-knowledge-induced reshaping of visual perception emerges gradually, and late in childhood. To isolate the effects of prior knowledge on vision, we presented 4-to-12-year-olds and adults with two-tone images, which are degraded photos that are hard to recognise on first viewing. In adults, seeing the original photo causes a perceptual reorganisation leading to sudden, mandatory recognition of the two-tone version - a well-documented process relying on top-down signalling from higher-order brain areas to early visual cortex. We find that children younger than 7 to 9 years, however, do not experience this knowledge-guided shift, despite viewing the original photo immediately before each two-tone. To assess potential computations underlying this development we compared human performance to three state-of-the-art neural networks with varying architectures. We found that the best-performing architecture behaved much like 4- to 5-year-old humans, who display a feature-based rather than holistic processing strategy akin to neural networks. Our results reveal a striking age-related shift in the reconciliation of prior knowledge with sensory input, which may underpin the development of many perceptual abilities.

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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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bioRxiv2017

Population receptive field tuning properties of visual cortex during childhood

Improvements in visuospatial perception such as contrast sensitivity and Vernier acuity continue until late in childhood, but the neural mechanisms driving these age-related changes are currently unclear. One contributing factor could be the protracted development of spatial tuning of neuronal populations across the visual cortex. Here we tested this possibility using population receptive field (pRF) mapping (Dumoulin and Wandell, 2008) in 6-to 12-year-old children and adults. We fitted pRF models to BOLD signals measured in areas V1-V4 and V3a during fMRI whilst participants watched wedge and ring stimuli traversing the visual field. Cortical magnification and the width of pRF tuning functions changed with viewing eccentricity in all participants. However, there were no age-related changes in pRF size, shape, cortical magnification, or map consistency across any of the visual areas measured. These results suggest that visuospatial perception in late childhood beyond age 6 years is not substantially limited by low-level spatial tuning properties of neuronal populations in visual cortex. Instead, performance improvements in this period may reflect more efficient use of the spatial information available in the visual system when forming perceptual judgments. These findings are an important step towards disentangling which neural mechanisms contribute to the eventual emergence of mature spatial vision, and for understanding the processes that determine the scope for visual plasticity at different stages of life.

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