Biology subjects

Schwarzkopf, D. S.

Publications and source records attributed to Schwarzkopf, D. S..

At least 19 recordsLinked to original sources

bioRxiv2026

A comparison of sharpening and dampening accounts of the role of expectation in shaping the neural fidelity of early visual representations

The natural environment is spatiotemporally structured, and the brain exploits this regularity to predict and prepare for upcoming sensory stimuli. Such predictive processing is thought to increase neural efficiency by reducing metabolic expenditure and altering the fidelity with which newly encountered stimuli are encoded. Competing theoretical frameworks propose this is achieved either through sharpening, whereby expected events are encoded more precisely, or dampening, whereby expected events are suppressed and encoded less precisely. Despite clear, opposing predictions, evidence in humans for each account remains mixed due to methodological and analytical inconsistencies. Here we addressed these issues using probabilistic visual paradigm combined with functional magnetic resonance imaging (fMRI) and electroencephalography (EEG). We used population receptive field (pRF) mapping of fMRI data and inverted encoding of EEG data to compare the fidelity and timecourse of activity in visual areas in response to expected, unexpected, and random stimuli. Both methods produced a consistent pattern of results. Post hoc analysis of EEG data revealed that the apparent effect of expectancy was better explained by local spatiotemporal stimulus properties than the global expectancy manipulation. Although this pattern resembled sensory adaptation, it was more consistent with an expectation of temporal stability combined with dampening, in which both the aggregate response to expected features and their representational fidelity are suppressed. Taken together, our findings suggest that predictive processing may operate through dampening, with ecological advantages for high-fidelity encoding of unexpected sensory events.

neuroscience↗

bioRxiv2026

Neuroendocrine influences on dynamic cerebrovascular function and implications for functional MRI

To fully profile how ovarian hormones interact with cerebrovascular function, it is vital to consider, not just resting physiology, but also dynamic aspects of cerebrovasculature that support neural activity. This study uses hypercapnic cerebrovascular reactivity (CVR) and the visually-evoked haemodynamic response function (HRF) to investigate the influence of menstrual-related changes in oestradiol and progesterone on dynamic aspects of the cerebrovascular system. 20 menstruating females (age mean[SD]=23.01[4.01]years) completed a 3T MRI scanning session during the early follicular, late follicular, and mid-luteal phases of their menstrual cycle. Circulating hormones were measured via blood samples. Simultaneous blood oxygen level dependant (BOLD)-CVR and cerebral blood flow (CBF)-CVR data were collected using a pseudocontinuous arterial spin labelling (pCASL) acquisition using a dual-excitation (DEXI) readout during periods of hypercapnia (5% CO2). The HRF was estimated using a whole brain EPI scan during high-contrast radial checkerboard presentation. Both oestradiol and additional progesterone variance were associated with increased CVR (both BOLD-CVR and CBF-CVR; p<0.001) and altered HRF shape (p<0.005). No statistically significant regional effects were found. A secondary experiment investigated the impact of using either canonical or individually mapped HRF in a standard fMRI processing pipeline; namely, population receptive field (pRF) mapping. Results across phases suggest that neither hormone was associated with pRF size when modelled using a canonical HRF (both p>0.05). However, a significant neuroendocrine influence on pRF sizes was discovered when using individually measured HRFs (p<0.001). This study found evidence that dynamic cerebrovascular functions are sensitive to menstrual-related ovarian hormones, which may be a potential mechanism underlying menstrual symptomatology and has implications for fMRI studies that assume intact neurovascular coupling processes in women, regardless of menstrual staging, to make inferences about neural activity.

neuroscience↗

bioRxiv2025

People report having consistent idiosyncratic diets of imagined sensations when they re-experience the past, and pre-experience the future

To some extent, humans can re-experience the sensations of past events and pre-experience the future. These capacities are inter-related. But there are substantial individual differences. At the extremes, small minorities of people report that they either cannot have imagined experiences at all, or that their imagined sensations are as real to them as their actual experiences of the physical world. We wanted to know if such individual differences are uniform across different types of imagined experience (e.g. vision, audio, taste and smell), or if people generally have idiosyncratic patterns of different types (vision, audio, taste and smell) of imagined experiences. We find that people report having idiosyncratic diets of different types of imagined sensation, characterised by differences in salience. One person might have more salient imagined visual than taste experiences, while another reports the reverse. Moreover, these propensities are consistent across peoples attempts to re-experience the past, and to pre-experience the future, and they predict peoples experience and usage of different types of imagined sensation in their everyday lives.

neuroscience↗

bioRxiv2025

Predicting functional topography of the human visual cortex from cortical anatomy at scale

Topographic organization, whereby neighboring cortical locations encode neighboring features in sensory or cognitive space, is a fundamental principle of brain function. Existing approaches for obtaining individual-specific topographic maps either require resource-intensive functional neuroimaging or, when relying on population atlases, lack precision for individual-level inference. Here, we introduce deepRetinotopy toolbox, a deep learning-based application for predicting the functional topographic organization of human visual cortex from cortical anatomy alone. DeepRetinotopy toolbox produces accurate retinotopic maps across diverse experimental conditions, imaging sites, and scanner types. We demonstrate how predicted maps can be utilized to automatically generate individual-specific visual area boundaries, overcoming common biases in manual annotations. Finally, we applied our method to 11,060 anatomical scans, which allowed us to quantify age-related changes in the functional organization of visual cortex predictable from anatomy alone, underscoring the methods broad utility for scalable, anatomy-based functional brain mapping.

neuroscience↗

bioRxiv2025

Hemodynamic modelling improves population receptive field estimates

Population receptive field (pRF) modelling is a ubiquitous tool in sensory neuroscience for estimating the functional architecture of the human brain. Most pRF models assume a canonical hemodynamic response function (HRF) to account for neurovascular effects. But how does this assumption affect results in practice? Here, we concurrently fit the HRF within the pRF model. Using simulations, we demonstrate that this algorithm accurately estimated different ground truth HRFs used for synthesizing data. Moreover, concurrent fitting improves the accuracy of pRF estimates, especially for the complex Difference-of-Gaussian model. Next, we reanalyzed empirical datasets with different stimulus paradigms and scanning parameters. Concurrent fitting substantially reduced the proportion of implausibly small pRFs. Importantly, the best-fitting HRF differed substantially from canonical HRFs or from those measured independently. HRFs also peaked earlier in higher than early visual cortex, suggesting response nonlinearities. This variability need not only be hemodynamic but could result from neural factors. Critically, all these differences could produce spurious pRF estimates and alter the interpretation of reported findings.

neuroscience↗

bioRxiv2025

Vividness of mental imagery is a broad trait measure of internally generated visual experiences

Research on mental visual imagery typically relies on vividness ratings. However, vividness is ill-defined as it lacks an objective reference. Here, we present survey results that suggest vividness is nevertheless a robust measure. It explains individual differences of a broad range of subjective experiences, from the detail of mental imagery, the propensity to report having other internally generated visual experiences, and the vividness of visual dreams. Critically, simple vividness ratings can replace the protracted questionnaires commonly used for this purpose and reduce methodological issues with these instruments. We further find that vividness is closely linked with the experience of "seeing" mental images or projecting them into the external world. People who report seeing mental images with their eyes shut are also more likely to experience externally projected imagery. Nevertheless, many people report having mental depictions but without seeing. Overall, our results indicate we should redefine visual aphantasia to distinguish individuals with faint or unseen visual images from those completely lacking a pictorial representation.

neuroscience↗

bioRxiv2025

Modeling 2D Spatio-Tactile Population Receptive Fields of the Fingertip in Human Primary Somatosensory Cortex

Tactile fingertip sensations are critical for everyday life. Accordingly, tactile fingertip maps have been extensively studied in human primary somatosensory cortex. However, the fine-grained functional architecture of these maps remains largely unknown. To uncover this architecture, we sought to estimate 2D spatio-tactile population receptive fields (pRFs) of the tip of the index finger in human Brodmann area 3b (BA3b). Using functional magnetic resonance imaging at 7T and submillimeter resolution along with prospective motion correction, we recorded brain responses whilst participants sensed a row of vibrotactile pins sweeping along cardinal axes over a portion of the fingertip. To estimate pRF position and size, we initially fit a 2D Gaussian pRF model to the data, which, however, produced largely implausible pRF estimates. Simulations indicated that this likely occurred because the size of pRFs in BA3b surpasses the portion of the fingertip we stimulated, resulting in an incomplete mapping of pRFs. To address this issue, we constrained the fitting procedure and refined the 2D Gaussian pRF model by keeping pRF size constant. Our results for pRF position then revealed that the ulnar-to-radial axis spanning the fingertip maps onto a superior-to-inferior axis in BA3b. Both the putatively large pRF size (relative to the mapping area) and the pRF position gradient we uncover here appear compatible with receptive field properties quantified in monkeys. Our study provides the first comprehensive investigation into the fine-grained functional architecture of human fingertip maps and brings us one step closer to a thorough understanding thereof.

neuroscience↗

bioRxiv2024

Retinotopic mapping data permit accurate matching of participants across different datasets

Public sharing of neuroimaging data is becoming increasingly common for the advancement and validation of scientific research. However, this sharing poses challenges regarding privacy and data safety, and associated questions about the ownership of research data. Here we show that a simple pattern correlation algorithm can match nominally deidentified participants across two separate functional magnetic resonance imaging (fMRI) experiments. This re-identification procedure is effective despite functional maps being spatially warped to a common template brain. This work highlights the need for appropriate safeguards against possible misuse of shared neuroimaging data.

neuroscience↗

bioRxiv2024

Imagery priming of binocular rivalry is not a reliable metric of individual differences in the subjective vividness of visualisations.

Most people can imagine images that they experience within their minds eye. However, there are marked individual differences, with some people reporting that they are unable to construct mental images at all (aphantasics), and others who report having imagined experiences that are as realistic as seeing (hyper-phantasics). The vividness of imagined images is most often measured via subjective self-report. Chang and Pearson (2018), however, have suggested that a binocular rivalry (BR) protocol can be used as an objective measure. They found that pre-imagining a moving input could enhance performance on an objective probe detection task when probes were embedded in imagery consistent inputs, as opposed to imagery inconsistent inputs. To date, nobody has assessed if this type of objective imagery priming can be used to predict the vividness of different peoples visualisations. Here, we report that imagery priming of objective sensitivity to probes within static BR inputs does not correlate with the typical ratings people use to describe the vividness of their visualisations (a between participants effect). However, objective priming of sensitivity to probes embedded in BR inputs was greater on trials when participants reported that their pre-imagined experience had been relatively vivid (a within participants effect). Overall, our data suggest that while imagery can prime objective sensitivity to probes during BR, there is currently no strong evidence that this effect can be used as an objective method of predicting the subjective vividness of different peoples visualisations. Highlights[tpltrtarr] There are marked individual differences in the subjective intensity of visual imagery [tpltrtarr]Most measures of the subjective intensity of visual imagery are subjective [tpltrtarr]A binocular rivalry probe-detection protocol has been suggested as an objective measure [tpltrtarr]The authors find that there is only a very small predictive relationship between outcomes in a binocular rivalry task and participants reported imagery strength [tpltrtarr]We still lack an objective measure that reliably predicts the subjective intensity of visual imagery

animal behavior and cognition↗

bioRxiv2024

Individual differences distinguish mechanisms underlying contextual size illusions

Previous research suggests the magnitudes of the Ebbinghaus, Delboeuf, Ponzo, and tilt illusions all depend on the cortical distance between the neural representations of target stimuli and the surrounding context. However, several psychophysical studies found no compelling association between these illusions, calling this hypothesis into question. Here we ask if these discrepant reports could arise from methodological differences between these studies. We ran a battery of visual size illusion and basic discrimination tasks with carefully matched geometric properties, using a classic forced-choice design. In our small, homogenous sample, the Ebbinghaus and Delboeuf illusion magnitudes were strongly correlated, consistent with the idea that they reflect the same underlying mechanism when other sources of individual differences are minimised. Ponzo illusion magnitude also correlated with these two illusions, although less strongly in the case of the Ebbinghaus. Interestingly, the classic arrowhead version of the Mueller-Lyer illusion did not correlate with any of the other illusions or even with the objective ability to discriminate line length. This suggests that an altogether separate process underlies this perceptual effect. We further demonstrate that presenting stimuli briefly with central fixation critically affects measurements of the Ebbinghaus illusion. Additionally, we found that measuring illusion magnitude via adjustment is less reliable compared to two-alternative forced-choice procedures. Taken together, our findings suggest that different tasks probe separable processes determining illusion measurements. They further highlight the importance of the experimental design when testing relationships between perceptual effects and their links to neural processing.

neuroscience↗

bioRxiv2024

Spatial frequency adaptation modulates population receptive field sizes

The spatial tuning of neuronal populations in the early visual cortical regions is related to the spatial frequency (SF) selectivity of neurons. However, there has been no direct investigation into how this relationship is reflected in population receptive field (pRF) sizes despite the common application of pRF mapping in visual neuroscience. We hypothesised that adaptation to high/low SF would decrease the sensitivity of neurons with respectively small/large receptive field sizes, resulting in a change in pRF sizes as measured by functional magnetic resonance imaging (fMRI). To test this hypothesis, we first quantified the SF aftereffect using a psychophysical paradigm where observers made SF judgments following adaptation to high/low SF noise patterns. We then incorporated the same adaptation technique into a standard pRF mapping procedure, to investigate the spatial tuning of the early visual cortex following SF adaptation. Results showed that adaptation to a low/high SF resulted in smaller/larger pRFs respectively, as hypothesised. Our results provide the most direct evidence to date that the spatial tuning of the visual cortex, as measured by pRF mapping, is related to the SF selectivity of visual neural populations. This has implications for various domains of visual processing, including size perception and visual acuity.

neuroscience↗

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.

neuroscience↗

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.

neuroscience↗

bioRxiv2023

A layer-specific model of cortical sensory aging

The segregation of processes into cortical layers is a convergent feature in animal evolution. However, how changes in the cortical layer architecture interact with sensory system function and dysfunction remains unclear. We conducted functional and structural layer-specific in-vivo 7T-MRI of the primary somatosensory cortex in two cohorts of healthy younger and older adults. Input layer IV is enlarged and more myelinated in older adults, and associated with extended sensory input signals. Age-related cortical thinning is driven by deep layers and accompanied by increased myelination, but there is no clear evidence for reduced inhibition. Calcium imaging and histology in younger and older mice reveal increased sensory-evoked neuronal activity accompanied by increased parvalbumin expression as a potential inhibitory balance, with dynamic changes in layer-specific myelination across age groups. Using multimodal imaging, we demonstrate that middle and deep layers show specific sensitivity to aging across species.

neuroscience↗

bioRxiv2023

Predicting the subjective intensity of imagined sensory experiences from electrophysiological measures of oscillatory brain activity.

Most people can conjure images and sounds that they experience in their minds. There are, however, marked individual differences. Some people report that they cannot generate imagined sensory experiences at all (aphantasics) and others report that they have unusually intense imagined experiences (hyper-phantasics). These individual differences have been linked to activity in sensory brain regions, driven by feedback. We would therefore expect imagined experiences to be associated with specific frequencies of oscillatory brain activity, as these can be a hallmark of neural interactions within and across regions of the brain. Replicating a number of other studies, relative to meditation we find that the act of engaging in imagining auditory or visual sensations is linked to reductions in the power of oscillatory brain activity across a broad range of frequencies, with prominent peaks in the alpha band (8-12 Hz). This oscillatory activity, however, did not predict individual differences in the subjective intensity of imagined experiences. For imagined audio experiences, these were rather predicted by reductions within the theta (6 - 9 Hz) and gamma (33 - 38 Hz) bands, and by increases in beta (15 - 17Hz) band activity. For imagined visual experiences these were predicted by reductions in lower (14 - 16Hz) and upper (29 - 32 Hz) beta band activity, and by an increase in mid-beta band (24 - 26 Hz) activity. Our data suggest that there is sufficient ground truth to the subjective reports that people use to describe the intensity of their imagined sensory experiences such that these can be predicted by the power of distinct rhythms of brain activity.

neuroscience↗

bioRxiv2023

A simple statistical framework for small sample studies

Most studies in psychology, neuroscience, and life science research make inferences about how strong an effect is on average in the population. Yet, many research questions could instead be answered by testing for the universality of the phenomenon under investigation. By using reliable experimental designs that maximise both sensitivity and specificity of individual experiments, each participant or subject can be treated as an independent replication. This approach is common in certain subfields. To date, there is however no formal approach for calculating the evidential value of such small sample studies and to define a priori evidence thresholds that must be met to draw meaningful conclusions. Here we present such a framework, based on the ratio of binomial probabilities between a model assuming the universality of the phenomenon versus the null hypothesis that any incidence of the effect is sporadic. We demonstrate the benefits of this approach, which permits strong conclusions from samples as small as 2-5 participants and the flexibility of sequential testing. This approach will enable researchers to preregister experimental designs based on small samples and thus enhance the utility and credibility of such studies.

neuroscience↗

bioRxiv2023

Extrastriate activity reflects the absence of local retinal input

The physiological blind spot corresponds to the optic disc where the retina contains no light-detecting photoreceptor cells. Our perception seemingly fills in this gap in input. Here we suggest that rather than an active process, such perceptual filling-in could instead be a consequence of the integration of visual inputs at higher stages of processing discounting the local absence of retinal input. Using functional brain imaging, we resolved the retinotopic representation of the physiological blind spot in early human visual cortex and measured responses while participants perceived filling-in. Responses in early visual areas simply reflected the absence of visual input. In contrast, higher extrastriate regions responded more to stimuli in the eye containing the blind spot than the fellow eye. However, this signature was independent of filling-in. We argue that these findings agree with philosophical accounts that posit that the concept of filling-in of absent retinal input is unnecessary.

neuroscience↗

bioRxiv2023

Retinotopic connectivity maps of human visual cortex with unconstrained eye movements

Human visual cortex contains topographic visual field maps whose organization can be revealed with retinotopic mapping. Unfortunately, constraints posed by standard mapping hinders its use in patients, atypical subject groups, and individuals at either end of the lifespan. This severely limits the conclusions we can draw about visual processing in such individuals. Here we present a novel data-driven method to estimate connective fields, fine-grained maps of the functional connectivity between brain areas. We find that inhibitory connectivity fields accompany, and often surround, facilitatory fields. The visual field extent of these inhibitory subfields falls off with cortical magnification. We further show that our method is robust to large eye movements and myopic defocus. Importantly, freed from the controlled stimulus conditions in standard mapping experiments, using entertaining stimuli and unconstrained eye movements our approach can generate retinotopic maps, including the periphery visual field hitherto only possible to map with special stimulus displays. Generally, our results show that the connective field method can gain knowledge about retinotopic architecture of visual cortex in patients and participants where this is at best difficult and confounded, if not impossible, with current methods.

neuroscience↗