Biology subjects

Castaldi, E.

Publications and source records attributed to Castaldi, E..

8 recordsLinked to original sources

bioRxiv2026

Late cortical dynamics mediate the symmetry-induced numerosity illusion

Numerosity perception enables us to quickly estimate the number of objects in a visual scene, a fundamental skill supporting efficient interaction with the environment. In this study, we tracked how the human brain transforms physical inputs into numerical percepts. Using EEG we measured the neural dynamics to assess whether numerosity is perceived directly or inferred from correlated non-numerical features, and how these signals are integrated with contextual cues that bias perception. We leveraged on a visual illusion in which dot arrays appear less numerous when arranged symmetrically rather than randomly. Participants viewed dot arrays of varying physical or perceived numerosity, illusorily altered by the dots spatial arrangement. Both univariate and multivariate analyses revealed that physical numerosity was decodable from occipitoparietal electrodes as early as [~]50 ms after stimulus onset, independently of low-level features such as dot size or convex hull. Spatial arrangement was represented later, from [~]150 ms, and perceived numerosity, biased by symmetry-induced grouping, emerged at a similar latency. These results indicate that early neural signals encode physical numerosity directly, whereas the symmetry-induced underestimation arises from later grouping processes that integrate spatial information to shape perceived numerosity.

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bioRxiv2025

Numerosity Is Directly Sensed and Dynamically Transformed in the Human Brain: Evidence from MEG-MRI Fusion

Humans can estimate the number of objects in a scene within a fraction of a second, suggesting that numerosity is encoded rapidly and directly by the visual system. Yet how this encoding unfolds over time and interacts with other visual features remains unclear. Here, we combined magnetoencephalography (MEG) with time-resolved representational similarity analysis (RSA) and MEG-fMRI fusion to track how numerosity is represented in the brain over time. We also used multidimensional scaling (MDS) to visualize the evolving patterns of neural activity. Two main findings emerged. First, numerosity exhibited the hallmarks of a primary perceptual attribute: its neural signature appeared rapidly after stimulus onset, preceding the encoding of non-numeric features that could otherwise define number. Second, Visualization of the neural patterns using MDS suggested a temporal transformation in representational geometry, reflecting the engagement of two distinct coding schemes- an early, linear number line, consistent with a "summation code", dominating activity in occipital regions, and a later, curved number line, consistent with "numerosity-tuned code", emerging more strongly in associative areas along the dorsal stream. Together, these findings demonstrate that numerosity processing is encoded directly from the visual image and unfolds through a rapid hierarchical transformation, from a broad quantity signal to a finely tuned, number-specific code, linking perceptual encoding to higher-level numerical abstraction.

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bioRxiv2024

Feature selective adaptation of numerosity perception

Perceptual adaptation has been used to infer the existence of numerosity detectors, which allow humans to quickly estimate the number of objects in a scene. While adaptation was originally thought to affect numerosity perception regardless of the low-level features of the items, a recent study demonstrated that adaptation is more pronounced when the adapting and adapted (test) stimuli share the same color, compared to when they were colored differently. In this study we explored whether such adaptation reduction depends on a novelty effect induced by changes in stimulus features or whether this effect is observed only when implying an identity change of the stimuli. To this aim, we performed six experiments in which numerosity adaptation was investigated in conditions in which adapting and adapted stimuli were either matched or differed for several low-level (color, luminance, shape, and motion) or high-level (letters identity, face emotions) features. Numerosity adaptation was consistently observed across all conditions, but it was reduced when adaptor and test differed in color, luminance and shape. However, when stimuli differed in their motion profile, a very salient perceptual change that does not imply a change in items identity, adaptation selectivity vanished. Moreover, adaptation selectivity was not observed when items identity was changed by spatial rotations of the same stimulus (letters) or when stimuli were matched for the global configuration (face outline) but differed for the arrangement of local features (mouth, nose, eyes). Interestingly, image dissimilarity between test and adaptor, as quantified by Gabor filters simulating a simplified model of the primary visual cortex, nicely predicted the strength of numerosity adaptation across all conditions. Overall, changes in stimulus identity defined by low-level features, rather than novelty in general, determined the strength of the adaptation effects, provided that the changes were readily noticeable. Our findings suggest that numerosity mechanisms may be able to operate on segregated and categorized visual items in addition to the total quantity of the set, with part of the aftereffects induced by numerosity adaptation occurring after feature-binding.

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bioRxiv2023

Neural codes for visual numerosity independent of other quantities are present both in the dorsal and in the ventral stream of the human brain

Visual numerosity, a feature of visual sets traditionally associated with the parietal cortex, has recently been suggested to be encoded in a broader range of cortical regions, including early visual and associative cortices of both streams. However, a clear picture of how numerosity information is represented alongside other non-numeric features across the cortex is still lacking. To address this, we conducted a whole-brain 3T fMRI study with thirty-one adult subjects (eighteen females) engaged in a numerosity estimation task involving visual sets varying in number, item size, total item area, total field area, and density, thus allowing us to perform a tight control of the stimulus properties. Using model-based semipartial correlation representational similarity analyses, we found that numerosity is represented independently of other visual properties already in early visual areas and amplified in both retinotopic and non-retinotopic associative areas along both the dorsal and ventral streams. Dimensionality reduction analyses of BOLD activity patterns revealed clearly distinct neural representational geometries of numerosity across brain regions: a one-dimensional representation of numerical rank in early visual and in associative retinotopic regions of the ventral stream, and a curved structure encoding rank-order together with distance to end-point in associative regions of both the dorsal and ventral stream. These findings indicate that numerosity is represented over and above other visual features broadly in the brain but with substantially different neural coding schemes across regions, and are discussed in light of two non necessarily exclusive interpretations.

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bioRxiv2020

Excessive visual crowding effects in developmental dyscalculia

Visual crowding refers to the inability to identify objects when surrounded by other similar items. Crowding-like mechanisms are thought to play a key role in numerical perception by determining the sensory mechanisms through which ensembles are perceived. Enhanced visual crowding might hence prevent the normal development of a system involved in segregating and perceiving discrete numbers of items and ultimately the acquisition of more abstract numerical skills. Here, we investigated whether excessive crowding occurs in developmental dyscalculia (DD), a neurodevelopmental disorder characterized by difficulty in learning the most basic numerical and arithmetical concepts, and whether it is found independently of associated major reading and attentional difficulties. We measured spatial crowding in two groups of adult individuals with DD and control subjects. In separate experiments, participants were asked to discriminate the orientation of a Gabor patch either in isolation or under spatial crowding. Orientation discrimination thresholds were comparable across groups when stimuli were shown in isolation, yet they were much higher for the DD group with respect to the control group when the target was crowded by closely neighbouring flanking gratings. The difficulty in discriminating orientation (as reflected by the combination of accuracy and reaction times) in the DD compared to the control group persisted over several larger target flanker distances. Finally, we found that the degree of such spatial crowding correlated with impairments in mathematical abilities even when controlling for visual attention and reading skills. These results suggest that excessive crowding effects might be a characteristic of DD, independent of other associated neurodevelopmental disorders. Bullet pointsO_LIPeople with DD have difficulty learning about numbers and arithmetics. C_LIO_LIPerception of non-symbolic number seems to be modulated by visual crowding. C_LIO_LICan stronger than normal crowding effects contribute to the origin of DD? C_LIO_LIWe measured crowding with orientation discrimination tasks using Gabor gratings. C_LIO_LIAbnormal crowding characterizes DD independently of other developmental deficits. C_LI

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bioRxiv2019

Mapping numerical perception and operations in relation to functional and anatomical landmarks of human parietal cortex

Human functional imaging has identified the middle part of the intraparietal sulcus (IPS) as an important brain substrate for different types of numerical tasks. This area is often equated with the macaque ventral intraparietal area (VIP) where neuronal selectivity for non-symbolic numbers is found. However, the low spatial resolution and whole-brain averaging analysis performed in most fMRI studies limit the extent to which an exact correspondence of activation in different numerical tasks with specific sub-regions of the IPS can be established. Here we disentangled the functional neuroanatomy of numerical perception and operations (comparison and calculation) by acquiring high-resolution 7T fMRI data in a group of human adults, and relating the activations in different numerical contrasts to anatomical and functional landmarks on the cortical surface. Our results reveal a functional heterogeneity within human intraparietal cortex where the visual field map representations in superior/medial parts of IPS and superior parietal gyrus are involved predominantly in numerosity perception, whereas numerical operations predominantly recruit lateral/inferior parts of IPS. Since calculation and comparison-related activity fell mainly outside the field map representations considered the functional equivalent of the monkey VIP/LIP complex, the areas most activated during such numerical operations in humans are likely different from VIP.

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bioRxiv2019

Attentional amplification of neural codes for number independent of other quantities along the dorsal visual stream

Humans and other animals base important decisions on estimates of number, and intraparietal cortex is thought to provide a crucial substrate of this ability. However, it remains debated whether an independent neuronal processing mechanism underlies this 'number sense', or whether number is instead judged indirectly on the basis of other quantitative features. We performed high-resolution 7 Tesla fMRI while adult human volunteers attended either to the numerosity or to an orthogonal dimension (average item size) of visual dot arrays. Numerosity explained a significant amount of variance in activation patterns, above and beyond non-numerical dimensions. Its representation was progressively enhanced along the dorsal visual pathway and was selectively amplified by attention when task relevant. These results reveal a dedicated extraction mechanism for numerosity that operates independently of other quantitative dimensions of the stimuli, and suggest that later stages along the dorsal stream are most important for the explicit manipulation of numerical quantity.

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bioRxiv2018

Asymmetrical interference between number and item size perception provide evidence for a domain specific impairment in dyscalculia

Dyscalculia, a specific learning disability that impacts arithmetical skills, has previously been associated to a deficit in the precision of the system that estimates the approximate number of objects in visual scenes (the so called number sense system). However, because in tasks involving numerosity comparisons dyscalculics judgements appears disproportionally affected by continuous quantitative dimensions (such as the size of the items), an alternative view linked dyscalculia to a domain-general difficulty in inhibiting task-irrelevant responses.\n\nTo arbitrate between these views, we evaluated the degree of reciprocal interference between numerical and non-numerical quantitative dimensions in adult dyscalculics and matched controls. We used a novel stimulus set orthogonally varying in mean item size and numerosity, putting particular attention into matching both features perceptual discriminability. Participants compared those stimuli based on each of the two dimensions. While control subjects showed no significant size interference when judging numerosity, dyscalculics numerosity judgments were strongly biased by the unattended size dimension. Importantly however, both groups showed the same degree of interference from number when judging mean size. Moreover, only the ability to discard the irrelevant size information when comparing numerosity (but not the reverse) significantly predicted calculation ability across subjects.\n\nOverall, our results show that numerosity discrimination is less prone to interference than discrimination of another quantitative feature (mean item size) when the perceptual discriminability of these features is matched, as here in control subjects. By quantifying, for the first time, dyscalculic subjects degree of interference on another orthogonal dimension of the same stimuli, we are able to exclude a domain-general inhibition deficit as explanation for their poor / biased numerical judgement. We suggest that enhanced reliance on non-numerical cues during numerosity discrimination can represent a strategy to cope with a less precise number sense.

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