Cortical Magnification Factor Research Articles

The role of extrinsic and intrinsic factors in perceptual filling-in of the blind-spot with variegated color and texture stimuli

Vision scientists dedicated their efforts to unraveling the mechanism of filling-in at the blind-spot (BS) through numerous psychophysical experiments. The prevalent interpretation, emphasizing active filling-in, has spurred extensive research endeavors. In a parallel vein, a pertinent study highlighted the predominance of the nasal Visual Field (VF) over the temporal one and postulated the role of the Cortical Magnification Factor (CMF) in explaining the asymmetry of filling-in. In this study, we first replicated this experiment and then conducted BS-specific psychophysical experiments employing various bi-colored and bi-textured (patterned) stimuli. We observed that nasal dominance is not persistent in the context of the spread of perception for BS filling-in. We posit that the visual information processing priority index (VIPPI), comprising the CMF (an intrinsic factor unaffected by stimulus characteristics) and relative luminance (an extrinsic factor dependent on stimulus characteristics), governs the spread of perception for filling-in in case of diverse neighborhoods of the BS.

Visual field asymmetries in visual word form identification

Visual performance across the visual fields interacts with visual tasks and visual stimuli, and visual resolution decreases as a function of eccentricity, varying at isoeccentric locations. In this study, we investigated the extent of asymmetry and the rate of change in visual acuity threshold for visual word form (VWF) identification at horizontal and vertical azimuths across the fovea, and at eccentricities of 1°, 2°, 4°, 6° and 8° for 10%, 20%, 40%, and 80% contrast levels, to determine whether and how the eccentricities, meridians, and contrasts modulated the VWF identification acuity threshold. The stimuli were 16 traditional Chinese characters of similar legibility. Participants pressed a key to indicate the character presented, either monocularly or binocularly, at one of 21 randomly selected locations. A staircase procedure was used to determine the threshold, and a multiple linear regression model was used to fit the linear cortical magnification factor (CMF). We found that (1) the asymmetry was most pronounced on the vertical and superior azimuths, (2) the asymmetry between the right and left azimuths was not significant, (3) the CMF was significantly smaller on the vertical azimuth than on the horizontal azimuth, (4) the CMF was smaller on the superior vertical azimuth than on the inferior azimuth, and (5) monocular viewing and low contrast enhanced the CMF difference between azimuths. In conclusion, vertical and horizontal azimuths, location of eccentricity, contrast levels of word symbols, and monocular/binocular viewing have different effects on visual field asymmetry and cortical magnification factors.

Adaptive smoothing of retinotopic maps based on Teichmüller parametrization

Retinotopic mapping, the mapping between visual inputs on the retina and neural responses on the cortical surface, is one of the fundamental topics in visual neuroscience. In human studies, retinotopic maps are conventionally constructed and processed by decoding blood oxygenation-level dependent (BOLD) functional magnetic resonance imaging (fMRI) responses to designed visual stimuli on the cortical surface. However, these methods frequently generate retinotopic maps that do not preserve topology, contradicting a fundamental property of retinotopic maps observed in neurophysiology. To address this problem, we propose an integrated approach to simultaneously refine the flattening from the 3D cortical surface to the 2D parametric space and adaptively smooth retinotopic perception centers in the visual space to make the retinotopic maps topological. One key element of the approach is the enhanced error tolerant Teichmüller mapping, which refines the parametrization by minimizing angle distortions and maximizing alignment to noisy landmarks. We validated our overall approach with synthetic and real retinotopic mapping datasets and applied it to compute cortical magnification factor (CMF). The results showed that the proposed approach was superior to other conventional retinotopic mapping methods in predicting BOLD fMRI time series and preserving topology.

Early visual experience refines the retinotopic organization within and across visual cortical regions

Early visual areas are retinotopically organized in human and non-human primates. Population receptive field (pRF) size increases with eccentricity and from lower- to higher-level visual areas. Furthermore, the cortical magnification factor (CMF), a measure of how much cortical space is devoted to each degree of visual angle, is typically larger for foveal as opposed to peripheral regions of the visual field. Whether this fine-scale organization within and across visual areas depends on early visual experience has yet been unknown. Here, we employed 7T functional magnetic resonance imaging pRF mapping to assess the retinotopic organization of early visual regions (i.e., V1, V2, and V3) in eight sight recovery individuals with a history of congenital blindness until a maximum of 4 years of age. Compared with sighted controls, foveal pRF sizes in these individuals were larger, and pRF sizes did not show the typical increase with eccentricity and down the visual cortical processing stream (V1-V2-V3). Cortical magnification was overall diminished and decreased less from foveal to parafoveal visual field locations. Furthermore, cortical magnification correlated with visual acuity in sight recovery individuals. The results of this study suggest that early visual experience is essential for refining a presumably innate prototypical retinotopic organization in humans within and across visual areas, which seems to be crucial for acquiring full visual capabilities.

Central-peripheral dichotomy: color-motion and luminance-motion binding show stronger top-down feedback in central vision.

Recently a theory (Zhaoping, Vision Research, 136, 32-49, 2017) proposed that top-down feedback from higher to lower visual cortical areas, to aid visual recognition, is stronger in the central than in the peripheral visual fields. Since top-down feedback helps feature binding, a critical visual recognition process, this theory predicts that insufficient feedback in the periphery should make feature misbinding more likely. To test this prediction, this study assessed binding between color and motion features, or between luminance and motion features, at different visual field eccentricities. We first used color-motion stimuli containing equiluminant red and green dots moving in opposite directions, for example, red dots moved leftward while green dots moved rightward. Such stimuli were shown in both a central reference strip and a peripheral test strip; participants reported whether it was the first or second interval in a trial in which the dots of each color moved in the opposite directions between the two strips. The center of the test strip was at 4° or 15° away from the gaze fixation. Participants' performance was much worse when the test strip was more peripheral, suggesting that feature misbinding occurred more frequently there. This held even when the size and density of the dots were adjusted by eccentricity-dependent cortical magnification factors, and even when red/green dots were replaced by yellow/blue dots or black/white dots to suit the retinal input sampling peripherally. Our findings support that top-down feedback is more directed to central vision, which can resolve ambiguities in feature binding at more central visual locations.

On the cortical mapping function – Visual space, cortical space, and crowding

The retino-cortical visual pathway is retinotopically organized: Neighbourhood relationships on the retina are preserved in the mapping. Size relationships in that mapping are also highly regular: The size of a patch in the visual field that maps onto a cortical patch of fixed size follows, along any radius and over a wide range, simply a linear function with retinal eccentricity. As a consequence, the mapping of retinal to cortical locations follows a logarithmic function along that radius. While this has already been shown by Fischer (1973, Vision Research, 13, 2113–2120), the link between the linear function – which describes the local behaviour by the cortical magnification factor M – and the logarithmic location function for the global behaviour, has never been made explicit. The present paper provides such a link as a set of ready-to-use equations using Levi and Klein’s E2 nomenclature, and examples for their validity and applicability in the mapping literature are discussed. The equations allow estimating M in the retinotopic centre; values thus derived from the literature show enormous, hitherto unnoticed, variability. A new structural parameter, d2, is proposed to characterize the cortical map, as a counterpart to E2; it shows much more stability. One pitfall is discussed and spelt out, namely the common myth that a pure logarithmic function, without constant term, will give an adequate map. The correct equations are finally extended to describe the cortical map of Bouma’s law on visual crowding. The result contradicts recent suggestions that critical crowding distance corresponds to constant cortical distance.

Asymmetries around the visual field: From retina to cortex to behavior.

Visual performance varies around the visual field. It is best near the fovea compared to the periphery, and at iso-eccentric locations it is best on the horizontal, intermediate on the lower, and poorest on the upper meridian. The fovea-to-periphery performance decline is linked to the decreases in cone density, retinal ganglion cell (RGC) density, and V1 cortical magnification factor (CMF) as eccentricity increases. The origins of polar angle asymmetries are not well understood. Optical quality and cone density vary across the retina, but recent computational modeling has shown that these factors can only account for a small percentage of behavior. Here, we investigate how visual processing beyond the cone photon absorptions contributes to polar angle asymmetries in performance. First, we quantify the extent of asymmetries in cone density, midget RGC density, and V1 CMF. We find that both polar angle asymmetries and eccentricity gradients increase from cones to mRGCs, and from mRGCs to cortex. Second, we extend our previously published computational observer model to quantify the contribution of phototransduction by the cones and spatial filtering by mRGCs to behavioral asymmetries. Starting with photons emitted by a visual display, the model simulates the effect of human optics, cone isomerizations, phototransduction, and mRGC spatial filtering. The model performs a forced choice orientation discrimination task on mRGC responses using a linear support vector machine classifier. The model shows that asymmetries in a decision maker's performance across polar angle are greater when assessing the photocurrents than when assessing isomerizations and are greater still when assessing mRGC signals. Nonetheless, the polar angle asymmetries of the mRGC outputs are still considerably smaller than those observed from human performance. We conclude that cone isomerizations, phototransduction, and the spatial filtering properties of mRGCs contribute to polar angle performance differences, but that a full account of these differences will entail additional contribution from cortical representations.

Topological dominance in peripheral vision.

The question of what peripheral vision is good for, especially in pattern recognition, is one of the most important and controversial issues in cognitive science. In a series of experiments, we provide substantial evidence that observers’ behavioral performance in the periphery is consistently superior to central vision for topological change detection, while nontopological change detection deteriorates with increasing eccentricity. These experiments generalize the topological account of object perception in the periphery to different kinds of topological changes (i.e., including introduction, disappearance, and change in number of holes) in comparison with a broad spectrum of geometric properties (e.g., luminance, similarity, spatial frequency, perimeter, and shape of the contour). Moreover, when the stimuli were scaled according to cortical magnification factor and the task difficulty was well controlled by adjusting luminance of the background, the advantage of topological change detection in the periphery remained. The observed advantage of topological change detection in the periphery supports the view that the topological definition of objects provides a coherent account for object perception in peripheral vision, allowing pattern recognition with limited acuity.

Shape discrimination in peripheral vision: Addressing pragmatic limitations of M-scaling radial frequency patterns

Peripheral worsening in shape discrimination (SD) can be compensated by size-scaling of peripheral stimuli. However, such scaling results in production of large stimuli that occupy a vast range of eccentricities. We used six proportionally decreasing spatial scales to address this pragmatic limitation and to explore how shape discrimination varies with radius in the nasal visual field. Five participants with normal vision discriminated circles and radial frequency (RF) patterns presented nasally to the fixation point at 5°, 10°, 15° and 20°. Stimuli were scaled with the nasal cortical magnification factor (nCMF) from a central stimulus in six spatial scales, which varied from 0.125 to 1, where 1 corresponded to 1.2° radius. Thresholds expressed in Weber fractions remained constant at eccentricities up to 20° regardless of the spatial scale. Weber fractions for the smaller spatial scales (0.125–0.5) were higher and more variable than for the larger spatial scales (0.75–1), yet still constant across periphery. The results provide evidence that peripheral shape discrimination is constrained by low-level properties, such as eccentricity, and can be predicted by the cortical magnification theory. However, above the peripheral modulation resolution limits, RF shape discrimination is based on the proportion between the modulation amplitude and the radius for larger scales (0.75–1), and demonstrates peripheral scale invariance for these stimuli. For eccentric shape discrimination tests, stimuli with low spatial frequency, high contrast, and radii corresponding to SS 0.75–0.875 should be used to ensure constant Weber fractions, small variability, and peripheral stimuli that are not excessively magnified.

Somatotopic Arrangement of the Human Primary Somatosensory Cortex Derived From Functional Magnetic Resonance Imaging.

Functional magnetic resonance imaging (fMRI) was used to estimate neuronal activity in the primary somatosensory cortex of six participants undergoing cutaneous tactile stimulation on skin areas spread across the entire body. Differences between the accepted somatotopic maps derived from Penfield's work and those generated by this fMRI study were sought, including representational transpositions or replications across the cortex. MR-safe pneumatic devices mimicking the action of a Wartenberg wheel supplied touch stimuli in eight areas. Seven were on the left side of the body: foot, lower, and upper leg, trunk beneath ribcage, anterior forearm, middle fingertip, and neck above the collarbone. The eighth area was the glabella. Activation magnitude was estimated as the maximum cross-correlation coefficient at a certain phase shift between ideal time series and measured blood oxygen level dependent (BOLD) time courses on the cortical surface. Maximally correlated clusters associated with each cutaneous area were calculated, and cortical magnification factors were estimated. Activity correlated to lower limb stimulation was observed in the paracentral lobule and superomedial postcentral region. Correlations to upper extremity stimulation were observed in the postcentral area adjacent to the motor hand knob. Activity correlated to trunk, face and neck stimulation was localized in the superomedial one-third of the postcentral region, which differed from Penfield's cortical homunculus.

ВОЗМОЖНОСТИ И ОГРАНИЧЕНИЯ ЭКСТРАФОВЕАЛЬНОГО ВОСПРИЯТИЯ: АНАЛИТИЧЕСКИЙ ОБЗОР

The article presents an analytical review of studies in the field of extrafoveal perception. The region of extrafoveal vision combines the parafovea and periphery of the retina, so extrafoveal perception is the perception of objects which projections are outside of the fovea. For a long time, it has been believed that extrafoveal vision, in contrast to foveal, has a lower acuity and resolution and is used mainly for preliminary analysis of the visual field and selection of relevant objects for their more thorough analysis in fovea. However, the literature shows that extrafoveal perception is much more interesting and autonomous process as it has been previously considered. The paper analyzes a number of works showing that it is possible to identify both specific features of simple stimuli and rather complex objects, such as faces or whole scenes, up to the possibility of their semantic analysis, even in extrafoveal vision. The review considers the history of studies on extrafoveal perception, from the earliest works (Hueck, 1840; Aubert, Foerster, 1857) to the most recent ones of the last 5 years. These works have analyzed the main factors influencing the effectiveness of extrafoveal vision, for example, cortical magnification factor, which reflects differences in the number of neurons in the visual cortex responsible for processing stimuli depending on the region of the retina: the closer the object is to the fovea, the more neurons are involved in its processing, and vice versa. Other factors determining the efficacy of extrafoveal perception include the following: crowding effect when the target object on the periphery surrounded by distractors is identified worse than a separately located stimulus; specific characteristics of a target and distractors (for example, contexts evoking pop-out effect). Crowding effect is also related to the question of correlating two forms of processing extrafoveal information: preattentive processing (parallel “bottom up” processing) and covert attention (moving the point of deeper analysis along the visual field without eye movements) which can be controlled up to some degree. The other factors influencing the effectiveness of extrafoveal perception concern the context of a task (categorical search in laboratory conditions, analysis of the real world scenes, reactions to extrafoveal stimuli during definite activity) and individual differences. Additionally, we have analyzed the works on the possibilities of training extrafoveal perception, which can affect both higher-level processes, for example, identification of the context of complex scenes, perception of emotions and categorical visual search, and lower-level visual functions, such as identification of spatial orientation, contrast perception and reduction of crowding effect.

Spatial clustering of orientation preference in primary visual cortex of the large rodent agouti.

SummaryAll rodents investigated so far possess orientation-selective neurons in the primary visual cortex (V1) but – in contrast to carnivores and primates – no evidence of periodic maps with pinwheel-like structures. Theoretical studies debating whether phylogeny or universal principles determine development of pinwheels point to V1 size as a critical constraint. Thus, we set out to study maps of agouti, a big diurnal rodent with a V1 size comparable to cats'. In electrophysiology, we detected interspersed orientation and direction-selective neurons with a bias for horizontal contours, corroborated by homogeneous activation in optical imaging. Compatible with spatial clustering at short distance, nearby neurons tended to exhibit similar orientation preference. Our results argue against V1 size as a key parameter in determining the presence of periodic orientation maps. They are consistent with a phylogenetic influence on the map layout and development, potentially reflecting distinct retinal traits or interspecies differences in cortical circuitry.

Postnatal development of subfields in the core region of the mouse auditory cortex

The core region of the rodent auditory cortex has two subfields: the primary auditory area (A1) and the anterior auditory field (AAF). Although the postnatal development of A1 has been studied in several mammalian species, few studies have been conducted on the postnatal development of AAF. Using a voltage-sensitive-dye-based imaging method, we examined and compared the postnatal development of AAF and A1 in mice from postnatal day 11 (P11) to P40. We focused on the postnatal development of tonotopy, the relative position between A1 and AAF, and the properties of tone-evoked responses in the subfields. Tone-evoked responses in the mouse auditory cortex were first observed at P12, and tonotopy was found in both A1 and AAF at this age. Quantification of tonotopy using the cortical magnification factor (CMF; octave difference per unit cortical distance) revealed a rapid change from P12 to P14 in both A1 and AAF, and a stable level from P14. A similar time course of postnatal development was found for the distance between the 4 kHz site in A1 and AAF, the distance between the 16 kHz site in A1 and AAF, and the angle between the frequency axis of A1 and AAF. The maximum amplitude and rise time of tone-evoked signals in both A1 and AAF showed no significant change from P12 to P40, but the latency of the responses to both the 4 kHz and 16 kHz tones decreased during this period, with a more rapid decrease in the latency to 16 kHz tones in both subfields. The duration of responses evoked by 4 kHz tones in both A1 and AAF showed no significant postnatal change, but the duration of responses to 16 kHz tones decreased exponentially in both subfields. The cortical area activated by 4 kHz tones in AAF was always larger than that in A1 at all ages (P12-P40). Our results demonstrated that A1 and AAF developed in parallel postnatally, showing a rapid maturation of tonotopy, slow maturation of response latency and response duration, and a dorsal-to-ventral order (high-frequency site to low-frequency site) of functional maturation.

Geometry and Geodesy on the Primary Visual Cortex as a Surface of Revolution

Biological mapping of the visual field from the eye retina to the primary visual cortex, also known as occipital area V1, is central to vision and eye movement phenomena and research. That mapping is critically dependent on the existence of cortical magnification factors. Once unfolded, V1 has a convex three-dimensional shape, which can be mathematically modeled as a surface of revolution embedded in three-dimensional Euclidean space. Thus, we solve the problem of differential geometry and geodesy for the mapping of the visual field to V1, involving both isotropic and non-isotropic cortical magnification factors of a most general form. We provide illustrations of our technique and results that apply to V1 surfaces with curve profiles relevant to vision research in general and to visual phenomena such as ‘crowding’ effects and eye movement guidance in particular. From a mathematical perspective, we also find intriguing and unexpected differential geometry properties of V1 surfaces, discovering that geodesic orbits have alternative prograde and retrograde characteristics, depending on the interplay between local curvature and global topology.

Cortical Electrocorticogram (ECoG) Is a Local Signal.

Electrocorticogram (ECoG), obtained by low-pass filtering the brain signal recorded from a macroelectrode placed on the cortex, is extensively used to find the seizure focus in drug-resistant epilepsy and is of growing importance in cognitive and brain-machine-interfacing studies. To accurately estimate the epileptogenic cortex or to make inferences about cognitive processes, it is important to determine the "spatial spread" of ECoG (i.e., the extent of cortical tissue that contributes to its activity). However, the ECoG spread is currently unknown; even the spread of local field potential (LFP) obtained from microelectrodes is debated, with estimates ranging from a few hundred micrometers to several millimeters. Spatial spread can be estimated by measuring the receptive field (RF) and multiplying by the cortical magnification factor, but this method overestimates the spread because RF size gets inflated due to several factors. This issue can be partially addressed using a model that compares the RFs of two measures, such as LFP and multi-unit activity (MUA). To use this approach for ECoG, we designed a customized array containing both microelectrodes and ECoG electrodes to simultaneously map MUA, LFP, and ECoG RFs from the primary visual cortex of awake monkeys (three female Macaca radiata). The spatial spread of ECoG was surprisingly local (diameter ∼3 mm), only 3 times that of the LFP. Similar results were obtained using a model to simulate ECoG as a sum of LFPs of varying electrode sizes. Our results further validate the use of ECoG in clinical and basic cognitive research.SIGNIFICANCE STATEMENT Brains signals capture different attributes of the neural network depending on the size and location of the recording electrode. Electrocorticogram (ECoG), obtained by placing macroelectrodes (typically 2-3 mm diameter) on the exposed surface of the cortex, is widely used by neurosurgeons to identify the source of seizures in drug-resistant epileptic patients. The brain area responsible for seizures is subsequently surgically removed. Accurate estimation of the epileptogenic cortex and its removal requires the estimation of spatial spread of ECoG. Here, we estimated the spatial spread of ECoG in five behaving monkeys using two different approaches. Our results suggest that ECoG is a local signal (diameter of ∼3 mm), which can provide a useful tool for clinical, cognitive neuroscience, and brain-machine-interfacing applications.

The Organization and Connections of Second Somatosensory Cortex in the Agouti.

In order to understand how the mammalian sensory cortex has been structured during evolution, it is necessary to compare data from different species across distinct mammalian lineages. Here, we investigated the organization of the secondary somatosensory area (S2) in the agouti (Dasyprocta aguti), a medium-sized Amazonian rodent, using microelectrode mapping techniques and neurotracer injections. The topographic map obtained from multiunit electrophysiological recordings were correlated with both cytochrome oxidase (CO) histochemistry and with patterns of corticocortical connections in tangential sections. The electrophysiological mapping of the lateral strip of parietal cortex adjacent to the primary somatosensory area (S1) revealed that S2 displays a mirror-reversed topographical representation of S1, but with a smaller cortical magnification factor. The caudal border of S2 is surrounded by sensory fields which also respond to auditory stimulation. BDA injections into the forelimb representation of S2 revealed a dense homotopic ipsilateral projection to S1, supplemented by a less dense projection to the caudolateral cortex located near the rhinal sulcus (parietal rhinal area) and to a frontal region probably associated with the motor cortex. Our findings were similar to those described in other mammalian species, reinforcing the existence of a common plan of organization for S2 in the mammalian parietal cortex.

Connectivity of neuronal populations within and between areas of primate somatosensory cortex

Functions of the cerebral cortex emerge via interactions of horizontally distributed neuronal populations within and across areas. However, the connectional underpinning of these interactions is not well understood. The present study explores the circuitry of column-size cortical domains within the hierarchically organized somatosensory cortical areas 3b and 1 using tract tracing and optical intrinsic signal imaging (OIS). The anatomical findings reveal that feedforward connections exhibit high topographic specificity, while intrinsic and feedback connections have a more widespread distribution. Both intrinsic and inter-areal connections are topographically oriented across the finger representations. Compared to area 3b, the low clustering of connections and small cortical magnification factor supports that the circuitry of area 1 scaffolds a sparse functional representation that integrates peripheral information from a large area that is fed back to area 3b. Fast information exchange between areas is ensured by thick axons forming a topographically organized, reciprocal pathway. Moreover, the highest density of projecting neurons and groups of axon arborization patches corresponds well with the size and locations of the functional population response reported by OIS. The findings establish connectional motifs at the mesoscopic level that underpin the functional organization of the cerebral cortex.

Parallel, exhaustive processing underlies logarithmic search functions: Visual search with cortical magnification.

Our lab recently found evidence that efficient visual search (with a fixed target) is characterized by logarithmic Reaction Time (RT) × Set Size functions whose steepness is modulated by the similarity between target and distractors. To determine whether this pattern of results was based on low-level visual factors uncontrolled by previous experiments, we minimized the possibility of crowding effects in the display, compensated for the cortical magnification factor by magnifying search items based on their eccentricity, and compared search performance on such displays to performance on displays without magnification compensation. In both cases, the RT × Set Size functions were found to be logarithmic, and the modulation of the log slopes by target-distractor similarity was replicated. Consistent with previous results in the literature, cortical magnification compensation eliminated most target eccentricity effects. We conclude that the log functions and their modulation by target-distractor similarity relations reflect a parallel exhaustive processing architecture for early vision.

Spatial Frequency Selectivity Is Impaired in Dopamine D2 Receptor Knockout Mice.

Dopamine is a neurotransmitter implicated in several brain functions, including vision. In the present study, we investigated the impacts of the lack of D2 dopamine receptors on the structure and function of the primary visual cortex (V1) of D2-KO mice using optical imaging of intrinsic signals. Retinotopic maps were generated in order to measure anatomo-functional parameters such as V1 shape, cortical magnification factor, scatter, and ocular dominance. Contrast sensitivity and spatial frequency selectivity (SF) functions were computed from responses to drifting gratings. When compared to control mice, none of the parameters of the retinotopic maps were affected by D2 receptor loss of function. While the contrast sensitivity function of D2-KO mice did not differ from their wild-type counterparts, SF selectivity function was significantly affected as the optimal SF and the high cut-off frequency (p < 0.01) were higher in D2-KO than in WT mice. These findings show that the lack of function of D2 dopamine receptors had no influence on cortical structure whereas it had a significant impact on the spatial frequency selectivity and high cut-off. Taken together, our results suggest that D2 receptors play a specific role on the processing of spatial features in early visual cortex while they do not seem to participate in its development.

Radial asymmetries in population receptive field size and cortical magnification factor in early visual cortex

Human visual cortex does not represent the whole visual field with the same detail. Changes in receptive field size, population receptive field (pRF) size and cortical magnification factor (CMF) with eccentricity are well established, and associated with changes in visual acuity with eccentricity. Visual acuity also changes across polar angle. However, it remains unclear how RF size, pRF size and CMF change across polar angle. Here, we examine differences in pRF size and CMF across polar angle in V1, V2 and V3 using pRF modeling of human fMRI data. In these visual field maps, we find smaller pRFs and larger CMFs in horizontal (left and right) than vertical (upper and lower) visual field quadrants. Differences increase with eccentricity, approximately in proportion to average pRF size and CMF. Similarly, we find larger CMFs in the lower than upper quadrant, and again differences increase with eccentricity. However, pRF size differences between lower and upper quadrants change direction with eccentricity. Finally, we find slightly smaller pRFs in the left than right quadrants of V2 and V3, though this difference is very small, and we find no differences in V1 and no differences in CMF. Moreover, differences in pRF size and CMF vary gradually with polar angle and are not limited to the meridians or visual field map discontinuities. PRF size and CMF differences do not consistently follow patterns of cortical curvature, despite the link between cortical curvature and polar angle in V1. Thus, the early human visual cortex has a radially asymmetric representation of the visual field. These asymmetries may underlie consistent reports of asymmetries in perceptual abilities.