An investigation into the relationship between stimulus property, neural response and its manifestation in the visual evoked potential involving retinal resolution

The visual evoked potential (VEP) has been shown to reflect the size of the neural population activated by a processing mechanism selective to the temporal - and spatial luminance contrast property of a stimulus. We set out to better understand how the factors determining the neural response associated with these mechanisms. To do so we recorded the VEP from 14 healthy volunteers viewing two series of pattern reversing stimuli with identical temporal-and spatial luminance contrast properties. In one series the size of the elements increased towards the edge of the image, in the other it decreased. In the former element size was congruent with receptive field size across eccentricity, in the later it was incongruent. P100 amplitude to the incongruent series exceeded that obtained to the congruent series. Using electric dipoles due the excitatory neural response we accounted for this using dipole cancellation of electric dipoles of opposite polarity originating in supra- and infragranular layers of V1. The phasic neural response in granular lamina of V1 exhibited magnocellular characteristics, the neural response outside of the granular lamina exhibited parvocellular characteristics and was modulated by re-entrant projections. Using electric current density, we identified areas of the dorsal followed by areas of the ventral stream as the source of the re-entrant signal modulating infragranular activity. Our work demonstrates that the VEP does not signal reflect the overall level of a neural response but is the result of an interaction between electric dipoles originating from neural responses in different lamina of V1.

Topographic connectivity reveals task-dependent retinotopic processing throughout the human brain

The human visual system is organized as a hierarchy of maps that share the topography of the retina. Known retinotopic maps have been identified using simple visual stimuli under strict fixation, conditions different from everyday vision which is active, dynamic, and complex. This means that it remains unknown how much of the brain is truly visually organized. Here I demonstrate widespread stable visual organization beyond the traditional visual system, in default-mode network and hippocampus. Detailed topographic connectivity with primary visual cortex during movie-watching, resting-state, and retinotopic-mapping experiments revealed that visual-spatial representations throughout the brain are warped by cognitive state. Specifically, traditionally visual regions alternate with default-mode network and hippocampus in preferentially representing the center of the visual field. This visual role of default-mode network and hippocampus would allow these regions to interface between abstract memories and concrete sensory impressions. Together, these results indicate that visual-spatial organization is a fundamental coding principle that structures the communication between distant brain regions.

A Probabilistic Functional Atlas of Human Occipito-Temporal Visual Cortex

Human visual cortex contains many retinotopic and category-specific regions. These brain regions have been the focus of a large body of functional magnetic resonance imaging research, significantly expanding our understanding of visual processing. As studying these regions requires accurate localization of their cortical location, researchers perform functional localizer scans to identify these regions in each individual. However, it is not always possible to conduct these localizer scans. Here, we developed and validated a functional region of interest (ROI) atlas of early visual and category-selective regions in human ventral and lateral occipito-temporal cortex. Results show that for the majority of functionally defined ROIs, cortex-based alignment results in lower between-subject variability compared to nonlinear volumetric alignment. Furthermore, we demonstrate that 1) the atlas accurately predicts the location of an independent dataset of ventral temporal cortex ROIs and other atlases of place selectivity, motion selectivity, and retinotopy. Next, 2) we show that the majority of voxel within our atlas is responding mostly to the labeled category in a left-out subject cross-validation, demonstrating the utility of this atlas. The functional atlas is publicly available (download.brainvoyager.com/data/visfAtlas.zip) and can help identify the location of these regions in healthy subjects as well as populations (e.g., blind people, infants) in which functional localizers cannot be run.

Projection of Orthogonal Tiling from the Retina to the Visual Cortex

In higher mammals, the primary visual cortex (V1) is organized into diverse tuning maps of visual features. The topography of these maps intersects orthogonally, but it remains unclear how such a systematic relationship can develop. Here, we show that the orthogonal organization already exists in retinal ganglion cell (RGC) mosaics, providing a blueprint of the organization in V1. From analysis of the RGC mosaics data in monkeys and cats, we find that the ON-OFF RGC distance and ON-OFF angle of neighboring RGCs are organized into a topographic tiling across mosaics, analogous to the orthogonal intersection of cortical tuning maps. Our model simulation shows that the ON-OFF distance and angle in RGC mosaics correspondingly initiate ocular dominance/spatial frequency tuning and orientation tuning, resulting in the orthogonal intersection of cortical tuning maps. These findings suggest that the regularly structured ON-OFF patterns mirrored from the retina initiate the uniform representation of combinations of map features over the visual space.

Spatial readout of visual looming in the central brain of Drosophila

Visual systems can exploit spatial correlations in the visual scene by using retinotopy, the organizing principle by which neighboring cells encode neighboring spatial locations. However, retinotopy is often lost, such as when visual pathways are integrated with other sensory modalities. How is spatial information processed outside of strictly visual brain areas? Here, we focused on visual looming responsive LC6 cells in Drosophila, a population whose dendrites collectively cover the visual field, but whose axons form a single glomerulus-a structure without obvious retinotopic organization-in the central brain. We identified multiple cell types downstream of LC6 in the glomerulus and found that they more strongly respond to looming in different portions of the visual field, unexpectedly preserving spatial information. Through EM reconstruction of all LC6 synaptic inputs to the glomerulus, we found that LC6 and downstream cell types form circuits within the glomerulus that enable spatial readout of visual features and contralateral suppression-mechanisms that transform visual information for behavioral control.

Spatially Specific Working Memory Activity in the Human Superior Colliculus

Theoretically, working memory (WM) representations are encoded by population activity of neurons with distributed tuning across the stored feature. Here, we leverage computational neuroimaging approaches to map the topographic organization of human superior colliculus (SC) and model how population activity in SC encodes WM representations. We first modeled receptive field properties of voxels in SC, deriving a detailed topographic organization resembling that of the primate SC. Neural activity within human (5 male and 1 female) SC persisted throughout a retention interval of several types of modified memory-guided saccade tasks. Assuming an underlying neural architecture of the SC based on its retinotopic organization, we used an encoding model to show that the pattern of activity in human SC represents locations stored in WM. Our tasks and models allowed us to dissociate the locations of visual targets and the motor metrics of memory-guided saccades from the spatial locations stored in WM, thus confirming that human SC represents true WM information. These data have several important implications. They add the SC to a growing number of cortical and subcortical brain areas that form distributed networks supporting WM functions. Moreover, they specify a clear neural mechanism by which topographically organized SC encodes WM representations.

SIGNIFICANCE STATEMENT Using computational neuroimaging approaches, we mapped the topographic organization of human superior colliculus (SC) and modeled how population activity in SC encodes working memory (WM) representations, rather than simpler visual or motor properties that have been traditionally associated with the laminar maps in the primate SC. Together, these data both position the human SC into a distributed network of brain areas supporting WM and elucidate the neural mechanisms by which the SC supports WM.

Axonal Projections from Middle Temporal Area to the Pulvinar in the Common Marmoset

The pulvinar, the largest thalamic nucleus in the primate brain, has connections with a variety of cortical areas and is involved in many aspects of higher brain functions. Among cortico-pulvino-cortical systems, the connection between the middle temporal area (MT) and the pulvinar has been thought to contribute significantly to complex motion recognition. Recently, the common marmoset (Callithrix jacchus), has become a valuable model for a variety of neuroscience studies, including visual neuroscience and translational research of neurological and psychiatric disorders. However, information on projections from MT to the pulvinar in the marmoset brain is scant. We addressed this deficiency by injecting sensitive anterograde viral tracers into MT to examine the distribution of labeled terminations in the pulvinar. The injection sites were placed retinotopically according to visual field coordinates mapped by optical intrinsic imaging. All injections produced anterograde terminal labeling, which was densest in the medial nucleus of the inferior pulvinar (PIm), sparser in the central nucleus of the inferior pulvinar, and weakest in the lateral pulvinar. Within each subnucleus, terminations formed separate retinotopic fields. Most labeled terminals were small but these comingled with a few large terminals, distributed mainly in the dorsomedial part of the PIm. Our results further delineate the organization of projections from MT to the pulvinar in the marmoset as forming parallel complex networks, which may differentially contribute to motion processing. It is interesting that the densest projections from MT target the PIm, the subnucleus recently reported to preferentially receive direct retinal projections.

Investigating the Reliability of Population Receptive Field Size Estimates Using fMRI

In functional MRI (fMRI), population receptive field (pRF) models allow a quantitative description of the response as a function of the features of the stimuli that are relevant for each voxel. The most popular pRF model used in fMRI assumes a Gaussian shape in the features space (e.g., the visual field) reducing the description of the voxel’s pRF to the Gaussian mean (the pRF preferred feature) and standard deviation (the pRF size). The estimation of the pRF mean has been proven to be highly reliable. However, the estimate of the pRF size has been shown not to be consistent within and between subjects. While this issue has been noted experimentally, here we use an optimization theory perspective to describe how the inconsistency in estimating the pRF size is linked to an inherent property of the Gaussian pRF model. When fitting such models, the goodness of fit is less sensitive to variations in the pRF size than to variations in the pRF mean. We also show how the same issue can be considered from a bias-variance perspective. We compare different estimation procedures in terms of the reliability of their estimates using simulated and real fMRI data in the visual (using the Human Connectome Project database) and auditory domain. We show that, the reliability of the estimate of the pRF size can be improved considering a linear combination of those pRF models with similar goodness of fit or a permutation based approach. This increase in reliability of the pRF size estimate does not affect the reliability of the estimate of the pRF mean and the prediction accuracy.

Mapping Short Association Fibers in the Early Cortical Visual Processing Stream Using In Vivo Diffusion Tractography

Short association fibers (U-fibers) connect proximal cortical areas and constitute the majority of white matter connections in the human brain. U-fibers play an important role in brain development, function, and pathology but are underrepresented in current descriptions of the human brain connectome, primarily due to methodological challenges in diffusion magnetic resonance imaging (dMRI) of these fibers. High spatial resolution and dedicated fiber and tractography models are required to reliably map the U-fibers. Moreover, limited quantitative knowledge of their geometry and distribution makes validation of U-fiber tractography challenging. Submillimeter resolution diffusion MRI—facilitated by a cutting-edge MRI scanner with 300 mT/m maximum gradient amplitude—was used to map U-fiber connectivity between primary and secondary visual cortical areas (V1 and V2, respectively) in vivo. V1 and V2 retinotopic maps were obtained using functional MRI at 7T. The mapped V1–V2 connectivity was retinotopically organized, demonstrating higher connectivity for retinotopically corresponding areas in V1 and V2 as expected. The results were highly reproducible, as demonstrated by repeated measurements in the same participants and by an independent replication group study. This study demonstrates a robust U-fiber connectivity mapping in vivo and is an important step toward construction of a more complete human brain connectome.

Origins of Functional Organization in the Visual Cortex

How are the complex maps for orientation selectivity (OS) created in the primary visual cortex (V1)? Rodents and rabbits have a random distribution of OS preferences across V1 while in cats, ferrets, and all primates cells with similar OS preferences cluster together into relatively wide cortical columns. Given other clear similarities in the organization of the visual pathways, why is it that maps coding OS preferences are so radically different? Prominent models have been created of cortical OS mapping that incorporate Hebbian plasticity, intracortical interactions, and the properties of growing axons. However, these models suggest that the maps arise primarily through intracortical interactions. Here we focus on several other features of the visual system and brain that may influence V1 structure. These are: eye divergence, the total number of cells in V1, the thalamocortical networks, the topography of the retina and phylogeny. We outline the evidence for and against these factors contributing to map formation. One promising theory is that the central-to-peripheral ratio (CP ratio) of retinal cell density can be used to predict whether or not a species has pinwheel maps. Animals with high CP ratios (>7) have orientation columns while those with low CP ratios (<4) have random OS maps. The CP ratio is related to the total number of cells in cortex, which also appears to be a reasonable contributing factor. However, while these factors correlate with map structure to some extent, there is a gray area where certain species do not fit elegantly into the theory. A problem with the existing literature is that OS maps have been investigated in only a small number of mammals, from a small fraction of the mammalian phylogenetic tree. We suggest four species (agouti, fruit bat, sheep, and wallaby) that have a range of interesting characteristics, which sit at intermediate locations between primates and rodents, that make them good targets for filling in the missing gaps in the literature. We make predictions about the map structures of these species based on the organization of their brains and visual systems and, in doing so, set possible paths for future research.

Retinotopic specializations of cortical and thalamic inputs to area MT

Retinotopic specializations in the ventral visual stream, especially foveal adaptations, provide primates with high-acuity vision in the central visual field. However, visual field specializations have not been studied in the dorsal visual stream, dedicated to processing visual motion and visually guided behaviors. To investigate this, we injected retrograde neuronal tracers occupying the whole visuotopic representation of the middle temporal (MT) visual area in marmoset monkeys and studied the distribution and morphology of the afferent primary visual cortex (V1) projections. Contrary to previous reports, we found a heterogeneous population of V1-MT projecting neurons distributed in layers 3C and 6. In layer 3C, spiny stellate neurons were distributed mainly in foveal representations, while pyramidal morphologies were characteristic of peripheral eccentricities. This primate adaptation of the V1 to MT pathway is arranged in a way that we had not previously understood, with abundant stellate projection neurons in the high-resolution foveal portions, suggesting rapid relay of motion information to visual area MT. We also describe that the medial portion of the inferior pulvinar (PIm), which is the main thalamic input to area MT, shows a retinotopic organization, likely reflecting the importance of this pathway during development and the establishment of area MT topography.

Primary visual cortical thickness in correlation with visual field defects in patients with pituitary macroadenomas: a structural 7-Tesla retinotopic analysis

OBJECTIVE

Vision loss remains a debilitating complication of pituitary adenomas, although there is considerable variability in visual impairment before and after decompression surgery. Growing evidence suggests secondary damage to remote visual structures may contribute to vision loss in patients with chiasmatic compression. The present study leverages ultrahigh field 7-T MRI to study the retinotopic organization of the primary visual cortex (V1), and correlates visual defects with cortical thinning in V1 to characterize consequences of pituitary adenomas on the posterior visual system.

METHODS

Eight patients (4 males and 4 females, mean age 44.3 years) with pituitary adenomas who exhibited chiasmatic compression and visual field defects, as well as 8 matched healthy controls (4 males and 4 females, mean age 43.3 years), were scanned at 7-T MRI for this prospective study. Whole-brain cortical thickness was calculated using an automated algorithm. A previously published surface-based algorithm was applied to associate the eccentricity and polar angle with each position in V1. Cortical thickness was calculated at each point in the retinotopic organization, and a cortical thickness ratio was generated against matched controls for each point in the visual fields. Patients with adenoma additionally underwent neuroophthalmological examination including 24-2 Humphrey automated visual field perimetry. Pattern deviation (PD) of each point in the visual field, i.e., the deviation in point detection compared with neurologically healthy controls, was correlated with cortical thickness at corresponding polar and eccentricity angles in V1.

RESULTS

Whole-brain cortical thickness was successfully derived for all patients and controls. The mean tumor volume was 19.4 cm3. The median global thickness of V1 did not differ between patients (mean ± SD 2.21 ± 0.12 cm), compared with controls (2.06 ± 0.13 cm, p > 0.05). Surface morphometry-based retinotopic maps revealed that all 8 patients with adenoma showed a significant positive correlation between PD and V1 thickness ratios (r values ranged from 0.31 to 0.53, p < 0.05). Mixed-procedure analysis revealed that PD = -8.0719 + 5.5873*[Median V1 Thickness Ratio].

CONCLUSIONS

All 8 patients showed significant positive correlations between V1 thickness and visual defect. These findings provide retinotopic maps of localized V1 cortical neurodegeneration spatially corresponding to impairments in the visual field. These results further characterize changes in the posterior visual pathway associated with chiasmatic compression, and may prove useful in the neuroophthalmological workup for patients with pituitary macroadenoma.

Spatial specificity of alpha oscillations in the human visual system

Alpha oscillations are strongly modulated by spatial attention. To what extent, the generators of cortical alpha oscillations are spatially distributed and have selectivity that can be related to retinotopic organization is a matter of continuous scientific debate. In the present report, neuromagnetic activity was quantified by means of spatial location tuning functions from 30 participants engaged in a visuospatial attention task. A cue presented briefly in one of 16 locations directing covert spatial attention resulted in a robust modulation of posterior alpha oscillations. The distribution of the alpha sources approximated the retinotopic organization of the human visual system known from hemodynamic studies. Better performance in terms of target identification was associated with a more spatially constrained alpha modulation. The present findings demonstrate that the generators of posterior alpha oscillations are retinotopically organized when modulated by spatial attention.

X-Chromosome Insufficiency Alters Receptive Fields across the Human Early Visual Cortex

Here, we investigated processing by receptive fields, a fundamental property of neurons in the visual system, using fMRI and population receptive field (pRF) mapping in 20 human females with monosomic Turner syndrome (TS) (mean age, 10.3 ± 2.0 years) versus 22 age- and sex-matched controls (mean age, 10.4 ± 1.9 years). TS, caused by X-chromosome haploinsufficiency in females, is associated with well-recognized effects on visuospatial processing, parieto-occipital cortical anatomy, and parietal lobe function. However, it is unknown whether these effects are related to altered brain structure and function in early visual areas (V1-V3) versus downstream parietal cortical regions. Results show that girls with TS have the following: (1) smaller volume of V1-V3, (2) lower average pRF eccentricity in early visual areas, and (3) sparser pRF coverage in the periphery of the visual field. Further, we examined whether the lower volume of early visual areas, defined using retinotopic mapping, in TS is due to smaller surface area or thinner cortex. Results show that girls with TS had a general reduction in surface area relative to controls in bilateral V1 and V2. Our data suggest the possibility that the smaller cortical surface area of early visual areas in girls with TS may be associated with a lower number of neurons, which in turn, leads to lesser coverage of the peripheral visual field compared to controls. These results indicate that X-chromosome haploinsufficiency associated with TS affects the functional neuroanatomy of early visual areas, and suggest that investigating pRFs in TS may shed insights into their atypical visuospatial processing.SIGNIFICANCE STATEMENT Turner syndrome is caused by the absence of one of the two X-chromosomes in females. Using functional neuroimaging and population receptive field mapping, we find that chromosome dosage variation (X-monosomy) associated with Turner syndrome affects the functional neuroanatomy of the visual cortex. Specifically, girls with Turner syndrome have smaller early visual areas that provide lesser coverage of the peripheral visual field compared with healthy controls. Our observations provide compelling evidence that the X-chromosome affects not only parietal cortex, as described in previous studies, but also affects early visual areas. These findings suggest a paradigm change in understanding the effect of X-monosomy on the development of visuospatial abilities in humans.

Submillimeter fMRI reveals a layout of dorsal visual cortex in macaques, remarkably similar to New World monkeys

The primate visual system encompasses >30 visual areas. Characterizing each area, and the interactions between them, is a prerequisite to understanding the visual system. This fundamental task, however, requires a precise parcellation of the visual cortex. Nonetheless, already at the earliest visual processing stages, i.e., just rostral to V2, the number and exact definition of areas are heavily contested. Here, we map the macaque visual cortex using fMRI at unprecedented high resolution. We show a substantially different retinotopic organization of dorsal third and fourth visual areas in macaques compared with widely accepted models, yet a remarkably similar layout in Old and New World monkeys. This organization largely reconciles most reported discrepancies concerning the visuotopic organization of nonhuman primate caudo-dorsal occipital cortex.

The macaque dorsal occipital cortex is generally thought to contain an elongated third visual area, V3d, extending along most of the rostral border of area V2. In contrast, our submillimeter retinotopic fMRI maps (0.6-mm isotropic voxels, achieved by implanted phased-array receive coils) consistently show three sectors anterior to V2d. The dorsal (mirror image) sector complies with the traditional V3d definition, and the middle (nonmirror image) sector with V3A. The ventral (mirror image) sector bends away from V2d, as does the ventrolateral posterior area (VLP) in marmosets and the dorsolateral posterior area (DLP) in owl monkeys, and represents the entire contralateral hemifield as V3A does. Its population-receptive field size, however, suggests that this ventral sector is another area at the same hierarchical level as V4d. Hence, contrary to prevailing views, the retinotopic organization of cortex rostral to V2d differs substantially from widely accepted models. Instead, it is evolutionarily largely conserved in Old and New World monkeys given its surprisingly similar overall visuotopic organization.

Bayesian analysis of retinotopic maps

Human visual cortex is organized into multiple retinotopic maps. Characterizing the arrangement of these maps on the cortical surface is essential to many visual neuroscience studies. Typically, maps are obtained by voxel-wise analysis of fMRI data. This method, while useful, maps only a portion of the visual field and is limited by measurement noise and subjective assessment of boundaries. We developed a novel Bayesian mapping approach which combines observation–a subject’s retinotopic measurements from small amounts of fMRI time–with a prior–a learned retinotopic atlas. This process automatically draws areal boundaries, corrects discontinuities in the measured maps, and predicts validation data more accurately than an atlas alone or independent datasets alone. This new method can be used to improve the accuracy of retinotopic mapping, to analyze large fMRI datasets automatically, and to quantify differences in map properties as a function of health, development and natural variation between individuals.

Retinotopic Responses in the Visual Cortex Elicited by Epiretinal Electrical Stimulation in Normal and Retinal Degenerate Rats

Purpose

Electronic retinal prostheses restore vision in people with outer retinal degeneration by electrically stimulating the inner retina. We characterized visual cortex electrophysiologic response elicited by electrical stimulation of retina in normally sighted and retinal degenerate rats.

Methods

Nine normally sighted Long Evans and 11 S334ter line 3 retinal degenerate (rd) rats were used to map cortical responses elicited by epiretinal electrical stimulation in four quadrants of the retina. Six normal and six rd rats were used to compare the dendritic spine density of neurons in the visual cortex.

Results

The rd rats required higher stimulus amplitudes to elicit responses in the visual cortex. The cortical electrically evoked responses (EERs) for both healthy and rd rats show a dose-response characteristic with respect to the stimulus amplitude. The EER maps in healthy rats show retinotopic organization. For rd rats, cortical retinotopy is not well preserved. The neurons in the visual cortex of rd rats show a 10% higher dendritic spine density than in the healthy rats.

Conclusions

Cortical activity maps, produced when epiretinal stimulation is applied to quadrants of the retina, exhibit retinotopy in normal but not rd rats. This is likely due to a combination of degeneration of the retina and increased stimulus thresholds in rd, which broadens the activated area of the retina.

Translational Relevance

Loss of retinotopy is evident in rd rats. If a similar loss of retinotopy is present in humans, retinal prostheses design must include flexibility to account for patient specific variability.

Unexplained Progressive Visual Field Loss in the Presence of Normal Retinotopic Maps

Lesions of primary visual cortex or its primary inputs typically result in retinotopically localized scotomas. Here we present an individual with unexplained visual field loss and deficits in visual perception in the absence of structural damage to the early visual pathway or lesions in visual cortex. The subject, monocular from an early age, underwent repeated perimetry tests over 8 years demonstrating severe anopia of the lower hemifield, and a clockwise progression of the loss through her upper left visual field. Her visual impairment was evident in a number of standardized tests and psychophysics, especially in tasks assessing spatial integration using illusory contours. However, her intellectual ability was intact and her performance in some other tasks assessing color vision or object detection in scenes was normal. We employed functional magnetic resonance imaging (fMRI), electroretinography and visually evoked potentials. Surprisingly, in contrast to the participant's severe anopia, we found no evidence of abnormal function of her early visual pathways. Specifically, we performed retinotopic mapping using population receptive field (pRF) analysis to map the functional organization of visual cortex in the anopic participant and three control participants on two occasions three and a half years apart. Despite the behavioral visual field loss, her retinotopic maps and pRF parameters in visual areas V1-V3 were qualitatively normal. Further behavioral experiments confirmed that this discrepancy was not trivially explained by the difference between stimuli used for retinotopic mapping and perimetry. Structural T1 scans were normal at both time points, and volumetric analysis of white and gray matter tissue on the segmented T1 volumes did not reveal any abnormalities or deterioration over time. Our findings suggest that normal functional organization of early visual cortex without evident structural damage to the early visual pathway as disclosed by the techniques employed in this study does not necessarily guarantee conscious perception across the visual field.

Idiosyncratic, Retinotopic Bias in Face Identification Modulated by Familiarity

The perception of gender and age of unfamiliar faces is reported to vary idiosyncratically across retinal locations such that, for example, the same androgynous face may appear to be male at one location but female at another. Here, we test spatial heterogeneity for the recognition of the identity of personally familiar faces in human participants. We found idiosyncratic biases that were stable within participants and that varied more across locations for low as compared to high familiar faces. These data suggest that like face gender and age, face identity is processed, in part, by independent populations of neurons monitoring restricted spatial regions and that the recognition responses vary for the same face across these different locations. Moreover, repeated and varied social interactions appear to lead to adjustments of these independent face recognition neurons so that the same familiar face is eventually more likely to elicit the same recognition response across widely separated visual field locations. We provide a mechanistic account of this reduced retinotopic bias based on computational simulations.

Local-Global Parcellation of the Human Cerebral Cortex from Intrinsic Functional Connectivity MRI

A central goal in systems neuroscience is the parcellation of the cerebral cortex into discrete neurobiological "atoms". Resting-state functional magnetic resonance imaging (rs-fMRI) offers the possibility of in vivo human cortical parcellation. Almost all previous parcellations relied on 1 of 2 approaches. The local gradient approach detects abrupt transitions in functional connectivity patterns. These transitions potentially reflect cortical areal boundaries defined by histology or visuotopic fMRI. By contrast, the global similarity approach clusters similar functional connectivity patterns regardless of spatial proximity, resulting in parcels with homogeneous (similar) rs-fMRI signals. Here, we propose a gradient-weighted Markov Random Field (gwMRF) model integrating local gradient and global similarity approaches. Using task-fMRI and rs-fMRI across diverse acquisition protocols, we found gwMRF parcellations to be more homogeneous than 4 previously published parcellations. Furthermore, gwMRF parcellations agreed with the boundaries of certain cortical areas defined using histology and visuotopic fMRI. Some parcels captured subareal (somatotopic and visuotopic) features that likely reflect distinct computational units within known cortical areas. These results suggest that gwMRF parcellations reveal neurobiologically meaningful features of brain organization and are potentially useful for future applications requiring dimensionality reduction of voxel-wise fMRI data. Multiresolution parcellations generated from 1489 participants are publicly available (https://github.com/ThomasYeoLab/CBIG/tree/master/stable_projects/brain_parcellation/Schaefer2018_LocalGlobal).

Emergence of symmetry selectivity in the visual areas of the human brain: fMRI responses to symmetry presented in both frontoparallel and slanted planes

Symmetry is effortlessly perceived by humans across changes in viewing geometry. Here, we re-examined the network subserving symmetry processing in the context of up-to-date retinotopic definitions of visual areas. Responses in object selective cortex, as defined by functional localizers, were also examined. We further examined responses to both frontoparallel and slanted symmetry while manipulating attention both toward and away from symmetry. Symmetry-specific responses first emerge in V3 and continue across all downstream areas examined. Of the retinotopic areas, ventral occipital VO1 showed the strongest symmetry response, which was similar in magnitude to the responses observed in object selective cortex. Neural responses were found to increase with both the coherence and folds of symmetry. Compared to passive viewing, drawing attention to symmetry generally increased neural responses and the correspondence of these neural responses with psychophysical performance. Examining symmetry on the slanted plane found responses to again emerge in V3, continue through downstream visual cortex, and be strongest in VO1 and LOB. Both slanted and frontoparallel symmetry evoked similar activity when participants performed a symmetry-related task. However, when a symmetry-unrelated task was performed, fMRI responses to slanted symmetry were reduced relative to their frontoparallel counterparts. These task-related changes provide a neural signature that suggests slant has to be computed ahead of symmetry being appropriately extracted, known as the "normalization" account of symmetry processing. Specifically, our results suggest that normalization occurs naturally when attention is directed toward symmetry and orientation, but becomes interrupted when attention is directed away from these features.

A Retinotopic Spiking Neural Network System for Accurate Recognition of Moving Objects Using NeuCube and Dynamic Vision Sensors

This paper introduces a new system for dynamic visual recognition that combines bio-inspired hardware with a brain-like spiking neural network. The system is designed to take data from a dynamic vision sensor (DVS) that simulates the functioning of the human retina by producing an address event output (spike trains) based on the movement of objects. The system then convolutes the spike trains and feeds them into a brain-like spiking neural network, called NeuCube, which is organized in a three-dimensional manner, representing the organization of the primary visual cortex. Spatio-temporal patterns of the data are learned during a deep unsupervised learning stage, using spike-timing-dependent plasticity. In a second stage, supervised learning is performed to train the network for classification tasks. The convolution algorithm and the mapping into the network mimic the function of retinal ganglion cells and the retinotopic organization of the visual cortex. The NeuCube architecture can be used to visualize the deep connectivity inside the network before, during, and after training and thereby allows for a better understanding of the learning processes. The method was tested on the benchmark MNIST-DVS dataset and achieved a classification accuracy of 92.90%. The paper discusses advantages and limitations of the new method and concludes that it is worth exploring further on different datasets, aiming for advances in dynamic computer vision and multimodal systems that integrate visual, aural, tactile, and other kinds of information in a biologically plausible way.

Development of Concurrent Retinotopic Maps in the Fly Motion Detection Circuit

Understanding how complex brain wiring is produced during development is a daunting challenge. In Drosophila, information from 800 retinal ommatidia is processed in distinct brain neuropiles, each subdivided into 800 matching retinotopic columns. The lobula plate comprises four T4 and four T5 neuronal subtypes. T4 neurons respond to bright edge motion, whereas T5 neurons respond to dark edge motion. Each is tuned to motion in one of the four cardinal directions, effectively establishing eight concurrent retinotopic maps to support wide-field motion. We discovered a mode of neurogenesis where two sequential Notch-dependent divisions of either a horizontal or a vertical progenitor produce matching sets of two T4 and two T5 neurons retinotopically coincident with pairwise opposite direction selectivity. We show that retinotopy is an emergent characteristic of this neurogenic program and derives directly from neuronal birth order. Our work illustrates how simple developmental rules can implement complex neural organization.

Spatial elongation of population receptive field profiles revealed by model-free fMRI back-projection

Estimates of visual field topographies in human visual cortex obtained through fMRI traveling wave techniques usually provide the parameters of population receptive field (pRF) location (polar angle, eccentricity) and receptive field size. These parameters are obtained by fitting the recorded data to a standard model population receptive field. In this work, pRF profiles are measured directly by back-projecting preprocessed fMRI time-series to sweeps of a bar across the visual field in different angles. The current data suggest that the model-free pRF profiles contain information not only about receptive field location and size but also about the pRF shape characteristics. The elongation (ellipticity) of pRFs decreases along the early visual hierarchy to a different degree for the ventral and the dorsal stream. Furthermore, ellipticity changes as a function of eccentricity. pRF orientation shows a high degree of collinearity with its angular position within the visual field. Using model-free pRF measurements, the traveling wave technique provides additional characteristics of pRF topographies that are not restricted to size and provide robust measures within the single subject.

Differential Sampling of Visual Space in Ventral and Dorsal Early Visual Cortex

A fundamental feature of cortical visual processing is the separation of visual processing for the upper and lower visual fields. In early visual cortex (EVC), the upper visual field is processed ventrally, with the lower visual field processed dorsally. This distinction persists into several category-selective regions of occipitotemporal cortex, with ventral and lateral scene-, face-, and object-selective regions biased for the upper and lower visual fields, respectively. Here, using an elliptical population receptive field (pRF) model, we systematically tested the sampling of visual space within ventral and dorsal divisions of human EVC in both male and female participants. We found that (1) pRFs tend to be elliptical and oriented toward the fovea with distinct angular distributions for ventral and dorsal divisions of EVC, potentially reflecting a radial bias; and (2) pRFs in ventral areas were larger (∼1.5×) and more elliptical (∼1.2×) than those in dorsal areas. These differences potentially reflect a tendency for receptive fields in ventral temporal cortex to overlap the fovea with less emphasis on precise localization and isotropic representation of space compared with dorsal areas. Collectively, these findings suggest that ventral and dorsal divisions of EVC sample visual space differently, likely contributing to and/or stemming from the functional differentiation of visual processing observed in higher-level regions of the ventral and dorsal cortical visual pathways.SIGNIFICANCE STATEMENT The processing of visual information from the upper and lower visual fields is separated in visual cortex. Although ventral and dorsal divisions of early visual cortex (EVC) are commonly assumed to sample visual space equivalently, we demonstrate systematic differences using an elliptical population receptive field (pRF) model. Specifically, we demonstrate that (1) ventral and dorsal divisions of EVC exhibit diverging distributions of pRF angle, which are biased toward the fovea; and (2) ventral pRFs exhibit higher aspect ratios and cover larger areas than dorsal pRFs. These results suggest that ventral and dorsal divisions of EVC sample visual space differently and that such differential sampling likely contributes to different functional roles attributed to the ventral and dorsal pathways, such as object recognition and visually guided attention, respectively.