Visual Cortical Area MT Is Required for Development of the Dorsal Stream and Associated Visuomotor Behaviors.

The middle temporal (MT) area of the extrastriate visual cortex has long been studied in adulthood for its distinctive physiological properties and function as a part of the dorsal stream, yet interestingly it possesses a similar maturation profile as the primary visual cortex (V1). Here, we examined whether an early-life lesion in MT of marmoset monkeys (six female, two male) altered the dorsal stream development and the behavioral precision of reaching-to-grasp sequences. We observed permanent changes in the anatomy of cortices associated with both reaching (parietal and medial intraparietal areas) and grasping (anterior intraparietal area), as well as in reaching-and-grasping behaviors. In addition, we observed a significant impact on the anatomy of V1 and the direction sensitivity of V1 neurons in the lesion projection zone. These findings indicate that area MT is a crucial node in the development of primate vision, affecting both V1 and areas in the dorsal visual pathway known to mediate visually guided manual behaviors.SIGNIFICANCE STATEMENT Previous studies have identified a role for the MT area of the visual cortex in perceiving motion, yet none have examined its central role in the development of the visual cortex and in the establishment of visuomotor behaviors. To address this, we used a unilateral MT lesion model in neonatal marmosets before examining the anatomic, physiological, and behavioral consequences. In adulthood, we observed perturbations in goal-orientated reach-and-grasp behavior, altered direction selectivity of V1 neurons, and changes in the cytoarchitecture throughout dorsal stream areas. This study highlights the importance of MT as a central node in visual system development and consequential visuomotor activity.

Visual responses in the dorsolateral frontal cortex of marmoset monkeys.

The marmoset monkey (Callithrix jacchus) has gained attention in neurophysiology research as a new primate model for visual processing and behavior. In particular, marmosets have a lissencephalic cortex, making multielectrode, optogenetic, and calcium-imaging techniques more accessible than other primate models. However, the degree of homology of brain circuits for visual behavior with those identified in macaques and humans is still being ascertained. For example, whereas the location of the frontal eye fields (FEF) within the dorsolateral frontal cortex has been proposed, it remains unclear whether neurons in the corresponding areas show visual responses-an important characteristic of FEF neurons in other species. Here, we provide the first description of receptive field properties and neural response latencies in the marmoset dorsolateral frontal cortex, based on recordings using Utah arrays in anesthetized animals. We find brisk visual responses in specific regions of the dorsolateral prefrontal cortex, particularly in areas 8aV, 8C, and 6DR. As in macaque FEF, the receptive fields were typically large (10°-30° in diameter) and the median responses latency was brisk (60 ms). These results constrain the possible interpretations about the location of the marmoset FEF and suggest that the marmoset model's significant advantages for the use of physiological techniques may be leveraged in the study of visuomotor cognition.NEW & NOTEWORTHY Behavior and cognition in humans and other primates rely on networks of brain areas guided by the frontal cortex. The marmoset offers exciting new opportunities to study links between brain physiology and behavior, but the functions of frontal cortex areas are still being identified in this species. Here, we provide the first evidence of visual receptive fields in the marmoset dorsolateral frontal cortex, an important step toward future studies of visual cognitive behavior.

Topographic Organization of the 'Third-Tier' Dorsomedial Visual Cortex in the Macaque.

The boundaries of the visual areas located anterior to V2 in the dorsomedial region of the macaque cortex remain contentious. This region is usually conceptualized as including two functional subdivisions: the dorsal component of area V3 (V3d) laterally and another area named the parietooccipital area (PO) or V6 medially. However, the nature of the putative border between V3d and PO/V6 has remained undefined. We recorded the receptive fields of multiunit clusters in male macaques and reconstructed the locations of recording sites using histological sections and computer-generated maps. Immediately adjacent to dorsomedial V2, we observed a representation of the lower contralateral quadrant that represented the vertical meridian at its rostral border. This region formed a simple eccentricity gradient from ∼<5° in the annectant gyrus to >60° in the parietooccipital medial sulcus. There was no topographic reversal where one would expect to find the border between V3d and PO/V6. Rather, near the midline, this lower quadrant map continued directly into a representation of the peripheral upper visual field without an intervening lower quadrant representation. Therefore, cortex previously assigned to the medial part of V3d and to PO/V6 forms a single map that includes parts of both quadrants. Together with previous observations that V3d and PO/V6 are densely myelinated relative to adjacent cortex and share similar input from V1, these results suggest that they are parts of a single area (for which we suggest the designation V6), which is distinct from the one forming the ventral component of the third-tier complex.SIGNIFICANCE STATEMENT The primate visual cortex has a large number of areas. Knowing the extent of each visual area and how they can be distinguished from each other is essential for the interpretation of experiments aimed at understanding visual processing. Currently, there are conflicting models of the organization of the dorsomedial visual cortex rostral to area V2 (one of the earliest stages of cortical processing of vision). By conducting large-scale electrophysiological recordings, we found that what were originally thought to be distinct areas in this region (dorsal V3 and the parietooccipital area PO/V6), together form a single map of the visual field. This will help to guide future functional studies and the interpretation of the outcomes of lesions involving the dorsal visual cortex.

Correlated Variability in the Neurons With the Strongest Tuning Improves Direction Coding.

Sensory perception depends on neuronal populations creating an accurate representation of the external world. The amount of information that a population can represent depends on the tuning of individual neurons and the trial-by-trial variability shared among neurons. Although on average, pairwise spike-count correlations between neurons are positive, the distribution is wide, and the relationship between correlations and encoding is not straightforward. Here, we examine how single-neuron and population-level factors impact the efficacy of the neural code. We recorded responses to moving visual stimuli from motion-sensitive neurons in the middle temporal area of anesthetized marmosets (Callithrix jacchus) and trained decoders to assess how correlated and uncorrelated populations encoded stimulus motion direction. We found that the most responsive, direction-selective, and least variable neurons are the most relied-upon neurons in an uncorrelated population. In correlated populations, the same neurons do the most to shape the shared variability across the population in a way that facilitates decoding, and decoding is improved by the presence of temporally stable correlations. This suggests that the least variable neurons with the strongest stimulus representations enhance the population code by providing a strong signal and shaping correlations in variability orthogonally to the locus defined by the mean response.

Robust Visual Responses and Normal Retinotopy in Primate Lateral Geniculate Nucleus following Long-term Lesions of Striate Cortex.

Lesions of striate cortex (V1) trigger massive retrograde degeneration of neurons in the LGN. In primates, these lesions also lead to scotomas, within which conscious vision is abolished. Mediation of residual visual capacity within these regions (blindsight) has been traditionally attributed to an indirect visual pathway to the extrastriate cortex, which involves the superior colliculus and pulvinar complex. However, recent studies have suggested that preservation of the LGN is critical for behavioral evidence of blindsight, raising the question of what type of visual information is channeled by remaining neurons in this structure. A possible contribution of LGN neurons to blindsight is predicated on two conditions: that the neurons that survive degeneration remain visually responsive, and that their receptive fields continue to represent the region of the visual field inside the scotoma. We tested these conditions in male and female marmoset monkeys (Callithrix jacchus) with partial V1 lesions at three developmental stages (early postnatal life, young adulthood, old age), followed by long recovery periods. In all cases, recordings from the degenerated LGN revealed neurons with well-formed receptive fields throughout the scotoma. The responses were consistent and robust, and followed the expected eye dominance and retinotopy observed in the normal LGN. The responses had short latencies and preceded those of neurons recorded in the extrastriate middle temporal area. These findings suggest that the pathway that links LGN neurons to the extrastriate cortex is physiologically viable and can support residual vision in animals with V1 lesions incurred at various ages.SIGNIFICANCE STATEMENT Patients with a lesion of the primary visual cortex (V1) can retain certain visually mediated behaviors, particularly if the lesion occurs early in life. This phenomenon ("blindsight") not only sheds light on the nature of consciousness, but also has implications for studies of brain circuitry, development, and plasticity. However, the pathways that mediate blindsight have been the subject of debate. Recent studies suggest that projections from the LGN might be critical, but this finding is puzzling given that the lesions causes severe cell death in the LGN. Here we demonstrate in monkeys that the surviving LGN neurons retain a remarkable level of visual function and could therefore be the source of the visual information that supports blindsight.

Rapid Adaptation Induces Persistent Biases in Population Codes for Visual Motion.

Each visual experience changes the neural response to subsequent stimuli. If the brain is unable to incorporate these encoding changes, the decoding, or perception, of subsequent stimuli is biased. Although the phenomenon of adaptation pervades the nervous system, its effects have been studied mainly in isolation, based on neuronal encoding changes induced by an isolated, prolonged stimulus. To understand how adaptation-induced biases arise and persist under continuous, naturalistic stimulation, we simultaneously recorded the responses of up to 61 neurons in the marmoset (Callithrix jacchus) middle temporal area to a sequence of directions that changed every 500 ms. We found that direction-specific adaptation following only 0.5 s of stimulation strongly affected encoding for up to 2 s by reducing both the gain and the spike count correlations between pairs of neurons with preferred directions close to the adapting direction. In addition, smaller changes in bandwidth and preferred direction were observed in some animals. Decoding individual trials of adaptation-affected activity in simultaneously recorded neurons predicted repulsive biases that are consistent with the direction aftereffect. Surprisingly, removing spike count correlations by trial shuffling did not impact decoding performance or bias. When adaptation had the largest effect on encoding, the decoder made the most errors. This suggests that neural and perceptual repulsion is not a mechanism to enhance perceptual performance but is instead a necessary consequence of optimizing neural encoding for the identification of a wide range of stimulus properties in diverse temporal contexts. SIGNIFICANCE STATEMENT: Although perception depends upon decoding the pattern of activity across a neuronal population, the encoding properties of individual neurons are unreliable: a single neuron's response to repetitions of the same stimulus is variable, and depends on both its spatial and temporal context. In this manuscript, we describe the complete cascade of adaptation-induced effects in sensory encoding and show how they predict population decoding errors consistent with perceptual biases. We measure the time course of adaptation-induced changes to the response properties of neurons in isolation, and to the correlation structure across pairs of simultaneously recorded neurons. These results provide novel insight into how and for how long adaptation affects the neural code, particularly during continuous, naturalistic vision.

A conserved pattern of differential expansion of cortical areas in simian primates.

The layout of areas in the cerebral cortex of different primates is quite similar, despite significant variations in brain size. However, it is clear that larger brains are not simply scaled up versions of smaller brains: some regions of the cortex are disproportionately large in larger species. It is currently debated whether these expanded areas arise through natural selection pressures for increased cognitive capacity or as a result of the application of a common developmental sequence on different scales. Here, we used computational methods to map and quantify the expansion of the cortex in simian primates of different sizes to investigate whether there is any common pattern of cortical expansion. Surface models of the marmoset, capuchin, and macaque monkey cortex were registered using the software package CARET and the spherical landmark vector difference algorithm. The registration was constrained by the location of identified homologous cortical areas. When comparing marmosets with both capuchins and macaques, we found a high degree of expansion in the temporal parietal junction, the ventrolateral prefrontal cortex, and the dorsal anterior cingulate cortex, all of which are high-level association areas typically involved in complex cognitive and behavioral functions. These expanded maps correlated well with previously published macaque to human registrations, suggesting that there is a general pattern of primate cortical scaling.

Visually evoked responses in extrastriate area MT after lesions of striate cortex in early life.

Lesions of striate cortex [primary visual cortex (V1)] in adult primates result in blindness. In contrast, V1 lesions in neonates typically allow much greater preservation of vision, including, in many human patients, conscious perception. It is presently unknown how this marked functional difference is related to physiological changes in cortical areas that are spared by the lesions. Here we report a study of the middle temporal area (MT) of adult marmoset monkeys that received unilateral V1 lesions within 6 weeks of birth. In contrast with observations after similar lesions in adult monkeys, we found that virtually all neurons in the region of MT that was deprived of V1 inputs showed robust responses to visual stimulation. These responses were very similar to those recorded in neurons with receptive fields outside the lesion projection zones in terms of firing rate, signal-to-noise ratio, and latency. In addition, the normal retinotopic organization of MT was maintained. Nonetheless, we found evidence of a very specific functional deficit: direction selectivity, a key physiological characteristic of MT that is known to be preserved in many cells after adult V1 lesions, was absent. These results demonstrate that lesion-induced reorganization of afferent pathways is sufficient to develop robust visual function in primate extrastriate cortex, highlighting a likely mechanism for the sparing of vision after neonatal V1 lesions. However, they also suggest that interactions with V1 in early postnatal life are critical for establishing stimulus selectivity in MT.

Uniformity and diversity of response properties of neurons in the primary visual cortex: selectivity for orientation, direction of motion, and stimulus size from center to far periphery.

Although the primary visual cortex (V1) is one of the most extensively studied areas of the primate brain, very little is known about how the far periphery of visual space is represented in this area. We characterized the physiological response properties of V1 neurons in anaesthetized marmoset monkeys, using high-contrast drifting gratings. Comparisons were made between cells with receptive fields located in three regions of V1, defined by eccentricity: central (3-5°), near peripheral (5-15°), and far peripheral (>50°). We found that orientation selectivity of individual cells was similar from the center to the far periphery. Nonetheless, the proportion of orientation-selective neurons was higher in central visual field representation than in the peripheral representations. In addition, there were similar proportions of cells representing all orientations, with the exception of the representation of the far periphery, where we detected a bias favoring near-horizontal orientations. The proportions of direction-selective cells were similar throughout V1. When the center/surround organization of the receptive fields was tested with gratings with varying diameters, we found that the population of neurons that was suppressed by large gratings was smaller in the far periphery, although the strength of suppression in these cells tended to be stronger. In addition, the ratio between the diameters of the excitatory centers and suppressive surrounds was similar across the entire visual field. These results suggest that, superimposed on the broad uniformity of V1, there are subtle physiological differences, which indicate that spatial information is processed differently in the central versus far peripheral visual fields.

Contrasting patterns of cortical input to architectural subdivisions of the area 8 complex: a retrograde tracing study in marmoset monkeys.

Contemporary studies recognize 3 distinct cytoarchitectural and functional areas within the Brodmann area 8 complex, in the caudal prefrontal cortex: 8b, 8aD, and 8aV. Here, we report on the quantitative characteristics of the cortical projections to these areas, using injections of fluorescent tracers in marmoset monkeys. Area 8b was distinct from both 8aD and 8aV due to its connections with medial prefrontal, anterior cingulate, superior temporal polysensory, and ventral midline/retrosplenial areas. In contrast, areas 8aD and 8aV received the bulk of the projections from posterior parietal cortex and dorsal midline areas. In the frontal lobe, area 8aV received projections primarily from ventrolateral areas, while both 8aD and 8b received dense inputs from areas on the dorsolateral surface. Whereas area 8aD received the most significant auditory projections, these were relatively sparse, in comparison with those previously reported in macaques. Finally, area 8aV was distinct from both 8aD and 8b by virtue of its widespread input from the extrastriate visual areas. These results are compatible with a homologous organization of the prefrontal cortex in New and Old World monkeys, and suggest significant parallels between the present pathways, revealed by tract-tracing, and networks revealed by functional connectivity analysis in Old World monkeys and humans.

Cortical input to the frontal pole of the marmoset monkey.

We used fluorescent tracers to map the pattern of cortical afferents to frontal area 10 in marmosets. Dense projections originated in several subdivisions of orbitofrontal cortex, in the medial frontal cortex (particularly areas 14 and 32), and in the dorsolateral frontal cortex (particularly areas 8Ad and 9). Major projections also stemmed, in variable proportions depending on location of the injection site, from both the inferior and superior temporal sensory association areas, suggesting a degree of audiovisual convergence. Other temporal projections included the superior temporal polysensory cortex, temporal pole, and parabelt auditory cortex. Medial area 10 received additional projections from retrosplenial, rostral calcarine, and parahippocampal areas, while lateral area 10 received small projections from the ventral somatosensory and premotor areas. There were no afferents from posterior parietal or occipital areas. Most frontal connections were balanced in terms of laminar origin, giving few indications of an anatomical hierarchy. The pattern of frontopolar afferents suggests an interface between high-order representations of the sensory world and internally generated states, including working memory, which may subserve ongoing evaluation of the consequences of decisions as well as other cognitive functions. The results also suggest the existence of functional differences between subregions of area 10.

Estimating Global Visual Field Indices in Glaucoma by Combining Macula and Optic Disc OCT Scans Using 3-Dimensional Convolutional Neural Networks.

PURPOSE: To evaluate the accuracy at which visual field global indices could be estimated from OCT scans of the retina using deep neural networks and to quantify the contributions to the estimates by the macula (MAC) and the optic nerve head (ONH). DESIGN: Observational cohort study. PARTICIPANTS: A total of 10 370 eyes from 109 healthy patients, 697 glaucoma suspects, and 872 patients with glaucoma over multiple visits (median = 3). METHODS: Three-dimensional convolutional neural networks were trained to estimate global visual field indices derived from automated Humphrey perimetry (SITA 24-2) tests (Zeiss, Dublin, CA), using OCT scans centered on MAC, ONH, or both (MAC + ONH) as inputs. MAIN OUTCOME MEASURES: Spearman's rank correlation coefficients, Pearson's correlation coefficient, and absolute errors calculated for 2 indices: visual field index (VFI) and mean deviation (MD). RESULTS: The MAC + ONH achieved 0.76 Spearman's correlation coefficient and 0.87 Pearson's correlation for VFI and MD. Median absolute error was 2.7 for VFI and 1.57 decibels (dB) for MD. Separate MAC or ONH estimates were significantly less correlated and less accurate. Accuracy was dependent on the OCT signal strength and the stage of glaucoma severity. CONCLUSIONS: The accuracy of global visual field indices estimate is improved by integrating information from MAC and ONH in advanced glaucoma, suggesting that structural changes of the 2 regions have different time courses in the disease severity spectrum.

Visual response characteristics of neurons in the second visual area of marmosets.

The physiological characteristics of the marmoset second visual area (V2) are poorly understood compared with those of the primary visual area (V1). In this study, we observed the physiological response characteristics of V2 neurons in four healthy adult marmosets using intracortical tungsten microelectrodes. We recorded 110 neurons in area V2, with receptive fields located between 8° and 15° eccentricity. Most (88.2%) of these neurons were orientation selective, with half-bandwidths typically ranging between 10° and 30°. A significant proportion of neurons (28.2%) with direction selectivity had a direction index greater than 0.5. The vast majority of V2 neurons had separable spatial frequency and temporal frequency curves and, according to this criterion, they were not speed selective. The basic functional response characteristics of neurons in area V2 resemble those found in area V1. Our findings show that area V2 together with V1 are important in primate visual processing, especially in locating objects in space and in detecting an object's direction of motion. The methods used in this study were approved by the Monash University Animal Ethics Committee, Australia (MARP 2009-2011) in 2009.

Connections of the dorsomedial visual area: pathways for early integration of dorsal and ventral streams in extrastriate cortex.

The dorsomedial area (DM), a subdivision of extrastriate cortex characterized by heavy myelination and relative emphasis on peripheral vision, remains the least understood of the main targets of striate cortex (V1) projections in primates. Here we placed retrograde tracer injections encompassing the full extent of this area in marmoset monkeys, and performed quantitative analyses of the numerical strengths and laminar patterns of its afferent connections. We found that feedforward projections from V1 and from the second visual area (V2) account for over half of the inputs to DM, and that the vast majority of the remaining connections come from other topographically organized visual cortices. Extrastriate projections to DM originate in approximately equal proportions from adjacent medial occipitoparietal areas, from the superior temporal motion-sensitive complex centered on the middle temporal area (MT), and from ventral stream-associated areas. Feedback from the posterior parietal cortex and other association areas accounts for <10% of the connections. These results do not support the hypothesis that DM is specifically associated with a medial subcircuit of the dorsal stream, important for visuomotor integration. Instead, they suggest an early-stage visual-processing node capable of contributing across cortical streams, much as V1 and V2 do. Thus, although DM may be important for providing visual inputs for guided body movements (which often depend on information contained in peripheral vision), this area is also likely to participate in other functions that require integration across wide expanses of visual space, such as perception of self-motion and contour completion.

Spatial and temporal frequency tuning in striate cortex: functional uniformity and specializations related to receptive field eccentricity.

In light of anatomical evidence suggesting differential connection patterns in central vs. peripheral representations of cortical areas, we investigated the extent to which the response properties of cells in the primary visual area (V1) of the marmoset change as a function of eccentricity. Responses to combinations of the spatial and temporal frequencies of visual stimuli were quantified for neurons with receptive fields ranging from 3 degrees to 70 degrees eccentricity. Optimal spatial frequencies and stimulus speeds reflected the expectation that the responses of cells throughout V1 are essentially uniform, once scaled according to the cortical magnification factor. In addition, temporal frequency tuning was similar throughout V1. However, spatial frequency tuning curves depended both on the cell's optimal spatial frequency and on the receptive field eccentricity: cells with peripheral receptive fields showed narrower bandwidths than cells with central receptive fields that were sensitive to the same optimal spatial frequency. Although most V1 cells had separable spatial and temporal frequency tuning, the proportion of neurons displaying significant spatiotemporal interactions increased in the representation of far peripheral vision (> 50 degrees). In addition, of the fewer than 5% of V1 cells that showed robust (spatial frequency independent) selectivity to stimulus speed, most were concentrated in the representation of the far periphery. Spatiotemporal interactions in the responses of many cells in the peripheral representation of V1 reduced the ambiguity of responses to high-speed (> 30 degrees/s) signals. These results support the notion of a relative specialization for motion processing in the far peripheral representations of cortical areas, including V1.

A simple method for creating wide-field visual stimulus for electrophysiology: mapping and analyzing receptive fields using a hemispheric display.

Modern neurophysiological and psychophysical studies of vision are typically based on computer-generated stimuli presented on flat screens. While this approach allows precise delivery of stimuli, it suffers from a fundamental limitation in terms of the maximum achievable spatial coverage. This constraint becomes important in studies that require stimulation of large expanses of the visual field, such as those involving the mapping of receptive fields throughout the extent of a cortical area or subcortical nucleus, or those comparing neural response properties across a wide range of eccentricities. Here we describe a simple and highly cost-effective method for the projection of computer-generated stimuli on a hemispheric screen, which combines the advantages of computerized control and wide-field (100° × 75°) delivery, without the requirement of highly specialized hardware. The description of the method includes programming techniques for the generation of stimuli in spherical coordinates and for the quantitative determination of receptive field sizes and shapes. The value of this approach is demonstrated by quantitative electrophysiological data obtained in the far peripheral representations of various cortical areas, including automated mapping of receptive field extents in cortex that underwent plasticity following lesions.

A twisted visual field map in the primate dorsomedial cortex predicted by topographic continuity.

Adjacent neurons in visual cortex have overlapping receptive fields within and across area boundaries, an arrangement theorized to minimize wiring cost. This constraint is traditionally thought to create retinotopic maps of opposing field signs (mirror and nonmirror visual field representations) in adjacent areas, a concept that has become central in current attempts to subdivide the extrastriate cortex. We simulated the formation of retinotopic maps using a model that balances constraints imposed by smoothness in the representation within an area and by congruence between areas. As in the primate cortex, this model usually leads to alternating mirror and nonmirror maps. However, we found that it can also produce a more complex type of map, consisting of sectors with opposing field sign within a single area. Using fully quantitative electrode array recordings, we then demonstrate that this type of inhomogeneous map exists in the controversial dorsomedial region of the primate extrastriate cortex.

Neuronal degeneration in the dorsal lateral geniculate nucleus following lesions of primary visual cortex: comparison of young adult and geriatric marmoset monkeys.

Neuronal loss in the lateral geniculate nucleus (LGN) is a consequence of lesions of the primary visual cortex (V1). Despite the importance of this phenomenon in understanding the residual capacities of the primate visual system following V1 damage, few quantitative studies are available, and the effect of age at the time of lesion remains unknown. We compared the volume, neuronal number, and neuronal density in the LGN, 6-21 months after unilateral V1 lesions in marmoset monkeys. Stereological sampling techniques and neuronal nuclei (NeuN) staining were used to assess the effects of similar-sized lesions in adult (2-4 years) and geriatric (10-14 years) animals. We found that lesions involving the opercular and caudal calcarine parts of V1 caused robust loss of neurons in topographically corresponding regions of the ipsilateral LGN (lesion projection zones), concomitant with a substantial reduction in the volume of this nucleus. Neuronal density was markedly reduced in the lesion projection zones, relative to the corresponding regions of the contralateral LGN, or the LGN in non-lesioned animals. Moreover, the percentage decrease in neuronal density within the lesion projection zones was significantly greater in the geriatric group, compared with the adult groups. The volume and neuronal density in the contralateral LGN of lesioned adult and geriatric marmosets were similar to those in non-lesioned animals. These results show that the primate LGN becomes more vulnerable to degeneration with advancing age. However, even in geriatric primates there is a population of LGN neurons which survives degeneration, and which could play a role in blindsight.

Whole-brain metallomic analysis of the common marmoset (Callithrix jacchus).

Despite the importance of transition metals for normal brain function, relatively little is known about the distribution of these elemental species across the different tissue compartments of the primate brain. In this study, we employed laser ablation-inductively coupled plasma-mass spectrometry on PFA-fixed brain sections obtained from two adult common marmosets. Concurrent cytoarchitectonic, myeloarchitectonic, and chemoarchitectonic measurements allowed for identification of the major neocortical, archaecortical, and subcortical divisions of the brain, and precise localisation of iron, manganese, and zinc concentrations within each division. Major findings across tissue compartments included: (1) differentiation of white matter tracts from grey matter based on manganese and zinc distribution; (2) high iron concentrations in the basal ganglia, cortex, and substantia nigra; (3) co-localization of high concentrations of iron and manganese in the primary sensory areas of the cerebral cortex; and (4) high manganese in the hippocampus. The marmoset has become a model species of choice for connectomic, aging, and transgenic studies in primates, and the application of metallomics to these disciplines has the potential to yield high translational and basic science value.

Natural motion trajectory enhances the coding of speed in primate extrastriate cortex.

The ability to estimate the speed of an object irrespective of size or texture is a crucial function of the visual system. However, previous studies have suggested that the neuronal coding of speed in the middle temporal area (MT, a key cortical area for motion analysis in primates) is ambiguous, with most neurons changing their speed tuning depending on the spatial frequency (SF) of a visual pattern. Here we demonstrate that the ability of MT neurons to encode speed is markedly improved when stimuli follow a trajectory across the visual field, prior to entering their receptive fields. We also show that this effect is much less marked in the primary visual area. These results indicate that MT neurons build up on computations performed at earlier levels of the visual system to provide accurate coding of speed in natural situations, and provide additional evidence that nonlinear pooling underlie motion processing.