Contrast sensitivity and spatial frequency response of primate cortical neurons in and around the cytochrome oxidase blobs

Nonuniform and pathway-specific laminar processing of spatial frequencies in the primary visual cortex of primates

The neocortex comprises six cortical layers that play a crucial role in information processing; however, it remains unclear whether laminar processing is consistent across all regions within a single cortex. In this study, we demonstrate diverse laminar response patterns in the primary visual cortex (V1) of three male macaque monkeys when exposed to visual stimuli at different spatial frequencies (SFs). These response patterns can be categorized into two groups. One group exhibit suppressed responses in the output layers for all SFs, while the other type shows amplified responses specifically at high SFs. Further analysis suggests that both magnocellular (M) and parvocellular (P) pathways contribute to the suppressive effect through feedforward mechanisms, whereas amplification is specific to local recurrent mechanisms within the parvocellular pathway. These findings highlight the non-uniform distribution of neural mechanisms involved in laminar processing and emphasize how pathway-specific amplification selectively enhances representations of high-SF information in primate V1.

Different rules for binocular combination of luminance flicker in cortical and subcortical pathways

How does the human brain combine information across the eyes? It has been known for many years that cortical normalization mechanisms implement 'ocularity invariance': equalizing neural responses to spatial patterns presented either monocularly or binocularly. Here, we used a novel combination of electrophysiology, psychophysics, pupillometry, and computational modeling to ask whether this invariance also holds for flickering luminance stimuli with no spatial contrast. We find dramatic violations of ocularity invariance for these stimuli, both in the cortex and also in the subcortical pathways that govern pupil diameter. Specifically, we find substantial binocular facilitation in both pathways with the effect being strongest in the cortex. Near-linear binocular additivity (instead of ocularity invariance) was also found using a perceptual luminance matching task. Ocularity invariance is, therefore, not a ubiquitous feature of visual processing, and the brain appears to repurpose a generic normalization algorithm for different visual functions by adjusting the amount of interocular suppression.

Mechanisms of Surround Suppression Effect on the Contrast Sensitivity of V1 Neurons in Cats

Surround suppression (SS) is a phenomenon that a neuron’s response to visual stimuli within the classical receptive field (cRF) is suppressed by a concurrent stimulation in the surrounding receptive field (sRF) beyond the cRF. Studies show that SS affects neuronal response contrast sensitivity in the primary visual cortex (V1). However, the underlying mechanisms remain unclear. Here, we examined SS effect on the contrast sensitivity of cats’ V1 neurons with different preferred SFs using external noise-masked visual stimuli and perceptual template model (PTM) analysis at the system level. The contrast sensitivity was evaluated by the inverted threshold contrast of neurons in response to circular gratings of different contrasts in the cRF with or without an annular grating in the sRF. Our results showed that SS significantly reduced the contrast sensitivity of cats’ V1 neurons. The SS-induced reduction of contrast sensitivity was not correlated with SS strength but was dependent on neuron’s preferred SF, with a larger reduction for neurons with low preferred SFs than those with high preferred SFs. PTM analysis of threshold versus external noise contrast (TvC) functions indicated that SS decreased contrast sensitivity by increasing both the internal additive noise and impact of external noise for neurons with low preferred SFs, but improving only internal additive noise for neurons with high preferred SFs. Furthermore, the SS effect on the contrast-response function of low- and high-SF neurons also exhibited different mechanisms in contrast gain and response gain. Collectively, these results suggest that the mechanisms of SS effect on neuronal contrast sensitivity may depend on neuronal populations with different SFs.

Effects of top-down influence suppression on behavioral and V1 neuronal contrast sensitivity functions in cats

To explore the relative contributions of higher-order and primary visual cortex (V1) to visual perception, we compared cats' behavioral and V1 neuronal contrast sensitivity functions (CSF) and threshold versus external noise contrast (TvC) functions before and after top-down influence of area 7 (A7) was modulated with transcranial direct current stimulation (tDCS). We found that suppressing top-down influence of A7 with cathode-tDCS, but not sham-tDCS, reduced behavioral and neuronal contrast sensitivity in the same range of spatial frequencies and increased behavioral and neuronal contrast thresholds in the same range of external noise levels. The neuronal CSF and TvC functions were highly correlated with their behavioral counterparts both before and after the top-down suppression. Analysis of TvC functions using the Perceptual Template Model (PTM) indicated that top-down influence of A7 increased both behavioral and V1 neuronal contrast sensitivity by reducing internal additive noise and the impact of external noise.

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Top-down suppression lowers both behavioral and V1 neuronal CSF functions

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Top-down suppression raises both behavioral and V1 neuronal TvC functions

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The neuronal CSFs and TvCs are highly correlated with their behavioral counterparts

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Top-down influence lowers internal additive noise and impact of external noise in V1

Functional organization of spatial frequency tuning in macaque V1 revealed with two-photon calcium imaging

V1 neurons are functionally organized in orientation columns in primates. Whether spatial frequency (SF) columns also exist is less clear because mixed results have been reported. A definitive solution would be SF functional maps at single-neuron resolution. Here we used two-photon calcium imaging to construct first cellular SF maps in V1 superficial layers of five awake fixating macaques, and studied SF functional organization properties and neuronal tuning characteristics. The SF maps (850 × 850 μm²) showed weak horizontal SF clustering (median clustering index = 1.43 vs. unity baseline), about one sixth as strong as orientation clustering in the same sets of neurons, which argues against a meaningful orthogonal relationship between orientation and SF functional maps. These maps also displayed nearly absent vertical SF clustering between two cortical depths (150 & 300 μm), indicating a lack of SF columnar structures within the superficial layers. The underlying causes might be that most neurons were tuned to a narrow two-octave range of medium frequencies, and many neurons with different SF preferences were often spatially mixed, which disallowed finer grouping of SF tuning. In addition, individual SF tuning functions were often asymmetric, having wider lower frequency branches, which may help encode low SF information for later decoding.

Chromatic micromaps in primary visual cortex

The clustering of neurons with similar response properties is a conspicuous feature of neocortex. In primary visual cortex (V1), maps of several properties like orientation preference are well described, but the functional architecture of color, central to visual perception in trichromatic primates, is not. Here we used two-photon calcium imaging in macaques to examine the fine structure of chromatic representation and found that neurons responsive to spatially uniform, chromatic stimuli form unambiguous clusters that coincide with blobs. Further, these responsive groups have marked substructure, segregating into smaller ensembles or micromaps with distinct chromatic signatures that appear columnar in upper layer 2/3. Spatially structured chromatic stimuli revealed maps built on the same micromap framework but with larger subdomains that go well beyond blobs. We conclude that V1 has an architecture for color representation that switches between blobs and a combined blob/interblob system based on the spatial content of the visual scene.

Stimulus feature maps are found in primary visual cortex of many species. Here the authors show color maps in trichromatic primates containing segregated ensembles of neurons with distinct chromatic signatures that associate with cortical modules known as blobs.

Immunoreactivity of Vesicular Glutamate Transporter 2 Corresponds to Cytochrome Oxidase-Rich Subcompartments in the Visual Cortex of Squirrel Monkeys

Cytochrome oxidase (CO) histochemistry has been used to reveal the cytoarchitecture of the primate brain, including blobs/puffs/patches in the striate cortex (V1), and thick, thin and pale stripes in the middle layer of the secondary visual cortex (V2). It has been suggested that CO activity is coupled with the spiking activity of neurons, implying that neurons in these CO-rich subcompartments are more active than surrounding regions. However, we have discussed possibility that CO histochemistry represents the distribution of thalamo-cortical afferent terminals that generally use vesicular glutamate transporter 2 (VGLUT2) as their main glutamate transporter, and not the activity of cortical neurons. In this study, we systematically compared the labeling patterns observed between CO histochemistry and immunohistochemistry (IHC) for VGLUT2 from the system to microarchitecture levels in the visual cortex of squirrel monkeys. The two staining patterns bore striking similarities at all levels of the visual cortex, including the honeycomb structure of V1 layer 3Bβ (Brodmann's layer 4A), the patchy architecture in the deep layers of V1, the superficial blobs of V1, and the V2 stripes. The microarchitecture was more evident in VGLUT2 IHC, as expected. VGLUT2 protein expression that produced specific IHC labeling is thought to originate from the thalamus since the lateral geniculate nucleus (LGN) and the pulvinar complex both show high expression levels of VGLUT2 mRNA, but cortical neurons do not. These observations support our theory that the subcompartments revealed by CO histochemistry represent the distribution of thalamo-cortical afferent terminals in the primate visual cortex.

Uniform spatial pooling explains topographic organization and deviation from receptive-field scale invariance in primate V1

Receptive field (RF) size and preferred spatial frequency (SF) vary greatly across the primary visual cortex (V1), increasing in a scale invariant fashion with eccentricity. Recent studies reveal that preferred SF also forms a fine-scale periodic map. A fundamental open question is how local variability in preferred SF is tied to the overall spatial RF. Here, we use two-photon imaging to simultaneously measure maps of RF size, phase selectivity, SF bandwidth, and orientation bandwidth—all of which were found to be topographically organized and correlate with preferred SF. Each of these newly characterized inter-map relationships strongly deviate from scale invariance, yet reveal a common motif—they are all accounted for by a model with uniform spatial pooling from scale invariant inputs. Our results and model provide novel and quantitative understanding of the output from V1 to downstream circuits.

Two-photon imaging in macaque V1 captured maps of tuning selectivity for four spatial parameters, all of which correlated with peak spatial frequency. These inter-map relationships reveal a common motif—they are described by uniform spatial pooling from a family of scale invariant Gabor receptive fields.

Anatomy and Physiology of Macaque Visual Cortical Areas V1, V2, and V5/MT: Bases for Biologically Realistic Models

The cerebral cortex of primates encompasses multiple anatomically and physiologically distinct areas processing visual information. Areas V1, V2, and V5/MT are conserved across mammals and are central for visual behavior. To facilitate the generation of biologically accurate computational models of primate early visual processing, here we provide an overview of over 350 published studies of these three areas in the genus Macaca, whose visual system provides the closest model for human vision. The literature reports 14 anatomical connection types from the lateral geniculate nucleus of the thalamus to V1 having distinct layers of origin or termination, and 194 connection types between V1, V2, and V5, forming multiple parallel and interacting visual processing streams. Moreover, within V1, there are reports of 286 and 120 types of intrinsic excitatory and inhibitory connections, respectively. Physiologically, tuning of neuronal responses to 11 types of visual stimulus parameters has been consistently reported. Overall, the optimal spatial frequency (SF) of constituent neurons decreases with cortical hierarchy. Moreover, V5 neurons are distinct from neurons in other areas for their higher direction selectivity, higher contrast sensitivity, higher temporal frequency tuning, and wider SF bandwidth. We also discuss currently unavailable data that could be useful for biologically accurate models.

Influence of Spatial and Chromatic Noise on Luminance Discrimination

Pseudoisochromatic figures are designed to base discrimination of a chromatic target from a background solely on the chromatic differences. This is accomplished by the introduction of luminance and spatial noise thereby eliminating these two dimensions as cues. The inverse rationale could also be applied to luminance discrimination, if spatial and chromatic noise are used to mask those cues. In this current study estimate of luminance contrast thresholds were conducted using a novel stimulus, based on the use of chromatic and spatial noise to mask the use of these cues in a luminance discrimination task. This was accomplished by presenting stimuli composed of a mosaic of circles colored randomly. A Landolt-C target differed from the background only by the luminance. The luminance contrast thresholds were estimated for different chromatic noise saturation conditions and compared to luminance contrast thresholds estimated using the same target in a non-mosaic stimulus. Moreover, the influence of the chromatic content in the noise on the luminance contrast threshold was also investigated. Luminance contrast threshold was dependent on the chromaticity noise strength. It was 10-fold higher than thresholds estimated from non-mosaic stimulus, but they were independent of colour space location in which the noise was modulated. The present study introduces a new method to investigate luminance vision intended for both basic science and clinical applications.

Local Circuits of V1 Layer 4B Neurons Projecting to V2 Thick Stripes Define Distinct Cell Classes and Avoid Cytochrome Oxidase Blobs

Decades of anatomical studies on the primate primary visual cortex (V1) have led to a detailed diagram of V1 intrinsic circuitry, but this diagram lacks information about the output targets of V1 cells. Understanding how V1 local processing relates to downstream processing requires identification of neuronal populations defined by their output targets. In primates, V1 layers (L)2/3 and 4B send segregated projections to distinct cytochrome oxidase (CO) stripes in area V2: neurons in CO blob columns project to thin stripes while neurons outside blob columns project to thick and pale stripes, suggesting functional specialization of V1-to-V2 CO streams. However, the conventional diagram of V1 shows all L4B neurons, regardless of their soma location in blob or interblob columns, as projecting selectively to CO blobs in L2/3, suggesting convergence of blob/interblob information in L2/3 blobs and, possibly, some V2 stripes. However, it is unclear whether all L4B projection neurons show similar local circuitries. Using viral-mediated circuit tracing, we have identified the local circuits of L4B neurons projecting to V2 thick stripes in macaque. Consistent with previous studies, we found the somata of this L4B subpopulation to reside predominantly outside blob columns; however, unlike previous descriptions of local L4B circuits, these cells consistently projected outside CO blob columns in all layers. Thus, the local circuits of these L4B output neurons, just like their extrinsic projections to V2, preserve CO streams. Moreover, the intra-V1 laminar patterns of axonal projections identify two distinct neuron classes within this L4B subpopulation, including a rare novel neuron type, suggestive of two functionally specialized output channels.

SIGNIFICANCE STATEMENT

Conventional diagrams of primate primary visual cortex (V1) depict neuronal connections within and between different V1 layers, but lack information about the cells' downstream targets. This information is critical to understanding how local processing in V1 relates to downstream processing. We have identified the local circuits of a population of cells in V1 layer (L)4B that project to area V2. These cells' local circuits differ from classical descriptions of L4B circuits in both the laminar and functional compartments targeted by their axons, and identify two neuron classes. Our results demonstrate that both local intra-V1 and extrinsic V1-to-V2 connections of L4B neurons preserve CO-stream segregation, suggesting that across-stream integration occurs downstream of V1, and that output targets dictate local V1 circuitry.

Efficient Receptive Field Tiling in Primate V1

The primary visual cortex (V1) encodes a diverse set of visual features, including orientation, ocular dominance (OD), and spatial frequency (SF), whose joint organization must be precisely structured to optimize coverage within the retinotopic map. Prior experiments have only identified efficient coverage based on orthogonal maps. Here we used two-photon calcium imaging to reveal an alternative arrangement for OD and SF maps in macaque V1; their gradients run parallel but with unique spatial periods, whereby low-SF regions coincide with monocular regions. Next we mapped receptive fields and found surprisingly precise micro-retinotopy that yields a smaller point-image and requires more efficient inter-map geometry, thus underscoring the significance of map relationships. While smooth retinotopy is constraining, studies suggest that it improves both wiring economy and the V1 population code read downstream. Altogether, these data indicate that connectivity within V1 is finely tuned and precise at the level of individual neurons.

Contrast and response gain control depend on cortical map architecture

Visual cortical neurons are sensitive to visual stimulus contrast and most cells adapt their sensitivity to the prevailing visual environment. Specifically, they match the steepest region of their contrast response function to the prevailing contrast (contrast gain control), and reduce spike rates to limit saturation (response gain control). Most neurons are also tuned for stimulus orientation, and neurons with similar orientation preference are clustered together into iso-orientation zones arranged around pinwheels, i.e. points where all orientations are represented. Here we investigated the relationship between the contrast adaptation properties of neurons and their location relative to pinwheels in the orientation preference map. We measured orientation preference maps in cat cortex using optical intrinsic signal imaging. We then characterized the contrast adaptation properties of single neurons located close to pinwheels, in iso-orientation zones, and at regions in between. We found little evidence of differential contrast sensitivity of neurons adapted to zero contrast. However, after adaptation to their preferred orientation at high contrast, changes in both contrast and response gain were greater for neurons near pinwheels compared with other map regions. Therefore, in the adapted state, which is probably typical during natural viewing, there is a spatial map of contrast sensitivity that is associated with the orientation preference map. This differential adaptation revealed a new dimension of cortical functional organization, linking the contrast adaptation of cells with the orientation preference of their nearest neighbours.

Relationship between size summation properties, contrast sensitivity and response latency in the dorsomedial and middle temporal areas of the primate extrastriate cortex

Analysis of the physiological properties of single neurons in visual cortex has demonstrated that both the extent of their receptive fields and the latency of their responses depend on stimulus contrast. Here, we explore the question of whether there are also systematic relationships between these response properties across different cells in a neuronal population. Single unit recordings were obtained from the middle temporal (MT) and dorsomedial (DM) extrastriate areas of anaesthetized marmoset monkeys. For each cell, spatial integration properties (length and width summation, as well as the presence of end- and side-inhibition within 15° of the receptive field centre) were determined using gratings of optimal direction of motion and spatial and temporal frequencies, at 60% contrast. Following this, contrast sensitivity was assessed using gratings of near-optimal length and width. In both areas, we found a relationship between spatial integration and contrast sensitivity properties: cells that summated over smaller areas of the visual field, and cells that displayed response inhibition at larger stimulus sizes, tended to show higher contrast sensitivity. In a sample of MT neurons, we found that cells showing longer latency responses also tended to summate over larger expanses of visual space in comparison with neurons that had shorter latencies. In addition, longer-latency neurons also tended to show less obvious surround inhibition. Interestingly, all of these effects were stronger and more consistent with respect to the selectivity for stimulus width and strength of side-inhibition than for length selectivity and end-inhibition. The results are partially consistent with a hierarchical model whereby more extensive receptive fields require convergence of information from larger pools of “feedforward” afferent neurons to reach near-optimal responses. They also suggest that a common gain normalization mechanism within MT and DM is involved, the spatial extent of which is more evident along the cell’s preferred axis of motion.

Neural correlates of stimulus spatial frequency-dependent contrast detection

Psychophysical studies on human and non-human vertebrate species have shown that visual contrast sensitivity function (CSF) peaks at a certain stimulus spatial frequency and declines in both lower and higher spatial frequencies. The underlying neural substrate and mechanisms remain in debate. Here, we investigated the role of primary visual cortex (V1: area 17) in spatial frequency-dependent contrast detection in cats. Perceptual CSFs of three cats were measured using a two-alternative forced-choice task. The responses of V1 neurons to their optimal visual stimuli in a range of luminance contrast levels (from 0 to 1.0) were recorded subsequently using in vivo extracellular single-unit recording techniques. The contrast sensitivity of each neuron was determined. The neuronal CSF for each cat was constructed from the mean contrast sensitivity of neurons with different preferred stimulus spatial frequencies.

RESULT

(1) The perceptual and neuronal CSFs of each of the three cats exhibited a similar shape with peak amplitude near 0.4 cpd. (2) The neuronal CSF of each cat was highly correlated with its perceptual CSF. (3) V1 neurons with different preferred stimulus spatial frequencies had different contrast gains.

CONCLUSION

(1) Contrast detection of visual stimuli with different spatial frequencies may likely involve population coding of V1 neurons with different preferred stimulus spatial frequencies. (2) Difference in contrast gain may underlie the observed contrast sensitivity variation of V1 neurons with different preferred stimulus spatial frequencies, possibly from either evolution or postnatal visual experiences.

Integrate-and-fire vs Poisson models of LGN input to V1 cortex: noisier inputs reduce orientation selectivity

One of the reasons the visual cortex has attracted the interest of computational neuroscience is that it has well-defined inputs. The lateral geniculate nucleus (LGN) of the thalamus is the source of visual signals to the primary visual cortex (V1). Most large-scale cortical network models approximate the spike trains of LGN neurons as simple Poisson point processes. However, many studies have shown that neurons in the early visual pathway are capable of spiking with high temporal precision and their discharges are not Poisson-like. To gain an understanding of how response variability in the LGN influences the behavior of V1, we study response properties of model V1 neurons that receive purely feedforward inputs from LGN cells modeled either as noisy leaky integrate-and-fire (NLIF) neurons or as inhomogeneous Poisson processes. We first demonstrate that the NLIF model is capable of reproducing many experimentally observed statistical properties of LGN neurons. Then we show that a V1 model in which the LGN input to a V1 neuron is modeled as a group of NLIF neurons produces higher orientation selectivity than the one with Poisson LGN input. The second result implies that statistical characteristics of LGN spike trains are important for V1's function. We conclude that physiologically motivated models of V1 need to include more realistic LGN spike trains that are less noisy than inhomogeneous Poisson processes.

Orthogonal micro-organization of orientation and spatial frequency in primate primary visual cortex

Orientation and spatial frequency tuning are highly salient properties of neurons in primary visual cortex (V1). The combined organization of these particular tuning properties in the cortical space will strongly shape the V1 population response to different visual inputs, yet it is poorly understood. In this study, we used two-photon imaging in macaque monkey V1 to demonstrate the three-dimensional cell-by-cell layout of both spatial frequency and orientation tuning. We first found that spatial frequency tuning was organized into highly structured maps that remained consistent across the depth of layer II/III, similarly to orientation tuning. Next, we found that orientation and spatial frequency maps were intimately related at the fine spatial scale observed with two-photon imaging. Not only did the map gradients tend notably toward orthogonality, but they also co-varied negatively from cell to cell at the spatial scale of cortical columns.

Mapping of contextual modulation in the population response of primary visual cortex

We review the evidence of long-range contextual modulation in V1. Populations of neurons in V1 are activated by a wide variety of stimuli outside of their classical receptive fields (RF), well beyond their surround region. These effects generally involve extra-RF features with an orientation component. The population mapping of orientation preferences to the upper layers of V1 is well understood, as far as the classical RF properties are concerned, and involves organization into pinwheel-like structures. We introduce a novel hypothesis regarding the organization of V1's contextual response. We show that RF and extra-RF orientation preferences are mapped in related ways. Orientation pinwheels are the foci of both types of features. The mapping of contextual features onto the orientation pinwheel has a form that recapitulates the organization of the visual field: an iso-orientation patch within the pinwheel also responds to extra-RF stimuli of the same orientation. We hypothesize that the same form of mapping applies to other stimulus properties that are mapped out in V1, such as colour and contrast selectivity. A specific consequence is that fovea-like properties will be mapped in a systematic way to orientation pinwheels. We review the evidence that cytochrome oxidase blobs comprise the foci of this contextual remapping for colour and low contrasts. Neurodynamics and motion in the visual field are argued to play an important role in the shaping and maintenance of this type of mapping in V1.

Hyperbolic planforms in relation to visual edges and textures perception

We propose to use bifurcation theory and pattern formation as theoretical probes for various hypotheses about the neural organization of the brain. This allows us to make predictions about the kinds of patterns that should be observed in the activity of real brains through, e.g., optical imaging, and opens the door to the design of experiments to test these hypotheses. We study the specific problem of visual edges and textures perception and suggest that these features may be represented at the population level in the visual cortex as a specific second-order tensor, the structure tensor, perhaps within a hypercolumn. We then extend the classical ring model to this case and show that its natural framework is the non-Euclidean hyperbolic geometry. This brings in the beautiful structure of its group of isometries and certain of its subgroups which have a direct interpretation in terms of the organization of the neural populations that are assumed to encode the structure tensor. By studying the bifurcations of the solutions of the structure tensor equations, the analog of the classical Wilson and Cowan equations, under the assumption of invariance with respect to the action of these subgroups, we predict the appearance of characteristic patterns. These patterns can be described by what we call hyperbolic or H-planforms that are reminiscent of Euclidean planar waves and of the planforms that were used in previous work to account for some visual hallucinations. If these patterns could be observed through brain imaging techniques they would reveal the built-in or acquired invariance of the neural organization to the action of the corresponding subgroups.

Our visual perception of the world is remarkably stable despite the fact that we move our gaze and body. This must be the effect of the neuronal organization of the visual areas of our brains that succeed in maintaining in our consciouness a representation that seems to be protected from brutal variations. We propose a theory to account for an invariance that pertains to such image features as edges and textures. It is based on the simple assumption that the spatial variations of the image intensity, its derivatives, are extracted and represented in some visual brain areas by populations of neurons that excite and inhibit each other according to the values of these derivatives. Geometric transformations of the retinal image, caused say by eye movements, affect these derivatives. Assuming that their representations are invariant to these transformations, we predict the appearance of specific patterns of activity which we call hyperbolic planforms. It is surprising that the geometry that emerges from our investigations is not the usual Euclidean geometry but the much less familiar hyperbolic, non-Euclidean, geometry. We also propose some preliminary ideas for putting our theory to the test by actual measurements of brain activity.

Ocular dominance columns: enigmas and challenges

In some mammalian species, geniculocortical afferents serving each eye are segregated in layer 4C of striate cortex into stripes called ocular dominance columns. Having described the complete pattern of ocular dominance columns in the human brain, the authors enumerate here the principal enigmas that confront future investigators. Probably the overarching challenge is to explain the function, if any, of ocular dominance columns and why they are present in some species and not others. A satisfactory solution must account for the enormous natural variation, even within the same species, among individuals in column expression, pattern, periodicity, and alignment with other components of the functional architecture. Another major priority is to explain the development of ocular dominance columns. It has been established clearly that they form without visual experience, but the innate signals that guide their segregation and maturation are unknown. Experiments addressing the role of spontaneous retinal activity have yielded contradictory data. These studies must be reconciled, to pave the way for new insights into how columnar structure is generated in the cerebral cortex.

Parallel processing strategies of the primate visual system

Incoming sensory information is sent to the brain along modality-specific channels corresponding to the five senses. Each of these channels further parses the incoming signals into parallel streams to provide a compact, efficient input to the brain. Ultimately, these parallel input signals must be elaborated on and integrated in the cortex to provide a unified and coherent percept. Recent studies in the primate visual cortex have greatly contributed to our understanding of how this goal is accomplished. Multiple strategies including retinal tiling, hierarchical and parallel processing and modularity, defined spatially and by cell type-specific connectivity, are used by the visual system to recover the intricate detail of our visual surroundings.

Physiological differences between neurons in layer 2 and layer 3 of primary visual cortex (V1) of alert macaque monkeys

The physiological literature does not distinguish between the superficial layers 2 and 3 of the primary visual cortex even though these two layers differ in their cytoarchitecture and anatomical connections. To distinguish layer 2 from layer 3, we have analysed the response characteristics of neurons recorded during microelectrode penetrations perpendicular to the cortical surface. Extracellular responses of single neurons to sweeping bars were recorded while macaque monkeys performed a fixation task. Data were analysed from penetrations where cells could be localized to specific depths in the cortex. Although the most superficial cells (depth, 145-371 microm; presumably layer 2) responded preferentially to particular stimulus orientations, they were less selective than cells encountered immediately beneath them (depth, 386-696 microm; presumably layer 3). Layer 2 cells had smaller spikes, higher levels of ongoing activity, larger receptive field activating regions, and less finely tuned selectivity for stimulus orientation and length than layer 3 cells. Direction selectivity was found only in layer 3. These data suggest that layer 3 is involved in generating and transmitting precise, localized information about image features, while the lesser selectivity of layer 2 cells may participate in top-down influences from higher cortical areas, as well as modulatory influences from subcortical brain regions.

Functional organization of color domains in V1 and V2 of macaque monkey revealed by optical imaging

Areas V1 and V2 of Macaque monkey visual cortex are characterized by unique cytochrome-oxidase (CO)-staining patterns. Initial electrophysiological studies associated CO blobs in V1 with processing of surface properties such as color and brightness and the interblobs with contour information processing. However, many subsequent studies showed controversial results, some supporting this proposal and others failing to find significant functional differences between blobs and interblobs. In this study, we have used optical imaging to map color-selective responses in V1 and V2. In V1, we find striking "blob-like" patterns of color response. Fine alignment of optical maps and CO-stained tissue revealed that color domains in V1 strongly associate with CO blobs. We also find color domains in V1 align along centers of ocular dominance columns. Furthermore, color blobs in V1 have low orientation selectivity and do not overlap with centers of orientation domains. In V2, color domains coincide with thin stripes; orientation-selective domains coincide with thick and pale stripes. We conclude that color and orientation-selective responses are preferentially located in distinct CO compartments in V1 and V2. We propose that the term "blob" encompasses both the concept of "CO blob" and "color domain" in V1.

Optical imaging of contrast response in Macaque monkey V1 and V2

Our studies on brightness information processing in Macaque monkey visual cortex suggest that the thin stripes in the secondary visual area (V2) are preferentially activated by brightness stimuli (such as full field luminance modulation and illusory edge-induced brightness modulation). To further examine this possibility, we used intrinsic signal optical imaging to examine contrast response of different functional domains in primary and secondary visual areas (V1 and V2). Color and orientation stimuli were used to map functional domains in V1 (color domains, orientation domains) and V2 (thin stripes, thick/pale stripes). To examine contrast response, sinusoidal gratings at different contrasts and spatial frequencies were presented. We find that, consistent with previous studies, the optical signal increased systematically with contrast level. Unlike single-unit responses, optical signals for both color domains and orientation domains in V1 exhibit linear contrast response functions, thereby providing a large dynamic range for V1 contrast response. In contrast to domains in V1, domains in V2 exhibit nonlinear responses, characterized by high gain at low contrasts, saturating at a mid-high contrast levels. At high contrasts, thin stripes exhibit increasing response, whereas thick/pale stripes saturate, consistent with a strong parvocellular input to thin stripes. These findings suggest that, with respect to contrast encoding, thin stripes have a larger dynamic range than thick/pale stripes and further support a role for thin stripes in processing of brightness information.

How do functional maps in primary visual cortex vary with eccentricity?

It is important to understand whether functional maps of primary visual cortex (V1) are organized differently at the representation of different eccentricities. By using optical imaging of intrinsic signals, we compared the maps of orientation and spatial frequency (SF) preference between central (0-3 degrees ) and paracentral (4-8 degrees ) V1 in the prosimian bush baby (Otolemur garnetti). No differences related to eccentricity were found for orientation selectivity or magnitude between central and paracentral V1. We found, however, that cardinal orientations were overrepresented in central but not in paracentral V1 and that isoorientation domain size tended to be smaller in the central representation. We demonstrated that spatial frequency was represented continuously across V1, and that the map of SF preference exhibited eccentricity-dependent variations, with more territory devoted to higher SFs in central than in paracentral V1. Although there were no spatial relationships between orientation domains and cytochrome oxidase (CO) blobs or interblobs, CO blobs tended to prefer lower SFs than interblobs. Taken together with previous research, our data indicate that functional domains in V1 show eccentricity-related differences in organization and also support the idea that different maps (with or without specific geometrical relationships) are organized for adequate coverage of each feature in visual space.

Hue maps in primate striate cortex

The macaque striate cortex (V1) contains neurons that respond preferentially to various hues. The properties of these hue-selective neurons have been studied extensively at the single-unit level, but it is unclear how stimulus hue is represented by the distribution of activity across neuronal populations in V1. Here we use the intrinsic optical signal to image V1 responses to spatially uniform stimuli of various hues. We found that (1) each of these stimuli activates an array of patches in the supragranular layers of the parafoveal V1; (2) the patches activated by different hues overlapped partially; 3) the peak locations of these patches were determined by stimulus hue. The peaks associated with various hues form well-separated clusters, in which nearby peaks represent perceptually similar hues. Each cluster represents a full gamut of hue in a small cortical area ( approximately 160 microm long). The hue order is preserved within each peak cluster, but the clusters have various geometrical shapes. These clusters were co-localized with regions that responded preferentially to chromatic gratings compared with achromatic ones. Our results suggest that V1 contains an array of hue maps, in which the hue of a stimulus is represented by the location of the peak response to the stimulus. The orderly, organized hue maps in V1, together with the recently discovered hue maps in the extrastriate cortical area V2, are likely to play an important role in hue perception in primates.

Attention induces synchronization-based response gain in steady-state visual evoked potentials

When attention is voluntarily directed to a spatial location, visual sensitivity increases at that location. What causes this improved sensitivity? Studies of single neuron spike rates in monkeys have provided mixed results in regard to whether attending to a stimulus increases its effective contrast (contrast gain) or multiplicatively boosts stimulus-driven neural responses (response or activity gain). We monitored frequency-tagged steady-state visual evoked potentials (SSVEPs) in humans and found that voluntary sustained attention multiplicatively increased stimulus-driven population electrophysiological activity. Analyses of intertrial phase coherence showed that this attentional response gain was at least partially due to the increased synchronization of SSVEPs to stimulus flicker. These results suggest that attention operates in a complementary manner at different levels; attention seems to increase single-neuron spike rates in a variety of ways, including contrast, response and activity gains, while also inducing a multiplicative boost on neural population activity via enhanced response synchronization.

Coding of color and form in the geniculostriate visual pathway (invited review)

We review how neurons in the principal pathway connecting the retina to the visual cortex represent information about the chromatic and spatial characteristics of the retinal image. Our examination focuses particularly on individual neurons: what are their visual properties, how might these properties arise, what do these properties tell us about visual signal transformations, and how might these properties be expressed in perception? Our discussion is inclined toward studies on old-world monkeys and where possible emphasizes quantitative work that has led to or illuminates models of visual signal processing.

The circuitry of V1 and V2: integration of color, form, and motion

Primary and secondary visual cortex (V1 and V2) form the foundation of the cortical visual system. V1 transforms information received from the lateral geniculate nucleus (LGN) and distributes it to separate domains in V2 for transmission to higher visual areas. During the past 20 years, schemes for the functional organization of V1 and V2 have been based on a tripartite framework developed by Livingstone & Hubel (1988) . Since then, new anatomical data have accumulated concerning V1's input, its internal circuitry, and its output to V2. These new data, along with physiological and imaging studies, now make it likely that the visual attributes of color, form, and motion are not neatly segregated by V1 into different stripe compartments in V2. Instead, there are just two main streams, originating from cytochrome oxidase patches and interpatches, that project to V2. Each stream is composed of a mixture of magno, parvo, and konio geniculate signals. Further studies are required to elucidate how the patches and interpatches differ in the output they convey to extrastriate cortex.

High Response Reliability of Neurons in Primary Visual Cortex (V1) of Alert, Trained Monkeys

The reliability of neuronal responses determines the resources needed to represent the external world and constrains the nature of the neural code. Studies of anesthetized animals have indicated that neuronal responses become progressively more variable as information travels from the retina to the cortex. These results have been interpreted to indicate that perception must be based on pooling across relatively large numbers of cells. However, we find that in alert monkeys, responses in primary visual cortex (V1) are as reliable as the inputs from the retina and the thalamus. Moreover, when the effects of fixational eye movements were minimized, response variability (variance/mean - Fano factor, FF) in all V1 layers was low. When presenting optimal stimuli, the median FF was 0.3. High variability, FF approximately 1, was found only near threshold. Our results suggest that in natural vision, suprathreshold perception can be based on small numbers of optimally stimulated cells.

Cortical processing of a brightness illusion

Several brightness illusions indicate that borders can affect the perception of surfaces dramatically. In the Cornsweet illusion, two equiluminant surfaces appear to be different in brightness because of the contrast border between them. Here, we report the existence of cells in monkey visual cortex that respond to such an "illusory" brightness. We find that luminance responsive cells are located in color-activated regions (cytochrome oxidase blobs and bridges) of primary visual cortex (V1), whereas Cornsweet responsive cells are found preferentially in the color-activated regions (thin stripes) of second visual area (V2). This colocalization of brightness and color processing within V1 and V2 suggests a segregation of contour and surface processing in early visual pathways and a hierarchy of brightness information processing from V1 to V2 in monkeys.

Functional organization of visual cortex in the owl monkey

In this study, we compared the organization of orientation preference in visual areas V1, V2, and V3. Within these visual areas, we also quantified the relationship between orientation preference and cytochrome oxidase (CO) staining patterns. V1 maps of orientation preference contained both pinwheels and linear zones. The location of CO blobs did not relate in a systematic way to maps of orientation; although, as in other primates, there were approximately twice as many pinwheels as CO blobs. V2 contained bands of high and low orientation selectivity. The bands of high orientation selectivity were organized into pinwheels and linear zones, but iso-orientation domains were twice as large as those in V1. Quantitative comparisons between bands containing high or low orientation selectivity and CO dark and light bands suggested that at least four functional compartments exist in V2, CO dense bands with either high or low orientation selectivity, and CO light bands with either high or low selectivity. We also demonstrated that two functional compartments exist in V3, with zones of high orientation selectivity corresponding to CO dense areas and zones of low orientation selectivity corresponding to CO pale areas. Together with previous findings, these results suggest that the modular organization of V1 is similar across primates and indeed across most mammals. V2 organization in owl monkeys also appears similar to that of other simians but different from that of prosimians and other mammals. Finally, V3 of owl monkeys shows a compartmental organization for orientation selectivity that remains to be demonstrated in other primates.

Feedforward, feedback and inhibitory connections in primate visual cortex

Visual cortical circuits are organized at multiple levels of complexity including cortical areas, layers and columns, and specific cell types within these modules. Making sense of the functions of these circuits from anatomical observations requires linking these circuits to function at each of these levels of complexity. Observations of these relationships have become increasingly sophisticated over the last several decades, beginning with correlations between the connectivities and functions of various visual cortical areas and progressing toward cell type-specificity. These studies have informed current views about the functional interactions between cortical areas and modules and the mechanisms by which fine scale microcircuits influence interactions at more coarse levels of organization.

A theory of the Benham Top based on center-surround interactions in the parvocellular pathway

A model color-opponent neuron was used to investigate the subjective colors evoked by the Benham Top (BT). Color-opponent inputs from cone-selective parvocellular (P) pathway neurons with center-surround receptive fields were subtracted with a short relative delay, yielding a small transient input in response to a white spot. This transient input was amplified by BT-like stimuli, modeled as a thin dark bar followed by full-field illumination. The narrow bar produced maximal activation of the P-pathway surrounds but only partial activation of the P-pathway centers. Due to saturation, subsequent removal of the bar had little effect on the P-pathway surrounds, whereas the transient input from the P-pathway centers was amplified via disinhibition. Responses to BT-like stimuli became weaker as surround sensitivity recovered, producing an effect analogous to the progression of perceived BT colors. Our results suggest that the BT-illusion arises because cone-selective neurons convey information about both color and luminance contrast, allowing the two signals become confounded.

Intrinsic connections in tree shrew V1 imply a global to local mapping

The local-global map hypothesis states that locally organized response properties--such as orientation preference--result from visuotopically organized local maps of non-retinotopic response properties. In the tree shrew, the lateral extent of horizontal patchy connections is as much as 80-100% of V1 and is consistent with the length summation property. We argue that neural signals can be transmitted across the entire extent of V1 and this allows the formation of maps at the local scale that are visuotopically organized. We describe mechanisms relevant to the formation of local maps and report modeling results showing the same patterns of horizontal connectivity, and relationships to orientation preference, seen in vivo. The structure of the connectivity that emerges in the simulations reveals a 'hub and spoke' organization. Singularities form the centers of local maps, and linear zones and saddle-points arise as smooth border transitions between maps. These findings are used to present the case for the local-global map hypothesis for tree shrew V1.

The functional geometry of local and horizontal connections in a model of V1

A mathematical model of interacting hypercolumns in primary visual cortex (V1) is presented that incorporates details concerning the geometry of local and long-range horizontal connections. Each hypercolumn is modeled as a network of interacting excitatory and inhibitory neural populations with orientation and spatial frequency preferences organized around a pair of pinwheels. The pinwheels are arranged on a planar lattice, reflecting the crystalline-like structure of cortex. Local interactions within a hypercolumn generate orientation and spatial frequency tuning curves, which are modulated by horizontal connections between different hypercolumns on the lattice. The symmetry properties of the local and long-range connections play an important role in determining the types of spontaneous activity patterns that can arise in cortex.

Projections from primary visual cortex to cytochrome oxidase thin stripes and interstripes of macaque visual area 2

It has been controversial whether the cytochrome oxidase (CO)-dense blobs in primate primary visual cortex (V1) and CO-dense thin stripes in visual area 2 (V2) are parts of a cortical color-processing stream that is segregated from other functional streams. One of the key pieces of evidence for the segregated color stream is the previous report of specific connections between blobs and thin stripes, which is parallel to the connections between interblobs and interstripes. To study the degree of the segregation between the proposed different streams, in the current study, anatomical tracers were injected into different V2 compartments with the functional guidance of optical imaging. The spatial relationship between each labeled cell and the CO blobs in V1 were analyzed quantitatively. After tracer injections in the color-preferring modules in CO thin stripes, equal amounts of labeled cells were found in the blobs and interblobs. However, the density of the labeled cells was more than twice as high in the blobs as that in the interblobs, and most of the clusters of labeled cells partially overlapped with the blobs. Tracer injections in the interstripes labeled cells predominantly in the interblobs. Our results suggest that both the blobs and interblobs project to the thin stripes and call into question the proposition that different CO compartments in V1 and V2 are connected in parallel to form highly segregated functional streams.

Independent projection streams from macaque striate cortex to the second visual area and middle temporal area

The interareal wiring of the neocortex is usually depicted as a network of single point-to-point connections, often side-stepping the possibility that some neurons may project to multiple cortical areas. The prevalence of such neurons is unknown; if they are abundant, cortical circuits are more likely to be connectionally diffuse. We used a dual-tracer approach to determine whether single neurons in the macaque primary visual cortex (V1) project to two extrastriate areas, the second visual area (V2) and the middle temporal area (MT). We found two large intermingled groups of single-labeled neurons in layer 4B of V1 projecting independently to either V2 or MT. A third, sparser group of double-labeled neurons projected to both areas; we termed these manifold neurons. We also found that MT-projecting cells were distributed indiscriminately with respect to cytochrome oxidase compartment in layer 4B, revealing a subpopulation that provides a potential source of patch input from V1 to MT. The results demonstrate that primary sensory cortices can use multiple projection strategies to distribute signals to higher areas, and suggest that feedforward projections may route signals with more specificity than feedback pathways.

Striate cortex in dichromatic and trichromatic marmosets: neurochemical compartmentalization and geniculate input

The superficial layers of primate striate cortex (V1) contain a regular pattern of dense staining for cytochrome oxidase (CO) reactivity ("blobs") that receive direct input from the koniocellular layers of the lateral geniculate nucleus. It has been suggested that the blob regions are dedicated to processing color information. Here, the neurochemical compartmentalization of blobs and their input from the lateral geniculate nucleus (LGN) was measured in marmosets (Callithrix jacchus) identified as having either dichromatic or trichromatic color vision. In all animals, layer III of V1 showed a patchy distribution of CO. The spatial density of CO blobs (mean, 4.6 blobs/mm(2); range, 3.9-5.5), blob diameter, and the proportion of cortical area within blobs was not significantly different in dichromats and trichromats. The LGN input was studied by injecting retrograde tracer into V1. The koniocellular layers of the LGN contribute 11% of all relay cells, and form the only geniculate input to upper layer III of V1. Only half of all relay cells in the KC layers express calbindin. There is no obvious difference between dichromats and trichromats in the pattern of the geniculate projection to V1. It is concluded that the trichromatic phenotype is not associated with changes in the gross anatomy, neurochemistry, or organization of the geniculate afferents to the superficial layers of V1.

Color processing in macaque striate cortex: relationships to ocular dominance, cytochrome oxidase, and orientation

We located clusters of color-selective neurons in macaque striate cortex, as mapped with optical imaging and confirmed with electrophysiological recordings. By comparing responses to an equiluminant red/green stimulus versus a high-contrast luminance stimulus, we were able to reveal a patchy distribution of color selectivity. Other color imaging protocols, when compared with electrophysiological data, did not reliably indicate the location of functional structures. The imaged color patches were compared with other known functional subdivisions of striate cortex. There was a high degree of overlap of the color patches with the cytochrome-oxidase (CO) blobs. The patches were often larger than a single blob in size, however, and in some instances spanned two neighboring blobs. More than one-half (56%) of the color-selective patches seen in optical imaging were not confined to one ocular dominance (OD) column. Almost one-quarter of color patches (23%) extended across OD columns to encompass two blobs of different eye preference. We also compared optical images of orientation selectivity to maps of color selectivity. Results indicate that the layout of orientation and color selectivity are not directly related. Specifically, despite having similar scales and distributions, the maps of orientation and color selectivity were not in consistent alignment or registration. Further, we find that the maps of color selectivity and of orientation are each only loosely related to maps of OD. This description stands in contrast to a common depiction of color-selective regions as identical to CO blobs, appearing as pegs in the centers of OD columns in the classical "ice cube" model. These results concerning the pattern of color selectivity in V1 support the view (put forth in previous imaging studies of the organization of orientation and ocular dominance) that there is not a fundamental registration of functional hypercolumns in V1.

Color processing in macaque striate cortex: electrophysiological properties

We have shown in the accompanying paper that optical imaging of macaque striate cortex reveals patches that are preferentially activated by equiluminant chromatic gratings compared with luminance gratings. These imaged color patches are highly correlated, although not always in one-to-one correspondence, with the cytochrome-oxidase (CO) blobs. In the present study, we have investigated the electrophysiological properties of neurons in the imaged color patches and the CO blobs. Our results indicate that individual blobs tend to contain cells of only one type of color opponency: either red/green or blue/yellow. Individual imaged color patches, however, can bridge blobs of similar opponency or differing opponency. When imaged color patches contain two blobs of differing opponency, the cells in the bridge region exhibit mixed color properties that are not opponent along the two cardinal color axes (either red/green or blue/yellow). Two blobs within a single imaged color patch receive input from the same eye or from different eyes. In the latter case, the bridge region between blobs contains binocular cells that are color selective. Because the cells recorded in imaged color patches were more color selective and unoriented than cells outside of color patches, color properties appear to be organized in a clustered and segregated fashion in primate V1.

Visual field defects and neural losses from experimental glaucoma

Glaucoma is a relatively common disease in which the death of retinal ganglion cells causes a progressive loss of sight, often leading to blindness. Typically, the degree of a patient's visual dysfunction is assessed by clinical perimetry, involving subjective measurements of light-sense thresholds across the visual field, but the relationship between visual and neural losses is inexact. Therefore, to better understand of the effects of glaucoma on the visual system, a series of investigations involving psychophysics, electrophysiology, anatomy, and histochemistry were conducted on experimental glaucoma in monkeys. The principal results of the studies showed that, (1) the depth of visual defects with standard clinical perimetry are predicted by a loss of probability summation among retinal detection mechanisms, (2) glaucomatous optic atrophy causes a non-selective reduction of metabolism of neurons in the afferent visual pathway, and (3) objective electrophysiological methods can be as sensitive as standard clinical perimetry in assessing the neural losses from glaucoma. These experimental findings from glaucoma in monkeys provide fundamental data that should be applicable to improving methods for assessing glaucomatous optic neuropathy in patients.

Tactile discrimination of edge shape: limits on spatial resolution imposed by parameters of the peripheral neural population

When the flat faces of a coin are grasped between thumb and index finger, a "curved edge" is felt. Analogous curved edges were generated by our stimuli, which comprised the flat face of segments of annuli applied passively to immobilized fingers. Humans could scale the curvature of the annulus and could discriminate changes in curvature of approximately 20 m(-1). The responses of single slowly adapting type I afferents (SAIs) recorded in anesthetized monkeys could be quantified by the product of two factors: their sensitivity and a spatial profile dependent only on the radius of the annulus. This allowed us to reconstruct realistic SAI population responses that included noise, variation in fiber sensitivity, and varying innervation patterns. The critical question was how relatively small populations ( approximately 70 active fibers) can encode edge curvature with such precision. A template-matching approach was used to establish the accuracy of edge representation in the population. The known large interfiber variability in sensitivity had no effect on curvature resolution. Neural resolution was superior to human performance until large levels of central noise were present showing that, unlike simple detection, spatial processing is limited centrally. In contrast to the behavior of mean response codes, neural resolution improved with increasing covariance in noise. Surprisingly, resolution for any single population varied considerably with small changes in the position of the stimulus relative to the SAI matrix. Overall innervation density was not as critical as the spacing of receptive fields at right angles to the edge.

A two-layer sparse coding model learns simple and complex cell receptive fields and topography from natural images

The classical receptive fields of simple cells in the visual cortex have been shown to emerge from the statistical properties of natural images by forcing the cell responses to be maximally sparse, i.e. significantly activated only rarely. Here, we show that this single principle of sparseness can also lead to emergence of topography (columnar organization) and complex cell properties as well. These are obtained by maximizing the sparsenesses of locally pooled energies, which correspond to complex cell outputs. Thus, we obtain a highly parsimonious model of how these properties of the visual cortex are adapted to the characteristics of the natural input.

A comparison of koniocellular, magnocellular and parvocellular receptive field properties in the lateral geniculate nucleus of the owl monkey (Aotus trivirgatus)

1. By analogy to previous work on lateral geniculate nucleus (LGN) magnocellular (M) and parvocellular (P) cells our goal was to construct a physiological profile of koniocellular (K) cells that might be linked to particular visual perceptual attributes. 2. Extracellular recordings were used to study LGN cells, or their axons, in silenced primary visual cortex (V1), in nine anaesthetized owl monkeys injected with a neuromuscular blocker. Receptive field centre-surround organization was examined using flashing spots. Spatial and temporal tuning and contrast responses were examined using drifting sine-wave gratings; counterphase sine-wave gratings were used to examine linearity of spatial summation. 3. Receptive fields of 133 LGN cells and 10 LGN afferent axons were analysed at eccentricities ranging from 2.8 to 31.3 deg. Thirty-four per cent of K cells and only 9 % of P and 6 % of M cells responded poorly to drifting gratings. K, P and M cells showed increases in centre size with eccentricity, but K cells showed more scatter. All cells, except one M cell, showed linearity in spatial summation. 4. At matched eccentricities, K cells exhibited lower spatial and intermediate temporal resolution compared with P and M cells. K contrast thresholds and gains were more similar to those of M than P cells. M cells showed lower spatial and higher temporal resolution and contrast gains than P cells. 5. K cells in different K LGN layers differed in spatial, temporal and contrast characteristics, with K3 cells having higher spatial resolution and lower temporal resolution than K1/K2 cells. 6. Taken together with previous results these findings suggest that the K cells consist of several classes, some of which could contribute to conventional aspects of spatial and temporal resolution.

Tactile discrimination of gaps by slowly adapting afferents: effects of population parameters and anisotropy in the fingerpad

The aim of this study was to determine the acuity of the peripheral tactile system for gaps and to determine how stimulus orientation may impact on this. We quantified the ability of humans to discriminate small differences in gap width using a forced-choice task. Stimuli were presented passively to the distal fingerpad in a region where the skin ridges all run approximately in the same direction. Two standard gap widths were used (2 and 2.9 mm), and the comparison gap widths were larger than the standard gaps. With the gap axis parallel to the skin ridges, the average difference limen was approximately 0.2 mm for both standards. We examined the effect of stimulus orientation by asking subjects to discriminate between a smooth surface and a grating (ridge width, 1.5 mm; groove width, 0. 75 mm). They were able to discriminate the two surfaces when the axis of the grooves was parallel to the skin ridges, but performance was below threshold in the orthogonal orientation. The underlying neural mechanisms were investigated using the gap stimuli to activate single slowly adapting type I mechanoreceptive afferents (SAIs) innervating the fingerpads of anesthetized monkeys. The edges of the gap produced response peaks, and the gap resulted in a trough in the receptive field profiles. The response magnitude at the peaks was greater, and at the troughs was smaller, for larger gap widths and also when the axis of the gap was parallel to the skin ridges as compared with the orthogonal orientation. Simulated SAI population responses showed that response profiles were distorted by variation in afferent sensitivity and by neural noise. Using signal detection theory, based on a neural measure of the gaps computed over the active population, the acuity of the SAIs under realistic population conditions was compared with human performance. These analyses showed how parameters like afferent sensitivity, the pattern and density of innervation, and noise impact on performance and why their impact is different for the two stimulus orientations investigated. The greatest limitation was imposed by noise that is independent of response magnitude, and this effect was greater for stimuli oriented orthogonal to the skin ridges than for the parallel orientation.

Deficits in visual feature binding under isoluminant conditions

The contribution of the magnocellular stream to visual feature binding was examined psychophysically through the use of isoluminant stimuli. Subjects were presented with three briefly flashed colored letters arranged in an array and asked to identify the shape and color of the center letter. The rate of illusory conjunctions was much higher when the letters were isoluminant with a gray background, compared to when the letters were either brighter or dimmer. Over 90% of conjunction errors involved pairing the wrong shape with the correct color, rather than vice versa. Directing attention to the target location with a nonisoluminant cue did not reduce illusory conjunctions. High rates of binding errors under isoluminance are interpreted here in terms of abnormalities in visual form processing rather than an attentional effect. In another experiment designed to examine the role of synchrony in feature binding, the rate of illusory conjunctions was highest when flanking letters were presented before the central target letter and not synchronously.

Functional coupling shows stronger stimulus dependency for fast oscillations than for low-frequency components in striate cortex of awake monkey

It has been argued that coupling among the neural signals activated by a visual object supports binding of local features into a coherent object perception. During visual stimulation by a grating texture we studied functional coupling by calculating spectral coherence among pairs of signals recorded in the striate cortex of awake monkeys. Multiple unit activity (MUA) and local field potentials (LFP, 1-140 Hz) were extracted from seven parallel broad band recordings. Spectral coherence was dominated by high-frequency oscillations in the range 35-50 Hz and often by additional low-frequency components (0-12 Hz). Functional coupling among separate cortical sites was more stimulus specific for MUA than for LFP: MUA coherence at high and low frequencies depended highly significantly on: (i) the similarity of the preferred orientations at the two sites - the more similar the higher the coherence; (ii) the orientation of the stimulus grating - with highest coherence at half angle between the preferred orientations at the two sites; (iii) cortical distance - coherence decreases to noise levels at approximately 3 mm (MUA) and 6 mm (LFP). Coherence of fast oscillations did not depend on the degree of coaxiality of the orientation-sensitive receptive fields, whereas low frequencies showed significant dependency. This indicates that different frequency components can engage different coupling networks in the striate cortex which probably support different coding tasks. Changes in average oscillation frequency with stimulus orientation were highly significant for fast oscillations while there was no dependency for low frequencies. Finally, stimulus-related spectral power and coherence of fast oscillations were considerably higher than of low frequency components. Fast oscillations may therefore contribute more to feature binding and coding of object continuity than low-frequency components, at least for texture surfaces as analysed here.

Diversity and cell type specificity of local excitatory connections to neurons in layer 3B of monkey primary visual cortex

In the primary visual cortex of macaque monkeys, laminar and columnar axonal specificity are correlated with functional differences between locations. We describe evidence that embedded within this anatomical framework is finer specificity of functional connections. Photostimulation-based mapping of functional input to 31 layer 3B neurons revealed that input sources to individual cells were highly diverse. Although some input differences were correlated with neuronal anatomy, no 2 neurons received excitatory input from the same cortical layers. Thus, input diversity reveals far more cell types than does anatomical diversity. This implies relatively little functional redundancy; despite trends related to laminar or columnar position, pools of neurons contributing uniquely to visual processing are likely relatively small. These results also imply that similarities in the anatomy of circuits in different cortical areas or species may not indicate similar functional connectivity.