The Organization of Feedback Connections from Area V2 (18) to V1 (17). In: Peters A, Rockland KS, editors. Primary Visual Cortex in Primates. Boston: Springer US; 1994. pp. 261-99. [DOI: 10.1007/978-1-4757-9628-5_6]

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.

Circuits and Mechanisms for Surround Modulation in Visual Cortex

Surround modulation (SM) is a fundamental property of sensory neurons in many species and sensory modalities. SM is the ability of stimuli in the surround of a neuron's receptive field (RF) to modulate (typically suppress) the neuron's response to stimuli simultaneously presented inside the RF, a property thought to underlie optimal coding of sensory information and important perceptual functions. Understanding the circuit and mechanisms for SM can reveal fundamental principles of computations in sensory cortices, from mouse to human. Current debate is centered over whether feedforward or intracortical circuits generate SM, and whether this results from increased inhibition or reduced excitation. Here we present a working hypothesis, based on theoretical and experimental evidence, that SM results from feedforward, horizontal, and feedback interactions with local recurrent connections, via synaptic mechanisms involving both increased inhibition and reduced recurrent excitation. In particular, strong and balanced recurrent excitatory and inhibitory circuits play a crucial role in the computation of SM.

The anatomical and physiological properties of the visual cortex argue against cognitive penetration

We are consciously aware of visual objects together with the minute details that characterize each object. Those details are perceived instantaneously and in parallel. V1 is the only visual area with spatial resolution and topographical exactitude matching perceptual abilities. For cognition to penetrate perception, it needs to affect V1 image representation. That is unlikely because of the detailed parallel V1 organization and the nature of top-down connections, which can influence only large parts of the visual field.

Anatomy of hierarchy: feedforward and feedback pathways in macaque visual cortex

The laminar location of the cell bodies and terminals of interareal connections determines the hierarchical structural organization of the cortex and has been intensively studied. However, we still have only a rudimentary understanding of the connectional principles of feedforward (FF) and feedback (FB) pathways. Quantitative analysis of retrograde tracers was used to extend the notion that the laminar distribution of neurons interconnecting visual areas provides an index of hierarchical distance (percentage of supragranular labeled neurons [SLN]). We show that: 1) SLN values constrain models of cortical hierarchy, revealing previously unsuspected areal relations; 2) SLN reflects the operation of a combinatorial distance rule acting differentially on sets of connections between areas; 3) Supragranular layers contain highly segregated bottom-up and top-down streams, both of which exhibit point-to-point connectivity. This contrasts with the infragranular layers, which contain diffuse bottom-up and top-down streams; 4) Cell filling of the parent neurons of FF and FB pathways provides further evidence of compartmentalization; 5) FF pathways have higher weights, cross fewer hierarchical levels, and are less numerous than FB pathways. Taken together, the present results suggest that cortical hierarchies are built from supra- and infragranular counterstreams. This compartmentalized dual counterstream organization allows point-to-point connectivity in both bottom-up and top-down directions.

Differential expression of vesicular glutamate transporters 1 and 2 may identify distinct modes of glutamatergic transmission in the macaque visual system

Glutamate is the primary neurotransmitter utilized by the mammalian visual system for excitatory neurotransmission. The sequestration of glutamate into synaptic vesicles, and the subsequent transport of filled vesicles to the presynaptic terminal membrane, is regulated by a family of proteins known as vesicular glutamate transporters (VGLUTs). Two VGLUT proteins, VGLUT1 and VGLUT2, characterize distinct sets of glutamatergic projections between visual structures in rodents and prosimian primates, yet little is known about their distributions in the visual system of anthropoid primates. We have examined the mRNA and protein expression patterns of VGLUT1 and VGLUT2 in the visual system of macaque monkeys, an Old World anthropoid primate, in order to determine their relative distributions in the superior colliculus, lateral geniculate nucleus, pulvinar complex, V1 and V2. Distinct expression patterns for both VGLUT1 and VGLUT2 identified architectonic boundaries in all structures, as well as anatomical subdivisions of the superior colliculus, pulvinar complex, and V1. These results suggest that VGLUT1 and VGLUT2 clearly identify regions of glutamatergic input in visual structures, and may identify common architectonic features of visual areas and nuclei across the primate radiation. Additionally, we find that VGLUT1 and VGLUT2 characterize distinct subsets of glutamatergic projections in the macaque visual system; VGLUT2 predominates in driving or feedforward projections from lower order to higher order visual structures while VGLUT1 predominates in modulatory or feedback projections from higher order to lower order visual structures. The distribution of these two proteins suggests that VGLUT1 and VGLUT2 may identify class 1 and class 2 type glutamatergic projections within the primate visual system (Sherman and Guillery, 2006).

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.

Synaptic connection from cortical area V4 to V2 in macaque monkey

The major target of the V4 projection in V2 is layer 1, where it forms a tangential spread of asymmetric (excitatory) synapses. This is characteristic of a "feedback" projection. Some axons formed discrete clusters of bouton terminaux between lengths of myelinated axon, while others were unbranched and formed a continuous distribution of en passant boutons with no intercalated myelin. Minor projections were found in layers 2/3 and 6. Dendritic spines were the most frequently encountered targets of the V4 projection (80% in layer 1 and layer 2/3, 94% in layer 6). The remaining targets were dendritic shafts. In layer 1, 69% of target dendrites (12% of all targets) had characteristics identifying them as smooth (GABAergic) cells. In layer 2/3 and layer 6 virtually all the shaft synapses were on smooth dendrites (86% and 100%, respectively). Multisynaptic boutons were rare (mean 1.1 synapses per bouton). Synapses formed in layer 6 were smaller than those of layer 1 (mean area 0.073 microm(2) vs. 0.117 microm(2)). Synapses formed with spines had a more complex postsynaptic density than those formed with dendritic shafts. With respect to targets and synaptic type and size and morphology of synapses, the feedback projection from V4 to V2 resembles those of feedforward projections. The principal difference between the feedforward and feedback projection is in the lamina location of their terminal boutons. The concentration of the V4 projection on layer 1, where it forms asymmetric synapses mainly with spines, suggests that it excites the distal apical dendrites of pyramidal cells.

Attentional integration between anatomically distinct stimulus representations in early visual cortex

Vision often requires attending to, and integrating information from, distant parts of the visual field. However, the neural basis for such long-range integration is not clearly understood. Here, we demonstrate a specific neural signature of attentional integration between stimuli in different parts of the visual field. Using functional MRI, we found that a task requiring the integration of information between two attended but spatially separated stimuli actively modulated the degree of functional integration (in terms of effective connectivity) between their retinotopic representations in visual cortical areas V1, V2, and V4. Spatial attention enhanced long-distance coupling between distinct neuronal populations that represented the attended visual stimuli, even at the earliest stages of cortical processing. In contrast, unattended stimulus representations were decoupled both from attended representations and particularly strongly from each other. Furthermore, enhanced functional integration between cortical representations was associated with enhanced behavioral performance. Attention may thus serve to "bind" together cortical loci at multiple levels of the visual hierarchy that are commonly involved in processing attended stimuli, promoting integration between otherwise functionally isolated cortical loci.

Fine Modulation in Network Activation during Motor Execution and Motor Imagery

Motor imagery, the 'mental rehearsal of motor acts without overt movements', involves either a visual representation (visual imagery, VI) or mental simulation of movement, associated with a kinesthetic feeling (kinetic imagery, KI). Previous brain imaging work suggests that patterns of brain activation differ when comparing execution (E) with either type of imagery but the functional connectivity of the participating networks has not been studied. Using functional magnetic resonance imaging (fMRI) and structural equation modeling, this study elucidates the inter-relationships among the relevant areas for each of the three motor behaviors. Our results suggest that networks underlying these behaviors are not identical, despite the extensive overlap between E and KI. Inputs to M1, which are facilitatory during E, have the opposite effect during KI, suggesting a physiological mechanism whereby the system prevents overt movements. Finally, this study highlights the role of the connection of superior parietal lobule to the supplementary motor area in both types of motor imagery.

Perirhinal and parahippocampal cortices of the macaque monkey: projections to the neocortex

We investigated the topographic and laminar organization of the efferent cortical projections of the perirhinal and parahippocampal cortices. Area 36 of the perirhinal cortex projects preferentially to areas TE and TEO, whereas area TF of the parahippocampal cortex projects preferentially to the posterior parietal cortex and area V4. Area TF projects to many regions of the frontal lobe, whereas area 36 projects mainly to the orbital surface. The insular and cingulate cortices receive projections from areas 36 and TF, whereas only area TF projects to the retrosplenial cortex. Projections to the superior temporal gyrus, including the dorsal bank of the superior temporal sulcus, arise predominantly from area TF. Area 36 projects only to rostral levels of the superior temporal gyrus. Area TF has, in general, reciprocal connections with the neocortex, whereas area 36 has more asymmetric connections. Area 36, for example, projects to more restricted regions of the frontal cortex and superior temporal sulcus than it receives inputs from. In contrast, it projects to larger portions of areas TE and TEO than it receives inputs from. The efferent projections of areas 36 and TF are primarily directed to the superficial layers of the neocortex, a laminar organization consistent with connections of the feedback type. Projections to unimodal visual areas terminate in large expanses of the cortex, but predominantly in layer I. Projections to other sensory and polymodal areas, in contrast, terminate in a columnar manner predominantly in layers II and III. In all areas receiving heavy projections, the projections extend throughout most cortical layers, largely avoiding layer IV. We discuss these findings in relation to current theories of memory consolidation.

Extrastriate feedback to primary visual cortex in primates: a quantitative analysis of connectivity

Knowledge-based or top-down influences on primary visual cortex (area V1) are believed to originate from information conveyed by extrastriate feedback axon connections. Understanding how this information is communicated to area V1 neurons relies in part on elucidating the quantitative as well as the qualitative nature of extrastriate pathway connectivity. A quantitative analysis of the connectivity based on anatomical data regarding the feedback pathway from extrastriate area V2 to area V1 in macaque monkey suggests (i) a total of around ten million or more area V2 axons project to area V1; (ii) the mean number of synaptic inputs from area V2 per upper-layer pyramidal cell in area V1 is less than 6% of all excitatory inputs; and (iii) the mean degree of convergence of area V2 afferents may be high, perhaps more than 100 afferent axons per cell. These results are consistent with empirical observations of the density of radial myelinated axons present in the upper layers in macaque area V1 and the proportion of excitatory extrastriate feedback synaptic inputs onto upper-layer neurons in rat visual cortex. Thus, in primate area V1, extrastriate feedback synapses onto upper-layer cells may, like geniculocortical afferent synapses onto layer IVC neurons, form only a small percentage of the total excitatory synaptic input.

Myelinated axons and the pyramidal cell modules in monkey primary visual cortex

In addition to the horizontal bands of myelinated axons that produce the line of Gennari and the inner band of Baillarger, the macaque primary visual cortex contains prominent vertical bundles of myelinated axons. In tangential sections through layer IVC, these axon bundles are regularly arranged. They have a mean center-to-center spacing of about 23 microns, and each one contains an average of 34 (S.D. +/- 13) myelinated axons. These bundles seem to be largely composed of efferent fibers, because in material in which pyramidal cells have been labelled in layer II/III and in layers IVA and IVB the axons of these neurons descend towards the white matter in bundles. However, it is doubtful whether all of the descending myelinated axons from the superficial layers emerge from the cortex, since counts show that the bundles contain maximum numbers of myelinated axons at the level of layer IVC, and that in layers V and VI their number is reduced by about 30%. Perhaps some of the axons enter the line of Baillarger, in layer V. When the bundles of myelinated axons and the clusters of apical dendrites of the layer V pyramidal cells are visualized simultaneously within layer IVC in electron microscopic preparations, it is apparent that their center-to-center spacing is similar, namely, about 23 microns and that a bundle of axons has a cluster of apical dendrites lying adjacent to it. Because of this association, and because axons from layer III pyramidal cells have been shown to enter the bundles, it is suggested that the myelinated axon bundles contain the efferent axons from the projection neurons in the individual pyramidal cell modules. However, in addition to the myelinated axons, the bundles contain unmyelinated axons, so that they also probably serve as the conduits for axons forming connections between layers. It is proposed that the pyramidal cell modules are the basic, functional neuronal units of the visual cortex, and since the neurons within a particular module can be expected to have slightly different inputs and response properties from those in neighboring modules, the individual axon bundles that emerge from each module would be expected to carry a unique set of efferent information.