Local circuit neurons of macaque monkey striate cortex: II. Neurons of laminae 5B and 6

Motor Control of Distinct Layer 6 Corticothalamic Feedback Circuits

Layer 6 corticothalamic (L6 CT) neurons provide massive input to the thalamus, and these feedback connections enable the cortex to influence its own sensory input by modulating thalamic excitability. However, the functional role(s) feedback serves during sensory processing is unclear. One hypothesis is that CT feedback is under the control of extra-sensory signals originating from higher-order cortical areas, yet we know nothing about the mechanisms of such control. It is also unclear whether such regulation is specific to CT neurons with distinct thalamic connectivity. Using mice (either sex) combined with in vitro electrophysiology techniques, optogenetics, and retrograde labeling, we describe studies of vibrissal primary motor cortex (vM1) influences on different CT neurons in the vibrissal primary somatosensory cortex (vS1) with distinct intrathalamic axonal projections. We found that vM1 inputs are highly selective, evoking stronger postsynaptic responses in Dual ventral posterior medial nucleus (VPm) and posterior medial nucleus (POm) projecting CT neurons located in lower L6a than VPm-only projecting CT cells in upper L6a. A targeted analysis of the specific cells and synapses involved revealed that the greater responsiveness of Dual CT neurons was due to their distinctive intrinsic membrane properties and synaptic mechanisms. These data demonstrate that vS1 has at least two discrete L6 CT subcircuits distinguished by their thalamic projection patterns, intrinsic physiology, and functional connectivity with vM1. Our results also provide insights into how a distinct CT subcircuit may serve specialized roles specific to contextual modulation of tactile-related sensory signals in the somatosensory thalamus during active vibrissa movements.

SIGNIFICANCE STATEMENT

Layer 6 corticothalamic (L6 CT) feedback circuits are ubiquitous across mammalian species and modalities, and their activities have a strong influence on thalamic excitability and information throughput to the neocortex. Despite clear evidence of CT effects on the thalamus, we know relatively little about how CT cells themselves are regulated. Our results show that input from the primary motor cortex strongly excites a subclass of CT neurons in the primary somatosensory cortex that innervate both core and higher-order somatosensory nuclei rather than those exclusively targeting core somatosensory thalamus. The cortico-cortico-thalamic pathway formed by these connections establishes a circuit-level substrate for supporting CT influence operating under the guidance of ongoing motor activities.

Computational components of visual predictive coding circuitry

If a full visual percept can be said to be a 'hypothesis', so too can a neural 'prediction' - although the latter addresses one particular component of image content (such as 3-dimensional organisation, the interplay between lighting and surface colour, the future trajectory of moving objects, and so on). And, because processing is hierarchical, predictions generated at one level are conveyed in a backward direction to a lower level, seeking to predict, in fact, the neural activity at that prior stage of processing, and learning from errors signalled in the opposite direction. This is the essence of 'predictive coding', at once an algorithm for information processing and a theoretical basis for the nature of operations performed by the cerebral cortex. Neural models for the implementation of predictive coding invoke specific functional classes of neuron for generating, transmitting and receiving predictions, and for producing reciprocal error signals. Also a third general class, 'precision' neurons, tasked with regulating the magnitude of error signals contingent upon the confidence placed upon the prediction, i.e., the reliability and behavioural utility of the sensory data that it predicts. So, what is the ultimate source of a 'prediction'? The answer is multifactorial: knowledge of the current environmental context and the immediate past, allied to memory and lifetime experience of the way of the world, doubtless fine-tuned by evolutionary history too. There are, in consequence, numerous potential avenues for experimenters seeking to manipulate subjects' expectation, and examine the neural signals elicited by surprising, and less surprising visual stimuli. This review focuses upon the predictive physiology of mouse and monkey visual cortex, summarising and commenting on evidence to date, and placing it in the context of the broader field. It is concluded that predictive coding has a firm grounding in basic neuroscience and that, unsurprisingly, there remains much to learn.

The primary visual cortex of Cetartiodactyls: organization, cytoarchitectonics and comparison with perissodactyls and primates

Cetartiodactyls include terrestrial and marine species, all generally endowed with a comparatively lateral position of their eyes and a relatively limited binocular field of vision. To this day, our understanding of the visual system in mammals beyond the few studied animal models remains limited. In the present study, we examined the primary visual cortex of Cetartiodactyls that live on land (sheep, Père David deer, giraffe); in the sea (bottlenose dolphin, Risso’s dolphin, long-finned pilot whale, Cuvier’s beaked whale, sperm whale and fin whale); or in an amphibious environment (hippopotamus). We also sampled and studied the visual cortex of the horse (a closely related perissodactyl) and two primates (chimpanzee and pig-tailed macaque) for comparison. Our histochemical and immunohistochemical results indicate that the visual cortex of Cetartiodactyls is characterized by a peculiar organization, structure, and complexity of the cortical column. We noted a general lesser lamination compared to simians, with diminished density, and an apparent simplification of the intra- and extra-columnar connections. The presence and distribution of calcium-binding proteins indicated a notable absence of parvalbumin in water species and a strong reduction of layer 4, usually enlarged in the striated cortex, seemingly replaced by a more diffuse distribution in neighboring layers. Consequently, thalamo-cortical inputs are apparently directed to the higher layers of the column. Computer analyses and statistical evaluation of the data confirmed the results and indicated a substantial correlation between eye placement and cortical structure, with a markedly segregated pattern in cetaceans compared to other mammals. Furthermore, cetacean species showed several types of cortical lamination which may reflect differences in function, possibly related to depth of foraging and consequent progressive disappearance of light, and increased importance of echolocation.

A Quantitative Comparison of Inhibitory Interneuron Size and Distribution between Mouse and Macaque V1, Using Calcium-Binding Proteins

The mouse is a useful and popular model for studying of visual cortical function. To facilitate the translation of results from mice to primates, it is important to establish the extent of cortical organization equivalence between species and to identify possible differences. We focused on the different types of interneurons as defined by calcium-binding protein (CBP) expression in the layers of primary visual cortex (V1) in mouse and rhesus macaque. CBPs parvalbumin (PV), calbindin (CB), and calretinin (CR) provide a standard, largely nonoverlapping, labeling scheme in macaque, with preserved corresponding morphologies in mouse, despite a slightly higher overlap. Other protein markers, which are relevant in mouse, are not preserved in macaque. We fluorescently tagged CBPs in V1 of both species, using antibodies raised against preserved aminoacid sequences. Our data demonstrate important similarities between the expression patterns of interneuron classes in the different layers between rodents and primates. However, in macaque, expression of PV and CB is more abundant, CR expression is lower, and the laminar distribution of interneuron populations is more differentiated. Our results reveal an integrated view of interneuron types that provides a basis for translating results from rodents to primates, and suggest a reconciliation of previous results.

The Synaptic Organization of Layer 6 Circuits Reveals Inhibition as a Major Output of a Neocortical Sublamina

The canonical cortical microcircuit has principally been defined by interlaminar excitatory connections among the six layers of the neocortex. However, excitatory neurons in layer 6 (L6), a layer whose functional organization is poorly understood, form relatively rare synaptic connections with other cortical excitatory neurons. Here, we show that the vast majority of parvalbumin inhibitory neurons in a sublamina within L6 send axons through the cortical layers toward the pia. These interlaminar inhibitory neurons receive local synaptic inputs from both major types of L6 excitatory neurons and receive stronger input from thalamocortical afferents than do neighboring pyramidal neurons. The distribution of these interlaminar interneurons and their synaptic connectivity further support a functional subdivision within the standard six layers of the cortex. Positioned to integrate local and long-distance inputs in this sublayer, these interneurons generate an inhibitory interlaminar output. These findings call for a revision to the canonical cortical microcircuit.

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.

Most calbindin-immunoreactive neurons, but few calretinin-immunoreactive neurons, express the m1 acetylcholine receptor in the middle temporal visual area of the macaque monkey

INTRODUCTION

Release of the neuromodulator acetylcholine into cortical circuits supports cognition, although its precise role and mechanisms of action are not well understood. Little is known about functional differences in cholinergic modulatory effects across cortical model systems, but anatomical evidence suggests that such differences likely exist because, for example, the expression of cholinergic receptors differs profoundly both within and between species.

METHODS

In the primary visual cortex (V1) of macaque monkeys, cholinergic receptors are strongly expressed by inhibitory interneurons. Using dual-immunofluorescence confocal microscopy, we examine m1 muscarinic acetylcholine receptor expression by two subclasses of inhibitory interneurons-identified by their expression of the calcium-binding proteins calbindin and calretinin-in the middle temporal extrastriate area (MT) of the macaque.

RESULTS AND CONCLUSIONS

We find that the majority of calbindin-immunoreactive neurons (55%) and only few calretinin-immunoreactive neurons (10%) express the m1 acetylcholine receptor. These results differ from the pattern observed in V1 of the same species, lending further support to the notion that cholinergic modulation in the cortex is tuned such that different cortical compartments will respond to acetylcholine release in different ways.

Morphological heterogeneity among corticogeniculate neurons in ferrets: quantification and comparison with a previous report in macaque monkeys

The corticogeniculate (CG) pathway links the visual cortex with the lateral geniculate nucleus (LGN) of the thalamus and is the first feedback connection in the mammalian visual system. Whether functional connections between CG neurons and LGN relay neurons obey or ignore the separation of feedforward visual signals into parallel processing streams is not known. Accordingly, there is some debate about whether CG neurons are morphologically heterogeneous or homogenous. Here we characterized the morphology of CG neurons in the ferret, a visual carnivore with distinct feedforward parallel processing streams, and compared the morphology of ferret CG neurons with CG neuronal morphology previously described in macaque monkeys [Briggs et al. (2016) Neuron, 90, 388]. We used a G-deleted rabies virus as a retrograde tracer to label CG neurons in adult ferrets. We then reconstructed complete dendritic morphologies for a large sample of virus-labeled CG neurons. Quantification of CG morphology revealed three distinct CG neuronal subtypes with striking similarities to the CG neuronal subtypes observed in macaques. These findings suggest that CG neurons may be morphologically diverse in a variety of highly visual mammals in which feedforward visual pathways are organized into parallel processing streams. Accordingly, these results provide support for the notion that CG feedback is functionally parallel stream-specific in ferrets and macaques.

Cornu Ammonis Regions-Antecedents of Cortical Layers?

Studying neocortex and hippocampus in parallel, we are struck by the similarities. All three to four layered allocortices and the six layered mammalian neocortex arise in the pallium. All receive and integrate multiple cortical and subcortical inputs, provide multiple outputs and include an array of neuronal classes. During development, each cell positions itself to sample appropriate local and distant inputs and to innervate appropriate targets. Simpler cortices had already solved the need to transform multiple coincident inputs into serviceable outputs before neocortex appeared in mammals. Why then do phylogenetically more recent cortices need multiple pyramidal cell layers? A simple answer is that more neurones can compute more complex functions. The dentate gyrus and hippocampal CA regions-which might be seen as hippocampal antecedents of neocortical layers-lie side by side, albeit around a tight bend. Were the millions of cells of rat neocortex arranged in like fashion, the surface area of the CA pyramidal cell layers would be some 40 times larger. Even if evolution had managed to fold this immense sheet into the space available, the distances between neurones that needed to be synaptically connected would be huge and to maintain the speed of information transfer, massive, myelinated fiber tracts would be needed. How much more practical to stack the "cells that fire and wire together" into narrow columns, while retaining the mechanisms underlying the extraordinary precision with which circuits form. This demonstrably efficient arrangement presents us with challenges, however, not the least being to categorize the baffling array of neuronal subtypes in each of five "pyramidal layers." If we imagine the puzzle posed by this bewildering jumble of apical dendrites, basal dendrites and axons, from many different pyramidal and interneuronal classes, that is encountered by a late-arriving interneurone insinuating itself into a functional circuit, we can perhaps begin to understand why definitive classification, covering every aspect of each neurone's structure and function, is such a challenge. Here, we summarize and compare the development of these two cortices, the properties of their neurones, the circuits they form and the ordered, unidirectional flow of information from one hippocampal region, or one neocortical layer, to another.

Multiple Transmitter Receptors in Regions and Layers of the Human Cerebral Cortex

We measured the densities (fmol/mg protein) of 15 different receptors of various transmitter systems in the supragranular, granular and infragranular strata of 44 areas of visual, somatosensory, auditory and multimodal association systems of the human cerebral cortex. Receptor densities were obtained after labeling of the receptors using quantitative in vitro receptor autoradiography in human postmortem brains. The mean density of each receptor type over all cortical layers and of each of the three major strata varies between cortical regions. In a single cortical area, the multi-receptor fingerprints of its strata (i.e., polar plots, each visualizing the densities of multiple different receptor types in supragranular, granular or infragranular layers of the same cortical area) differ in shape and size indicating regional and laminar specific balances between the receptors. Furthermore, the three strata are clearly segregated into well definable clusters by their receptor fingerprints. Fingerprints of different cortical areas systematically vary between functional networks, and with the hierarchical levels within sensory systems. Primary sensory areas are clearly separated from all other cortical areas particularly by their very high muscarinic M₂ and nicotinic α₄β₂ receptor densities, and to a lesser degree also by noradrenergic α₂ and serotonergic 5-HT₂ receptors. Early visual areas of the dorsal and ventral streams are segregated by their multi-receptor fingerprints. The results are discussed on the background of functional segregation, cortical hierarchies, microstructural types, and the horizontal (layers) and vertical (columns) organization in the cerebral cortex. We conclude that a cortical column is composed of segments, which can be assigned to the cortical strata. The segments differ by their patterns of multi-receptor balances, indicating different layer-specific signal processing mechanisms. Additionally, the differences between the strata-and area-specific fingerprints of the 44 areas reflect the segregation of the cerebral cortex into functionally and topographically definable groups of cortical areas (visual, auditory, somatosensory, limbic, motor), and reveals their hierarchical position (primary and unimodal (early) sensory to higher sensory and finally to multimodal association areas).

Highlights

Densities of transmitter receptors vary between areas of human cerebral cortex.

Multi-receptor fingerprints segregate cortical layers.

The densities of all examined receptor types together reach highest values in the supragranular stratum of all areas.

The lowest values are found in the infragranular stratum.

Multi-receptor fingerprints of entire areas and their layers segregate functional systems

Cortical types (primary sensory, motor, multimodal association) differ in their receptor fingerprints.

The effect of simultaneous flickering light stimulation on global form and motion perception thresholds

The question regarding the exact function of the primary visual cortex (V1) in vision has been around ever since the description of residual vision after damage to this cortical area by Riddoch in 1917. In 2002, Schoenfeld and colleagues proposed that V1 can be saturated by flashes of light, by which the function of V1-bypassing visual pathways can be "unmasked". The Schoenfeld group found that light flashes applied on stimulus onset led to the elevation of brightness increment detection thresholds, but left motion detection thresholds unaltered. Although the proposed method (i.e. the use of light flashes to induce refractoriness in V1) could be a simple, cheap and elegant way of exploring V1 functions, no study has followed up on this. Therefore it is not known if it works at all with other types of stimuli. For that reason, we decided to revisit the idea in a modified form. Global form and motion perception thresholds were assessed with static Glass pattern stimuli and random dot kinematograms, with and without 12Hz flickering light stimulation. Global motion thresholds were almost unaltered by flickering stimulation, while a significant threshold elevation was caused in the global form perception task. The strongest conclusion allowed by our data is that simultaneous flickering photostimulation elevates global form perception thresholds but not global motion perception thresholds. This is in some way related to the refractoriness generated in an unsatisfactorily defined part of V1. We suggest that this does not necessarily reflect the activity of V1-bypassing pathways, and propose that the application of light flashes is a method that deserves more attention in the exploration of the V1-dependent and independent elements of visual consciousness in human subjects.

Towards a unified scheme of cortical lamination for primary visual cortex across primates: insights from NeuN and VGLUT2 immunoreactivity

Primary visual cortex (V1) is clearly distinguishable from other cortical areas by its distinctive pattern of neocortical lamination across mammalian species. In some mammals, primates in particular, the layers of V1 are further divided into a number of sublayers based on their anatomical and functional characteristics. While these sublayers are easily recognizable across a range of primates, the exact number of divisions in each layer and their relative position within the depth of V1 has been inconsistently reported, largely due to conflicting schemes of nomenclature for the V1 layers. This conflict centers on the definition of layer 4 in primate V1, and the subdivisions of layer 4 that can be consistently identified across primate species. Brodmann’s (1909) laminar scheme for V1 delineates three subdivisions of layer 4 in primates, based on cellular morphology and geniculate inputs in anthropoid monkeys. In contrast, Hässler’s (1967) laminar scheme delineates a single layer 4 and multiple subdivisions of layer 3, based on comparisons of V1 lamination across the primate lineage. In order to clarify laminar divisions in primate visual cortex, we performed NeuN and VGLUT2 immunohistochemistry in V1 of chimpanzees, Old World macaque monkeys, New World squirrel, owl, and marmoset monkeys, prosimian galagos and mouse lemurs, and non-primate, but highly visual, tree shrews. By comparing the laminar divisions identified by each method across species, we find that Hässler’s (1967) laminar scheme for V1 provides a more consistent representation of neocortical layers across all primates, including humans, and facilitates comparisons of V1 lamination with non-primate species. These findings, along with many others, support the consistent use of Hässler’s laminar scheme in V1 research.

Inhibitory interneuron classes express complementary AMPA-receptor patterns in macaque primary visual cortex

Glutamate receptors mediate excitatory neurotransmission. A very prevalent type of glutamate receptor in the neocortex is the AMPA receptor (AMPAR). AMPARs mediate fast synaptic transmission and their functionality depends on the subunit composition. In primary visual cortex (area V1), the density and subunit composition of AMPARs differ among cortical layers and among cell types. The AMPARs expressed by the different types of inhibitory interneurons, which are crucial for network function, have not yet been characterized systematically. We investigated the distribution of AMPAR subunits in macaque V1 for three distinct subpopulations of inhibitory interneurons: parvalbumin-immunoreactive (PV-IR) interneurons, calbindin-immunoreactive (CB-IR) interneurons, and calretinin-immunoreactive (CR-IR) interneurons. We found that PV-IR cells, which have previously been identified as fast spiking, show high expression of the GluA2 and GluA3 subunits. In contrast, CB-IR and CR-IR cells, which tend to be intermediate spiking, show high expression of the GluA1 and GluA4 subunits. Thus, our data demonstrate that the expression of AMPARs divides inhibitory interneurons in macaque V1 into two categories that are compatible with existing classification methods based on calcium-binding proteins and firing behavior. Moreover, our findings suggest new approaches to target the different inhibitory interneuron classes pharmacologically in vivo.

Anatomical constraints on lateral competition in columnar cortical architectures

Competition is a well-studied and powerful mechanism for information processing in neuronal networks, providing noise rejection, signal restoration, decision making and associative memory properties, with relatively simple requirements for network architecture. Models based on competitive interactions have been used to describe the shaping of functional properties in visual cortex, as well as the development of functional maps in columnar cortex. These models require competition within a cortical area to occur on a wider spatial scale than cooperation, usually implemented by lateral inhibitory connections having a longer range than local excitatory connections. However, measurements of cortical anatomy reveal that the spatial extent of inhibition is in fact more restricted than that of excitation. Relatively few models reflect this, and it is unknown whether lateral competition can occur in cortical-like networks that have a realistic spatial relationship between excitation and inhibition. Here we analyze simple models for cortical columns and perform simulations of larger models to show how the spatial scales of excitation and inhibition can interact to produce competition through disynaptic inhibition. Our findings give strong support to the direct coupling effect-that the presence of competition across the cortical surface is predicted well by the anatomy of direct excitatory and inhibitory coupling and that multisynaptic network effects are negligible. This implies that for networks with short-range inhibition and longer-range excitation, the spatial extent of competition is even narrower than the range of inhibitory connections. Our results suggest the presence of network mechanisms that focus on intra-rather than intercolumn competition in neocortex, highlighting the need for both new models and direct experimental characterizations of lateral inhibition and competition in columnar cortex.

Translaminar inhibitory cells recruited by layer 6 corticothalamic neurons suppress visual cortex

In layer 6 (L6), a principal output layer of the mammalian cerebral cortex, a population of excitatory neurons defined by the NTSR1-Cre mouse line inhibit cortical responses to visual stimuli. Here we show that of the two major types of excitatory neurons existing in L6, the NTSR1-Cre line selectively targets those whose axons innervate both cortex and thalamus and not those whose axons remain within the cortex. These corticothalamic neurons mediate widespread inhibition across all cortical layers by recruiting fast-spiking inhibitory neurons whose cell body resides in deep cortical layers yet whose axons arborize throughout all layers. This study reveals a circuit by which L6 modulates cortical activity and identifies an inhibitory neuron able to regulate the strength of cortical responses throughout cortical depth.

Towards a circuit mechanism for movement tuning in motor cortex

The firing rates of neurons in primate motor cortex have been related to multiple parameters of voluntary movement. This finding has been corroborated by stimulation-based studies that have mapped complex movements in rodent and primate motor cortex. However, it has been difficult to link the movement tuning of a neuron with its role within the cortical microcircuit. In sensory cortex, neuronal tuning is largely established by afferents delivering information from tuned receptors in the periphery. Motor cortex, which lacks the granular input layer, may be better understood by analyzing its efferent projections. As a primary source of cortical output, layer 5 neurons represent an ideal starting point for this line of experimentation. It is in these deep output layers that movements can most effectively be evoked by intracortical microstimulation and recordings can obtain the most useful signals for the control of motor prostheses. Studies focused on layer 5 output neurons have revealed that projection identity is a fundamental property related to the laminar position, receptive field and ion channel complement of these cells. Given the variety of brain areas targeted by layer 5 output neurons, knowledge of a neuron's downstream connectivity may provide insight into its movement tuning. Future experiments that relate motor behavior to the activity of neurons with a known projection identity will yield a more detailed understanding of the function of cortical microcircuits.

Morphology of superior colliculus- and middle temporal area-projecting neurons in primate primary visual cortex

Layers 5 and 6 of primate primary visual cortex (V1) harbor morphologically diverse cell groups that have corticocortical and corticosubcortical projections. Layer 6 middle temporal area (MT)-projecting neurons are particularly interesting, as they are the only deep-layer cortical neurons that provide both corticocortical feedforward inputs (to area MT) and corticosubcortical feedback projections (to superior colliculus [SC]) (Fries et al. [1985] Exp Brain Res 58:613-616). However, due to limitations in anatomical tracing techniques, little is known about the detailed morphologies of these cells. We therefore applied a genetically modified rabies virus as a retrograde tracer to fill the dendrites of projection neurons with green fluorescent protein (GFP) (Wickersham et al. [2007] Nat Methods 4:47-49). We injected virus into SC or area MT of macaque monkeys and examined labeled cells in V1. Two-thirds of labeled neurons following SC injections were found in layer 5, consisting of "tall-tufted" and "nontufted" cells; the remaining cells were layer 6 "nontufted." Area MT injections labeled neurons in layers 4B and 6, as previously described (Shipp and Zeki [1989] Eur J Neurosci 1:309-332). The layer 6 neurons projecting to MT were remarkably similar to the layer 6 SC-projecting neurons. In contrast to the dense and focused dendritic arbors of layer 5 "tall-tufted" pyramids, all "nontufted" cells had sparse, but unusually long basal dendrites. The nontufted cells closely resemble Meynert cells (le Gros Clark [1942] J Anat 76:369-376; Winfield et al. [1983] Proc R Soc Lond B Biol Sci 217:129-139), but the full in vivo reconstructions presented here show that their basal dendrites can extend much further (up to 1.2 mm) and are less asymmetric than inferred from Golgi reconstructions.

Neocortical layer 6, a review

This review attempts to summarise some of the major areas of neocortical research as it pertains to neocortical layer 6. After a brief summary of the development of this intriguing layer, the major pyramidal cell classes to be found in layer 6 are described and compared. The connections made and received by these different classes of neurones are then discussed and the possible functions of these connections, with particular reference to the shaping of responses in visual cortex and thalamus. Inhibition in layer 6 is discussed where appropriate, but not in great detail. Many types of interneurones are to be found in each cortical layer and layer 6 is no exception, but the functions of each type remain to be elucidated (Gonchar et al., 2007).

Neocortical layer 6, a review

This review attempts to summarise some of the major areas of neocortical research as it pertains to neocortical layer 6. After a brief summary of the development of this intriguing layer, the major pyramidal cell classes to be found in layer 6 are described and compared. The connections made and received by these different classes of neurones are then discussed and the possible functions of these connections, with particular reference to the shaping of responses in visual cortex and thalamus. Inhibition in layer 6 is discussed where appropriate, but not in great detail. Many types of interneurones are to be found in each cortical layer and layer 6 is no exception, but the functions of each type remain to be elucidated (Gonchar et al., 2007).

Organizing principles of cortical layer 6

Neurons in the deepest layer of mammalian cerebral cortex are morphologically and physiological diverse and are situated in a strategic position to modulate neuronal activity locally and in other structures. The variety of neuronal circuits within which layer 6 neurons participate differs across species and cortical regions. However even amidst this diversity, common organizational features emerge. Examination of the anatomical and physiological characteristics of different classes of layer 6 neuron, each specialized to participate in distinct circuits, provides insight into the functional contributions of layer 6 neurons toward cortical information processing.

Inter- and Intralaminar Subcircuits of Excitatory and Inhibitory Neurons in Layer 6a of the Rat Barrel Cortex

Approximately half the excitatory neurons in layer 6 (L6) of the rat barrel cortex project to the thalamus with axon collaterals ramifying in the granular L4; the remaining project within cortex with collaterals restricted to infragranular laminae. In analogy, L6 inhibitory neurons also include locally arborizing and inter-laminar projecting neurons. We examined whether L6 neurons participating in different laminar interactions were also morphologically and electrically distinct. Corticothalamic (CT) neurons were labeled by in vivo injections of a retrogradely transported fluorescent tracer into the primary thalamic nucleus. Whole cell current-clamp recordings were performed from labeled and unlabeled L6 neurons in brain slices of juvenile rats; the morphology of cells was subsequently recovered and reconstructed. Corticocortical (CC) neurons were distinguished from CT cells based on the absence of a subcortical projection and the predominantly infragranular arborization of their axon collaterals. Two morphological CC subtypes could be further distinguished based on the structure of their apical dendrite. Electrically, CT neurons had shorter membrane time-constants and action potential (AP) durations and higher rheobase currents. CC neurons fired high-frequency spike doublets or triplets on sustained depolarization; the burst frequency also distinguished the two morphological CC subtypes. Among inhibitory L6 cells, the L4-projecting (L6iL4) and local (L6iL6) inhibitory neurons also had contrasting firing properties; L6iL4 neurons had broader APs and lower maximal firing rates. We propose that L6 excitatory and inhibitory neurons projecting to L4 constitute specialized subcircuits distinct from the infragranular network in their connectivity and firing patterns.

The processing of feature discontinuities for different cue types in primary visual cortex

This study examines whether neurons in the primary visual cortex (V1) of the cat (also referred to as area 17) are sensitive to boundaries that are delineated by a difference in features other than luminance contrast. Most research on this issue has concentrated on the responses to texture borders (e.g. 'illusory contours') and has found neurons that are sensitive to such borders in V2 and to a lesser extent in V1. Here neurons in cat area 17 (V1) were exposed to borders that were oblique to the orientation preference of the neuron and that were created by differences in phase, orientation or direction of motion of two drifting sinewave gratings. Nearby phase borders evoked increased firing in 15 out of 98 neurons, orientation borders in 18 out of 98, and direction borders in 15 out of 70 neurons recorded in area 17 (V1) of anesthetized cats. The firing rates of these neurons were enhanced when a feature border was presented close to their receptive field, partly independent of the cue involved. Control experiments with a contrast border showed that the enhanced firing was due to a release of suppression rather than facilitation. A conceptual model is presented that can describe the data and uncovers a peculiarity of the phase domain compared to the orientation and direction domain. The model unifies the knowledge gained here about orientation-specific center-surround interactions, contextual effects, and end-stopping. The data and model suggest that these phenomena are part of a single mechanism that enables the brain to detect feature discontinuities across a range of features.

Telencephalon: Neocortex

The neocortex is an ultracomplex, six-layered structure that develops from the dorsal palliai sector of the telencephalic hemispheres (Figs. 2.24, 2.25, 11.1). All mammals, including monotremes and marsupials, possess a neocortex, but in reptiles, i.e. the ancestors of mammals, only a three-layered neocortical primordium is present [509, 511]. The term neocortex refers to its late phylogenetic appearance, in comparison to the “palaeocortical” olfactory cortex and the “archicortical” hippocampal cortex, both of which are present in all amniotes [509].

Functional maps of neocortical local circuitry

This review aims to summarize data obtained with different techniques to provide a functional map of the local circuit connections made by neocortical neurones, a reference for those interested in cortical circuitry and the numerical information required by those wishing to model the circuit. A brief description of the main techniques used to study circuitry is followed by outline descriptions of the major classes of neocortical excitatory and inhibitory neurones and the connections that each layer makes with other cortical and subcortical regions. Maps summarizing the projection patterns of each class of neurone within the local circuit and tables of the properties of these local circuit connections are provided.This review relies primarily on anatomical studies that have identified the classes of neurones and their local and long distance connections and on paired intracellular and whole-cell recordings which have documented the properties of the connections between them. A large number of different types of synaptic connections have been described, but for some there are only a few published examples and for others the details that can only be obtained with paired recordings and dye-filling are lacking. A further complication is provided by the range of species, technical approaches and age groups used in these studies. Wherever possible the range of available data are summarised and compared. To fill some of the more obvious gaps for the less well-documented cases, data obtained with other methods are also summarized.

Reelin immunoreactivity and morphological analysis of the human visual cortex

Reelin is a secreted glycoprotein with a crucial role in development of the Central Nervous System. In adults, its function remains unclear but it may be involved in the modulation of synaptic plasticity. Having in mind this possible property of reelin, the authors decided to study the distribution of reelin immunoreactivity in the neurons of the human adult primary visual cortex and compare the findings with morphological analysis (Golgi method) of neuronal networks. The distribution of reelin in the primary visual cortex is different from other cortical area examined; reelin is mostly present in the neurons of second and sixth layer.

Loose-patch–juxtacellular recording in vivo—A method for functional characterization and labeling of neurons in macaque V1

We describe a method that uses a modified version of juxtacellular labeling [Pinault D. A novel single-cell staining procedure performed in vivo under electrophysiological control: morpho-functional features of juxtacellularly labeled thalamic cells and other central neurons with biocytin or neurobiotin. J Neurosci Meth 1996;65:113-36], which allows us to functionally characterize and subsequently label single neurons in vivo in macaque V1. The method is generally applicable in acute in vivo preparations. Extracellular recording is made with a patch electrode when the electrode is attached to the cell membrane. Initially a 'blind' search method is used as a guide to obtaining a cell attached configuration that we refer to as a loose-patch (LP). The neuron's receptive field properties are functionally characterized, the neuron is labeled and then characterization is confirmed, all in the LP configuration. There are a number of advantages of the method that we describe over other methods. First, we have found that we can obtain stable extracellular recordings for periods of hours that enable us to make a relatively comprehensive visual functional characterization of a neuron's receptive field properties. Second, because the electrode is closely apposed to the cell we obtain excellent isolation of the extracellular spike. Third, the method provides labeling that gives complete dendritic and axonal filling that survives over a number of days, which is an important feature in acute primate experiments. Fourth, the in vivo method of labeling and reconstructing neurons gives complete three-dimensional structure of the neuron including its intra-cortical axonal arbor. These features overcome known limits of the established methods of studying neuronal morphology including the Golgi stain (limited when adult tissue is used) and in vitro whole cell methods (incomplete axonal filling due to limited slice thickness). They also overcome the known limits of the established method of combined function-morphology studies i.e. intracellular recording in vivo. The modified juxtacellular method provides a reliable alternative to the difficult method of characterization by extracellular recording and subsequent intracellular labeling [Anderson JC, Martin KAC, Whitteridge D. Form, function and intracortical projections of neurons in the striate cortex of the monkey Macacus nemestrinus. Cerebral Cortex 1993;3:412-20]. We show the method can be used to record at a range of depths through V1 cortex allowing for sampling of neurons in the different layers and functional subpopulations. Links can then be made with existing knowledge about the anatomical organization of V1, the various morphological classes of neurons found therein, their functional connectivity and visual response properties.

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.

On the fight between excitation and inhibition: location is everything

In the intact brain, neurons are constantly subjected to both excitatory and inhibitory inputs to their dendritic trees. Although it is accepted that the overall response of a neuron--its train of output spikes--depends on the balance of excitation and inhibition, we continue to lack specific knowledge of the rules that govern how excitatory and inhibitory inputs interact in space and time within the confines of individual neurons. In a recent paper, Liu starts by providing evidence that the relative locations and numbers of excitatory and inhibitory synapses are tightly regulated in cultured neurons from the hippocampus. This is consistent with findings in other labs that suggest neurons work hard, and in a variety of different ways, to maintain their inputs in proper balance and their outputs within appropriate ranges. On this backdrop, Liu's most important finding of a functional nature is that inhibition appears to act quite locally; that is, an inhibitory synapse effectively opposes an excitatory synapse only when it is very close by within the same dendritic branch (Fig. 1). This finding provides further support for the view--anticipated by neural theorists more than 20 years ago--that the brain's principal neurons contain a potentially large number of separate computational subunits.

Laminar patterns of local excitatory input to layer 5 neurons in macaque primary visual cortex

Layer 5 neurons in primary visual cortex make putative reciprocal feedback connections to the superficial layers. To test this hypothesis, we employed scanning laser photostimulation combined with intracellular dye injection to examine local functional excitatory inputs to and axonal projections from individual layer 5 neurons in brain slices from monkey V1. In contrast with previous studies of other V1 neurons, layer 5 neurons received significant input from nearly all of the cortical layers, suggesting individual layer 5 cells integrate information from a broad range of input sources. Nevertheless relative strengths of laminar inputs varied across neurons. Cluster analysis of relative strength of laminar inputs to individual layer 5 neurons revealed four discrete clusters representing recurring input patterns; each cluster included both excitatory and inhibitory neurons. Twenty-five of 40 layer 5 neurons fell into two clusters, both characterized by very strong input from superficial layers. These input patterns are consistent with layer 5 neurons providing feedback to superficial layers. The remaining 15 neurons received stronger input from deep layers. Differences in input from layer 4Calpha versus 4Cbeta also suggest specific associations of the magnocellular and parvocellular visual pathways, with populations receiving stronger input from deep versus superficial cortical layers.

Axon collaterals of Meynert cells diverge over large portions of area V1 in the macaque monkey

Patchy intrinsic connections, originating mainly from horizontal collaterals of pyramidal neurons, have been demonstrated in area V1 and many other cortical areas. In this article, we identify a network of intrinsic connections concentrated in layer 6 of area V1. These are visualized by extracellular injections of anterograde tracers in V1, which label small clusters of large terminal boutons in layer 6, in conjunction with thick axon segments. These segments can be traced back to infragranular Meynert cells (n = 10), which are retrogradely labeled from the injections. By using serial section analysis, we identified the following features of this distinctive system of Meynert cell collaterals: (1) terminal clusters are relatively small (<100 microm); (2) each cluster has a small number of rather large boutons (up to 3.0 microm); (3) there is typically a termination-free zone in the immediate vicinity (0.5-2.0 mm) of the cell body; (4) a single neuron has multiple branches that can extend up to 8.0 mm from the soma; and (5) the collaterals are concentrated in layer 6. These features are different from those of horizontal intrinsic connections in the supragranular layers of area V1. They are consistent with fast dynamics and a possible role in wide-field motion processing, such as has been associated with Meynert cells from other studies.

Vertical bias in dendritic trees of non-pyramidal neocortical neurons expressing GAD67-GFP in vitro

The neocortical neuropil has a strong vertical (orthogonal to pia) orientation, constraining the intracortical flow of information and forming the basis for the functional parcellation of the cortex into semi-independent vertical columns or 'modules'. Apical dendrites of excitatory pyramidal neurons are a major component of this vertical neuropil, but the extent to which inhibitory, GABAergic neurons conform to this structural and functional design is less well documented. We used a gene gun to transfect organotypic slice cultures of mouse and rat neocortex with the enhanced green fluorescent protein (eGFP) gene driven by the promoter for glutamic acid decarboxylase 67 (GAD67), an enzyme expressed exclusively in GABAergic cells. Many GAD67-GFP expressing cells were highly fluorescent, and their dendritic morphologies and axonal patterns, revealed in minute detail, were characteristic of GABAergic neurons. We traced 150 GFP-expressing neurons from confocal image stacks, and estimated the degree of vertical bias in their dendritic trees using a novel computational metric. Over 70% of the neurons in our sample had dendritic trees with a highly significant vertical bias. We conclude that GABAergic neurons make an important contribution to the vertical neocortical neuropil, and are likely to integrate synaptic inputs from axons terminating within their own module.

Layer V in cat primary auditory cortex (AI): cellular architecture and identification of projection neurons

The cytoarchitectonic organization and the structure of layer V neuronal populations in cat primary auditory cortex (AI) were analyzed in Golgi, Nissl, immunocytochemical, and plastic-embedded preparations from mature specimens. The major cell types were characterized as a prelude to identifying their connections with the thalamus, midbrain, and cerebral cortex using axoplasmic transport methods. The goal was to describe the structure and connections of layer V neurons more fully. Layer V has three sublayers based on the types of neuron and their sublaminar projections. Four types of pyramidal and three kinds of nonpyramidal cells were present. Classic pyramidal cells had a long apical dendrite, robust basal arbors, and an axon with both local and corticofugal projections. Only the largest pyramidal cell apical dendrites reached the supragranular layers, and their somata were found mainly in layer Vb. Three types departed from the classic pattern; these were the star, fusiform, and inverted pyramidal neurons. Nonpyramidal cells ranged from large multipolar neurons with radiating dendrites, to Martinotti cells, with smooth dendrites and a primary trunk oriented toward the white matter. Many nonpyramidal cells were multipolar, of which three subtypes (large, medium, and small) were identified; bipolar and other types also were seen. Their axons formed local projections within layer V, often near pyramidal neurons. Several features distinguish layer V from other layers in AI. The largest pyramidal neurons were in layer V. Layer V neuronal diversity aligns it with layer VI (Prieto and Winer [1999] J. Comp. Neurol. 404:332--358), and it is consistent with the many connectional systems in layer V, each of which has specific sublaminar and neuronal origins. The infragranular layers are the source for several parallel descending systems. There were significant differences in somatic size among these projection neurons. This finding implies that diverse corticofugal roles in sensorimotor processing may require a correspondingly wide range of neuronal architecture.

Contrast-sensitive perceptual grouping and object-based attention in the laminar circuits of primary visual cortex

Recent neurophysiological studies have shown that primary visual cortex, or V1, does more than passively process image features using the feedforward filters suggested by Hubel and Wiesel. It also uses horizontal interactions to group features preattentively into object representations, and feedback interactions to selectively attend to these groupings. All neocortical areas, including V1, are organized into layered circuits. We present a neural model showing how the layered circuits in areas V1 and V2 enable feedforward, horizontal, and feedback interactions to complete perceptual groupings over positions that do not receive contrastive visual inputs, even while attention can only modulate or prime positions that do not receive such inputs. Recent neurophysiological data about how grouping and attention occur and interact in V1 are simulated and explained, and testable predictions are made. These simulations show how attention can selectively propagate along an object grouping and protect it from competitive masking, and how contextual stimuli can enhance or suppress groupings in a contrast-sensitive manner.

A neuronal network model of macaque primary visual cortex (V1): orientation selectivity and dynamics in the input layer 4Calpha

In this paper, we offer an explanation for how selectivity for orientation could be produced by a model with circuitry that is based on the anatomy of V1 cortex. It is a network model of layer 4Calpha in macaque primary visual cortex (area V1). The model consists of a large number of integrate-and-fire conductance-based point neurons, both excitatory and inhibitory, which represent dynamics in a small patch of 4Calpha-1 mm(2) in lateral area-which contains four orientation hypercolumns. The physiological properties and coupling architectures of the model are derived from experimental data for layer 4Calpha of macaque. Convergent feed-forward input from many neurons of the lateral geniculate nucleus sets up an orientation preference, in a pinwheel pattern with an orientation preference singularity in the center of the pattern. Recurrent cortical connections cause the network to sharpen its selectivity. The pattern of local lateral connections is taken as isotropic, with the spatial range of monosynaptic excitation exceeding that of inhibition. The model (i) obtains sharpening, diversity in selectivity, and dynamics of orientation selectivity, each in qualitative agreement with experiment; and (ii) predicts more sharpening near orientation preference singularities.

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.

Extreme diversity among amacrine cells: implications for function

We report a quantitative survey of the population of amacrine cells present in the retina of the rabbit. The cells' dendritic shape and level of stratification were visualized by a photochemical method in which a fluorescent product was created within an individual cell by focal irradiation of that cell's nucleus. A systematically random sample of 261 amacrine cells was examined. Four previously known amacrine cells were revealed at their correct frequencies. Our central finding is that the heterogeneous collection of other amacrine cells is broadly distributed among at least 22 types: only one type of amacrine cell makes up more than 5% of the total amacrine cell population. With these results, the program of identification and classification of retinal neurons begun by Cajal is nearing completion. The complexity encountered has implications both for the retina and for the many regions of the central nervous system where less is known.

Local circuits in primary visual cortex of the macaque monkey

The basic laminar organization of excitatory local circuitry in the primary visual cortex of the macaque monkey is similar to that described previously in the cat's visual cortex (Gilbert 1983). This circuitry is described here in the context of a two-level model that distinguishes between feedforward and feedback connections. Embedded within this basic framework is a more complex organization. Within the strictly feedforward pathway, these circuits distribute unique combinations of magno-, parvo-, and koniocellular input from the lateral geniculate nucleus (LGN) to neurons in layers 2-4B. Their input is dependent on the extrastriate cortical areas they target. The local feedback connections from deep layers (5 and 6) arise from a diverse population of pyramidal neurons. Each type forms local connections with a unique relationship to more superficial layers. In the case of layer 6 neurons, these connections are closely related to layer 4 subdivisions receiving input from different functional streams.

The number of unidentified amacrine cells in the mammalian retina

The three largest known populations of amacrine cells in the rabbit retina were stained with fluorescent probes in whole mounts and counted at a series of retinal eccentricities. The retinas were counterstained using a fluorescent DNA-binding molecule and the total number of nuclei in the inner nuclear layer were counted in confocal sections. From the total number of inner nuclear layer cells and the known fraction of them occupied by amacrine cells, the fraction of amacrine cells made up by the stained populations could be calculated. Starburst cells made up 3%, indoleamine-accumulating cells made up 4%, and AII cells made up 11% of all amacrine cells. By referring four smaller populations of amacrine cells to the number of indoleamine-accumulating cells, they were estimated to make up 4% of all amacrine cells. Thus, 78% of all amacrine cells in the rabbit's retina are known only from isolated examples, if at all. This proportion is similar in the retinas of the mouse, cat, and monkey. It is likely that a substantial fraction of the local circuit neurons present in other regions of the central nervous system are also invisible as populations to current techniques.

The hierarchical development of monkey visual cortical regions as revealed by the maturation of parvalbumin-immunoreactive neurons

The prefrontal cortex is known to be involved in behavioral paradigms requiring decisions based on short-term working memory, and visually related areas of prefrontal cortex represent the final point in a proposed hierarchical sequence of visual signal processing that begins in primary visual cortex. This study asks if the development of at least certain aspects of the circuitry of each region involved in this hierarchy proceeds in a sequential fashion from primary to higher-order areas. The timing and patterns of expression of immunoreactivity for the calcium-binding protein parvalbumin were examined in areas V1, V2, TE, 7a, and 46 in two series of macaque monkeys ranging in age from embryonic day 132 to adult. The number and laminar distribution of parvalbumin-labeled neurons reached adult levels first in area V1 (primary visual cortex), followed by the adjacent visual association area V2, and then by the higher-order regions of the inferior temporal (TE), posterior parietal (7a) and prefrontal (46) cortices. The appearance of parvalbumin immunoreactivity in the axons of the two major classes of local circuit neurons that express this protein, basket and chandelier cells, followed a similar regional pattern. Furthermore, striking differences were present between these two neuronal populations in the laminar pattern and time course of parvalbumin labeling of their axons. These findings demonstrate that at least some aspects of the intrinsic circuitry of the neocortex mature in accordance with a functional hierarchy of cortical regions. In addition, they illustrate the complexity of cortical development in terms of the different timing of expression of even a single protein in different compartments within single neurons, in different cell types, in different laminae within a region, and across different cortical regions.

The neocortex. An overview of its evolutionary development, structural organization and synaptology

By way of introduction, an outline is presented of the origin and evolutionary development of the neocortex. A cortical formation is lacking in amphibians, but a simple three-layered cortex is present throughout the pallium of reptiles. In mammals, two three-layered cortical structures, i.e. the prepiriform cortex and the hippocampus, are separated from each other by a six-layered neocortex. Still small in marsupials and insectivores, this "new" structure attains amazing dimensions in anthropoids and cetaceans. Neocortical neurons can be allocated to one of two basic categories: pyramidal and nonpyramidal cells. The pyramidal neurons form the principal elements in neocortical circuitry, accounting for at least 70% of the total neocortical population. The evolutionary development of the pyramidal neurons can be traced from simple, "extraverted" neurons in the amphibian pallium, via pyramid-like neurons in the reptilian cortex to the fully developed neocortical elements designated by Cajal as "psychic cells". Typical mammalian pyramidal neurons have the following eight features in common: (1) spiny dendrites, (2) a stout radially oriented apical dendrite, forming (3) a terminal bouquet in the most superficial cortical layer, (4) a set of basal dendrites, (5) an axon descending to the subcortical white matter, (6) a number of intracortical axon collaterals, (7) terminals establishing synaptic contacts of the round vesicle/asymmetric variety, and (8) the use of the excitatory aminoacids glutamate and/or aspartate as their neurotransmitter. The pyramidal neurons constitute the sole output and the largest input system of the neocortex. They form the principal targets of the axon collaterals of other pyramidal neurons, as well as of the endings of the main axons of cortico-cortical neurons. Indeed, the pyramidal neurons constitute together a continuous network extending over the entire neocortex, justifying the generalization: the neocortex communicates first and foremost within itself. The typical pyramidal neurons represent the end stage of a progressive evolutionary process. During further development many of these elements have become transformed by reduction into various kinds of atypical or aberrant pyramidal neurons. Interestingly, none of the six morphological characteristics, mentioned above under 1-6, has appeared to be unassailable; pyramidal neurons lacking spines, apical dendrites, long axons and intracortical axon collaterals etc. have all been described. From an evolutionary point of view the typical pyramidal neurons represent not only the principal neocortical elements, but also the source of various excitatory local circuit neurons. The spiny stellate cells, which are abundant in highly specialized primary sensory areas, form a remarkable case in point.(ABSTRACT TRUNCATED AT 400 WORDS)

Spatial scale dependent in-phase and anti-phase directional biases in the perception of self-organized motion patterns

A long row of evenly spaced dots is displaced on successive frames by half the distance between the dots. Although these stimuli are directionally ambiguous, spatially and temporally coherent unidirectional and oscillatory motion patterns are perceived as a result of the temporal persistence of competing in-phase and anti-phase directional biases, respectively. The perceiver's spatial scale is critical is determining whether dots are near enough to favor an in-phase bias or far enough apart to favor an anti-phase bias. The results are explained by a differential-gradient model of cooperative interaction, which specifies that the strength of facilitating (excitatory) interactions among motion detectors with similar directional selectivity falls off with distance at a greater rate than the strength of competing inhibiting interactions.

Differential localization of two glutamic acid decarboxylases (GAD65 and GAD67) in adult monkey visual cortex

Adult monkey primary visual cortex contains a diverse population of stellate neurons that utilize the neurotransmitter gamma aminobutyric acid (GABA). Two glutamic acid decarboxylase (GAD) enzymes that synthesize GABA, GAD65 and GAD67, were localized within these stellate neurons by in situ hybridization of 35S or digoxigenin (DIG) labeled riboprobes. Double labels were done by using 35S GAD67 riboprobe and GABA immunocytochemistry on the same section to verify that the neuronal population identified by immunocytochemistry was the same one studied in the in situ hybridization experiments. We find that GAD65 mRNA and GAD67 mRNA are widely distributed in the cortex, with four bands of heavily labeled neurons in upper layer 2, lower 3, 4C, and 6. GAD67 labeled neurons were more obvious in layer 4C beta, while GAD65 containing neurons were common in layer 1 and white matter. Northern blots and in situ hybridization on sections with both 35S and DIG riboprobes indicate that cortical neurons typically contain more GAD67 mRNA. Cell counts show that 18% of all cortical neurons contain GAD67 mRNA and 13% contain GAD65 mRNA, suggesting that a small population of GABA neurons might lack GAD65. Cell bodies that contain high amounts of GAD65 mRNA are prominent in layers deep 3, 4B, 4C alpha, and 6 and often are the largest cells in their respective layers. Double labels demonstrate that 96% of all GABA+ neurons contain GAD67 mRNA. Neurons heavily labeled for GABA tend to have smaller cell bodies and contain less GAD67 mRNA, while lightly labeled GABA neurons are larger and contain more GAD67 mRNA. These data indicate that most GABA neurons in monkey striate cortex contain both GAD enzymes. Although the differences in GABA content, cell size, laminar distribution, and GAD mRNA concentration suggest different requirements for GAD67 and GAD65 in cortical circuits, our experiments do not reveal what different roles these two enzymes subserve within GABAergic stellate neurons.

Independence and merger of thalamocortical channels within macaque monkey primary visual cortex: anatomy of interlaminar projections

An important issue in understanding the function of primary visual cortex in the macaque monkey is how the several efferent neuron groups projecting to extrastriate cortex acquire their different response properties. To assist our understanding of this issue, we have compared the anatomical distribution of V1 intrinsic relays that carry information derived from magno- (M) and parvocellular (P) divisions of the dorsal lateral geniculate nucleus between thalamic recipient neurons and interareal efferent neuron groups within area V1. We used small, iontophoretic injections of biocytin placed in individual cortical laminae of area V1 to trace orthograde and retrograde inter- and intralaminar projections. In either the same or adjacent sections, the tissue was reacted for cytochrome oxidase (CO), which provides important landmarks for different efferent neuron populations located in CO rich blobs and CO poor interblobs in laminae 2/3, as well as defining clear boundaries for the populations of efferent neurons in laminae 4A and 4B. This study shows that the interblobs, but not the blobs, receive direct input from thalamic recipient 4C neurons; the interblobs receive relays from mid 4C neurons (believed to receive convergent M and P inputs), while blobs receive indirect inputs from either M or P (or both) pathways through layers 4B (which receives M relays from layer 4C alpha) and 4A (which receives P relays directly from the thalamus as well as from layer 4C beta). The property of orientation selectivity, most prominent in the interblob regions and in layer 4B, may have a common origin from oriented lateral projections made by mid 4C spiny stellate neurons. While layer 4B efferents may emphasize M characteristics and layer 4A efferents emphasize P characteristics, the dendrites of their constituent pyramidal neurons may provide anatomical access to the other channel since both blob and interblob regions in layers 2/3 have anatomical access to M and P driven relays, despite functional differences in the way these properties may be expressed in the two compartments.

The postnatal development of geniculocortical axon arbors in owl monkeys

To characterize the postnatal development of geniculocortical axon arbor morphology in owl monkeys at a series of ages from birth to adulthood, individual arbors were bulk-filled with HRP in brain slice preparations and were reconstructed from serial sections. At all ages, cortical layers and sublayers were obvious. Presumed M or magnocellular arbors were largely confined to layer IV alpha, but they also extended into layer IIIc (IVB of Brodmann, 1909); presumed P or parvocellular arbors were almost exclusively confined to layer IV beta. Other axons that may reflect feedback projections from MT terminated in layer IIIc. Overall, M axon arbors increased in size and complexity from birth to adulthood with mean surface-view arbor areas ranging from 0.08 +/- 0.01 mm2 in newborns to 0.24 +/- 0.02 mm2 in adults. The developing P arbor areas were, on average, as large or larger than adult (newborn = 0.07 +/- 0.01 mm2, adult = 0.047 +/- 0.01 mm2; n.s.) but the arbors were somewhat less complex. Since the brain and area 17 increase in size postnatally, the proportion of area 17 subserved by each P arbor would decrease in postnatal development. Terminal boutons with immature features were evident in both M and P populations at all developmental ages. The results indicate that, while both LGN axon types in monkeys undergo morphological changes postnatally, M arbors appear to mature by increasing arbor size and terminal branching complexity, whereas P arbors increase in complexity but not in size. These distinct programs of axon arbor development suggest that the periods of susceptibility of geniculocortical axon arbors to postnatal influences of the environment, and the types of plastic responses they potentially exhibit, are class-specific.