Polyneuronal innervation of spiny stellate neurons in cat visual cortex

The thalamus as a monitor of motor outputs

Many of the ascending pathways to the thalamus have branches involved in movement control. In addition, the recently defined, rich innervation of 'higher' thalamic nuclei (such as the pulvinar) from pyramidal cells in layer five of the neocortex also comes from branches of long descending axons that supply motor structures. For many higher thalamic nuclei the clue to understanding the messages that are relayed to the cortex will depend on knowing the nature of these layer five motor outputs and on defining how messages from groups of functionally distinct output types are combined as inputs to higher cortical areas. Current evidence indicates that many and possibly all thalamic relays to the neocortex are about instructions that cortical and subcortical neurons are contributing to movement control. The perceptual functions of the cortex can thus be seen to represent abstractions from ongoing motor instructions.

Target and temporal pattern selection at neocortical synapses

We attempt to summarize the properties of cortical synaptic connections and the precision with which they select their targets in the context of information processing in cortical circuits. High-frequency presynaptic bursts result in rapidly depressing responses at most inputs onto spiny cells and onto some interneurons. These 'phasic' connections detect novelty and changes in the firing rate, but report frequency of maintained activity poorly. By contrast, facilitating inputs to interneurons that target dendrites produce little or no response at low frequencies, but a facilitating-augmenting response to maintained firing. The neurons activated, the cells they in turn target and the properties of those synapses determine which parts of the circuit are recruited and in what temporal pattern. Inhibitory interneurons provide both temporal and spatial tuning. The 'forward' flow from layer-4 excitatory neurons to layer 3 and from 3 to 5 activates predominantly pyramids. 'Back' projections, from 3 to 4 and 5 to 3, do not activate excitatory cells, but target interneurons. Despite, therefore, an increasing complexity in the information integrated as it is processed through these layers, there is little 'contamination' by 'back' projections. That layer 6 acts both as a primary input layer feeding excitation 'forward' to excitatory cells in other layers and as a higher-order layer with more integrated response properties feeding inhibition to layer 4 is discussed.

Excitatory inputs to spiny cells in layers 4 and 6 of cat striate cortex

The principal target of lateral geniculate nucleus in the cat visual cortex is the stellate neurons of layer 4. In previously reported work with intracellular recording and extracellular stimulation in slices of visual cortex, three general classes of fast excitatory synaptic potentials (EPSPs) in layer 4a spiny stellate neurons were identified. One of these classes, characterized by large and relatively invariant amplitudes (mean 1.7 mV, average coefficient of variation (CV) 0.083) were attributed to the action of geniculate axons because, unlike the other two classes, they could not be matched by intracortical inputs, using paired recording. We have examined in detail the properties of this synaptic input in twelve examples, selecting for study those EPSPs where there was secure extracellular stimulation of the single fibre input to a pair of stimuli 50 ms apart. In our analysis, we conclude that the depression that these inputs show to the second stimulus is entirely postsynaptic, since the evidence strongly suggests that the probability of transmitter release at the synaptic site(s) remains 1.0 for both stimuli. We argue that the most plausible explanation for this postsynaptic depression is a reduction in the average probability of opening the synaptic channels. Using a simple biochemical analysis (c.f. Sigworth plot), it is then possible to calculate the number of synaptic channels and their probability of opening, for each of the 12 connections. The EPSPs had a mean amplitude of 1.91 mV (+/- 1.3 mV SD) and a mean CV of 0.067 (+/- 0.022). The calculated number of channels ranged from 20 to 158 (59.4 +/- 48.7) and their probability of opening to the first EPSP had an average of 0.83 (+/- 0.09), with an average depression of the probability to 0.60 for the second EPSP. Geniculate afferents also terminate in layer 6. Intracellular recordings were also made in the upper part of this layer and a total of 51 EPSPs were recorded from pyramidal cells of three principal types. Amongst this dataset we sought EPSPs with similar properties to those characterized in layer 4a. Three examples were found, which is a much lower percentage (6%) than the incidence of putative geniculate EPSPs found in layer 4a (42%).

The role of the thalamus in the flow of information to the cortex

The lateral geniculate nucleus is the best understood thalamic relay and serves as a model for all thalamic relays. Only 5-10% of the input to geniculate relay cells derives from the retina, which is the driving input. The rest is modulatory and derives from local inhibitory inputs, descending inputs from layer 6 of the visual cortex, and ascending inputs from the brainstem. These modulatory inputs control many features of retinogeniculate transmission. One such feature is the response mode, burst or tonic, of relay cells, which relates to the attentional demands at the moment. This response mode depends on membrane potential, which is controlled effectively by the modulator inputs. The lateral geniculate nucleus is a first-order relay, because it relays subcortical (i.e. retinal) information to the cortex for the first time. By contrast, the other main thalamic relay of visual information, the pulvinar region, is largely a higher-order relay, since much of it relays information from layer 5 of one cortical area to another. All thalamic relays receive a layer-6 modulatory input from cortex, but higher-order relays in addition receive a layer-5 driver input. Corticocortical processing may involve these corticothalamocortical 're-entry' routes to a far greater extent than previously appreciated. If so, the thalamus sits at an indispensable position for the modulation of messages involved in corticocortical processing.

Thalamocortical bursts trigger recurrent activity in neocortical networks: layer 4 as a frequency-dependent gate

Sensory information reaches the cortex via thalamocortical (TC) synapses in layer 4. Thalamic relay neurons that mediate information flow to cortex operate in two distinct modes, tonic and burst firing. Burst firing has been implicated in enhancing reliability of information flow between individual neurons. However, little is known about how local networks of neocortical neurons respond to different temporal patterns of TC activity. We studied cortical activity patterns evoked by stimulating TC afferents at different frequencies, using a combination of electrophysiology and calcium imaging in TC slices that allowed for the reconstruction of spatiotemporal activity with single-cell resolution. Stimulation of TC axons at low frequencies triggered action potentials in only a small number of layer 4 neurons. In contrast, brief high-frequency stimulus trains triggered widespread recurrent activity in populations of neurons in layer 4 and then spread into adjacent layers 2/3 and 5. Recurrent activity had a clear threshold, typically lasted 300 msec, and could be evoked repetitively at frequencies up to 0.5 Hz. Moreover, the spatial extent of recurrent activity was controlled by the TC pattern of activity. Recurrent activity triggered within the highly interconnected networks of layer 4 might act to selectively amplify and redistribute transient high-frequency TC inputs, filter out low-frequency inputs, and temporarily preserve a record of past sensory activity.

Spike-based synaptic plasticity and the emergence of direction selective simple cells: simulation results

Direction selectivity (DS) of simple cells in the primary visual cortex was recently suggested to arise from short-term synaptic depression in thalamocortical afferents (Chance F, Nelson S, Abbott L (1998), J. Neuroscience 18(12): 4785-4799). In the model, two groups of afferents with spatially displaced receptive fields project through either depressing and non-depressing synapses onto the V1 cell. The degree of synaptic depression determines the temporal phase advance of the response to drifting gratings. We show that the spatial displacement and the appropriate degree of synaptic depression required for DS can develop within an unbiased input scenario by means of temporally asymmetric spike-timing dependent plasticity (STDP) which modifies both the synaptic strength and the degree of synaptic depression. Moving stimuli of random velocities and directions break any initial receptive field symmetry and produce DS. Frequency tuning curves and subthreshold membrane potentials akin to those measured for non-directional simple cells are thereby changed into those measured for directional cells. If STDP is such that down-regulation dominates up-regulation the overall synaptic strength adapts in a self-organizing way such that eventually the postsynaptic response for the non-preferred direction becomes subthreshold. To prevent unlearning of the acquired DS by randomly changing stimulus directions an additional learning threshold is necessary. To further protect the development of the simple cell properties against noise in the stimulus, asynchronous and irregular synaptic inputs are required.

Contribution of GABAergic cortical circuitry in shaping somatosensory evoked scalp responses: specific changes after single-dose administration of tiagabine

OBJECTIVES

To determine whether conventional as well as high-frequency somatosensory evoked potentials (SEPs) to upper limb stimulation are influenced by GABAergic intracortical circuitry.

METHODS

We recorded SEPs from 6 healthy volunteers before and after a single-oral administration of tiagabine. Conventional low-frequency SEPs have been obtained after stimulation of the median nerve, as well as after stimulation of the first phalanx of the thumb, which selectively involves cutaneous finger inputs. Median nerve SEPs have been further analyzed after digital narrow-bandpass filtering, to selectively examine high-frequency responses. Lastly, in order to explain scalp SEP distribution before and after tiagabine administration, we performed the brain electrical source analysis (BESA) of raw data.

RESULTS

After tiagabine administration, conventional scalp SEPs showed a significant amplitude increase of parietal P24, frontal N24 and central P22 components. Similarly, BESA showed a significant strength increase of the second peak of activation of the first two perirolandic dipoles, which are likely to correspond to the N24/P24 and P22 generators. By contrast, no significant changes of high-frequency SEPs were induced by drug intake.

CONCLUSIONS

Our findings support the view that both N24/P24 and P22 SEP components are probably generated by deep spiny cell hyperpolarization, which is strongly increased by inhibitory inputs from GABAergic interneurons. By considering the clear influence of inhibitory circuitry in shaping these SEP components, conventional scalp SEP recording could be useful in the functional assessment of the somatosensory cortex in different physiological and pathological conditions. By contrast, intrinsic firing properties of the cell population generating high-frequency SEP responses are unaffected by the increase of recurrent GABAergic inhibition.

Collective bursting in layer IV. Synchronization by small thalamic inputs and recurrent connections

Layer IV is believed to be the cortical signal amplifier, for example, of thalamic signals. A previous spiny stellate recurrent network model of this layer is made more realistic by the addition of inhibitory basket neurons. We study the persistence and characteristics of previously observed collective firing behavior, and investigate what additional features would need to be implemented to generate in vivo type neuronal firing. It is shown that neuronal activity is only coarsely synchronized within the network. By applying methods of noise-cleaning, it emerges that the firing of individual neurons is of low-dimensional hyperchaotic nature, as found in the analysis of measured cat in vivo spike trains. In order to reproduce in vivo firing patterns, it is sufficient to have time-varying thalamic input. Conclusions from low-dimensional hyperchaotic behavior of network-embedded neurons are drawn. We interpret observed in vivo pattern-sharpening features of stimuli and outline possible connections to epilepsy. From our results, it follows that emergent global behavior is likely to be the result of the interaction between comparably simple neuronal components, driven by input specificity.

Synaptic physiology of the flow of information in the cat's visual cortex in vivo

Each stage of the striate cortical circuit extracts novel information about the visual environment. We asked if this analytic process reflected laminar variations in synaptic physiology by making whole-cell recording with dye-filled electrodes from the cat's visual cortex and thalamus; the stimuli were flashed spots. Thalamic afferents terminate in layer 4, which contains two types of cell, simple and complex, distinguished by the spatial structure of the receptive field. Previously, we had found that the postsynaptic and spike responses of simple cells reliably followed the time course of flash-evoked thalamic activity. Here we report that complex cells in layer 4 (or cells intermediate between simple and complex) similarly reprised thalamic activity (response/trial, 99 +/- 1.9 %; response duration 159 +/- 57 ms; latency 25 +/- 4 ms; average +/- standard deviation; n = 7). Thus, all cells in layer 4 share a common synaptic physiology that allows secure integration of thalamic input. By contrast, at the second cortical stage (layer 2+3), where layer 4 directs its output, postsynaptic responses did not track simple patterns of antecedent activity. Typical responses to the static stimulus were intermittent and brief (response/trial, 31 +/- 40 %; response duration 72 +/- 60 ms, latency 39 +/- 7 ms; n = 11). Only richer stimuli like those including motion evoked reliable responses. All told, the second level of cortical processing differs markedly from the first. At that later stage, ascending information seems strongly gated by connections between cortical neurons. Inputs must be combined in newly specified patterns to influence intracortical stages of processing.

Synaptic connections between layer 4 spiny neurone-layer 2/3 pyramidal cell pairs in juvenile rat barrel cortex: physiology and anatomy of interlaminar signalling within a cortical column

Whole-cell voltage recordings were obtained from 64 synaptically coupled excitatory layer 4 (L4) spiny neurones and L2/3 pyramidal cells in acute slices of the somatosensory cortex ('barrel' cortex) of 17- to 23-days-old rats. Single action potentials (APs) in the L4 spiny neurone evoked single unitary EPSPs in the L2/3 pyramidal cell with a peak amplitude of 0.7 +/- 0.6 mV. The average latency was 2.1 +/- 0.6 ms, the rise time was 0.8 +/- 0.3 ms and the decay time constant was 12.7 +/- 3.5 ms. The percentage of failures of an AP in a L4 spiny neurone to evoke a unitary EPSP in the L2/3 pyramidal cell was 4.9 +/- 8.8 % and the coefficient of variation (c.v.) of the unitary EPSP amplitude was 0.27 +/- 0.13. Both c.v. and percentage of failures decreased with increased average EPSP amplitude. Postsynaptic glutamate receptors (GluRs) in L2/3 pyramidal cells were of the N-methyl-D-aspartate (NMDA) receptor (NMDAR) and the non-NMDAR type. At -60 mV in the presence of extracellular Mg2+ (1 mM), 29 +/- 15 % of the EPSP voltage-time integral was blocked by NMDAR antagonists. In 0 Mg2+, the NMDAR/AMPAR ratio of the EPSC was 0.50 +/- 0.29, about half the value obtained for L4 spiny neurone connections. Burst stimulation of L4 spiny neurones showed that EPSPs in L2/3 pyramidal cells depressed over a wide range of frequencies (1-100 s(-1) ). However, at higher frequencies (30 s(-1)) EPSP summation overcame synaptic depression so that the summed EPSP was larger than the first EPSP amplitude in the train. The number of putative synaptic contacts established by the axonal collaterals of the L4 projection neurone with the target neurone in layer 2/3 varied between 4 and 5, with an average of 4.5 +/- 0.5 (n = 13 pairs). Synapses were established on basal dendrites of the pyramidal cell. Their mean geometric distance from the pyramidal cell soma was 67 +/- 34 microm (range, 16-196 microm). The results suggest that each connected L4 spiny neurone produces a weak but reliable EPSP in the pyramidal cell. Therefore transmission of signals to layer 2/3 is likely to have a high threshold requiring simultaneous activation of many L4 neurons, implying that L4 spiny neurone to L2/3 pyramidal cell synapses act as a gate for the lateral spread of excitation in layer 2/3.

Thalamic relay functions and their role in corticocortical communication: generalizations from the visual system

All neocortical areas receive thalamic inputs. Some thalamocortical pathways relay information from ascending pathways (first order thalamic relays) and others relay information from other cortical areas (higher order thalamic relays), thus serving a role in corticocortical communication. Most, possibly all, afferents reaching thalamus, ascending and cortical, are branches of axons that innervate lower (motor) centers, so that thalamocortical pathways can be viewed generally as monitors of ongoing motor instructions. In terms of numbers, the thalamic relay is dominated by synapses that modulate the relay functions. One of the roles of these modulatory pathways is to change the transfer of information through the thalamus, in accord with current attentional demands. Other roles remain to be explored. These modulatory functions can be expected to act on corticocortical communication in addition to their action on ascending pathways.

Thalamic relay functions

The lateral geniculate nucleus is the best understood thalamic relay. Only 5-10% of the inputs to geniculate relay cells derive from retina, which is the driving input. The rest, being modulatory, derive from local inhibitory inputs, descending inputs from visual cortex, and ascending inputs from brainstem. The nonretinal, modulatory inputs, which form the vast majority, dynamically control the nature of the geniculate relay. Among other actions, these modulatory inputs regulate membrane properties of relay cells and thereby control their mode of response to retinal inputs, and this dramatically affects the nature of information relayed to cortex. Our studies of the lateral geniculate nucleus of the cat lead to the speculation that this dynamic control depends on the animal's behavioral state and represents the neuronal substrate for many forms of visual attention. The lateral geniculate nucleus is a first-order relay, because it relays subcortical (i.e. retinal) information to cortex for the first time. In contrast, the other main thalamic relay of visual information, the pulvinar (and lateral posterior nucleus in carnivores), is largely a higher-order relay, since much of it seems to relay information from one cortical area to another. Much more corticocortical processing may involve these 're-entry' routes than has been hitherto appreciated. If so, the thalamus sits at an indispensable position for corticocortical processing.

Construction of complex receptive fields in cat primary visual cortex

In primary visual cortex, neurons are classified into simple cells and complex cells based on their response properties. Although the role of these two cell types in vision is still unknown, an attractive hypothesis is that simple cells are necessary to construct complex receptive fields. This hierarchical model puts forward two main predictions. First, simple cells should connect monosynaptically to complex cells. Second, complex cells should become silent when simple cells are inactivated. We have recently provided evidence for the first prediction, and here we do the same for the second. In summary, our results suggest that the receptive fields of most layer 2+3 complex cells are generated by a mechanism that requires simple cell inputs.

Surround suppression in primate V1

We investigated the spatial organization of surround suppression in primate primary visual cortex (V1). We utilized drifting stimuli, configured to extend either from within the classical receptive field (CRF) to surrounding visual space, or from surrounding visual space into the CRF or subdivided to generate direction contrast, to make a detailed examination of the strength, spatial organization, direction dependence, mechanisms, and laminar distribution of surround suppression. Most cells (99/105, 94%) through all cortical layers, exhibited suppression (mean reduction 67%) to uniform stimuli exceeding the CRF, and 43% exhibited a more than 70% reduction. Testing with an annulus revealed two different patterns of surround influence. Some cells (37% of cells), classical surround suppression (CSS) cells exhibited responses to an annulus encroaching on the CRF that were less than the plateau in the spatial summation curve. The majority (63%), center-gated surround suppression (CGSS) cells, showed responses to annuli that equaled or exceeded the plateau in the spatial summation curve. Analysis suggested the CSS mechanism was implemented in all cells while the CGSS mechanism was implemented in varying strength across the sample with the extreme reflected in cells that gave larger responses to annuli than to a center stimulus. Reversing the direction of motion of the portion of the stimulus surrounding the CRF revealed four different patterns of effect: no reduction in the degree of suppression (22% of cells), a reduction in surround suppression (41%), a facilitation of the response above the level to the inner stimulus alone (37%), and a facilitation of the response above that to the inner stimulus alone that also exceeded the values associated with an optimal inner stimulus. The facilitatory effects were only seen for reverse direction interfaces between the central and surrounding stimulus at diameters equal to or more than the CRF size. The zones driving the suppressive influences and the direction contrast facilitation were often spatially heterogeneous and for a number of cells bore strong comparison with the class of behavior reported for surround mechanisms in MT. This suggests a potential role, for example, in extracting information about motion contrast in the representation of the three dimensional structure of moving objects.

Rules of connectivity between geniculate cells and simple cells in cat primary visual cortex

Hundreds of thalamic axons ramify within a column of cat visual cortex; yet each layer 4 neuron receives input from only a fraction of them. We have examined the specificity of these connections by recording simultaneously from layer 4 simple cells and cells in the lateral geniculate nucleus with spatially overlapping receptive fields (n = 221 cell pairs). Because of the precise retinotopic organization of visual cortex, the geniculate axons and simple-cell dendrites of these cell pairs should have overlapped within layer 4. Nevertheless, monosynaptic connections were identified in only 33% of all cases, as estimated by cross-correlation analysis. The visual responses of monosynaptically connected geniculate cells and simple cells were closely related. The probability of connection was greatest when a geniculate center overlapped a strong simple-cell subregion of the same sign (ON or OFF) near the center of the subregion. This probability was further increased when the time courses of the visual responses were similar. In addition, the connections were strongest when the simple-cell subregion and the geniculate center were matched in position, sign, and size. The rules of connectivity between geniculate afferents and simple cells resemble those found for retinal afferents to geniculate cells. The connections along the retinogeniculocortical pathway, therefore, show a precision that goes beyond simple retinotopy to include many other response properties, such as receptive-field sign, timing, subregion strength, and size. This specificity in wiring emphasizes the need for developmental mechanisms (presumably correlation-based) that can select among afferents that differ only slightly in their response properties.

Intracortical origin of visual maps

Previous experiments indicate that the shape of maps of preferred orientation in the primary visual cortex does not depend on visual experience. We propose a network model that demonstrates that the orientation and direction selectivity of individual units and the structure of the corresponding angle maps could emerge from local recurrent connections. Our model reproduces the structure of preferred orientation and direction maps, and explains the origin of their interrelationship. The model also provides an explanation for the correlation between position shifts of receptive fields and changes of preferred orientations of single neurons across the surface of the cortex.

Does corticothalamic feedback control cortical velocity tuning?

The thalamus is the major gate to the cortex, and its contribution to cortical receptive field properties is well established. Cortical feedback to the thalamus is, in turn, the anatomically dominant input to relay cells, yet its influence on thalamic processing has been difficult to interpret. For an understanding of complex sensory processing, detailed concepts of the corticothalamic interplay need to be established. To study corticogeniculate processing in a model, we draw on various physiological and anatomical data concerning the intrinsic dynamics of geniculate relay neurons, the cortical influence on relay modes, lagged and nonlagged neurons, and the structure of visual cortical receptive fields. In extensive computer simulations, we elaborate the novel hypothesis that the visual cortex controls via feedback the temporal response properties of geniculate relay cells in a way that alters the tuning of cortical cells for speed.

Orientation preference patterns in mammalian visual cortex: a wire length minimization approach

In the visual cortex of many mammals, orientation preference changes smoothly along the cortical surface, with the exception of singularities such as pinwheels and fractures. The reason for the existence of these singularities has remained elusive, suggesting that they are developmental artifacts. We show that singularities reduce the length of intracortical neuronal connections for some connection rules. Therefore, pinwheels and fractures could be evolutionary adaptations keeping cortical volume to a minimum. Wire length minimization approach suggests that interspecies differences in orientation preference maps reflect differences in intracortical neuronal circuits, thus leading to experimentally testable predictions. We discuss application of our model to direction preference maps.

A neural model of how horizontal and interlaminar connections of visual cortex develop into adult circuits that carry out perceptual grouping and learning

A neural model suggests how horizontal and interlaminar connections in visual cortical areas V1 and V2 develop within a laminar cortical architecture and give rise to adult visual percepts. The model suggests how mechanisms that control cortical development in the infant lead to properties of adult cortical anatomy, neurophysiology and visual perception. The model clarifies how excitatory and inhibitory connections can develop stably by maintaining a balance between excitation and inhibition. The growth of long-range excitatory horizontal connections between layer 2/3 pyramidal cells is balanced against that of short-range disynaptic interneuronal connections. The growth of excitatory on-center connections from layer 6-to-4 is balanced against that of inhibitory interneuronal off-surround connections. These balanced connections interact via intracortical and intercortical feedback to realize properties of perceptual grouping, attention and perceptual learning in the adult, and help to explain the observed variability in the number and temporal distribution of spikes emitted by cortical neurons. The model replicates cortical point spread functions and psychophysical data on the strength of real and illusory contours. The on-center, off-surround layer 6-to-4 circuit enables top-down attentional signals from area V2 to modulate, or attentionally prime, layer 4 cells in area V1 without fully activating them. This modulatory circuit also enables adult perceptual learning within cortical area V1 and V2 to proceed in a stable way.

Spatial summation in lateral geniculate nucleus and visual cortex

We have compared the spatial summation characteristics of cells in the primary visual cortex with those of cells in the dorsal lateral geniculate nucleus (LGN) that provide the input to the cortex. We explored the influence of varying the diameter of a patch of grating centred over the receptive field and quantitatively determined the optimal summation diameter and the degree of surround suppression for cells at both levels of the visual system using the same stimulus parameters. The mean optimal summation size for LGN cells (0.90 degrees) was much smaller than that of cortical cells (3.58 degrees). Virtually all LGN cells exhibited strong surround suppression with a mean value of 74%+/-1.61% SEM for the population as a whole. This potent surround suppression in the cells providing the input to the cortex suggests that cortical cells must integrate their much larger summation fields from the low firing rates associated with the suppression plateau of the LGN cell responses. Our data suggest that the strongest input to cortical cells will arise from geniculate cells representing areas of visual space located at the borders of a visual stimulus. We suggest that analysis of response properties by patterns centred over the receptive fields of cells may give a misleading impression of the process of the representation. Analysis of pattern terminations or salient borders over the receptive field may provide much more insight into the processing algorithms involved in stimulus representation.

Neural mechanisms of orientation selectivity in the visual cortex

The origin of orientation selectivity in the responses of simple cells in cat visual cortex serves as a model problem for understanding cortical circuitry and computation. The feed-forward model posits that this selectivity arises simply from the arrangement of thalamic inputs to a simple cell. Much evidence, including a number of recent intracellular studies, supports a primary role of the thalamic inputs in determining simple cell response properties, including orientation tuning. This mechanism alone, however, cannot explain the invariance of orientation tuning to changes in stimulus contrast. Simple cells receive push-pull inhibition: ON inhibition in OFF subregions and vice versa. Addition of such inhibition to the feed-forward model can account for this contrast invariance, provided the inhibition is sufficiently strong. The predictions of "normalization" and "feedback" models are reviewed and compared with the predictions of this modified feed-forward model and with experimental results. The modified feed-forward and the feedback models ascribe fundamentally different functions to cortical processing.

Columnar organization of dendrites and axons of single and synaptically coupled excitatory spiny neurons in layer 4 of the rat barrel cortex

Cortical columns are the functional units of the neocortex that are particularly prominent in the "barrel" field of the somatosensory cortex. Here we describe the morphology of two classes of synaptically coupled excitatory neurons in layer 4 of the barrel cortex, spiny stellate, and star pyramidal cells, respectively. Within a single barrel, their somata tend to be organized in clusters. The dendritic arbors are largely confined to layer 4, except for the distal part of the apical dendrite of star pyramidal neurons that extends into layer 2/3. In contrast, the axon of both types of neurons spans the cortex from layer 1 to layer 6. The most prominent axonal projections are those to layers 4 and 2/3 where they are largely restricted to a single cortical column. In layers 5 and 6, a small fraction of axon collaterals projects also across cortical columns. Consistent with the dense axonal projection to layers 4 and 2/3, the total number and density of boutons per unit axonal length was also highest there. Electron microscopy combined with GABA postimmunogold labeling revealed that most (>90%) of the synaptic contacts were established on dendritic spines and shafts of excitatory neurons in layers 4 and 2/3. The largely columnar organization of dendrites and axons of both cell types, combined with the preferential and dense projections within cortical layers 4 and 2/3, suggests that spiny stellate and star pyramidal neurons of layer 4 serve to amplify thalamic input and relay excitation vertically within a single cortical column.

Distributions of synaptic vesicle proteins and GAD65 in deprived and nondeprived ocular dominance columns in layer IV of kitten primary visual cortex are unaffected by monocular deprivation

Two days of monocular deprivation (MD) of kittens during a critical period of development is known to produce a loss of visual responses in the primary visual cortex to stimulation of the nondeprived eye, and 7 days of deprivation results in retraction of axon branches and loss of presynaptic sites from deprived-eye geniculocortical arbors. The rapid loss of responsiveness to deprived-eye visual stimulation could be due to a decrease in intracortical excitatory input to deprived-eye ocular dominance columns (ODCs) relative to nondeprived-eye columns. Alternatively, deprived-eye visual responses could be suppressed by an increase in intracortical inhibition in deprived columns relative to nondeprived columns. We tested these hypotheses in critical period kittens by labeling ODCs in layer IV of primary visual cortex with injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) into lamina A of the lateral geniculate nucleus (LGN). After either 2 or 7 days of MD, densities of intracortical excitatory presynaptic sites within deprived relative to nondeprived ODCs were estimated by measuring synaptic vesicle protein (SVP) immunoreactivity (IR). Because most of the synapses within layer IV of primary visual cortex are excitatory inputs from other cortical neurons, levels of SVP-IR provide an estimate of the amount of intracortical excitatory input. We also measured levels of immunoreactivity of the inhibitory presynaptic terminal marker glutamic acid decarboxylase (GAD)65 in deprived relative to nondeprived ODCs. Monocular deprivation (either 2 or 7 days) had no effect on the distributions of either SVP- or GAD65-IR in deprived and nondeprived columns. Therefore, the rapid loss of deprived-eye visual responsiveness following MD is due neither to a decrease in intracortical excitatory presynaptic sites nor to an increase in intracortical inhibitory presynaptic sites in layer IV of deprived-eye ODCs relative to nondeprived columns.

Membrane mechanisms underlying contrast adaptation in cat area 17 in vivo

Contrast adaptation is a psychophysical phenomenon, the neuronal bases of which reside largely in the primary visual cortex. The cellular mechanisms of contrast adaptation were investigated in the cat primary visual cortex in vivo through intracellular recording and current injections. Visual cortex cells, and to a much less extent, dorsal lateral geniculate nucleus (dLGN) neurons, exhibited a reduction in firing rate during prolonged presentations of a high-contrast visual stimulus, a process we termed high-contrast adaptation. In a majority of cortical and dLGN cells, the period of adaptation to high contrast was followed by a prolonged (5-80 sec) period of reduced responsiveness to a low-contrast stimulus (postadaptation suppression), an effect that was associated, and positively correlated, with a hyperpolarization of the membrane potential and an increase in apparent membrane conductance. In simple cells, the period of postadaptation suppression was not consistently associated with a decrease in the grating modulated component of the evoked synaptic barrages (the F1 component). The generation of the hyperpolarization appears to be at least partially intrinsic to the recorded cells, because the induction of neuronal activity with the intracellular injection of current resulted in both a hyperpolarization of the membrane potential and a decrease in the spike response to either current injections or visual stimuli. Conversely, high-contrast visual stimulation could suppress the response to low-intensity sinusoidal current injection. We conclude that control of the membrane potential by intrinsic neuronal mechanisms contributes importantly to the adaptation of neuronal responsiveness to varying levels of contrast. This feedback mechanism, internal to cortical neurons, provides them with the ability to continually adjust their responsiveness as a function of their history of synaptic and action potential activity.

Synaptic efficacy and reliability of excitatory connections between the principal neurones of the input (layer 4) and output layer (layer 5) of the neocortex

A prerequisite for the understanding of how a cortical column functions is a description of small and defined neuronal circuits consisting of only a few identified neurones. Here we summarise, with particular reference to the barrel cortex, the morphological and physiological properties of two synaptic connections, namely those between pairs of spiny neurones in layer 4 and pairs of pyramidal cells in layer 5. While layer 4 spiny neurones are the cortical input neurones that amplify and relay incoming excitation from the periphery, layer 5 pyramidal cells integrate neuronal activity both within and across cortical columns and subsequently distribute it to both cortical and subcortical brain regions.

Laminar characteristics of functional connectivity in rat barrel cortex revealed by stimulation with caged-glutamate

In rodent somatosensory (barrel) cortex input is processed by whisker-related columns before the integrated output is fed into behaviorally-relevant circuits. The layer-specific activation patterns of the rat barrel cortex were examined with a set-up for scanning functional connectivity in brain slices. Flash-induced release of caged-glutamate at a large number of stimulation sites was used in combination with simultaneous field potential recordings from layers II to VI with five electrodes. The field potentials revealed striking differences between the cortical layers. Glutamate-release in layer IV and lower layer III was most effective in evoking excitation in all other cortical layers, whereas field potentials recorded from layer IV itself were caused by stimulation of a very restricted columnar zone only. Field potentials in layers II and III were strongly driven by stimulation in layer IV and less consistently and much weaker by layer V. Layer V was the only lamina capable of responding to stimulation of all other cortical layers, thus displaying the largest input maps. Layer VI possessed functional connectivity intrinsically and with layer V. These data lead us to suggest that thalamic input may be boosted by its main target layer IV to start a sequence of excitation in layer IV, passing to the supragranular layers and finally reaching the infragranular layers. This sequence is likely to be backed-up by other simultaneous steps of transmission including a layer IV-to-V interaction. We proposed that the increasing size of the receptive fields when sampling granular, supragranular and infragranular layers in vivo, might have its functional basis in the laminar interactions described here in an in vitro preparation.

Termination of the geniculocortical projection in the striate cortex of macaque monkey: a quantitative immunoelectron microscopic study

The goal of this present study was to derive a new estimate of the synaptic contribution of the dorsal lateral geniculate nucleus (dLGN) to the subdivisions of its main recipient layer, layer 4C, of striate cortex of macaque monkey. The projection from the dLGN and its terminal boutons within layer 4C were visualized by immunodetection of the calcium binding protein, parvalbumin (PV), which is expressed in relay cells of the dLGN. The proportion of asymmetric synapses formed by PV-positive boutons within the alpha and beta sublayers of 4C was estimated by using a nonbiased stereological counting method. The proportion of asymmetric synapses contributed by the PV-positive boutons to layer 4Calpha is 8.7%; to 4Cbeta is 6.9%. Assuming all the PV-positive asymmetric synapses derive from the dLGN relay cells, this gives a ratio of dLGN synapses per neuron of 192 in layer 4Calpha and 128 in layer 4Cbeta. Thus, the recurrent excitatory input from neighboring cortical neurons must play an important part in responses of the neurons lying at the input stage of the cortical circuit.

Optimal sizes of dendritic and axonal arbors in a topographic projection

I consider a topographic projection between two neuronal layers with different densities of neurons. Given the number of output neurons connected to each input neuron (divergence) and the number of input neurons synapsing on each output neuron (convergence), I determine the widths of axonal and dendritic arbors which minimize the total volume of axons and dendrites. Analytical results for one-dimensional and two-dimensional projections can be summarized qualitatively in the following rule: neurons of the sparser layer should have arbors wider than those of the denser layer. This agrees with the anatomic data for retinal, cerebellar, olfactory bulb, and neocortical neurons the morphology and connectivity of which are known. The rule may be used to infer connectivity of neurons from their morphology.

Dynamic properties of recurrent inhibition in primary visual cortex: contrast and orientation dependence of contextual effects

A fundamental feature of neural circuitry in the primary visual cortex (V1) is the existence of recurrent excitatory connections between spiny neurons, recurrent inhibitory connections between smooth neurons, and local connections between excitatory and inhibitory neurons. We modeled the dynamic behavior of intermixed excitatory and inhibitory populations of cells in V1 that receive input from the classical receptive field (the receptive field center) through feedforward thalamocortical afferents, as well as input from outside the classical receptive field (the receptive field surround) via long-range intracortical connections. A counterintuitive result is that the response of oriented cells can be facilitated beyond optimal levels when the surround stimulus is cross-oriented with respect to the center and suppressed when the surround stimulus is iso-oriented. This effect is primarily due to changes in recurrent inhibition within a local circuit. Cross-oriented surround stimulation leads to a reduction of presynaptic inhibition and a supraoptimal response, whereas iso-oriented surround stimulation has the opposite effect. This mechanism is used to explain the orientation and contrast dependence of contextual interactions in primary visual cortex: responses to a center stimulus can be both strongly suppressed and supraoptimally facilitated as a function of surround orientation, and these effects diminish as stimulus contrast decreases.

Synaptic density in geniculocortical afferents remains constant after monocular deprivation in the cat

Monocular eyelid closure in cats during a critical period in development produces both physiological plasticity, as indicated by a loss of responsiveness of primary visual cortical neurons to deprived eye stimulation, and morphological plasticity, as demonstrated by a decrease in the total length of individual geniculocortical arbors representing the deprived eye. Although the physiological plasticity appears maximal after 2 d of monocular deprivation (MD), the shrinkage of deprived-eye geniculocortical arbors is less than half-maximal at 4 d and is not maximal until 7 d of deprivation, at which time the deprived arbors are approximately half their previous size. To study this form of plasticity at the level of individual thalamocortical synapses rather than arbors, we developed a new double-label colocalization technique. First, geniculocortical afferent arbors serving either the deprived or nondeprived eye were labeled by injection of the anterograde tracer Phaseolus vulgaris leucoagglutinin into lamina A of the lateral geniculate nucleus. Then, using antibodies to synaptic vesicle proteins, we identified presynaptic terminals within the labeled arbors in layer IV of the primary visual cortex. Analysis of serial optical sections obtained using confocal microscopy allowed measurement of the numerical density of presynaptic sites and the relative amounts of synaptic vesicle protein in geniculocortical afferents after both 2 and 7 d of MD. We found that the density of synapses in geniculocortical axons was similar for deprived and nondeprived afferents, suggesting that this feature of the afferents is conserved even during periods in which synapse number is reduced by half in deprived-eye arbors. These results are not consistent with the hypothesis that a rapid loss of deprived-eye geniculocortical presynaptic sites is responsible for the prompt physiological effects of MD.

Functional neocortical microcircuitry demonstrated with intrinsic signal optical imaging in vitro

Intrinsic signal optical imaging was used to record the changes in light transmittance evoked by electrical stimulation in slices prepared from sensorimotor cortex of young adult rats. The spatial characteristics of the optical signal evoked by stimulation of layer II/III, IV, V, or VI were clearly different. Layer IV and V stimulation elicited a radially-oriented region of increased light transmittance which was "hourglass" shaped: its tangential extent was greatest in layers II/III and layer V, and least in layer IV. Layer VI stimulation also elicited a radially-oriented signal but the tangential extent of this signal was the same across layers II-VI--that is, it was column-shaped. Upper layer stimulation produced a signal whose tangential extent was much greater in the upper layers than its radial extent to the deeper layers. The spatial form of the stimulus-evoked intrinsic signal was not dependent on the cytoarchitectonic area in which it was elicited. The tangential and radial distribution of the signal evoked by stimulation of different layers appears to reflect the connectivity of cortex, particularly the horizontal connectivity present in layers II/III, V, and VI, and the interlaminar connections that exist between layers II/III and V and from layers VI to IV. The spatial characteristics of the intrinsic signal were independent of the strength of stimulation used. The idea that inhibitory mechanisms restrict the tangential extent of the signal was evaluated in experiments in which the intrinsic signal was recorded before and after the addition of 10 microM bicuculline methiodide. In all slices studied in this way (n = 12), bicuculline methiodide drastically increased the tangential extent of the signal. In 4/12 slices, the tangential spread of the signal was asymmetric with respect to the stimulus site. Asymmetric spread of the signal occurred for both layer V and layer VI stimulation and, in 2/4 of those cases, could be attributed to a cytoarchitectonic border whose presence appeared to restrict the spread of the signal across the border. Although increasing stimulation strength did not change the spatial characteristics of the radially-oriented signal evoked by layer V or VI stimulation, at maximal stimulus intensity the signal evoked from these layers was often accompanied by a band of decreased light transmittance in the most superficial layers (layers I and II). It is concluded that in vitro intrinsic optical signal imaging allows one to image a response attributable to activation of local subsets of cortical connections. In addition, the opposite effects of high-intensity deep layer stimulation on the superficial layers vs layers III-VI of the same column raise the possibility that the most superficial layers may respond differently to repetitive input drive than the rest of the cortical column.

Feedback connections to the lateral geniculate nucleus and cortical response properties

The cerebral cortex receives sensory input from the periphery by means of thalamic relay nuclei, but the flow of information goes both ways. Each cortical area sends a reciprocal projection back to the thalamus. In the visual system, the synaptic relations that govern the influence of thalamic afferents on orientation selectivity in the cortex have been studied extensively. It now appears that the connectivity of the corticofugal feedback pathway is also fundamentally linked to the orientation preference of the cortical cells involved.

Reliable synaptic connections between pairs of excitatory layer 4 neurones within a single 'barrel' of developing rat somatosensory cortex

1. Dual whole-cell recordings were made from pairs of synaptically coupled excitatory neurones in the 'barrel field' in layer (L) 4 in slices of young (postnatal day 12-15) rat somatosensory cortex. The majority of interconnected excitatory neurones were spiny stellate cells with an asymmetrical dendritic arborisation largely confined to a single barrel. The remainder were star pyramidal cells with a prominent apical dendrite terminating in L2/3 without forming a tuft. 2. Excitatory synaptic connections were examined between 131 pairs of spiny L4 neurones. Single presynaptic action potentials evoked unitary EPSPs with a peak amplitude of 1.59 +/- 1.51 mV (mean +/- s. d.), a latency of 0.92 +/- 0.35 ms, a rise time of 1.53 +/- 0.46 ms and a decay time constant of 17.8 +/- 6.3 ms. 3. At 34-36 C, the coefficient of variation (c.v.) of the unitary EPSP amplitude was 0. 37 +/- 0.16 and the percentage of failures to evoke an EPSP was 5.3 +/- 7.8 %. The c.v. and failure rate decreased with increasing amplitude of the unitary EPSP. 4. Postsynaptic glutamate receptors in spiny L4 neurones were of the AMPA and NMDA type. At -60 mV in the presence of 1 mM Mg2+, NMDA receptors contributed 39.3 +/- 12.5 % to the EPSP integral. In Mg2+-free solution, the NMDA receptor/AMPA receptor ratio of the EPSC was 0.86 +/- 0.64. 5. The number of putative synaptic contacts established by the projection neurone with the target neurone varied between two and five with a mean of 3.4 +/- 1.0 (n = 11). Synaptic contacts were exclusively found in the barrel in which the cell pair was located and were preferentially located on secondary to quarternary dendritic branches. Their mean geometric distance from the soma was 68.8 +/- 37.4 microm (range, 33.4-168.0 microm). The number of synaptic contacts and mean EPSP amplitude showed no significant correlation. 6. The results suggest that in L4 of the barrel cortex synaptic transmission between spiny neurones is largely restricted to a single barrel. The connections are very reliable, probably due to a high release probability, and have a high efficacy because of the compact structure of the dendrites and axons of spiny neurones. Intrabarrel connections thus function to amplify and distribute the afferent thalamic activity in the vertical directions of a cortical column.

Integrating neuromorphic action-oriented perceptual inputs to generate a navigation behaviour for a robot

We use neural networks with pointer map architectures to provide simple attentional processing in a robotic task. A pointer map comprises a map of neurons that encode a stimulus. Besides global feedback inhibition, the map receives feedback excitation via a small group of pointer neurons that encode the location of a salient stimulus on the map as a vectorial representation. The pointer neurons are able to apply selective processing to a particular region of the network. The robot uses these properties to manoeuver in relation to an attended object. We implemented a controller composed of two pointer maps, and a motor map. The first pointer map reports the direction of a salient obstacle in a one-dimensional map of distance derived from infrared sensors. The second pointer map reports the direction to potential obstacles in a two-dimensional edge-enhanced image derived from a forward looking CCD-camera. These outputs are applied to a motor map, where they bias the motor control signals issued to the robots wheels, according to navigational intentions.

Development and plasticity of the cerebral cortex: from molecules to maps

The role played by environmental influences in the development of the nervous system has been subject to intense study for the last three decades. Many laboratories are currently engaged in characterizing the exact contributions of activity-dependent or -independent processes to the development of the mammalian neocortex. Here we introduce a special issue devoted to the topic and briefly review recent progress in this exciting field. At the systems level, many investigators are now distinguishing between an "establishment" phase of cortical connections, where activity-dependent and independent mechanisms could operate, and a later "maintenance" phase, which appears to be controlled by neuronal activity. A particularly interesting recent example of the role of top-down vs. bottom-up influences in the development of cortical connections is the emergence of orientation selectivity in visual cortex: we propose a synthetic view highlighting the role of the thalamo-cortical reciprocal projection in this process. Finally, at the cellular level, NMDA receptors, neurotrophins and many other molecules contribute to activity-dependent rearrangement of cortical connections during appropriate critical periods of development.

The relationship between synchronization among neuronal populations and their mean activity levels

In the past decade the importance of synchronized dynamics in the brain has emerged from both empirical and theoretical perspectives. Fast dynamic synchronous interactions of an oscillatory or nonoscillatory nature may constitute a form of temporal coding that underlies feature binding and perceptual synthesis. The relationship between synchronization among neuronal populations and the population firing rates addresses two important issues: the distinction between rate coding and synchronization coding models of neuronal interactions and the degree to which empirical measurements of population activity, such as those employed by neuroimaging, are sensitive to changes in synchronization. We examined the relationship between mean population activity and synchronization using biologically plausible simulations. In this article, we focus on continuous stationary dynamics. (In a companion article, Chawla (forthcoming), we address the same issue using stimulus-evoked transients.) By manipulation parameters such as extrinsic input, intrinsic noise, synaptic efficacy, density of extrinsic connections, the voltage-sensitive nature of postsynaptic mechanisms, the number of neurons, and the laminar structure within the populations, we were able to introduce variations in both mean activity and synchronization under a variety of simulated neuronal architectures. Analyses of the simulated spike trains and local field potentials showed that in nearly every domain of the model's parameter space, mean activity and synchronization were tightly coupled. This coupling appears to be mediated by an increase in synchronous gain when effective membrane time constants are lowered by increased activity. These observations show that under the assumptions implicit in our models, rate coding and synchrony coding in neural systems with reciprocal interconnections are two perspectives on the same underlying dynamic. This suggests that in the absence of specific mechanisms decoupling changes in synchronization from firing levels, indexes of brain activity that are based purely on synaptic activity (e.g., functional magnetic resonance imaging) may also be sensitive to changes in synchronous coupling.

Feedback interactions between neuronal pointers and maps for attentional processing

Neural networks combining local excitatory feedback with recurrent inhibition are valuable models of neocortical processing. However, incorporating the attentional modulation observed in cortical neurons is problematic. We propose a simple architecture for attentional processing. Our network consists of two reciprocally connected populations of excitatory neurons; a large population (the map) processes a feedforward sensory input, and a small population (the pointer) modulates location and intensity of this processing in an attentional manner dependent on a control input to the pointer. This pointer-map network has rich dynamics despite its simple architecture and explains general computational features related to attention/intention observed in neocortex, making it interesting both theoretically and experimentally.

Efficacy of thalamocortical and intracortical synaptic connections: quanta, innervation, and reliability

Thalamocortical (TC) synapses carry information into the neocortex, but they are far outnumbered by excitatory intracortical (IC) synapses. We measured the synaptic properties that determine the efficacy of TC and IC axons converging onto spiny neurons of layer 4 in the mouse somatosensory cortex. Quantal events from TC and IC synapses were indistinguishable. However, TC axons had, on average, about 3 times more release sites than IC axons, and the mean release probability at TC synapses was about 1.5 times higher than that at IC synapses. Differences of innervation ratio and release probability make the average TC connection several times more effective than the average IC connection, and may allow small numbers of TC axons to dominate the activity of cortical layer 4 cells during sensory inflow.

The connection from cortical area V1 to V5: a light and electron microscopic study

Area V5 (middle temporal) in the superior temporal sulcus of macaque receives a direct projection from the primary visual cortex (V1). By injecting anterograde tracers (biotinylated dextran and Phaseolus vulgaris lectin) into V1, we have examined the synaptic boutons that they form in V5 in the electron microscope. Nearly 80% of the target cells in V5 were spiny (excitatory). The boutons formed asymmetric (Gray's type 1) synapses with spines (54%), dendrites (33%), and somata (13%). All somatic targets and some (26%) of the target dendritic shafts showed features characteristic of smooth (inhibitory) cells. Each bouton formed, on average, 1.7 synapses. The larger boutons formed multiple synapses with the same neuron and completely enveloped the entire spine head. On most dendritic shafts and all somata the postsynaptic density en face was disk-shaped but in about half the cases the reconstructed postsynaptic densities of synapses on spines appeared as complete or partial annuli. Even in the zones of densest innervation only 3% of the asymmetric synapses were formed by the labeled boutons. Although the V1 projection forms only a small minority of synapses in V5, its affect could be considerably amplified by local circuits in V5, in a way analogous to the amplification of the small thalamic input to area V1.

Role of feedback in mammalian vision: a new hypothesis and a computational model

This paper presents a novel hypothesis on the function of massive feedback pathways in mammalian visual systems. We propose that the cortical feature detectors compete not for the right to represent the output at a point, but for exclusive rights to abstract and represent part of the underlying input. Feedback can do this very naturally. A computational model that implements the above idea for the problem of detection is presented and based on that we suggest a functional role for the thalamo-cortical loop during perception of lines. We show that the model successfully tackles the so called Cross problem. Based on some recent experimental results, we discuss the biological plausibility of our model. We also comment on the relevance of our hypothesis (on the role of feedback) to general sensory information processing and recognition.

Synaptic integration in striate cortical simple cells

Simple cells in the visual cortex respond to the precise position of oriented contours (Hubel and Wiesel, 1962). This sensitivity reflects the structure of the simple receptive field, which exhibits two sorts of antagonism between on and off inputs. First, simple receptive fields are divided into adjacent on and off subregions; second, within each subregion, stimuli of the reverse contrast evoke responses of the opposite sign: push-pull (Hubel and Wiesel, 1962; Palmer and Davis, 1981; Jones and Palmer, 1987; Ferster, 1988). We have made whole-cell patch recordings from cat area 17 during visual stimulation to examine the generation and integration of excitation (push) and suppression (pull) in the simple receptive field. The temporal structure of the push reflected the pattern of thalamic inputs, as judged by comparing the intracellular cortical responses to extracellular recordings made in the lateral geniculate nucleus. Two mechanisms have been advanced to account for the pull-withdrawal of thalamic drive and active, intracortical inhibition (Hubel and Wiesel, 1962; Heggelund, 1968; Ferster, 1988). Our results suggest that intracortical inhibition is the dominant, and perhaps sole, mechanism of suppression. The inhibitory influences operated within a wide dynamic range. When inhibition was strong, the membrane conductance could be doubled or tripled. Furthermore, if a stimulus confined to one subregion was enlarged so that it extended into the next, the sign of response often changed from depolarizing to hyperpolarizing. In other instances, the inhibition modulated neuronal output subtly, by elevating spike threshold or altering firing rate at a given membrane voltage.

Functionally distinct NMDA receptors mediate horizontal connectivity within layer 4 of mouse barrel cortex

In sensory areas of neocortex, thalamocortical afferents project primarily onto the spiny stellate neurons of Layer 4. Anatomical evidence indicates that these cells receive most of their excitatory input from other cortical neurons, including other spiny stellate cells. Although this local network must play an important role in sensory processing, little is known about the properties of the neurons and synapses involved. We have produced a slice preparation of mouse barrel cortex that isolates Layer 4. We report that excitatory interaction between spiny stellate neurons is largely via N-methyl-D-aspartate receptors (NMDARs) and that a given neuron contains more than one type of NMDAR, as distinguished by voltage dependence. Thus, spiny stellate cells act as effective integrators of powerful and persistent NMDAR-mediated recurrent excitation.

Ascending projections of simple and complex cells in layer 6 of the cat striate cortex

Receptive field properties vary systematically across the different layers of the cat striate cortex. Understanding how these functional differences emerge requires a precise description of the interlaminar connections and the quality of information that they transmit. This study examines the contribution of the two physiological types of neuron in layer 6, simple and complex, to the cortical microcircuit. The approach was to make whole-cell recordings with dye-filled electrodes in vivo to correlate visual response property with intracortical projection pattern. The two simple cells we stained projected to layer 4, as previously reported (Gilbert and Wiesel, 1979; Martin and Whitteridge, 1984). Six of the eight complex cells that we labeled projected to the superficial layers, a pathway not previously described in the cat. The remaining two cells targeted the infragranular layers. Layer 4 is dominated by simple cells, whereas layers 5 and 2+3 are mainly composed of complex cells (Hubel and Wiesel, 1962; Gilbert, 1977). Hence, our results indicate that the ascending projections of simple cells in layer 6 target other simple cells. In parallel, the ascending projections of a population of complex cells in layer 6 favor other complex cells. Anatomical experiments in several species (Lund and Boothe,1975; Burkhalter,1989; Usrey and Fitzpatrick, 1996; Wiser and Callaway, 1996) had also demonstrated that layer 6 gives rise to two separate intracortical pathways. Pooling the results of these anatomical studies with our own suggests a common feature of the laminar organization: cells that project to different intracortical targets have distinct functional characteristics.

Different mechanisms underlie three inhibitory phenomena in cat area 17

Recently, it has been proposed that all suppressive phenomena observed in the primary visual cortex (V1) are mediated by a single mechanism, involving inhibition by pools of neurons, which, between them, represent a wide range of stimulus specificities. The strength of such inhibition would depend on the stimulus that produces it (particularly its contrast) rather than on the firing rate of the inhibited cell. We tested this hypothesis by measuring contrast-response functions (CRFs) of neurons in cat V1 for stimulation of the classical receptive field of the dominant eye with an optimal grating alone, and in the presence of inhibition caused by (1) a superimposed orthogonal grating (cross-orientation inhibition); (2) a surrounding iso-oriented grating (surround inhibition); and (3) an orthogonal grating in the other eye (interocular suppression). We fitted hyperbolic ratio functions and found that the effect of cross-orientation inhibition was best described as a rightward shift of the CRF ('contrast-gain control'), while surround inhibition and interocular suppression were primarily characterised as downward shifts of the CRF ('response-gain control'). However, the latter also showed a component of contrast-gain control. The two modes of suppression were differently distributed between the layers of cortex. Response-gain control prevailed in layer 4, whereas cells in layers 2/3, 5 and 6 mainly showed contrast-gain control. As in human observers, surround gratings caused suppression when the central grating was of high contrast, but in over a third of the cells tested, enhanced responses for low-contrast central stimuli, hence actually decreasing threshold contrast.

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.

Strength and orientation tuning of the thalamic input to simple cells revealed by electrically evoked cortical suppression

Is thalamic input to the visual cortex strong and well tuned for orientation, as predicted by Hubel and Wiesel's (1962) model of orientation selectivity in simple cells? We directly measured the size of the thalamic input to single simple cells intracellularly by combining electrical stimulation of the cortex with a briefly flashed visual stimulus. In nearby cells, the electrical stimulation evoked a long-lasting inhibition that prevented them from firing in response to the visual stimulus. The visually evoked excitatory postsynaptic potentials (EPSPs) recorded during the period of cortical suppression, therefore, reflected largely the thalamic input. In 16 neurons that received monosynaptic input from the thalamus, cortical suppression left 46% of normal visual response on average (12%-86% in range). In those cells tested, this remaining visual response was as well tuned for orientation as the normal response to the visual stimulus alone. We conclude that the thalamic input to cortical simple cells with monosynaptic input from the thalamus is strong and well tuned in orientation, and that the intracortical input does not appear to sharpen orientation tuning in these cells.