Cortical feedback increases visual information transmitted by monkey parvocellular lateral geniculate nucleus neurons

Role of Feedback Connections in Central Visual Processing

The physiological response properties of neurons in the visual system are inherited mainly from feedforward inputs. Interestingly, feedback inputs often outnumber feedforward inputs. Although they are numerous, feedback connections are weaker, slower, and considered to be modulatory, in contrast to fast, high-efficacy feedforward connections. Accordingly, the functional role of feedback in visual processing has remained a fundamental mystery in vision science. At the core of this mystery are questions about whether feedback circuits regulate spatial receptive field properties versus temporal responses among target neurons, or whether feedback serves a more global role in arousal or attention. These proposed functions are not mutually exclusive, and there is compelling evidence to support multiple functional roles for feedback. In this review, the role of feedback in vision will be explored mainly from the perspective of corticothalamic feedback. Further generalized principles of feedback applicable to corticocortical connections will also be considered.

Optogenetic activation of corticogeniculate feedback stabilizes response gain and increases information coding in LGN neurons

In spite of their anatomical robustness, it has been difficult to establish the functional role of corticogeniculate circuits connecting primary visual cortex with the lateral geniculate nucleus of the thalamus (LGN) in the feedback direction. Growing evidence suggests that corticogeniculate feedback does not directly shape the spatial receptive field properties of LGN neurons, but rather regulates the timing and precision of LGN responses and the information coding capacity of LGN neurons. We propose that corticogeniculate feedback specifically stabilizes the response gain of LGN neurons, thereby increasing their information coding capacity. Inspired by early work by McClurkin et al. (1994), we manipulated the activity of corticogeniculate neurons to test this hypothesis. We used optogenetic methods to selectively and reversibly enhance the activity of corticogeniculate neurons in anesthetized ferrets while recording responses of LGN neurons to drifting gratings and white noise stimuli. We found that optogenetic activation of corticogeniculate feedback systematically reduced LGN gain variability and increased information coding capacity among LGN neurons. Optogenetic activation of corticogeniculate neurons generated similar increases in information encoded in LGN responses to drifting gratings and white noise stimuli. Together, these findings suggest that the influence of corticogeniculate feedback on LGN response precision and information coding capacity could be mediated through reductions in gain variability.

Focal Gain Control of Thalamic Visual Receptive Fields by Layer 6 Corticothalamic Feedback

The projections between the thalamus and primary visual cortex (V1) are a key reciprocal neural circuit, relaying retinal signals to cortical layers 4 & 6 while being simultaneously regulated by massive layer 6 corticothalamic feedback. Effectively dissecting the influence of this corticothalamic feedback circuit in higher mammals remains a challenge for vision research. By pharmacologically increasing the focal gain of visually driven layer 6 responses of cat V1 in a controlled fashion, we examined the effects of such focal cortical changes on the response amplitudes and spatial structure of the receptive fields (RFs) of individual dorsal lateral geniculate nucleus (dLGN) cells. We found that enhancing visually driven cortical feedback could facilitate or suppress the overall responses of dLGN cells, and such an effect was linked to the orientation preference of the cortical neuron. Related to these selective retinotopic gain changes, enhanced feedback induced the RFs of dLGN cells to expand, contract or shift their spatial focus. Our results provide further evidence for a functional mechanism through which the cortex can selectively gate visual information flow from the thalamus back to the visual cortex.

Complex Effects on In Vivo Visual Responses by Specific Projections from Mouse Cortical Layer 6 to Dorsal Lateral Geniculate Nucleus

Understanding the role of corticothalamic projections in shaping visual response properties in the thalamus has been a longstanding challenge in visual neuroscience. Here, we take advantage of the cell-type specificity of a transgenic mouse line, the GN220-Ntsr1 Cre line, to manipulate selectively the activity of a layer 6 (L6) corticogeniculate population while recording visual responses in the dorsal lateral geniculate nucleus (dLGN). Although driving Ntsr1 projection input resulted in reliable reduction in evoked spike count of dLGN neurons, removing these same projections resulted in both increases and decreases in visually evoked spike count. Both increases and decreases are contrast dependent and the sign is consistent over the full range of contrasts. Tuning properties suggest wide convergence of Ntsr1 cells with similar spatial and temporal frequency tuning onto single dLGN cells and we did not find evidence that Ntsr1 cells sharpen spatiotemporal filtering. These nonspecific changes occur independently of changes in burst frequency, indicating that Ntsr1 corticogeniculate activity can result in both net excitation and net inhibition.

The impact of the lateral geniculate nucleus and corticogeniculate interactions on efficient coding and higher-order visual object processing

Principles of efficient coding suggest that the peripheral units of any sensory processing system are designed for efficient coding. The function of the lateral geniculate nucleus (LGN) as an early stage in the visual system is not well understood. Some findings indicate that similar to the retina that decorrelates input signals spatially, the LGN tends to perform a temporal decorrelation. There is evidence suggesting that corticogeniculate connections may account for this decorrelation in the LGN. In this study, we propose a computational model based on biological evidence reported by Wang et al. (2006), who demonstrated that the influence pattern of V1 feedback is phase-reversed. The output of our model shows how corticogeniculate connections decorrelate LGN responses and make an efficient representation. We evaluated our model using criteria that have previously been tested on LGN neurons through cell recording experiments, including sparseness, entropy, power spectra, and information transfer. We also considered the role of the LGN in higher-order visual object processing, comparing the categorization performance of human subjects with a cortical object recognition model in the presence and absence of our LGN input-stage model. Our results show that the new model that considers the role of the LGN, more closely follows the categorization performance of human subjects.

A neuron-inspired computational architecture for spatiotemporal visual processing: real-time visual sensory integration for humanoid robots

In this article, we present a neurologically motivated computational architecture for visual information processing. The computational architecture's focus lies in multiple strategies: hierarchical processing, parallel and concurrent processing, and modularity. The architecture is modular and expandable in both hardware and software, so that it can also cope with multisensory integrations - making it an ideal tool for validating and applying computational neuroscience models in real time under real-world conditions. We apply our architecture in real time to validate a long-standing biologically inspired visual object recognition model, HMAX. In this context, the overall aim is to supply a humanoid robot with the ability to perceive and understand its environment with a focus on the active aspect of real-time spatiotemporal visual processing. We show that our approach is capable of simulating information processing in the visual cortex in real time and that our entropy-adaptive modification of HMAX has a higher efficiency and classification performance than the standard model (up to ∼+6%).

The function of metabotropic glutamate receptors in thalamus and cortex

Metabotropic glutamate receptors (mGluRs) are found throughout thalamus and cortex and are clearly important to circuit behavior in both structures, and so considering only participation of ionotropic glutamate receptors (e.g., [R,S]-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA] and N-methyl-d-aspartate receptors [NMDA] receptors) in glutamatergic processing would be an unfortunate oversimplification. These mGluRs are found both postsynaptically, on target cells of glutamatergic afferents, and presynaptically, on various synaptic terminals themselves, and when activated, they produce prolonged effects lasting at least hundreds of msec to several sec and perhaps longer. Two main types exist: activation of group I mGluRs causes postsynaptic depolarization, and group II, hyperpolarization. Both types are implicated in synaptic plasticity, both short term and long term. Their evident importance in functioning of thalamus and cortex makes it critical to develop a better understanding of how these receptors are normally activated, especially because they also seem implicated in a wide range of neurological and cognitive pathologies.

Extended difference-of-Gaussians model incorporating cortical feedback for relay cells in the lateral geniculate nucleus of cat

A striking feature of the organization of the early visual pathway is the significant feedback from primary visual cortex to cells in the dorsal lateral geniculate nucleus (LGN). Despite numerous experimental and modeling studies, the functional role for this feedback remains elusive. We present a new firing-rate-based model for LGN relay cells in cat, explicitly accounting for thalamocortical loop effects. The established DOG model, here assumed to account for the spatial aspects of the feedforward processing of visual stimuli, is extended to incorporate the influence of thalamocortical loops including a full set of orientation-selective cortical cell populations. Assuming a phase-reversed push-pull arrangement of ON and OFF cortical feedback as seen experimentally, this extended DOG (eDOG) model exhibits linear firing properties despite non-linear firing characteristics of the corticothalamic cells. The spatiotemporal receptive field of the eDOG model has a simple algebraic structure in Fourier space, while the real-space receptive field, as well as responses to visual stimuli, are found by evaluation of an integral. As an example application we use the eDOG model to study effects of cortical feedback on responses to flashing circular spots and patch-grating stimuli and find that the eDOG model can qualitatively account for experimental findings.

A minimal mechanistic model for temporal signal processing in the lateral geniculate nucleus

The receptive fields of cells in the lateral geniculate nucleus (LGN) are shaped by their diverse set of impinging inputs: feedforward synaptic inputs stemming from retina, and feedback inputs stemming from the visual cortex and the thalamic reticular nucleus. To probe the possible roles of these feedforward and feedback inputs in shaping the temporal receptive-field structure of LGN relay cells, we here present and investigate a minimal mechanistic firing-rate model tailored to elucidate their disparate features. The model for LGN relay ON cells includes feedforward excitation and inhibition (via interneurons) from retinal ON cells and excitatory and inhibitory (via thalamic reticular nucleus cells and interneurons) feedback from cortical ON and OFF cells. From a general firing-rate model formulated in terms of Volterra integral equations, we derive a single delay differential equation with absolute delay governing the dynamics of the system. A freely available and easy-to-use GUI-based MATLAB version of this minimal mechanistic LGN circuit model is provided. We particularly investigate the LGN relay-cell impulse response and find through thorough explorations of the model's parameter space that both purely feedforward models and feedback models with feedforward excitation only, can account quantitatively for previously reported experimental results. We find, however, that the purely feedforward model predicts two impulse response measures, the time to first peak and the biphasic index (measuring the relative weight of the rebound phase) to be anticorrelated. In contrast, the models with feedback predict different correlations between these two measures. This suggests an experimental test assessing the relative importance of feedforward and feedback connections in shaping the impulse response of LGN relay cells.

Neural information processing with feedback modulations

Descending feedback connections, together with ascending feedforward ones, are the indispensable parts of the sensory pathways in the central nervous system. This study investigates the potential roles of feedback interactions in neural information processing. We consider a two-layer continuous attractor neural network (CANN), in which neurons in the first layer receive feedback inputs from those in the second one. By utilizing the intrinsic property of a CANN, we use a projection method to reduce the dimensionality of the network dynamics significantly. The simplified dynamics allows us to elucidate the effects of feedback modulation analytically. We find that positive feedback enhances the stability of the network state, leading to an improved population decoding performance, whereas negative feedback increases the mobility of the network state, inducing spontaneously moving bumps. For strong, negative feedback interaction, the network response to a moving stimulus can lead the actual stimulus position, achieving an anticipative behavior. The biological implications of these findings are discussed. The simulation results agree well with our theoretical analysis.

Contrast adaptation in cat lateral geniculate nucleus and influence of corticothalamic feedback

Contrast adaptation is a basic property of visual information processing. However, important questions about contrast adaptation in the lateral geniculate nucleus (LGN) remain. For example, it is unclear whether the different information channels have the same or distinct contrast adaptation properties and mechanisms. It has been recognized that the visual system is not a one-way ascending pathway, but also contains descending feedback projections. Although studies have explored the role of this feedback system, it is unclear whether corticothalamic feedback contributes to adaptation in the LGN. To investigate these questions, we studied contrast adaptation of LGN neurons in anesthetized and paralysed cats by measuring electrophysiological responses to visual test stimuli before and after adaptation induced by prolonged visual stimulation. After adaptation, contrast response functions were usually shifted towards higher contrasts, indicating decreased contrast gain, and the maximum response decreased. Also, contrast adaptation effects were stronger in Y-cells than in X-cells. Furthermore, adaptation effects were still observed in the LGN when the corticothalamic feedback was inactivated. Changes in the contrast gain of Y-cells were diminished in the absence of feedback, while contrast gain was largely unchanged in X-cells. Our observations confirm that contrast adaptation occurs in LGN neurons and furthermore demonstrate that Y-cells show stronger adaptation effects than X-cells. These results also provide an example of how corticothalamic feedback modulates contrast information processing distinctly in different information channels.

Dynamics of stimulus-evoked spike timing correlations in the cat lateral geniculate nucleus

Precisely synchronized neuronal activity has been commonly observed in the mammalian visual pathway. Spike timing correlations in the lateral geniculate nucleus (LGN) often take the form of phase synchronized oscillations in the high gamma frequency range. To study the relations between oscillatory activity, synchrony, and their time-dependent properties, we recorded activity from multiple single units in the cat LGN under stimulation by stationary spots of light. Autocorrelation analysis showed that approximately one third of the cells exhibited oscillatory firing with a mean frequency ∼80 Hz. Cross-correlation analysis showed that 30% of unit pairs showed significant synchronization, and 61% of these pairs consisted of synchronous oscillations. Cross-correlation analysis assumes that synchronous firing is stationary and maintained throughout the period of stimulation. We tested this assumption by applying unitary events analysis (UEA). We found that UEA was more sensitive to weak and transient synchrony than cross-correlation analysis and detected a higher incidence (49% of cell pairs) of significant synchrony (unitary events). In many unit pairs, the unitary events were optimally characterized at a bin width of 1 ms, indicating that neural synchrony has a high degree of temporal precision. We also found that approximately one half of the unit pairs showed nonstationary changes in synchrony that could not be predicted by the modulation of firing rates. Population statistics showed that the onset of synchrony between LGN cells occurred significantly later than that observed between retinal afferents and LGN cells. The synchrony detected among unit pairs recorded on separate tetrodes tended to be more transient and have a later onset than that observed between adjacent units. These findings show that stimulus-evoked synchronous activity within the LGN is often rhythmic, highly nonstationary, and modulated by endogenous processes that are not tightly correlated with firing rate.

Corticogeniculate feedback and visual processing in the primate

Corticogeniculate neurones make more synapses in the lateral geniculate nucleus (LGN) than retinal ganglion cells, yet we know relatively little about the functions of corticogeniculate feedback for visual processing. In primates, feedforward projections from the retina to the LGN and from the LGN to primary visual cortex are organized into anatomically and physiologically distinct parallel pathways. Recent work demonstrates a close relationship between these parallel streams of feedforward projections and the corticogeniculate feedback pathway. Here, we review the evidence for stream-specific feedback in the primate and consider the implications of parallel streams of feedback for vision.

The effect of orientation adaptation on responses of lateral geniculate nucleus neurons with high orientation bias in cats

Adaptation to stimulus orientation is assumed to have a cortical basis, but few studies have addressed whether it affects the activity of subcortical neurons. Using single-unit recording, we studied the effects of orientation adaptation on the responses of lateral geniculate nucleus (LGN) neurons with high orientation bias (OB) in anesthetized and paralyzed cats. Following adaptation to one stimulus orientation, the response at the adapting orientation was decreased, and the preferred orientation was shifted away from the adapting orientation. This phenomenon was similar to the effects observed for orientation adaptation in the primary visual cortex (V1), and was obvious when the adapting orientation was at an appropriate location relative to the original preferred orientation. Moreover, when the V1 was inactivated, the response at the adapting orientation was also decreased but the preferred orientation did not show a systematic shift after orientation adaptation in LGN. This result indicates that cortical feedback contributes to the effect of orientation adaptation on LGN neurons, which have a high OB. These data provide an example of how the corticothalamic loop modulates the processing of visual information, and suggest that the LGN is not only a simply passive relay but also a modulator of visual information.

State dependence of network output: modeling and experiments

Emerging experimental evidence suggests that both networks and their component neurons respond to similar inputs differently, depending on the state of network activity. The network state is determined by the intrinsic dynamical structure of the network and may change as a function of neuromodulation, the balance or stochasticity of synaptic inputs to the network, and the history of network activity. Much of the knowledge on state-dependent effects comes from comparisons of awake and sleep states of the mammalian brain. Yet, the mechanisms underlying these states are difficult to unravel. Several vertebrate and invertebrate studies have elucidated cellular and synaptic mechanisms of state dependence resulting from neuromodulation, sensory input, and experience. Recent studies have combined modeling and experiments to examine the computational principles that emerge when network state is taken into account; these studies are highlighted in this article. We discuss these principles in a variety of systems (mammalian, crustacean, and mollusk) to demonstrate the unifying theme of state dependence of network output.

Corticothalamic feedback enhances stimulus response precision in the visual system

There is a tightly coupled bidirectional interaction between visual cortex and visual thalamus [lateral geniculate nucleus (LGN)]. Using drifting sinusoidal grating stimuli, we compared the response of cells in the LGN with and without feedback from the visual cortex. Raster plots revealed a striking difference in the response pattern of cells with and without feedback. This difference was reflected in the results from computing vector sum plots and the ratio of zero harmonic to the fundamental harmonic of the fast Fourier transform (FFT) for these responses. The variability of responses assessed by using the Fano factor was also different for the two groups, with the cells without feedback showing higher variability. We examined the covariance of these measures between pairs of simultaneously recorded cells with and without feedback, and they were much more strongly positively correlated with feedback. We constructed orientation tuning curves from the central 5 ms in the raw cross-correlograms of the outputs of pairs of LGN cells, and these curves revealed much sharper tuning with feedback. We discuss the significance of these data for cortical function and suggest that the precision in stimulus-linked firing in the LGN appears as an emergent factor from the corticothalamic interaction.

Role of centrifugal projections to the olfactory bulb in olfactory processing

While there is evidence that feedback projections from cortical and neuromodulatory structures to the olfactory bulb are crucial for maintaining the oscillatory dynamics of olfactory bulb processing, it is not clear how changes in dynamics are related to odor perception. Using electrical lesions of the olfactory peduncle, sparing output from the olfactory bulb while decreasing feedback inputs to the olfactory bulb, we demonstrate here a role for feedback inputs to the olfactory bulb in the formation of odor-reward associations, but not for maintaining primary bulbar odor representations, as reflected by spontaneous odor discrimination.

Always returning: feedback and sensory processing in visual cortex and thalamus

Feedback projections are an integral part of the mammalian visual system. Although it is tempting to relegate them to a subsidiary role in visual processing, because their supposed latency and lag might appear to be unfavourable for an involvement in fast processing, this is a dangerous simplification. Certainly for the world in motion, feedback from higher motion areas can influence the transfer of ascending input when, or even before, the input arrives. Here, we consider the circuit formed by layer 6 feedback cells in the visual cortex and how this straddles the retinothalamic and thalamocortical transfer of visual input. We discuss its links to feedback from the cortical motion area MT (V5), and suggest that motion perception involves a dynamic interplay between MT, V1 and the thalamus. This review is part of the TINS special issue on The Neural Substrates of Cognition.

Exploring spike transfer through the thalamus using hybrid artificial-biological neuronal networks

We use dynamic clamp to construct "hybrid" thalamic circuits by connecting a biological neuron in situ to silicon- or software-generated "neurons" through artificial synapses. The purpose is to explore cellular sensory gating mechanisms that regulate the transfer efficiency of signals during different sleep-wake states. Hybrid technology is applied in vitro to different paradigms such as: (1) simulating interactions between biological thalamocortical neurons, artificial reticular thalamic inhibitory interneurons and a simulated sensory input, (2) grafting an artificial sensory input to a wholly biological thalamic network that generates spontaneous sleep-like oscillations, (3) injecting in thalamocortical neurons a background synaptic bombardment mimicking the activity of corticothalamic inputs. We show that the graded control of the strength of intrathalamic inhibition, combined with the membrane polarization and the fluctuating synaptic noise in thalamocortical neurons, is able to govern functional shifts between different input/output transmission states of the thalamic gate.

Contrast response of temporally sparse dichoptic multifocal visual evoked potentials

Temporally sparse stimuli have been found to produce larger multifocal visual evoked potentials than rapid contrast-reversal stimuli. We compared the contrast-response functions of conventional contrast-reversing (CR) stimuli and three grades of temporally sparse stimuli, examining both the changes in response amplitude and signal-to-noise ratio (SNR). All stimuli were presented dichoptically to normal adult human subjects. One stimulus variant, the slowest pattern pulse, had interleaved monocular and binocular stimuli. Response amplitudes and SNRs were similar for all stimuli at contrast 0.4 but grew faster with increasing contrast for the sparser stimuli. The best sparse stimulus provided an SNR improvement that corresponded to a recording time improvement of 2.6 times relative to that required for contrast reversing stimuli. Multiple regression of log-transformed response metrics characterized the contrast-response functions by fitting power-law relationships. The exponents for the two sparsest stimuli were significantly larger (P < 0.001) than for the CR stimuli, as were the mean response amplitudes and signal-to-noise ratios for these stimuli. The contrast-dependent response enhancement is discussed with respect to the possible influences of rapid retinal contrast gain control, or intracortical and cortico-geniculate feedback.

Anatomical pathways that link perception and action

Pathways linking action to perception are generally presented as passing from sensory pathways, through the thalamus, and then to a putative hierarchy of corticocortical links to motor outputs or to memory. Evidence for more direct sensorimotor links is now presented to show that cerebral cortex rarely, if ever, receives messages representing receptor activity only; thalamic inputs to cortex also carry copies of current motor instructions. Pathways afferent to the thalamus represent the primary input to neocortex. Generally they are made up of branching axons that send one branch to the thalamus and another to output centers of the brain stem or spinal cord. The information transmitted through the classical "sensory" pathways to the thalamus represents not only information about the environment and the body, but also about instructions currently on their way to motor centers. The proposed hierarchy of direct corticocortical connections of the sensory pathways is not the only possible hierarchy of cortical connections. There is also a hierarchy of the corticofugal pathways to motor centers in the midbrain, and there are transthalamic corticocortical pathways that may show a comparable hierarchy. The extent to which these hierarchies may match each other, and relate to early developmental changes are poorly defined at present, but are important for understanding mechanisms that can link action and perception in the developing brain.

Differences in projection patterns between large and small corticothalamic terminals

We injected tracer into wide regions of visual cortex in the cat to produce retrograde and orthograde labeling in the thalamus, chiefly in the lateral geniculate nucleus and lateral posterior-pulvinar complex (LP-Pulvinar). We used the electron microscope to measure the sizes of orthogradely labeled terminals in thalamus and used these measurements to help determine whether the terminals were "RL" (large, presumed excitatory) or "RS" (small, presumed excitatory). We also distinguished reciprocal regions, which were zones of corticothalamic feedback defined by the presence of many retrogradely labeled cell bodies and orthogradely labeled terminals, from nonreciprocal regions, which were zones of feedforward corticothalamic projections defined by the presence of orthogradely labeled terminals alone. The lateral geniculate nucleus, a reciprocal region, had retrogradely labeled cell bodies as well as labeled RS terminals. Likewise, reciprocal regions in LP-Pulvinar were dominated by labeled RS terminals. In contrast, nonreciprocal regions were dominated by labeled RL terminals. Based on other evidence of corticothalamic projections that RL and RS terminals derive, respectively, from layer 5 and layer 6, we suggest the same relationship here, leading to the conclusion that the corticothalamic input from layer 6 is largely feedback, whereas that from layer 5 is largely feedforward. This finding lends credence to a recent hypothesis that layer 5 corticothalamic axons represent the afferent limb of a cortico-thalamo-cortical pathway that is critical for corticocortical communication.

Somatosensory corticothalamic projections: distinguishing drivers from modulators

We used a juvenile mouse thalamocortical slice preparation with whole cell recording to investigate synaptic properties of corticothalamic inputs from somatosensory cortex to the ventral posterior medial and posterior medial nuclei (98 cells). We compared these data to those obtained from activating retinal and cortical inputs to the lateral geniculate nucleus (8 cells), the former representing a prototypical driver input and the latter, a typical modulator. Retinogeniculate activation evoked large, all-or-none excitatory postsynaptic potentials (EPSPs) that showed paired-pulse depression antagonized by N-methyl-d-aspartate (NMDA) and AMPA receptor blockers but with no sign of a metabotropic glutamate receptor (mGluR) component. Corticogeniculate activation evoked small, graded EPSPs showing paired-pulse facilitation, and the EPSPs showed both NMDA and AMPA receptor component plus an mGluR1 component. In the somatosensory thalamic relays, cortical stimulation elicited glutamatergic EPSPs in all thalamic cells, and these EPSPs fell into two groups. One, elicited from cortical layer 6 to cells of both nuclei, involved small, graded EPSPs with paired-pulse facilitation, and most also showed an mGluR1 component. The other, elicited from layer 5 to cells only of the posterior medial nucleus, involved large, all-or-none EPSPs with paired-pulse depression, and none showed an mGluR component. By analogy with results from the lateral geniculate nucleus, we conclude that the input from layer 6 to both nuclei acts as a modulator but that the layer 5 input to the posterior medial nucleus serves as a driver. Our data extend a common organizing principle from first-order nuclei to higher-order thalamic relays and further implicate the latter in corticocortical communication.

Quantitative analyses of principal and secondary compound parieto-occipital feedback pathways in cat

The purpose of our study was to quantify the magnitude of principal and secondary pathways emanating from the middle suprasylvian (MS) region of visuoparietal cortex and terminating in area 18 of primary visual cortex. These pathways transmit feedback signals from visuoparietal cortex to primary visual cortex. (1) WGA-HRP was injected into area 18 to identify inputs from visual structures. In terms of numbers of neurons, feedback projections to area 18 from MS sulcal cortex (areas PMLS, AMLS and PLLS) comprise 26% of inputs from all visual structures. Of these neurons, between 21% and 34.9% are located in upper layers 2-4 and the dominant numbers are located in deep layers 5 and 6. Areas 17 (11.8%) and 19 (11.2%) provide more modest cortical inputs, and another eight areas provide a combined total of 4.3% of inputs. The sum of neurons in all subcompartments of the lateral geniculate nucleus (LGN) accounts for another 34.8% of the input to area 18, whereas inputs from the lateral division of the lateral-posterior nucleus (LPl) account for the final 11.9%. (2) Injection of tritiated-((3)H)-amino acids into MS sulcal cortex revealed substantial direct projections from MS cortex that terminated in all layers of area 18, but with a markedly lower density in layer 4. Projections from MS cortex to both areas 17 and 19 are of similar density and characteristics, whereas those to other cortical targets have very low densities. Quantification also revealed minor-to-modest axon projections to all components of LGN and a massive projection throughout the LP-Pul complex. (3) Superposition of the labeled terminal and cell fields identified secondary, compound feedback pathways from MS cortex to area 18. The largest secondary pathway is massive and it includes the LPl nucleus. Much more modest secondary pathways include areas 17 and 19, and LGN. The relative magnitudes of the secondary pathways suggest that the one through LPl exerts a major influence on area 18, whereas the others exert more modest or minor influences. MS cortex in the contralateral hemisphere also innervates area 18 directly. These data are important for interpreting the impact of deactivating feedback projections from visuoparietal cortex on occipital cortex.

Selective elimination of corticogeniculate feedback abolishes the electroencephalogram dependence of primary visual cortical receptive fields and reduces their spatial specificity

The role of corticogeniculate feedback in the organization, function, and state dependence of visual responses and receptive fields (RFs) is not well understood. We investigated the contribution of the corticogeniculate loop to state-dependent changes of characteristics of the primary visual cortex response by using a novel approach of eliminating corticogeniculate projection neurons with targeted neuronal apoptosis. Experiments were performed in anesthetized cats (N2O plus halothane) with parallel recordings of single units from experimental (right) and control (left) hemispheres approximately 2 weeks after induction of apoptosis. Within the experimental hemispheres, neurons of area 17 and of the dorsal lateral geniculate nucleus (dLGN) showed an unusually enhanced and prolonged tonic visual response during episodes of synchronized (syn) EEG activity, whereas response levels during less synchronized states were almost normal. In addition, dLGN cells showed a reduced tendency for burst firing and a less regular spike interval distribution compared with those of controls. These changes are likely attributable to a tonic depolarization of dLGN relay neurons or, more likely, to a decreased responsiveness of thalamic inhibitory processes to cortical feedback. Cortical neurons also displayed an activity-dependent increase in RF size, in contrast to an almost activity-invariant RF size of controls, a phenomenon likely related to the elimination of collateral, intracortical projections of layer 6 neurons. Together, these results demonstrate that selective chronic elimination of corticogeniculate feedback results in the loss of EEG-correlated differences of visual processing in the remaining thalamocortical network, accompanied by a significant increase in excitability during syn EEG, at the expense of noticeably reduced spatial receptive-field specificity in the remaining cortical neurons.

The impact of corticothalamic feedback on the output dynamics of a thalamocortical neurone model: the role of synapse location and metabotropic glutamate receptors

The spatio-temporal integration of cortical excitatory postsynaptic potentials was investigated in a multi-compartment model of a thalamocortical neurone. Consistent with experimental data, cortical excitatory postsynaptic potentials contained a metabotropic glutamate receptor-mediated component and were generated by synapses located on distal dendrites. Within this framework, three synaptic distributions (each with equal maximal synaptic conductances) were compared: symmetric, with synapses distributed equally between all dendritic trees, single-dendrite, where synapses were allocated on all distal segments of one dendrite, and single-segment, which comprised one synapse on a single dendritic compartment. We addressed three main issues: (1) the propagation of cortical excitatory postsynaptic potentials to the soma, (2) the interaction of cortical excitatory postsynaptic potentials with proximally generated retinal excitatory postsynaptic potentials, and (3) the effectiveness of cortical excitatory postsynaptic potentials in entraining and perturbing the delta oscillation. The somatic and dendritic amplitudes of the cortical excitatory postsynaptic potentials depended on the distribution of the synapses, being largest and smallest, respectively, for the symmetric distribution, and smallest and largest, respectively, for the single-segment distribution. When a retinal excitatory postsynaptic potential followed a subthreshold cortical excitatory postsynaptic potential with a short (2-200 ms) delay, its ability to evoke action potentials was increased, with single-segment cortical excitatory postsynaptic potentials having the longest-lasting facilitatory effect. When a retinal excitatory postsynaptic potential arrived with a longer delay (210-400 ms), the effect of the cortical excitatory postsynaptic potential was to decrease the number of retinally evoked action potentials. These facilitatory and depressant effects of the cortical excitatory postsynaptic potentials were dependent on the presence of their metabotropic glutamate receptor, and were enhanced by increasing the strength of this glutamate receptor component. Axial resistivity and distal dendritic A-type current had little qualitative effect on these modulatory actions of the cortical excitatory postsynaptic potential. Cortical excitatory postsynaptic potentials were more effective than retinal excitatory postsynaptic potentials in perturbing the phase of the delta oscillation, indicating that they are ideally suited to entraining this form of rhythmic activity. Again, this effect was closely dependent on the presence of metabotropic glutamate receptor but was largely independent of synapse distribution. These results indicate that the distribution of activated synapses and the presence of metabotropic glutamate receptor are crucial factors in determining the effect of cortical feedback excitation on thalamocortical neurons. Moreover, the distinct postsynaptic receptor composition of cortical inputs renders them ideally suited to synchronising low-frequency oscillatory activity in thalamocortical neurons.

The influence of the corticothalamic projection on responses in thalamus and cortex

We review results on the in vivo properties of neurons in the dorsal lateral geniculate nucleus (dLGN) that receives its afferent input from the retina and projects to the visual cortex. In addition, the dLGN receives input from the brain stem and from a rather strong corticothalamic back-projection, which originates in layer 6 of the visual cortex. We compare the behaviour of dLGN cells during spontaneous changes of the frequency contents of the electroencephalograph (EEG) (which are mainly related to a changing brain stem influence), with those that are obtained when experimentally silencing the corticothalamic feedback. The spatial and temporal response properties of dLGN cells are compared during these two conditions, and we report that the neurons behave similarly during a synchronized EEG state and during inactive corticothalamic feedback. In both situations, dLGN cells are rather phasic and their remaining tonic activity is temporally dispersed, indicating a hyperpolarizing effect. By means of a novel method, we were able to chronically eliminate a large proportion of the corticothalamic projection neurons from the otherwise intact cortex. In this condition, we found that cortical cells also lose their EEG specific response differences but, in this instance, probably due to a facilitatory (depolarizing) plasticity reaction of the remaining network.

Adaptation in the corticothalamic loop: computational prospects of tuning the senses

The present article discusses computational hypotheses on corticothalamic feedback and modulation of cortical response properties. We have recently proposed that the two phenomena are related, hypothesizing that neuronal velocity preference in the visual cortex is altered by feedback to the lateral geniculate nucleus. We now contrast the common view that response adaptation to stimuli subserves a function of redundancy reduction with the idea that it may enhance cortical representation of objects. Our arguments lead to the concept that the corticothalamic loop is involved in reducing sensory input to behaviourally relevant aspects, a pre-attentive gating.

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.

Spike timing, synchronization and information processing on the sensory side of the central nervous system

To what extent is the variability of the neuronal responses compatible with the use of spike timing for sensory information processing by the central nervous system? In reviewing the state of the art of this question, I first analyze the characteristics of this variability with its three elements: synaptic noise, impact of ongoing activity and possible fluctuations in evoked responses. I then review the recent literature on the various sensory modalities: somato-sensory, olfactory, gustatory and visual and auditory processing. I emphasize that the conditions in which precise timing, at the millisecond level, is usually obtained, are conditions that usually require dynamic stimulation or sharp changes in the stimuli. By contrast, situations in which stimulation not belonging to the temporal domain is temporally encoded lead to much coarser temporal coding; although in both cases, neural networks transmit the signals with similarly high precision. Synchronization among neurons is an important tool in information processing in both cases but again seems to act either at millisecond or tens of millisecond levels. Information theory applied to both situations confirms that the average rate of information transmission is much higher in dynamic than in static situations. These facts suggest that channels of precise temporal encoding may exist in the brain but imply populations of neurons working in a yet to be discovered way.

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.

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.

Visual cortical mechanisms of perceptual grouping: interacting layers, networks, columns, and maps

The visual cortex has a laminar organization whose circuits form functional columns in cortical maps. How this laminar architecture supports visual percepts is not well understood. A neural model proposes how the laminar circuits of V1 and V2 generate perceptual groupings that maintain sensitivity to the contrasts and spatial organization of scenic cues. The model can decisively choose which groupings cohere and survive, even while balanced excitatory and inhibitory interactions preserve contrast-sensitive measures of local boundary likelihood or strength. In the model, excitatory inputs from lateral geniculate nucleus (LGN) activate layers 4 and 6 of V1. Layer 6 activates an on-center off-surround network of inputs to layer 4. Together these layer 4 inputs preserve analog sensitivity to LGN input contrasts. Layer 4 cells excite pyramidal cells in layer 2/3, which activate monosynaptic long-range horizontal excitatory connections between layer 2/3 pyramidal cells, and short-range disynaptic inhibitory connections mediated by smooth stellate cells. These interactions support inward perceptual grouping between two or more boundary inducers, but not outward grouping from a single inducer. These boundary signals feed back to layer 4 via the layer 6-to-4 on-center off-surround network. This folded feedback joins cells in different layers into functional columns while selecting winning groupings. Layer 6 in V1 also sends top-down signals to LGN using an on-center off-surround network, which suppresses LGN cells that do not receive feedback, while selecting, enhancing, and synchronizing activity of those that do. The model is used to simulate psychophysical and neurophysiological data about perceptual grouping, including various Gestalt grouping laws.

Cortical involvement in the induction, but not expression, of thalamic plasticity

The present study examined the role of the somatosensory cortex in the plasticity of thalamic sensory maps. Thalamic plasticity was induced by the disruption of hindlimb input by unilateral destruction of nucleus gracilis. Unilateral somatosensory cortex lesions were performed either on the same day as or a week after the removal of hindlimb input. Multiple electrode penetrations enabled us to measure the volume of somatosensory thalamus devoted to hindlimb, forepaw, and shoulder body regions. Cortical lesions alone did not change the volume of the shoulder, forepaw, or hindlimb representations in the thalamus relative to controls. However, these lesions blocked the increase in shoulder representation resulting from the nucleus gracilis lesion. In contrast, if thalamic reorganization caused by removal of hindlimb input was allowed to occur, subsequent somatosensory cortex lesions 1 week later did not prevent reorganization. Thus, an intact somatosensory cortex is necessary for the occurrence of sensory map reorganization at the thalamic level (induction) in response to nucleus gracilis lesions, but not for the maintenance of such changes once they are present (expression).

Single units and visual cortical organization

The visual system has a parallel and hierarchical organization, evident at every stage from the retina onwards. Although the general benefits of parallel and hierarchical organization in the visual system are easily understood, it has not been easy to discern the function of the visual cortical modules. I explore the view that striate cortex segregates information about different attributes of the image, and dispatches it for analysis to different extrastriate areas. I argue that visual cortex does not undertake multiple relatively independent analyses of the image from which it assembles a unified representation that can be interrogated about the what and where of the world. Instead, occipital cortex is organized so that perceptually relevant information can be recovered at every level in the hierarchy, that information used in making decisions at one level is not passed on to the next level, and, with one rather special exception (area MT), through all stages of analysis all dimensions of the image remain intimately coupled in a retinotopic map. I then offer some explicit suggestions about the analyses undertaken by visual areas in occipital cortex, and conclude by examining some objections to the proposals.

The influence of corticofugal feedback on the temporal structure of visual responses of cat thalamic relay cells

1. Visually driven single-unit activity was recorded in the dorsal lateral geniculate nucleus (dLGN) of the anaesthetized cat while inactivating or stimulating the corticofugal feedback from area 17/18 by means of cortical cooling or application of GABA (inactivation), or application of glutamate or quisqualate (Glu, Quis; stimulation) to layer VI. 2. Manipulations of the corticofugal feedback primarily affected the multimodal interspike interval pattern previously reported to be present in the tonic component of visual responses elicited by spot-like stimuli. 3. Sixty-three per cent of all neurons could be influenced, and temporally localized interspike interval distributions were measured which commonly consisted of one fundamental interval peak (leftmost peak) and integer multiples thereof (higher order peaks). During blockade of the corticofugal feedback, interspike intervals were redistributed into the higher order peaks in about 70 % of the cases, accompanied by a reduced mean firing rate. During stimulation the reverse effect occurred in 69 % of cases. 4. Increased synchronization of the EEG (increased power in the delta-wave range, 1-4 Hz) had an effect similar to cortex inactivation. The specificity of corticofugal effects was verified by consideration of these EEG effects and by dLGN double recordings with one dLGN cell topographically matched with the cortical inactivation/activation site and the second cell outside this area. Clear effects due to manipulation of the corticofugal feedback were found only for the matched dLGN site. 5. In addition we observed that the peaks of the interval distributions were narrower during active corticofugal feedback, such that the temporal dispersion of the signal transmission to the cortex was reduced. 6. The mechanisms underlying this effect were further analysed in a biophysically realistic model demonstrating that the timing of the spikes in the dLGN is improved as soon as the cortical feedback is active. The high degree of convergence/divergence between neurons along the closed feedback loop thereby leads to a temporal averaging effect which reduces the interval dispersion and also introduces synchronization between dLGN cells. 7. Such a mechanism may thus counteract the deterioration of spike timing accuracy which would otherwise occur as a consequence of synaptic noise and other uncorrelated sources of activity at a given neuron.

Evidence for spatial aliasing effects in the Y-like cells of the magnocellular visual pathway

Several lines of evidence are provided indicating that our visual percept can be dominated by spatial aliasing for viewing conditions near those needed to see the spatial frequency doubled illusion. The apparent aliasing effect indicates that the underlying sampling array has a density 15-30% of that of M-cells, in agreement with the known proportion of Y-like M-cells (M(y)-cells). The presence of aliasing indicates, that there is a separate irregular array of M(y)-cells, and that their role is to rapidly convey information on retinal gain control to the brain rather than to act primarily as inputs to image motion computation.

On the significance of temporally structured activity in the dorsal lateral geniculate nucleus (LGN)

Higher organisms perceive information about external or internal physical or chemical stimuli with specialized sensors that encode characteristics of that stimulus by a train of action potentials. Usually, the location and modality of the stimulus is represented by the location and specificity of the receptor and the intensity of the stimulus and its temporal modulation is thought to be encoded by the instantaneous firing rate. Recent studies have shown that, primarily in cortical structures, special features of a stimulus also are represented in the temporal pattern of spike activity. Typical attributes of this time structure are oscillatory patterns of activity and synchronous discharges in spatially distributed neurons that respond to inputs evoked by a coherent object. The origin and functional significance of this kind of activity is less clear. Cortical, subcortical and even very peripheral sources seem to be involved. Most of the relevant studies were devoted to the mammalian visual system and cortical findings on temporally structured activity were reviewed recently (Eckhorn, 1994, Progr. Brain Res., Vol. 102, pp. 405-426; Singer and Gray, 1995, Annu. Rev. Neurosci., Vol. 18, pp. 555-586). Therefore, this article is designed to give an overview, especially of those studies concerned with the temporal structure of visual activity in subcortical centers of the primary visual pathway, which are the retina and the dorsal lateral geniculate nucleus (LGN). We discuss the mechanisms that possibly contribute to the generation and modulation of the subcortical activity time structure and we try to relate to each other the subcortical and cortical patterns of sensory activity.

Visual brain and visual perception: how does the cortex do perceptual grouping?

How the brain generates visual percepts is a central problem in neuroscience. We propose a detailed neural model of how lateral geniculate nuclei and the interblob cortical stream through V1 and V2 generate context-sensitive perceptual groupings from visual inputs. The model suggests a functional role for cortical layers, columns, maps and networks, and proposes homologous circuits for V1 and V2 with larger-scale processing in V2. An integrated treatment of interlaminar, horizontal, orientational and endstopping cortical interactions and a role for corticogeniculate feedback in grouping are proposed. Modeled circuits simulate parametric psychophysical data about boundary grouping and illusory contour formation.

Reversible deactivation of cerebral network components

Reversible deactivation techniques have shown that the cerebral network: (1) is dynamic, its functions depending on contemporaneous processing elsewhere in the network; (2) is composed of single nodes that contribute to several behaviors; (3) possesses an inherent plasticity that tends to minimize lesion-induced deficits; and (4) comprises feedforward and lateral connections that contribute in different ways to network operations. The next major advances in understanding network operations will probably be made by applying a combination of behavioral, neuron-recording and deactivation techniques. The greatest near-term gains are likely to be made in understanding the contributions that feedback projections make to cerebral network function.

The influence of the visual cortex on the spatiotemporal response properties of lateral geniculate nucleus cells

Previous studies of the cortical input to the mammalian dorsal lateral geniculate nucleus (LGN) have identified a number of possible functions for the corticogeniculate pathway, including alteration of LGN spatial frequency selectivity and facilitation of both binocular interactions and orientation selectivity. These changes may be due to either a tonic or a phasic cortical facilitation or both. The temporal differences between each of these inputs suggests that their impact on LGN cell temporal tuning should be unique. To test this hypothesis, we reversibly blocked the visual cortex (VI) and measured the effects on several indices of the temporal properties of LGN cells, including peak frequency, bandwidth, and response phase. Macaque monkeys were anesthetized and paralyzed during single cell recording from the LGN while area VI was cryogenically deactivated. Single-cell responses were visually evoked with drifting, luminance-modulated, sine-wave gratings and discrete-Fourier analyzed. Cortical cooling produced statistically significant increases or decreases in response amplitude in 64% of cells recorded. In most cases, alterations in response amplitude occurred for stimuli that varied in spatial as well as temporal frequency. For those cells influenced by changes in stimulus temporal frequency, the majority showed changes over a broad range of frequencies. A minority of cells showed changes in either peak temporal tuning or temporal frequency bandwidth. Response phase angles for all temporal frequencies tested were unaffected by cortical cooling. Overall, these results suggest that the cortical input may alter the temporal response properties of LGN cells, perhaps by tonic, but not exclusively excitatory, corticofugal influences.

No binocular rivalry in the LGN of alert macaque monkeys

Orthogonal drifting gratings were presented binocularly to alert macaque monkeys in an attempt to find neural correlates of binocular rivalry. Gratings were centered over lateral geniculate nucleus (LGN) receptive fields and the corresponding points for the opposite eye. The only task of the monkey was to fixate. We found no difference between the responses of LGN neurons under rivalrous and nonrivalrous conditions, as determined by examining the ratios of their respective power spectra. There was, however, a curious "temporal afterimage" effect in which cell responses continued to be modulated at the drift frequency of the grating for several seconds after the grating disappeared.

Dual response modes in lateral geniculate neurons: mechanisms and functions

Relay cells of the lateral geniculate nucleus, like those of other thalamic nuclei, manifest two distinct response modes, and these represent two very different forms of relay of information to cortex. When relatively hyperpolarized, these relay cells respond with a low threshold Ca2+ spike that triggers a brief burst of conventional action potentials. These cells switch to tonic mode when depolarized, since the low threshold Ca2+ spike, being voltage dependent, is inactivated at depolarized levels. In this mode they relay information with much more fidelity. This switch can occur under the influence of afferents from the visual cortex or parabrachial region of the brain stem. It has been previously suggested that the tonic mode is characteristic of the waking state while the burst mode signals an interruption of the geniculate relay during sleep. This review surveys the key properties of these two response modes and discusses the implications of new evidence that the burst mode may also occur in the waking animal.

Functional interactions between areas V1 and V2 in the monkey

The role of feedback connections from area V2 to V1 was studied by reversible inactivation. When V2 was inactivated the responses of some V1 neurons to stimulation of the surround region were increased while responses to center stimulation were unchanged or decreased. Latencies to small flashing stimuli were also compared in areas V1 and V2. The distributions in the two areas overlap largely, with a 10 ms shift between the two. Neurons of V1 and V2 that are driven by the magnocellular layers of the LGN are activated 20 ms earlier than neurons of the parvocellular stream.

Context-dependent interactions and visual processing in V1

We examined the influence of stimulus context on the response of cells in primate V1 utilising both concentric and spatially discrete stimuli. The majority of cells (63/71) showed marked patch suppression, including non-oriented cells. This suppression was reduced or lost if there was an orientation discontinuity in the stimulus overlying the receptive field. Cross-oriented stimuli could exert strong facilitatory effects so that a cell's response to an optimally oriented stimulus over its receptive field was increased by the presence of an adjacent cross-oriented stimulus. This increase appeared to involve both disinhibition as well as a direct facilitation. The strength of the cross-orientation effects was such that for some cells it seemed appropriate to define a cross-oriented stimulus configuration as the 'optimal' stimulus. Effects following from orientation context could be strongly influenced by stimulus direction. Subcortical as well as cortical interactions may contribute to these observations. It is suggested that the properties of the network as a whole define the responses of individual cells and that the representation of discontinuities is an important component of network function in V1.

An intracellular study of space and time representation in primary visual cortical receptive fields

In contrast with previous knowledge based on extracellular recordings, the recent development of intracellular techniques in vivo (sharp electrode or 'blind patch') ideally allows experimenters to analyze and dissect the contribution of feedforward and lateral connectivity in the functional expression of a synaptic 'integration field'. We will present recent data which demonstrate that the visual receptive field of cortical neurons described at the level of subthreshold synaptic events extends over much larger regions of the visual field than previously thought, and that the capacity of cells to amplify subthreshold responses depends on the immediate past history of their membrane potential. Our data suggest that visual cortical receptive fields should not be considered as a fixed entity but more as a dynamic field of integration and association. Two types of dynamics can be argued for: 1) the spatial structure of the minimal discharge field (defined by suprathreshold activation of the cell) can be profoundly reorganized at least during development and most probably during selective phases of learning under the control of activity-dependent mechanisms. Adaptive changes in visual responses are thought to reflect long-lasting potentiation and/or depression of synaptic efficacies conveying ON- and OFF-center information; and 2) during sensory processing, reconfiguration of synaptic weights may be achieved on a much faster time-scale and linked to nor-linear properties of the postsynaptic membrane as well as that of recruited networks. Association of information available in the central part of the receptive field (RF) and of input coming from the reputedly 'unresponsive' regions surrounding it, or arising simultaneously from different parts of the visual field, might be suppressive in certain cases and capable of boosting hidden responses in other cases, depending on the global stimulus configuration.