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Y Frégnac

Publications and source records attributed to Y Frégnac.

At least 19 recordsLinked to original sources
PubMed1999

Horizontal propagation of visual activity in the synaptic integration field of area 17 neurons.

The receptive field of a visual neuron is classically defined as the region of space (or retina) where a visual stimulus evokes a change in its firing activity. At the cortical level, a challenging issue concerns the roles of feedforward, local recurrent, intracortical, and cortico-cortical feedback connectivity in receptive field properties. Intracellular recordings in cat area 17 showed that the visually evoked synaptic integration field extends over a much larger area than that established on the basis of spike activity. Synaptic depolarizing responses to stimuli flashed at increasing distances from the center of the receptive field decreased in strength, whereas their onset latency increased. These findings suggest that subthreshold responses in the unresponsive region surrounding the classical discharge field result from the integration of visual activation waves spread by slowly conducting horizontal axons within primary visual cortex.

Action Potentials↗
PubMed1999

Activity-dependent regulation of receptive field properties of cat area 17 by supervised Hebbian learning.

Most algorithms currently used to model synaptic plasticity in self-organizing cortical networks suppose that the change in synaptic efficacy is governed by the same structuring factor, i.e., the temporal correlation of activity between pre- and postsynaptic neurons. Functional predictions generated by such algorithms have been tested electrophysiologically in the visual cortex of anesthetized and paralyzed cats. Supervised learning procedures were applied at the cellular level to change receptive field (RF) properties during the time of recording of an individual functionally identified cell. The protocols were devised as cellular analogs of the plasticity of RF properties, which is normally expressed during a critical period of postnatal development. We summarize here evidence demonstrating that changes in covariance between afferent input and postsynaptic response imposed during extracellular and intracellular conditioning can acutely induce selective long-lasting up- and down-regulations of visual responses. The functional properties that could be modified in 40% of cells submitted to differential pairing protocols include ocular dominance, orientation selectivity and orientation preference, interocular orientation disparity, and the relative dominance of ON and OFF responses. Since changes in RF properties can be induced in the adult as well, our findings also suggest that similar activity-dependent processes may occur during development and during active phases of learning under the supervision of behavioral attention or contextual signals. Such potential for plasticity in primary visual cortical neurons suggests the existence of a hidden connectivity expressing a wider functional competence than the one revealed at the spiking level. In particular, in the spatial domain the sensory synaptic integration field is larger than the classical discharge field. It can be shaped by supervised learning and its subthreshold extent can be unmasked by the pharmacological blockade of intracortical inhibition.

Action Potentials↗
PubMed1998

Visual input evokes transient and strong shunting inhibition in visual cortical neurons.

The function and nature of inhibition of neurons in the visual cortex have been the focus of both experimental and theoretical investigations. There are two ways in which inhibition can suppress synaptic excitation. In hyperpolarizing inhibition, negative and positive currents sum linearly to produce a net change in membrane potential. In contrast, shunting inhibition acts nonlinearly by causing an increase in membrane conductance; this divides the amplitude of the excitatory response. Visually evoked changes in membrane conductance have been reported to be nonsignificant or weak, supporting the hyperpolarization mode of inhibition. Here we present a new approach to studying inhibition that is based on in vivo whole-cell voltage clamping. This technique allows the continuous measurement of conductance dynamics during visual activation. We show, in neurons of cat primary visual cortex, that the response to optimally orientated flashed bars can increase the somatic input conductance to more than three times that of the resting state. The short latency of the visually evoked peak of conductance, and its apparent reversal potential suggest a dominant contribution from gamma-aminobutyric acid ((GABA)A) receptor-mediated synapses. We propose that nonlinear shunting inhibition may act during the initial stage of visual cortical processing, setting the balance between opponent 'On' and 'Off' responses in different locations of the visual receptive field.

Animals↗
PubMed1997

Synaptic origin and stimulus dependency of neuronal oscillatory activity in the primary visual cortex of the cat.

1. We have studied the oscillatory activity of single neurons (91 recorded extracellularly and 76 intracellularly) in the primary visual cortex of cats and kittens to characterize its origins and its stimulus dependency. A new method for the detection of oscillations was developed in order to maximize the range of detectable frequencies in both types of recordings. Three types of activity were examined: spontaneous background activity, responses to intracellular current steps and visual responses. 2. During spontaneous activity, persistent oscillatory activity was very rare in both types of recordings. However, when intracellular records were made using KCl-filled micropipettes, spontaneous activity appeared rhythmic and contained repeated depolarizing events at a variety of frequencies, suggestive of tonic periodic inhibitory input normally masked at resting potential. 3. Patterns of firing activity in response to intracellular current steps allowed us to classify neurons as regular spiking, intrinsically bursting, and fast-spiking types, as described in vitro. In the case of rhythmically firing cells, the spike frequency increased with the amount of injected current. Subthreshold current-induced oscillations were rarely observed (2 out of 76 cells). 4. Visual stimulation elicited oscillations in one-third of the neurons (55 out of 167), predominantly in the 7-20 Hz frequency range in 93% of the cases. Rhythmicity was observed in both simple and complex cells, and appeared to be more prominent at 5 and 6 weeks of age. 5. Intracellular recordings in bridge mode and voltage clamp revealed that visually evoked oscillations were driven by synaptic activity and did not depend primarily on the intrinsic properties of recorded neurons. Hyperpolarizing the membrane led to an increase in the size of the rhythmic depolarizing events without a change in frequency. In voltage-clamped cells, current responses showed large oscillations at the same frequency as in bridge mode, independently of the actual value of the holding potential. 6. In fourteen intracellularly recorded neurons, oscillations consisted of excitatory events that could be superimposed on a depolarizing or a hyperpolarizing slow wave. In two other neurons, visual responses consisted of excitatory and inhibitory events, alternating with a constant phase shift. 7. Drifting bars were much more efficient in evoking oscillatory responses than flashed bars. Except in three cells, the frequency of the oscillation did not depend on the physical characteristics of the stimulus that were tested (contrast, orientation, direction, ocularity and position in the receptive field). No significant correlation was found between the intensity of the visual response and the strength of the rhythmic component. 8. Although it cannot be excluded that the dominant frequency of oscillations might be related to the type of anaesthetics used, no correlation was found between local EEG and the oscillatory activity elicited by visual stimulation. 9. We conclude that the oscillations observed in the present work are generated by synaptic activity. It is likely that they represent an important mode of transmission in sensory processing, resulting from periodic packets of synchronized activity propagated across recurrent circuits. Their relevance to perceptual binding is further discussed.

Animals↗
PubMed1996

Dynamics of functional connectivity in visual cortical networks: an overview.

The aim of the Royaumont Symposium was to review various dynamic aspects of adaptive changes in functional connectivity, expressed in cortical networks during development, learning, and possibly during recognition and cognitive processing. The link between the various experimental and theoretical models was the comparison of cellular and molecular mechanisms that could be involved in the up- and down-regulation of functional connectivity, over different time scales. These processes have been investigated using several approaches in parallel: 1) at the molecular/subcellular level, to identify postsynaptic receptors (NMDA, mGluR) and second messengers (calcium protein kinases and phosphatases) involved in the induction of synaptic potentiation and depression, and to characterize diffusible factors (NO), released pre- or postsynaptically, involved in the spatial generalization of local changes to neighboring synapses; 2) at the level of integrating networks, to develop electrophysiological (single and multiple recording), pharmacological and optical imaging techniques in order to compare the dynamics of adaptive processes put into play during the natural development of cortical specificity and connectivity, versus those triggered during forced regimes of temporal correlations between pre- and post synaptic activities. Both in vitro and in vivo approaches have been combined in the primary visual cortex of the developing and adult vertebrate (rat, guinea-pig, ferret, cat and monkey). The various forms of 'slow' synaptic plasticity, demonstrated during epigenesis and selective phases of learning in the adult, can be compared with 'fast' forms of functional coupling (or synchronous firing) shown to develop during the time span required for perception and cognitive processing Phenomenology of the dynamics in functional connectivity and their relative dependence on temporal correlation in neuronal activity have been analyzed in each of these situations. Experimental results have been compared at different levels of neuronal integration (synapse, column map and cell assembly) in order to gain a better understanding of functional grouping within cortical networks.

Adaptation, Physiological↗
PubMed1996

Voltage-clamp measurement of visually-evoked conductances with whole-cell patch recordings in primary visual cortex.

Whole cell patch recordings have been realized in the primary visual cortex of the anesthetized and paralyzed cat, in order to better characterize input resistance and time constant of visual cortical cells in vivo. Measurements of conductance changes evoked by visual stimulation were derived from voltage clamp recordings achieved in continuous mode at two or more different subthreshold holding potentials. They show that the magnitude of the conductance increase can reach up to 300% of the mean conductance at rest. The observation of similar changes for the preferred and antagonist responses, when flashing ON and OFF, a test stimulus in pure ON and OFF subfields supports the hypothesis of a role for shunting inhibition in the spatial organization of simple receptive fields.

Animals↗
PubMed1996

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.

Animals↗
PubMed1996

Synaptic integration fields and associative plasticity of visual cortical cells in vivo.

Two major constraints in connectivity decide the spatial extent of visual cortical receptive fields, both during development and adult functioning: 1) feedforward input, extrinsic to visual cortex, is organized in an orderly projection to form a point-to-point mapping of the retina onto the cortical mantle and constitutes the core of the minimal discharge field (MDF) after amplification by local intracortical circuits; and 2) a second type of connectivity consists of long distance horizontal intracortical connections which are thought to favor the binding of local visual operations occurring simultaneously in different parts of the visual field. We review here possible factors, intrinsic to the considered cortical cell, that may control the effectiveness of post-synaptic integration. Their expression during sensory recognition could depend on the spatio-temporal distribution of active inputs onto the target cell dendrite and on their interplay with non-linearities of the membrane properties. On the basis of intracellular recordings in cat area, 17, we have observed that peripheral responses (excitatory and inhibitory) could be boosted by coincident postsynaptic depolarization. This effect is lost if the response and the postsynaptic depolarization are mismatched by 1,000 ms, suggesting that temporal correlation of central and peripheral responses could regulate in a non-linear manner the gain of center-surround interactions. This temporal selectivity is compatible with up-regulation of composite potentials by the transient voltage-dependent activation of slowly inactivating conductances in visual cortical neurons. A direct consequence of these different considerations is that the receptive field (RF) of neurons in visual pathways should not be considered as a static hardwired window probing the outer environment, but as an active filter which may continuously adapt and be updated as a function of global context and past experience.

Animals↗
PubMed1995

Temporal constraints in associative synaptic plasticity in hippocampus and neocortex.

We present comparative experimental evidence for the induction of synaptic potentiation and depression in organotypic cultures of hippocampus and in visual cortex in vitro and in vivo. The effects of associative pairings on the efficacy of synaptic transmission are analyzed as a function of the temporal delay between presynaptic activity and post-synaptic changes imposed in membrane potential. Synchronous association at a low temporal frequency (< 0.5 Hz) between presynaptic input and postsynaptic depolarization resulted in homosynaptic potentiation of functionally identified postsynaptic potentials in the three types of preparation. Synchronous pairing of afferent activity with hyperpolarization of the postsynaptic cell resulted in homosynaptic depression in visual cortex in vivo and in vitro. An associative form of depression was induced in hippocampus when the test input was followed repeatedly with a fixed-delay postsynaptic depolarization imposed either by intracellular current injection or synaptically. The latter process might play a significant role in heterosynaptic plasticity in visual cortex in vivo and in vitro, if it is assumed that associative depression still operates in visual cortex a few seconds after the initial surge of calcium in the postsynaptic cell. We conclude that the precise timing between presynaptic activity and polarization changes in postsynaptic membrane potential up- and down-regulates the efficacy of active pathways.

Adult↗
PubMed1994

Temporal covariance of pre- and postsynaptic activity regulates functional connectivity in the visual cortex.

1. It has been suggested from mathematical models and in vivo experiments in the visual cortex that periods of temporal covariance of pre- and postsynaptic activity can lead to a potentiation or depression of synaptic efficacy. We directly tested this hypothesis in vitro in the guinea pig and cat visual cortex. 2. Intracellular recordings were made in brain slices from 63 neurons in layers 2-4 in bicuculline-free artificial cerebrospinal fluid. Twenty-nine cells (n = 25 from pigmented guinea pigs and 4 from cats) were taken through a complete series of control and test protocols to evaluate the covariance hypothesis. Some (n = 7) cells that were taken through the complete experimental protocols were also filled intracellularly with biocytin. Compound postsynaptic potentials (PSPs) were evoked by low-frequency (0.2-1.0 Hz), weak (20% of threshold intensity) stimulation of the cortical white matter and/or intracortical sites in layers 2-3. 3. In one series of experiments we paired PSPs with imposed coincident depolarizing (S+) or hyperpolarizing (S-) pulses (mean +/- 2.8 nA for 50-80 ms) of the postsynaptic neuron (n = 54 PSPs; > 1 pairing protocol was often run on an individual cell). Controls consisted of analyzing the same number of S+ or S- pairings but with long temporal delays [called fixed delay pairings (FDPs)] between the test pathway stimulation and the onset of the intracellular current pulse (120 ms) and pseudopairings (PP) consisting of evoked PSPs and delivery of intracellular current injection pulses in a phase-independent manner. Twenty-one of 54 PSPs subjected to pairing were significantly modified by the protocol. The S+ protocol significantly (P < 0.05, Kolmogorov-Smirnov test) increased the peak amplitudes of 8 of 22 PSPs (+20 to +55%); the S- protocol significantly decreased the peak amplitudes of 13 of 32 PSPs (-15 to -88%), whereas the FDP and PP protocols generally did not cause significant changes in the PSPs (0% and 4%, respectively). Significant changes in PSPs persisted in most cases for 10-20 min. 4. Another series of experiments consisted of evaluating for the same cell the effects of evoking a PSP from one stimulation site without concomitant postsynaptic activation and alternately evoking a PSP from the other stimulation site with S+ or S- pairing (n = 25 PSPs). Only the paired pathway showed the predicted effects on the PSP (S+ pairing causing an increase in peak PSP amplitude and S- pairing causing a decrease in peak PSP amplitude).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

Cellular analogs of visual cortical epigenesis. I. Plasticity of orientation selectivity.

A differential pairing procedure was applied in vivo to individual neurons in the primary visual cortex of anesthetized paralyzed cats, in order to produce changes in their relative orientation preference. While we recorded from a single cell, its visual response to a light bar was driven iontophoretically to a "high" level when stimulating with an initially nonpreferred orientation (S+), and alternately reduced to a "low" level when stimulating with the preferred orientation (S). This associative procedure was devised to test the possible role of neuronal coactivity in controlling the plasticity of orientation selectivity. Among 87 cells tested, 35 (40%) showed significant long-lasting changes, either in the relative orientation preference for the two "paired" stimuli S+ and S-, in the global orientation tuning profile, or in both. Measurements of relative orientation preference demonstrated significant effects in 27 cells (31%), all in favor of the positively reinforced orientation (S+). Modifications of orientation selectivity (studied over the entire orientation spectrum in 45 of the conditioned cells) usually consisted (21 out of 25 modified cells) of a competitive reorganization of the orientation tuning curve: the preferred orientation shifted toward S+, and a loss of relative visual responsiveness was observed for orientations close to the negatively reinforced orientation (S-). The largest changes were found in deprived kittens at the peak of the critical period, although the probability of inducing a significant change studied during the first year of postnatal life was independent of age. These functional modifications demonstrated at the cellular level are analogous to those induced by a global manipulation of the visual environment, when only a restricted spectrum of orientations is experienced during the critical period. Our results support the hypothesis that covariance levels between pre- and postsynaptic activity determine the sign and the amplitude of the modification of efficacy of cortical synapses.

Animals↗
PubMed1992

Cellular analogs of visual cortical epigenesis. II. Plasticity of binocular integration.

Two differential pairing procedures were applied in the primary visual cortex of anesthetized and paralyzed kittens and cats, to produce changes in ocular dominance and interocular orientation disparity (IOD) during the time of recording of a single neuron. A first experiment was devised to demonstrate plasticity in the balance of monocular responses. The visual activity of the cell was driven iontophoretically to either a "high" or a "low" level, depending on the ocularity of the visual stimulation. Ocular dominance measurements before and after pairing revealed significant long-lasting changes in 33% of cases. Relative ocular preference shifted in most cases (87.5%) in favor of the reinforced eye. Similar proportions of significant changes were observed in kitten and adult cortex. The amplitude of the functional modifications was not significantly related with age, although the largest changes in ocular dominance were recorded at the peak of the critical period. The second experiment more specifically addressed the plasticity of binocular interaction. The activity of a binocular cell was driven iontophoretically to either a "high" or a "low" level, depending on the orientation disparity between two oriented stimuli, presented simultaneously and separately in the receptive field of each eye. Significant long-lasting changes in binocular responses were induced in 40% of cases. The relative IOD preference generally shifted (67%) in favor of the reinforced disparity. In half of the modified cells, functional changes were expressed only in the dichoptic viewing condition used during the pairing procedure. These functional modifications of binocular integration, demonstrated at the cellular level, are analogous to those induced by global manipulations of the visual environment (Hubel and Wiesel, 1970; Shinkman and Bruce, 1977). They are interpreted as evidence for synaptic plasticity. Our results support the hypothesis that covariance levels between pre- and postsynaptic activities determine the sign and the amplitude of changes in synaptic efficacy.

Animals↗
PubMed1989

Lesion of the PGO pathways in the kitten. II. Impairment of physiological and morphological maturation of the lateral geniculate nucleus.

Suppression of the geniculate ponto-geniculo-occipital (PGO) waves by bilateral lesions of PGO pathways at the mesencephalic level in 15-day-old kittens has been shown to induce a significant reduction of the mean discharge frequency recorded in the lateral geniculate nucleus (LGN) during paradoxical sleep. The present paper reports that one month after the bilateral lesion (i.e., 6-7 weeks of age) important deficits in the maturation of the LGN were observed: (1) electrophysiologically, the latencies of the LGN cellular responses to stimulation of the optic chiasm were significantly longer than those of age-paired controls or of unilaterally lesioned animals, and the proportion of visual cells characterized as type X by stimulation of the visual field was smaller; and (2) morphologically, the volume of the LGN and the size of its neuronal somata were smaller than those in control. These data suggest that bilateral suppression of extraretinal PGO afferents to the LGN in the kitten induces a significant delay in the development of this nucleus.

Action Potentials↗
PubMed1988

A cellular analogue of visual cortical plasticity.

Neuronal activity plays an important role in the development of the visual pathway. The modulation of synaptic transmission by temporal correlation between pre- and postsynaptic activity is one mechanism which could underly visual cortical plasticity. We report here that functional changes in single neurons of area 17, analogous to those known to take place during epigenesis of visual cortex, can be induced experimentally during the time of recording. This was done by a differential pairing procedure, during which iontophoresis was used to artificially increase the visual response for a given stimulus, and to decrease (or block) the response for a second stimulus which differed in ocularity or orientation. Long-term modifications in ocular dominance and orientation selectivity were produced in 33% and 43% of recorded cells respectively. Neuronal selectivity was nearly always displaced towards the stimulus paired with the reinforced visual response. The largest changes were obtained at the peak of the critical period in normally reared and visually deprived kittens, but changes were also observed in adults. Our findings support the role of temporal correlation between pre- and postsynaptic activity in the induction of long-lasting modifications of synaptic transmission during development, and in associative learning.

Afferent Pathways↗