Synaptic depression and cortical gain control

Presynaptic short-term depression is maintained during regulation of transmitter release at a GABAergic synapse in rat hippocampus

To examine possible interactions between fast depression and modulation of inhibitory synaptic transmission in the hippocampus, we recorded from pairs of synaptically connected basket cells (BCs) and granule cells (GCs) in the dentate gyrus of rat brain slices at 34 degrees C. Multiple-pulse depression (MPD) was examined in trains of 5 or 10 inhibitory postsynaptic currents (IPSCs) evoked at frequencies of 10-100 Hz under several conditions that inhibit transmitter release: block of voltage-dependent Ca2+ channels by Cd2+ (10 microM), activation of gamma-amino-butyric acid type B receptors (GABA(B)Rs) by baclofen (10 microM) and activation of muscarinic acetylcholine receptors (mAchRs) by carbachol (2 microM). All manipulations led to a substantial inhibition of synaptic transmission, reducing the amplitude of the first IPSC in the train (IPSC1) by 72%, 61% and 29%, respectively. However, MPD was largely preserved under these conditions (0.34 in control versus 0.31, 0.50 and 0.47 in the respective conditions at 50 Hz). Similarly, a theta burst stimulation (TBS) protocol reduced IPSC1 by 54%, but left MPD unchanged (0.40 in control and 0.39 during TBS). Analysis of both fractions of transmission failures and coefficients of variation (CV) of IPSC peak amplitudes suggested that MPD had a presynaptic expression site, independent of release probability. In conclusion, different types of presynaptic modulation of inhibitory synaptic transmission converge on a reduction of synaptic strength, while short-term dynamics are largely unchanged.

Early consolidation in human primary motor cortex

Behavioural studies indicate that a newly acquired motor skill is rapidly consolidated from an initially unstable state to a more stable state, whereas neuroimaging studies demonstrate that the brain engages new regions for performance of the task as a result of this consolidation. However, it is not known where a new skill is retained and processed before it is firmly consolidated. Some early aspects of motor skill acquisition involve the primary motor cortex (M1), but the nature of that involvement is unclear. We tested the possibility that the human M1 is essential to early motor consolidation. We monitored changes in elementary motor behaviour while subjects practised fast finger movements that rapidly improved in movement acceleration and muscle force generation. Here we show that low-frequency, repetitive transcranial magnetic stimulation of M1 but not other brain areas specifically disrupted the retention of the behavioural improvement, but did not affect basal motor behaviour, task performance, motor learning by subsequent practice, or recall of the newly acquired motor skill. These findings indicate that the human M1 is specifically engaged during the early stage of motor consolidation.

Redundancy reduction and sustained firing with stochastic depressing synapses

Many synapses in the CNS transmit only a fraction of the action potentials that reach them. Although unreliable, such synapses do not transmit completely randomly, because the probability of transmission depends on the recent history of synaptic activity. We examine how a variety of spike trains, including examples recorded from area V1 of monkeys freely viewing natural scenes, are transmitted through a realistic model synapse with activity-dependent depression arising from vesicle depletion or postrelease refractoriness. The resulting sequences of transmitted spikes are significantly less correlated, and hence less redundant, than the presynaptic spike trains that generate them. The spike trains we analyze, which are typical of those recorded in a variety of brain regions, have positive autocorrelations because of the occurrence of variable length periods of sustained firing at approximately constant rates. Sustained firing may, at first, seem inconsistent with input from depressing synapses. We show, however, that such a pattern of activity can arise if the postsynaptic neuron is driven by a fixed population of direct, "feedforward" inputs accompanied by a variable number of delayed, "reverberatory" inputs. This leads to a prediction for the number and latency distribution of the inputs that typically drive a cortical neuron.

Revisiting the role of the hippocampal mossy fiber synapse

The mossy fiber pathway has long been considered to provide the major source of excitatory input to pyramidal cells of hippocampal area CA3. In this review we describe anatomical and physiological properties of this pathway that challenge this view. We argue that the mossy fiber pathway does not provide the main input to CA3 pyramidal cells, and that the short-term plasticity and amplitude variance of mossy fiber synapses may be more important features than their long-term plasticity or absolute input strength.

Experience-dependent plasticity without long-term depression by type 2 metabotropic glutamate receptors in developing visual cortex

Synaptic depression is thought to underlie the loss of cortical responsiveness to an eye deprived of vision. Here, we establish a fundamental role for type 2 metabotropic glutamate receptors (mGluR2) in long-term depression (LTD) of synaptic transmission within primary visual cortex. Direct mGluR2 activation by (2S,2'R,3'R-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) persistently depressed layer 2/3 field potentials in slices of mouse binocular zone when stimulated concomitantly. Chemical LTD was independent of N-methyl-d-aspartate (NMDA) receptors but occluded conventional LTD by low-frequency stimulation, indicating shared downstream events. Antagonists or targeted disruption of mGluR2 conversely prevented LTD induction by electrical low-frequency stimulation to layer 4. In contrast, Schaeffer collateral synapses did not exhibit chemical LTD, revealing hippocampal area CA1, naturally devoid of mGluR2, to be an inappropriate model for neocortical plasticity. Moreover, monocular deprivation remained effective in mice lacking mGluR2, and receptor expression levels were unchanged during the critical period in wild-type mice, indicating that experience-dependent plasticity is independent of LTD induction in visual cortex. Short-term depression that was unaffected by mGluR2 deletion may better reflect circuit refinement in vivo.

Control of neural information transmission by synaptic dynamics

The computational processing of a neural system is strongly influenced by the dynamical characteristics of the information transmission between neurons. In this work, the control of neural information transmission by synaptic dynamics is investigated by means of a master-equation-based stochastic model of pre-synaptic release of neurotransmitter-containing vesicles. The model incorporates facilitation of vesicle fusion with the pre-synaptic membrane due to intracellular calcium ions and depletion of readily releasable vesicles. The message to be transmitted is coded by the pre-synaptic firing sequence, and the received signal corresponds to the post-synaptic membrane potential response. At the sending end, the stochastic character of the vesicle release contributes to the entropy of the probability distribution of the number of vesicles released and represents noise with respect to information transmission. At the receiving end, the generation of post-synaptic membrane potentials is influenced by the temporal behaviour of ionic currents and membrane charging and is determined by means of a low-dimensional model. The rate and temporal types of neural coding are compatible with limiting cases of the synaptic information transmission as a function of initial vesicle release probability and pre-synaptic firing rate. The effects of the nonlinear dependencies of the vesicle release probability on intracellular calcium concentration and number of available vesicles are analysed. The model is compared with phenomenological and reduced models, a principal advantage being the capability of also determining fluctuations of dynamic variables

Synaptic depression mediates bistability in neuronal networks with recurrent inhibitory connectivity

When depressing synapses are embedded in a circuit composed of a pacemaker neuron and a neuron with no autorhythmic properties, the network can show two modes of oscillation. In one mode the synapses are mostly depressed, and the oscillations are dominated by the properties of the oscillating neuron. In the other mode, the synapses recover from depression, and the oscillations are primarily controlled by the synapses. We demonstrate the two modes of oscillation in a hybrid circuit consisting of a biological pacemaker and a model neuron, reciprocally coupled via model depressing synapses. We show that across a wide range of parameter values this network shows robust bistability of the oscillation mode and that it is possible to switch the network from one mode to the other by injection of a brief current pulse in either neuron. The underlying mechanism for bistability may be present in many types of circuits with reciprocal connections and synaptic depression.

Short-term plasticity at the calyx of Held

Synapses show widely varying degrees of short-term facilitation and depression. Several mechanisms have been proposed to underlie short-term plasticity, but the contributions of presynaptic mechanisms have been particularly difficult to study because of the small size of synaptic boutons in the mammalian brain. Here we review the functional properties of the calyx of Held, a giant nerve terminal that has shed new light on the general mechanisms that control short-term plasticity. The calyx of Held has also provided fresh insights into the strategies used by synapses to extend their dynamic range of operation and preserve the timing of sensory stimuli.

Subthreshold low frequency repetitive transcranial magnetic stimulation selectively decreases facilitation in the motor cortex

OBJECTIVE

To investigate the modulatory effect of a subthreshold low frequency repetitive transcranial magnetic stimulation (rTMS) train on motor cortex excitability.

METHODS

The study consisted of two separate experiments. Subjects received a 10 min long subthreshold 1 Hz rTMS train. In the first experiment, (single pulse paradigm), cortical excitability was assessed by measuring the amplitude of motor evoked potentials (MEPs) before and after the rTMS train. In the second experiment, a paired pulse paradigm was employed.

RESULTS

Corticospinal excitability, as measured by the MEP amplitude, was reduced by the rTMS train (experiment 1), with a significant effect lasting for about 10 min after the train completion. There was notable inter-individual as well as intra-individual variability in the effect. rTMS produced a significant decrease in intra-cortical facilitation as measured by the paired pulse paradigm (experiment 2). This effect lasted for up to 15 min following the train. Intra-cortical inhibition was not significantly affected.

CONCLUSIONS

Subthreshold low frequency rTMS depresses cortical excitability beyond the duration of the train. This effect seems primarily due to cortical dysfacilitation. These results have implications on the therapeutic use of rTMS.

A spiking neuron model for binocular rivalry

We present a biologically plausible model of binocular rivalry consisting of a network of Hodgkin-Huxley type neurons. Our model accounts for the experimentally and psychophysically observed phenomena: (1) it reproduces the distribution of dominance durations seen in both humans and primates, (2) it exhibits a lack of correlation between lengths of successive dominance durations, (3) variation of stimulus strength to one eye influences only the mean dominance duration of the contralateral eye, not the mean dominance duration of the ipsilateral eye, (4) increasing both stimuli strengths in parallel decreases the mean dominance durations. We have also derived a reduced population rate model from our spiking model from which explicit expressions for the dependence of the dominance durations on input strengths are analytically calculated. We also use this reduced model to derive an expression for the distribution of dominance durations seen within an individual.

Coding of temporal information by activity-dependent synapses

Synaptic transmission in the neocortex is dynamic, such that the magnitude of the postsynaptic response changes with the history of the presynaptic activity. Therefore each response carries information about the temporal structure of the preceding presynaptic input spike train. We quantitatively analyze the information about previous interspike intervals, contained in single responses of dynamic synapses, using methods from information theory applied to experimentally based deterministic and probabilistic phenomenological models of depressing and facilitating synapses. We show that for any given dynamic synapse, there exists an optimal frequency of presynaptic spike firing for which the information content is maximal; simple relations between this optimal frequency and the synaptic parameters are derived. Depressing neocortical synapses are optimized for coding temporal information at low firing rates of 0.5-5 Hz, typical to the spontaneous activity of cortical neurons, and carry significant information about the timing of up to four preceding presynaptic spikes. Facilitating synapses, however, are optimized to code information at higher presynaptic rates of 9-70 Hz and can represent the timing of over eight presynaptic spikes.

Calmodulin mediates rapid recruitment of fast-releasing synaptic vesicles at a calyx-type synapse

In many synapses, depletion and recruitment of releasable synaptic vesicles contribute to use-dependent synaptic depression and recovery. Recently it has been shown that high-frequency presynaptic stimulation enhances recovery from depression, which may be mediated by Ca2+. We addressed this issue by measuring quantal release rates at the calyx of Held synapse and found that transmission is mediated by a heterogeneous population of vesicles, with one subset releasing rapidly and recovering slowly and another one releasing reluctantly and recovering rapidly. Ca2+ promotes refilling of the rapidly releasing synaptic vesicle pool and calmodulin inhibitors block this effect. We propose that calmodulin-dependent refilling supports recovery from synaptic depression during high-frequency trains in concert with rapid recovery of the slowly releasing vesicles.

Anterior insular cortex: linking intestinal pathology and brain function in autism-spectrum subgroups

Autism includes deficits in communications skills and is associated with intestinal pathology. Numerous parents and some physicians report that an autistic child's attention and language improve in response to treatments which eliminate certain dietary antigens and/or which improve intestinal health. For at least some autism-spectrum children, the link between intestinal pathology, attention, and language may derive from shared neuroanatomic pathways within the anterior insular cortex (aIC); from a neurotrophic virus such as herpes simplex (HSV) migrating within afferents to the insular cortex; and/or from synaptic exhaustion in the aIC as induced by chronically inappropriate neuronal activity in the enteric nervous system and/or its vagal efferents.

Differential synaptic processing separates stationary from transient inputs to the auditory cortex

Sound features are blended together en route to the central nervous system before being discriminated for further processing by the cortical synaptic network. The mechanisms underlying this synaptic processing, however, are largely unexplored. Intracortical processing of the auditory signal was investigated by simultaneously recording from pairs of connected principal neurons in layer II/III in slices from A1 auditory cortex. Physiological patterns of stimulation in the presynaptic cell revealed two populations of postsynaptic events that differed in mean amplitude, failure rate, kinetics and short-term plasticity. In contrast, transmission between layer II/III pyramidal neurons in barrel cortex were uniformly of large amplitude and high success (release) probability (Pr). These unique features of auditory cortical transmission may provide two distinct mechanisms for discerning and separating transient from stationary features of the auditory signal at an early stage of cortical processing.

Synchronization of the neural response to noisy periodic synaptic input

The timing information contained in the response of a neuron to noisy periodic synaptic input is analyzed for the leaky integrate-and-fire neural model. We address the question of the relationship between the timing of the synaptic inputs and the output spikes. This requires an analysis of the interspike interval distribution of the output spikes, which is obtained in the gaussian approximation. The conditional output spike density in response to noisy periodic input is evaluated as a function of the initial phase of the inputs. This enables the phase transition matrix to be calculated, which relates the phase at which the output spike is generated to the initial phase of the inputs. The interspike interval histogram and the period histogram for the neural response to ongoing periodic input are then evaluated by using the leading eigenvector of this phase transition matrix. The synchronization index of the output spikes is found to increase sharply as the inputs become synchronized. This enhancement of synchronization is most pronounced for large numbers of inputs and lower frequencies of modulation and also for rates of input near the critical input rate. However, the mutual information between the input phase of the stimulus and the timing of output spikes is found to decrease at low input rates as the number of inputs increases. The results show close agreement with those obtained from numerical simulations for large numbers of inputs.

Computing the optimally fitted spike train for a synapse

Experimental data have shown that synapses are heterogeneous: different synapses respond with different sequences of amplitudes of postsynaptic responses to the same spike train. Neither the role of synaptic dynamics itself nor the role of the heterogeneity of synaptic dynamics for computations in neural circuits is well understood. We present in this article two computational methods that make it feasible to compute for a given synapse with known synaptic parameters the spike train that is optimally fitted to the synapse in a certain sense. With the help of these methods, one can compute, for example, the temporal pattern of a spike train (with a given number of spikes) that produces the largest sum of postsynaptic responses for a specific synapse. Several other applications are also discussed. To our surprise, we find that most of these optimally fitted spike trains match common firing patterns of specific types of neurons that are discussed in the literature. Hence, our analysis provides a possible functional explanation for the experimentally observed regularity in the combination of specific types of synapses with specific types of neurons in neural circuits.

An analysis of neural receptive field plasticity by point process adaptive filtering

Neural receptive fields are plastic: with experience, neurons in many brain regions change their spiking responses to relevant stimuli. Analysis of receptive field plasticity from experimental measurements is crucial for understanding how neural systems adapt their representations of relevant biological information. Current analysis methods using histogram estimates of spike rate functions in nonoverlapping temporal windows do not track the evolution of receptive field plasticity on a fine time scale. Adaptive signal processing is an established engineering paradigm for estimating time-varying system parameters from experimental measurements. We present an adaptive filter algorithm for tracking neural receptive field plasticity based on point process models of spike train activity. We derive an instantaneous steepest descent algorithm by using as the criterion function the instantaneous log likelihood of a point process spike train model. We apply the point process adaptive filter algorithm in a study of spatial (place) receptive field properties of simulated and actual spike train data from rat CA1 hippocampal neurons. A stability analysis of the algorithm is sketched in the. The adaptive algorithm can update the place field parameter estimates on a millisecond time scale. It reliably tracked the migration, changes in scale, and changes in maximum firing rate characteristic of hippocampal place fields in a rat running on a linear track. Point process adaptive filtering offers an analytic method for studying the dynamics of neural receptive fields.

Postsynaptic conversion of silent synapses during LTP affects synaptic gain and transmission dynamics

Synaptic transmission relies on both the gain and the dynamics of synapses. Activity-dependent changes in synaptic gain are well-documented at excitatory synapses and may represent a substrate for information storage in the brain. Here we examine the mechanisms of changes in transmission dynamics at excitatory synapses. We show that paired-pulse ratios (PPRs) of AMPAR and NMDAR EPSCs onto dentate gyrus granule cells are often different; this difference is reduced during LTP, reflecting PPR changes of AMPAR but not NMDAR EPSCs. Presynaptic manipulations, however, produce parallel changes in AMPAR and NMDAR EPSCs. LTP at these synapses reflects a reduction in the proportion of silent synapses lacking functional AMPARs. Changes in PPR during LTP therefore reflect the initial difference between PPRs of silent and functional synapses. Functional conversion of silent synapses permits postsynaptic sampling from additional release sites and thereby affects the dynamics and gain of signals conveyed between neurons.

Developmental inhibitory gate controls the relay of activity to the superficial layers of the visual cortex

A developmental reduction in the radial transmission of synaptic activity has been proposed to underlie the end of the critical period for experience-dependent modification in layers II/III of the visual cortex. Using paired-pulse stimulation, we investigated in visual cortical slices how the propagation of synaptic activity to the superficial layers changes during development and how this process is affected by sensory experience. The results can be summarized as follows. (1) Layers II/III responses to repetitive stimulation of the white matter become increasingly depressed between the third and sixth week of postnatal development, a time course that parallels the end of the critical period. (2) Paired-pulse depression is reduced after dark rearing and also by blocking inhibitory synaptic transmission. (3) Paired-pulse depression and its regulation by age and sensory experience is more pronounced when stimulation is applied to the white matter than when applied to layer IV. Together, these results are consistent with the idea that the maturation of intracortical inhibition reduces the capability of the cortex to relay incoming high-frequency patterns of activity to the supragranular layers.

Modeling of substance P and 5-HT induced synaptic plasticity in the lamprey spinal CPG: consequences for network pattern generation

Consequences of synaptic plasticity in the lamprey spinal CPG are analyzed by means of simulations. This is motivated by the effects substance P (a tachykinin) and serotonin (5-hydroxytryptamin; 5-HT) have on synaptic transmission in the locomotor network. Activity-dependent synaptic depression and potentiation have recently been shown experimentally using paired intracellular recordings. Although normally activity-dependent plasticity presumably does not contribute to the patterning of network activity, this changes in the presence of the neuromodulators substance P and 5-HT, which evoke significant plasticity. Substance P can induce a faster and larger depression of inhibitory connections but potentiation of excitatory inputs, whereas 5-HT induces facilitation of both inhibitory and excitatory inputs. Changes in the amplitude of the first postsynaptic potential are also seen. These changes could thus be a potential mechanism underlying the modulatory role these substances have on the rhythmic network activity. The aim of the present study has been to implement the activity dependent synaptic depression and facilitation induced by substance P and 5-HT into two alternative models of the lamprey spinal locomotor network, one relying on reciprocal inhibition for bursting and one in which each hemicord is capable of oscillations. The consequences of the plasticity of inhibitory and excitatory connections are then explored on the network level. In the intact spinal cord, tachykinins and 5-HT, which can be endogenously released, increase and decrease the frequency of the alternating left-right burst pattern, respectively. The frequency decreasing effect of 5-HT has previously been explained based on its conductance decreasing effect on K(Ca) underlying the postspike afterhyperpolarization (AHP). The present simulations show that short-term synaptic plasticity may have strong effects on frequency regulation in the lamprey spinal CPG. In the network model relying on reciprocal inhibition, the observed effects substance P and 5-HT have on network behavior (i.e., a frequency increase and decrease respectively) can to a substantial part be explained by their effects on the total extent and time dynamics of synaptic depression and facilitation. The cellular effects of these substances will in the 5-HT case further contribute to its network effect.

Contrast-dependent nonlinearities arise locally in a model of contrast-invariant orientation tuning

We study a recently proposed "correlation-based," push-pull model of the circuitry of layer 4 of cat visual cortex. This model was previously shown to explain the contrast-invariance of cortical orientation tuning. Here we show that it can simultaneously account for several contrast-dependent (c-d) "nonlinearities" in cortical responses. These include an advance with increasing contrast in the temporal phase of response to a sinusoidally modulated stimulus; a change in shape of the temporal frequency tuning curve, so that higher temporal frequencies may give little or no response at low contrast but reasonable responses at high contrast; and contrast saturation that occurs at lower contrasts in cortex than in the lateral geniculate nucleus (LGN). In the context of the model circuit, these properties arise from a mixture of nonlinear cellular and synaptic mechanisms: short-term synaptic depression, spike-rate adaptation, contrast-induced changes in cellular conductance, and the nonzero spike threshold. The former three mechanisms are sufficient to explain the experimentally observed increase in c-d phase advance in cortex relative to LGN. The c-d changes in temporal frequency tuning arise as a threshold effect: voltage modulations in response to higher-frequency inputs are only slightly above threshold at lower contrast, but become robustly suprathreshold at higher contrast. The other three nonlinear mechanisms also play a crucial role in this result, allowing contrast dependence of temporal frequency tuning to coexist with contrast-invariance of orientation tuning. Contrast saturation, and the observation that responses to stimuli of increasing temporal frequency saturate at increasingly high contrasts, can be induced both by the model's push-pull inhibition and by synaptic depression. Previous proposals explained these nonlinear response properties by assuming contrast-invariant orientation tuning as a starting point, and adding normalization by shunting inhibition derived equally from cells of all preferred orientations. The present proposal simultaneously explains both contrast-invariant orientation tuning and these contrast-dependent nonlinearities and requires only processing that is local in orientation, in agreement with intracellular measurements.

Synaptic heterogeneity and stimulus-induced modulation of depression in central synapses

Short-term plasticity is a pervasive feature of synapses. Synapses exhibit many forms of plasticity operating over a range of time scales. We develop an optimization method that allows rapid characterization of synapses with multiple time scales of facilitation and depression. Investigation of paired neurons that are postsynaptic to the same identified interneuron in the buccal ganglion of Aplysia reveals that the responses of the two neurons differ in the magnitude of synaptic depression. Also, for single neurons, prolonged stimulation of the presynaptic neuron causes stimulus-induced increases in the early phase of synaptic depression. These observations can be described by a model that incorporates two availability factors, e.g., depletable vesicle pools or desensitizing receptor populations, with different time courses of recovery, and a single facilitation component. This model accurately predicts the responses to novel stimuli. The source of synaptic heterogeneity is identified with variations in the relative sizes of the two availability factors, and the stimulus-induced decrement in the early synaptic response is explained by a slowing of the recovery rate of one of the availability factors. The synaptic heterogeneity and stimulus-induced modifications in synaptic depression observed here emphasize that synaptic efficacy depends on both the individual properties of synapses and their past history.

Experimental and computational analysis of monkey smooth pursuit eye movements

Smooth pursuit eye movements are guided by visual feedback and are surprisingly accurate despite the time delay between visual input and motor output. Previous models have reproduced the accuracy of pursuit either by using elaborate visual signals or by adding sources of motor feedback. Our goal was to constrain what types of signals drive pursuit by obtaining data that would discriminate between these two modeling approaches, represented by the "image motion model" and the "tachometer feedback" model. Our first set of experiments probed the visual properties of pursuit with brief square-pulse and sine-wave perturbations of target velocity. Responses to pulse perturbations increased almost linearly with pulse amplitude, while responses to sine wave perturbations showed strong saturation with increasing stimulus amplitude. The response to sine wave perturbations was strongly dependent on the baseline image velocity at the time of the perturbation. Responses were much smaller if baseline image velocity was naturally large, or was artificially increased by superimposing sine waves on pulse perturbations. The image motion model, but not the tachometer feedback model, could reproduce these features of pursuit. We used a revision of the image motion model that was, like the original, sensitive to both image velocity and image acceleration. Due to a saturating nonlinearity, the sensitivity to image acceleration declined with increasing image velocity. Inclusion of this nonlinearity was motivated by our experimental results, was critical in accounting for the responses to perturbations, and provided an explanation for the unexpected stability of pursuit in the presence of perturbations near the resonant frequency. As an emergent property, the revised image motion model was able to reproduce the frequency and damping of oscillations recorded during artificial feedback delays. Our second set of experiments replicated prior recordings of pursuit responses to multiple-cycle sine wave perturbations, presented over a range of frequencies. The image motion model was able to reproduce the responses to sine wave perturbations across all frequencies, while the tachometer feedback model failed at high frequencies. These failures resulted from the absence of image acceleration signals in the tachometer model. We conclude that visual signals related to image acceleration are important in driving pursuit eye movements and that the nonlinearity of these signals provides stability. Smooth pursuit thus illustrates that a plausible neural strategy for combating natural delays in sensory feedback is to employ information about the derivative of the sensory input.

Synaptic reverberation underlying mnemonic persistent activity

Stimulus-specific persistent neural activity is the neural process underlying active (working) memory. Since its discovery 30 years ago, mnemonic activity has been hypothesized to be sustained by synaptic reverberation in a recurrent circuit. Recently, experimental and modeling work has begun to test the reverberation hypothesis at the cellular level. Moreover, theory has been developed to describe memory storage of an analog stimulus (such as spatial location or eye position), in terms of continuous 'bump attractors' and 'line attractors'. This review summarizes new studies, and discusses insights and predictions from biophysically based models. The stability of a working memory network is recognized as a serious problem; stability can be achieved if reverberation is largely mediated by NMDA receptors at recurrent synapses.

Natural signal statistics and sensory gain control

We describe a form of nonlinear decomposition that is well-suited for efficient encoding of natural signals. Signals are initially decomposed using a bank of linear filters. Each filter response is then rectified and divided by a weighted sum of rectified responses of neighboring filters. We show that this decomposition, with parameters optimized for the statistics of a generic ensemble of natural images or sounds, provides a good characterization of the nonlinear response properties of typical neurons in primary visual cortex or auditory nerve, respectively. These results suggest that nonlinear response properties of sensory neurons are not an accident of biological implementation, but have an important functional role.

Short-term synaptic plasticity as a temporal filter

Synaptic efficacy can increase (synaptic facilitation) or decrease (synaptic depression) markedly within milliseconds after the onset of specific temporal patterns of activity. Recent evidence suggests that short-term synaptic depression contributes to low-pass temporal filtering, and can account for a well-known paradox - many low-pass neurons respond vigorously to transients and the onsets of high temporal-frequency stimuli. The use of depression for low-pass filtering, however, is itself a paradox; depression induced by ongoing high-temporal frequency stimuli could preclude desired responses to low-temporal frequency information. This problem can be circumvented, however, by activation of short-term synaptic facilitation that maintains responses to low-temporal frequency information. Such short-term plasticity might also contribute to spatio-temporal processing.

Acetylcholine-dependent induction and expression of functional plasticity in the barrel cortex of the adult rat

The involvement of acetylcholine (ACh) in the induction of neuronal sensory plasticity is well documented. Recently we demonstrated in the somatosensory cortex of the anesthetized rat that ACh is also involved in the expression of neuronal plasticity. Pairing stimulation of the principal whisker at a fixed temporal frequency with ACh iontophoresis induced potentiations of response that required re-application of ACh to be expressed. Here we fully characterize this phenomenon and extend it to stimulation of adjacent whiskers. We show that these ACh-dependent potentiations are cumulative and reversible. When several sensori-cholinergic pairings were applied consecutively with stimulation of the principal whisker, the response at the paired frequency was further increased, demonstrating a cumulative process that could reach saturation levels. The potentiations were specific to the stimulus frequency: if the successive pairings were done at different frequencies, then the potentiation caused by the first pairing was depotentiated, whereas the response to the newly paired frequency was potentiated. During testing, the potentiation of response did not develop immediately on the presentation of the paired frequency during application of ACh: the analysis of the kinetics of the effect indicates that this process requires the sequential presentation of several trains of stimulation at the paired frequency to be expressed. We present evidence that a plasticity with similar characteristics can be induced for responses to stimulation of an adjacent whisker, suggesting that this potentiation could participate in receptive field spatial reorganizations. The spatial and temporal properties of the ACh-dependent plasticity presented here impose specific constraints on the underlying cellular and molecular mechanisms.

Is the integrate-and-fire model good enough?--a review

We review some recent results on the behaviour of the integrate-and-fire (IF) model, the FitzHugh-Nagumo (FHN) model, a simplified version of the FHN (IF-FHN) model and the Hodgkin-Huxley (HH) model with correlated inputs. The effect of inhibitory inputs on the model behaviour is also taken into account. Here, inputs exclusively take the form of diffusion approximation and correlated inputs mean correlated synaptic inputs (Sections 2 and 3). It is found that the IF and HH models respond to correlated inputs in totally opposite ways, but the IF-FHN model shows similar behaviour to the HH model. Increasing inhibitory input to single neuronal models, such as the FHN model and the HH model can sometimes increase their firing rates, which we termed inhibition-boosted firing (IBF). Using the IF model and the IF-FHN model, we theoretically explore how and when IBF can happen. The computational complexity of the IF-FHN model is very similar to the conventional IF model, but the former captures some interesting and essential features of biophysical models and could serve as a better model for spiking neuron computation.

Distributed synchrony in a cell assembly of spiking neurons

We investigate the formation of a Hebbian cell assembly of spiking neurons, using a temporal synaptic learning curve that is based on recent experimental findings. It includes potentiation for short time delays between pre- and post-synaptic neuronal spiking, and depression for spiking events occurring in the reverse order. The coupling between the dynamics of synaptic learning and that of neuronal activation leads to interesting results. One possible mode of activity is distributed synchrony, implying spontaneous division of the Hebbian cell assembly into groups, or subassemblies, of cells that fire in a cyclic manner. The behavior of distributed synchrony is investigated both by simulations and by analytic calculations of the resulting synaptic distributions.

Spike-based strategies for rapid processing

Most experimental and theoretical studies of brain function assume that neurons transmit information as a rate code, but recent studies on the speed of visual processing impose temporal constraints that appear incompatible with such a coding scheme. Other coding schemes that use the pattern of spikes across a population a neurons may be much more efficient. For example, since strongly activated neurons tend to fire first, one can use the order of firing as a code. We argue that Rank Order Coding is not only very efficient, but also easy to implement in biological hardware: neurons can be made sensitive to the order of activation of their inputs by including a feed-forward shunting inhibition mechanism that progressively desensitizes the neuronal population during a wave of afferent activity. In such a case, maximum activation will only be produced when the afferent inputs are activated in the order of their synaptic weights.

LTD induction in adult visual cortex: role of stimulus timing and inhibition

One Hertz stimulation of afferents for 15 min with constant interstimulus intervals (regular stimulation) can induce long-term depression (LTD) of synaptic strength in the neocortex. However, it is unknown whether natural patterns of low-frequency afferent spike activity induce LTD. Although neurons in the neocortex can fire at overall rates as low as 1 Hz, the intervals between spikes are irregular. This irregular spike activity (and thus, presumably, irregular activation of the synapses of that neuron onto postsynaptic targets) can be approximated by stimulation with Poisson-distributed interstimulus intervals (Poisson stimulation). Therefore, if low-frequency presynaptic spike activity in the intact neocortex is sufficient to induce a generalized LTD of synaptic transmission, then Poisson stimulation, which mimics this spike activity, should induce LTD in slices. We tested this hypothesis by comparing changes in the strength of synapses onto layer 2/3 pyramidal cells induced by regular and Poisson stimulation in slices from adult visual cortex. We find that regular stimulation induces LTD of excitatory synaptic transmission as assessed by field potentials and intracellular postsynaptic potentials (PSPs) with inhibition absent. However, Poisson stimulation does not induce a net LTD of excitatory synaptic transmission. When the PSP contained an inhibitory component, neither Poisson nor regular stimulation induced LTD. We propose that the short bursts of synaptic activity that occur during a Poisson train have potentiating effects that offset the induction of LTD that is favored with regular stimulation. Thus, natural (i.e., irregular) low-frequency activity in the adult neocortex in vivo should not consistently induce LTD.

Minimizing synaptic depression by control of release probability

Transmission at the end-bulb synapse formed by auditory nerve terminals onto the soma of neurons in the avian nucleus magnocellularis is characterized by high transmitter release probability and strong synaptic depression. Activation of presynaptic GABA(B) receptors minimizes depression at this synapse and significantly enhances synaptic strength during high-frequency activity. Here we investigate synaptic mechanisms underlying this phenomenon. EPSC amplitudes evoked by 200 Hz trains increased more than twofold when release probability was reduced with Cd(2+) or baclofen. This effect was not exhibited by a transmitter depletion model of presynaptic depression, which predicts that EPSC amplitudes reach a common steady-state amplitude during high-frequency trains, despite alterations of initial release probability. However, an additional source of postsynaptic depression was sufficient to explain our findings. Aniracetam, a modulator of AMPA receptors that reduces desensitization, decreased the amount of synaptic depression during trains, indicating that desensitization occurred during trains of stimuli. However, this effect of aniracetam was absent when release probability was lowered with baclofen or Cd(2+). No effect of aniracetam on the NMDA component of the EPSC was seen, confirming a postsynaptic site of action of aniracetam. When desensitization was reduced with aniracetam, steady-state EPSC amplitudes during trains were found to converge over a wide range of release probabilities, as predicted by the depletion model. Additional evidence of AMPA receptor desensitization was provided by direct measurement of quantal amplitudes immediately after stimulus trains. Thus, presynaptic modulation by GABA(B) receptors regulates the extent of AMPA receptor desensitization and controls synaptic strength, thereby modulating the flow of information at an auditory synapse.

Rate coding versus temporal order coding: what the retinal ganglion cells tell the visual cortex

It is often supposed that the messages sent to the visual cortex by the retinal ganglion cells are encoded by the mean firing rates observed on spike trains generated with a Poisson process. Using an information transmission approach, we evaluate the performances of two such codes, one based on the spike count and the other on the mean interspike interval, and compare the results with a rank order code, where the first ganglion cells to emit a spike are given a maximal weight. Our results show that the rate codes are far from optimal for fast information transmission and that the temporal structure of the spike train can be efficiently used to maximize the information transfer rate under conditions where each cell needs to fire only one spike.

Presynaptic group II mGluR inhibition of short-term depression in the medial perforant path of the dentate gyrus in vitro

Inhibition of short-term plasticity by activation of presynaptic group II metabotropic glutamate receptors (group II mGluR) was investigated in the medial perforant path of the dentate gyrus in the hippocampus in vitro. Brief trains of stimulation (10 stimuli at 1--200 Hz) evoked short-term depression of field excitatory postsynaptic potentials (EPSPs). The steady-state level of depression, measured after 10 stimuli, was frequency dependent, increasing between 1 and 200 Hz. Activation of group II mGluR by the selective agonist LY354740 did not alter short-term depression evoked by frequencies up to 10 Hz, but did inhibit short-term depression evoked at higher frequencies in a frequency- and concentration-dependent manner. The time-averaged postsynaptic response (EPSP per unit time) was found to increase linearly with frequency up to approximately 20 Hz. At higher frequencies, the response plateaued, thereby becoming independent of frequency. Frequencies above this were differentiated only during the transient postsynaptic response that accompanies changes in firing rates. Activation of presynaptically located group II mGluR increased the frequency at which the EPSP per unit time plateaued up to 30-50 Hz.

Foci of orientation plasticity in visual cortex

Cortical areas are generally assumed to be uniform in their capacity for adaptive changes or plasticity. Here we demonstrate, however, that neurons in the cat striate cortex (V1) show pronounced adaptation-induced short-term plasticity of orientation tuning primarily at specific foci. V1 neurons are clustered according to their orientation preference in iso-orientation domains that converge at singularities or pinwheel centres. Although neurons in pinwheel centres have similar orientation tuning and responses to those in iso-orientation domains, we find that they differ markedly in their capacity for adaptive changes. Adaptation with an oriented drifting grating stimulus alters responses of neurons located at and near pinwheel centres to a broad range of orientations, causing repulsive shifts in orientation preference and changes in response magnitude. In contrast, neurons located in iso-orientation domains show minimal changes in their tuning properties after adaptation. The anisotropy of adaptation-induced orientation plasticity is probably mediated by inhomogeneities in local intracortical interactions that are overlaid on the map of orientation preference in V1.

Behavior of integrate-and-fire and Hodgkin-Huxley models with correlated inputs

We assess, both numerically and theoretically, how positively correlated Poisson inputs affect the output of the integrate-and-fire and Hodgkin-Huxley models. For the integrate-and-fire model the variability of efferent spike trains is an increasing function of input correlation, and of the ratio between inhibitory and excitatory inputs. Interestingly for the Hodgkin-Huxley model the variability of efferent spike trains is a decreasing function of input correlation, and for fixed input correlation it is almost independent of the ratio between inhibitory and excitatory inputs. In terms of the signal to noise ratio of efferent spike trains the integrate-and-fire model works better in an environment of asynchronous inputs, but the Hodgkin-Huxley model has an advantage for more synchronous (correlated) inputs. In conclusion the integrate-and-fire and Hodgkin-Huxley models respond to correlated inputs in totally opposite ways.

Extracellular calcium depletion as a mechanism of short-term synaptic depression

Recent experiments have demonstrated that normal neural activity can cause significant decrements in external calcium levels, and that these decrements mediate a form of short-term synaptic depression. These findings raise the possibility that certain forms of short-term synaptic depression at glutamatergic synapses throughout the mammalian CNS may be influenced by similar changes in external calcium. We use a computational model of the extracellular space, combined with experimental data on calcium consumption, to show that such short-term depression can be accounted for by changes in calcium just outside active synapses, provided that external calcium diffusion is restricted. Remarkably, the model suggests the novel possibility that synapses may possess private pools of external calcium that enforce some forms of short-term depression in a synapse-specific manner.