Paired-spike interactions and synaptic efficacy of retinal inputs to the thalamus

A model of the dynamics of retinal activity during natural visual fixation

During visual fixation, small eye movements keep the retinal image continuously in motion. It is known that neurons in the visual system are sensitive to the spatiotemporal modulations of luminance resulting from this motion. In this study, we examined the influence of fixational eye movements on the statistics of neural activity in the macaque's retina during the brief intersaccadic periods of natural visual fixation. The responses of parvocellular (P) and magnocellular (M) ganglion cells in different regions of the visual field were modeled while their receptive fields scanned natural images following recorded traces of eye movements. Immediately after the onset of fixation, wide ensembles of coactive ganglion cells extended over several degrees of visual angle, both in the central and peripheral regions of the visual field. Following this initial pattern of activity, the covariance between the responses of pairs of P and M cells and the correlation between the responses of pairs of M cells dropped drastically during the course of fixation. Cell responses were completely uncorrelated by the end of a typical 300-ms fixation. This dynamic spatial decorrelation of retinal activity is a robust phenomenon independent of the specifics of the model. We show that it originates from the interaction of three factors: the statistics of natural scenes, the small amplitudes of fixational eye movements, and the temporal sensitivities of ganglion cells. These results support the hypothesis that fixational eye movements, by shaping the statistics of retinal activity, are an integral component of early visual representations.

Interspike interval analysis of retinal ganglion cell receptive fields

The interspike interval (ISI) preceding a retinal spike has a strong influence on whether retinal spikes will drive postsynaptic responses in the lateral geniculate nucleus (LGN). This ISI-based filtering of retinal spikes could, in principle, be used as a mechanism for processing visual information en route from retina to cortex; however, this form of processing has not been previously explored. Using a white noise stimulus and reverse correlation analysis, we compared the receptive fields associated with retinal spikes over a range of ISIs (0-120 ms). Results showed that, although the location and sign of retinal ganglion cell receptive fields are invariant to ISI, the size and amplitude of receptive fields vary with ISI. These results support the notion that ISI-based filtering of retinal spikes can serve as a mechanism for shaping receptive fields.

Transmission of spike trains at the retinogeniculate synapse

Retinal spikes impinging on relay neurons in the lateral geniculate nucleus (LGN) generate synaptic potentials, which sometimes produce spikes sent to visual cortex. We examined how signal transmission is regulated in the macaque LGN by recording the retinal input to a single LGN neuron while stimulating the receptive field center with a naturalistic luminance sequence. After extracting the EPSPs, which are often partially merged with spike waveforms, we found that >95% of spikes were associated with an EPSP from a single retinal ganglion cell. Each spike within a "burst" train was generated by an EPSP, indicating that LGN bursts are inherited from retinal bursts. LGN neurons rarely fired unless at least two EPSPs summated within 40 ms. This facilitation in EPSP efficacy was followed by depression. If a spike was generated by the first EPSP in a pair, it did not alter the efficacy of the second EPSP. Hence, the timing of EPSPs arising from the primary retinal driver governs synaptic efficacy and provides the basis for successful retinogeniculate transmission.

Retinogeniculate connections: A balancing act between connection specificity and receptive field diversity

Retinogeniculate connections are one of the most striking examples of connection specificity within the visual pathway. In almost every connection there is one dominant afferent cell per geniculate cell, and both afferent and geniculate cells have very similar receptive fields. The remarkable specificity and strength of retinogeniculate connections have inspired comparisons of the lateral geniculate nucleus (LGN) with a simple relay that connects the retina with the visual cortex. However, because each retinal ganglion cell diverges to innervate multiple cells in the LGN, most geniculate cells must receive additional inputs from other retinal afferents that are not the dominant ones. These additional afferents make weaker connections and their receptive fields are not as perfectly matched with the geniculate target as the dominant afferent. We argue that these 'match imperfections' are important to create receptive field diversity among the cells that represent each point of visual space in the LGN. We propose that the convergence of dominant and weak retinal afferents in the LGN multiplexes the array of retinal ganglion cells by creating receptive fields that have a richer range of positions, sizes and response time courses than those available at the ganglion cell layer of the retina.

Brightness Induction: Rate Enhancement and Neuronal Synchronization as Complementary Codes

In cat visual cortex, we investigated with parallel recordings from multiple units the neuronal correlates of perceived brightness. The perceived brightness of a center grating was changed by varying the orientation or the relative spatial phase of a surrounding grating. Brightness enhancement by orientation contrast is associated with an increase of discharge rates of responses to the center grating but not with changes in spike synchronization. In contrast, if brightness enhancement is induced by phase offset, discharge rates are unchanged but synchronization increases between neurons responding to the center grating. The changes in synchronization correlate well with changes in perceived brightness that were assessed in parallel in human subjects using the same stimuli. These results indicate that in cerebral cortex the modulation of synchronicity of responses is used as a mechanism complementary to rate changes to enhance the saliency of neuronal responses.

Impaired firing and cell-specific compensation in neurons lacking nav1.6 sodium channels

The ability of neurons to fire precise patterns of action potentials is critical for encoding inputs and efficiently driving target neurons. At the axon initial segment and nodes of Ranvier, where nerve impulses are generated and propagated, a high density of Na(v)1.2 sodium channels is developmentally replaced by Na(v)1.6 channels. In retinal ganglion cells (GCs), this isoform switch coincides with the developmental transition from single spikes to repetitive firing. Also, Na(v)1.6 channels are required for repetitive spiking in cerebellar Purkinje neurons. These previous observations suggest that the developmental appearance of Na(v)1.6 underlies the transition to repetitive spiking in GCs. To test this possibility, we recorded from GCs of med (Na(v)1.6-null) and wild-type mice during postnatal development. By postnatal day 18, when the switch to Na(v)1.6 at GC initial segments is normally complete, the maximal sustained and instantaneous firing rates were lower in med than in wild-type GCs, demonstrating that Na(v)1.6 channels are necessary to attain physiologically relevant firing frequencies in GCs. However, the firing impairment was milder than that reported previously in med Purkinje neurons, which prompted us to look for differences in compensatory sodium channel expression. Both Na(v)1.2 and Na(v)1.1 channels accumulated at initial segments and nodes of med GCs, sites normally occupied by Na(v)1.6. In med Purkinje cells, only Na(v)1.1 channels were found at initial segments, whereas in other brain regions, only Na(v)1.2 was detected at med initial segments and nodes. Thus, compensatory mechanisms in channel isoform distribution are cell specific, which likely results in different firing properties.

Stimulus-dependent correlated firing in directionally selective retinal ganglion cells

Synchronous spiking has been postulated to be a meta-signal in visual cortex and other CNS loci that tags neuronal spike responses to a single entity. In retina, however, synchronized spikes have been postulated to arise via mechanisms that would largely preclude their carrying such a code. One such mechanism is gap junction coupling, in which synchronous spikes would be a by-product of lateral signal sharing. Synchronous spikes have also been postulated to arise from common-source inputs to retinal ganglion cells having overlapping receptive fields, and thus code for stimulus location in the overlap area. On-Off directionally selective ganglion cells of the rabbit retina exhibit a highly precise tiling pattern in which gap junction coupling occurs between some neighboring, same-preferred-direction cells. Depending on how correlated spikes arise, and for what purpose, one could postulate that synchronized spikes in this system (1) always arise in some subset of same-direction cells because of gap junctions, but never in non-same-preferred-directional cells; (2) never arise in same-directional cells because their receptive fields do not overlap, but arise only in different-directional cells whose receptive fields overlap, as a code for location in the overlap region; or (3) arise in a stimulus-dependent manner for both same- and different-preferred-direction cells for a function similar to that postulated for neurons in visual cortex. Simultaneous, extracellular recordings were obtained from neighboring On-Off directionally selective (DS) ganglion cells having the same and different preferred directions in an isolated rabbit retinal preparation. Stimulation by large flashing spots elicited responses from DS ganglion-cell pairs that typically showed little synchronous firing. Movement of extended bars, however, often produced synchronous spikes in cells having similar or orthogonal preferred directions. Surprisingly, correlated firing could occur for the opposite contrast polarity edges of moving stimuli when the leading edge of a sweeping bar excited the receptive field of one cell as its trailing edge stimulated another. Pharmacological manipulations showed that the spike synchronization is enhanced by excitatory cholinergic amacrine-cell inputs, and reduced by inhibitory GABAergic inputs, in a motion-specific manner. One possible interpretation is that this synchronous firing could be a signal to higher centers that the outputs of the two DS ganglion cells should be "bound" together as responding to a contour of a common object.

Synaptic interactions underlying song-selectivity in the avian nucleus HVC revealed by dual intracellular recordings

Stimulus-dependent synaptic interactions underlying selective sensory representations in neural circuits specialized for sensory processing and sensorimotor integration remain poorly understood. The songbird telencephalic nucleus HVC is a sensorimotor area essential to learned vocal control with one projection neuron (PN) type (HVC(RA)) innervating a song premotor pathway, another PN (HVC(X)) innervating a basal ganglia pathway essential to vocal plasticity, and interneurons (HVC(Int)). Playback of the bird's own song (BOS), but not other songs, evokes action potential bursts from both PNs, but HVC(RA) and HVC(X) display distinct BOS-evoked subthreshold responses. To characterize synaptic interactions underlying HVC's BOS-selective responses and assess stimulus-evoked changes in functional interactions between HVC neurons, we made simultaneous in vivo intracellular recordings from various HVC neuron pairs in urethan-anesthetized zebra finches. Spike-triggered averaging revealed that all HVC neuron types receive common excitation and that the onset of this excitation occurs during a narrower time window in projection neurons during BOS playback. To distinguish local from extrinsic contributions to HVC subthreshold response patterns, we inactivated the HVC local circuit with GABA or occluded inhibition in single HVC(X) cells. After either treatment, BOS-evoked responses in HVC(X) neurons became purely depolarizing and subthreshold responses of HVC(X) and HVC(RA) cells became remarkably similar to one another while retaining BOS selectivity. Therefore both PN types receive a common extrinsic source of BOS-selective excitation, and local inhibition specifically alters processing of auditory information in HVC(X) cells. In HVC, excitatory and inhibitory synaptic interactions are recruited in a stimulus-dependent fashion, affecting auditory representations of the BOS locally and in other song nuclei important to song learning and production.

Short-term depression in thalamocortical synapses of cat primary visual cortex

Neurons in primary visual cortex exhibit several nonlinearities in their responses to visual stimuli, including response decrements to repeated stimuli, contrast-dependent phase advance, contrast saturation, and cross-orientation suppression. Thalamocortical synaptic depression has been implicated in these phenomena but has not been examined directly in visual cortex in vivo. We assessed depression of visual thalamocortical synapses in vivo using 20-100 Hz trains of electrical stimuli delivered to the LGN. Cortical cells receiving direct input from the LGN, identified by short latency and low jitter of LGN-evoked PSPs, showed moderate reductions in PSP amplitude during the fastest trains. Cells receiving indirect input from the thalamus via other cortical excitatory neurons show a marked reduction in PSP amplitude during a train, which could be explained either by synaptic depression in corticocortical synapses or by an inhibition-mediated suppression of the firing of their afferents. Reducing spontaneous activity in the LGN (by retinal blockade) unmasked additional depression at the thalamocortical synapse but only for the first stimulus in the train. That is, the first PSP was increased in amplitude relative to the unblocked condition, but subsequent responses were essentially unchanged. Thus, the synapses are maintained at significant levels of depression by spontaneous activity. These findings constrain the role that thalamocortical depression can play in shaping cortical responses to visual stimuli.

Approaches to Information-Theoretic Analysis of Neural Activity

Understanding how neurons represent, process, and manipulate information is one of the main goals of neuroscience. These issues are fundamentally abstract, and information theory plays a key role in formalizing and addressing them. However, application of information theory to experimental data is fraught with many challenges. Meeting these challenges has led to a variety of innovative analytical techniques, with complementary domains of applicability, assumptions, and goals.

Comparing neuronal spike trains with inhomogeneous Poisson distribution: evaluation procedure and experimental application in cases of cyclic activity

A procedure is proposed to compare single-unit spiking activity elicited in repetitive cycles with an inhomogeneous Poisson process (IPP). Each spike sequence in a cycle is discretized and represented as a point process on a circle. The interspike interval probability density predicted for an IPP is computed on the basis of the experimental firing probability density; differences from the experimental interval distribution are assessed. This procedure was applied to spike trains which were repetitively induced by opening-closing movements of the distal article of a lobster leg. As expected, the density of short interspike intervals, less than 20-40 ms in length, was found to lie greatly below the level predicted for an IPP, reflecting the occurrence of the refractory period. Conversely, longer intervals, ranging from 20-40 to 100-120 ms, were markedly more abundant than expected; this provided evidence for a time window of increased tendency to fire again after a spike. Less consistently, a weak depression of spike generation was observed for longer intervals. A Monte Carlo procedure, implemented for comparison, produced quite similar results, but was slightly less precise and more demanding as concerns computation time.

Synaptic background activity controls spike transfer from thalamus to cortex

Characterizing the responsiveness of thalamic neurons is crucial to understanding the flow of sensory information. Typically, thalamocortical neurons possess two distinct firing modes. At depolarized membrane potentials, thalamic cells fire single action potentials and faithfully relay synaptic inputs to the cortex. At hyperpolarized potentials, the activation of T-type calcium channels promotes burst firing, and the transfer is less accurate. Our results suggest that this duality no longer holds if synaptic background activity is taken into account. By injecting stochastic conductances into guinea-pig thalamocortical neurons in slices, we show that the transfer function of these neurons is strongly influenced by conductance noise. The combination of synaptic noise with intrinsic properties gives a global responsiveness that is more linear, mixing single-spike and burst responses at all membrane potentials. Because in thalamic neurons, background synaptic input originates mainly from cortex, these results support a determinant role of corticothalamic feedback during sensory information processing.

Frequency-dependent modulation of retinogeniculate transmission by serotonin

The relay of visual information converging in the lateral geniculate nucleus (LGN) en route to the visual cortex is modulated by projections from brainstem nuclei. The release of serotonin, one mediator of these effects, has been shown to act at a presynaptic site to inhibit neurotransmitter release at the retinogeniculate synapse, the connection between retinal ganglion cells and thalamocortical relay neurons in the LGN. To understand how serotonergic inhibition of synaptic transmission influences the transfer of information at this synapse, we examined the EPSCs and firing responses of relay neurons to 5-carboxytryptamine (5-CT), a 5-HT1 receptor agonist that preferentially activates the presynaptic over postsynaptic modulatory effects of serotonin. Bath application of 5-CT inhibits synaptic strength, relieves synaptic depression, and reduces the total synaptic charge transferred at the retinogeniculate synapse in mouse LGN brain slices. In contrast, 5-CT does not significantly alter the membrane potential response of relay neurons to trains of intracellular current injections. Here we show that presynaptic serotonergic modulation results in a frequency-dependent inhibition of relay neuron firing. At low-frequency stimulation, 5-CT markedly reduces charge transfer at the retinogeniculate synapse, thus inhibiting relay neuron firing. However, inhibition of firing by 5-CT is diminished during high-frequency stimulation, because relief from synaptic depression partially offsets the reduction in charge transfer. Thus, presynaptic serotonergic inhibition plays a powerful role in modulating the frequency range of visual information transmitted via the retinogeniculate synapse such that high-frequency inputs are more reliably transmitted than low-frequency inputs.

Bursting as an effective relay mode in a minimal thalamic model

In recent years, accumulating evidence indicates that thalamic bursts are present during wakefulness and participate in information transmission as an effective relay mode with distinctive properties from the tonic activity. Thalamic bursts originate from activation of the low threshold calcium cannels via a local feedback inhibition, exerted by the thalamic reticular neurons upon the relay neurons. This article, examines if this simple mechanism is sufficient to explain the distinctive properties of thalamic bursting as an effective relay mode. A minimal model of thalamic circuit composed of a retinal spike train, a relay neuron and a reticular neuron is simulated to generate the tonic and burst firing modes. The integrate-and-fire-or-burst model is used to simulate the neurons. After discriminating the burst events with criteria based on inter-spike-intervals, statistical indices show that the bursts of the minimal model are stereotypic events. The relation between the rate of bursts and the parameters of the input spike train demonstrates marked nonlinearities. Burst response is shown to be selective to spike-silence-spike sequences in the input spike train. Moreover, burst events represent the input more reliably than the tonic spike in a considerable range of the parameters of the model. In conclusion, many of the distinctive properties of thalamic bursts such as stereotypy, nonlinear dependence on the sensory stimulus, feature selectivity and reliability are reproducible in the minimal model. Furthermore, the minimal model predicts that while the bursts are more frequent in the spike train of the off-center X relay neurons (corresponding to off-center X retinal ganglion cells), they are more reliable when generated by the on-center ones (corresponding to on-center X ganglion cells).

Dynamics of sensory thalamocortical synaptic networks during information processing states

The thalamocortical network consists of the pathways that interconnect the thalamus and neocortex, including thalamic sensory afferents, corticothalamic and thalamocortical pathways. These pathways are essential to acquire, analyze, store and retrieve sensory information. However, sensory information processing mostly occurs during behavioral arousal, when activity in thalamus and neocortex consists of an electrographic sign of low amplitude fast activity, known as activation, which is caused by several neuromodulator systems that project to the thalamocortical network. Logically, in order to understand how the thalamocortical network processes sensory information it is essential to study its response properties during states of activation. This paper reviews the temporal and spatial response properties of synaptic pathways in the whisker thalamocortical network of rodents during activated states as compared to quiescent (non-activated) states. The evidence shows that these pathways are differentially regulated via the effects of neuromodulators as behavioral contingencies demand. Thus, during activated states, the temporal and spatial response properties of pathways in the thalamocortical network are transformed to allow the processing of sensory information.

Dopamine receptor activation can reduce voltage-gated Na+ current by modulating both entry into and recovery from inactivation

We tested whether dopamine receptor activation modulates the voltage-gated Na+ current of goldfish retinal ganglion cells, using a fast voltage-clamp amplifier, perforated-patch whole cell mode, and a physiological extracellular Na+ concentration. As found in other cells, activators of D1-type dopamine receptors and of protein kinase A reduced the amplitude of current activated by depolarizations from resting potential without altering the current kinetics or activation range. However, D1-type dopamine receptor activation also accelerated the rate of entry into inactivation during subthreshold depolarizations and slowed the rate of recovery from inactivation after single, brief depolarizations. Our results provide the first evidence in any preparation that D1-type receptor activation can produce both of these latter effects.

Gamma Oscillation Maintains Stimulus Structure-Dependent Synchronization in Cat Visual Cortex

Visual cortical cells demonstrate both oscillation and synchronization, although the underlying causes and functional significance of these behaviors remain uncertain. We simultaneously recorded single-unit activity with microelectrode arrays in supragranular layers of area 17 of cats paralyzed and anesthetized with propofol and N2O. Rate-normalized autocorrelograms of 24 cells reveal bursting (100%) and gamma oscillation (63%). Renewal density analysis, used to explore the source of oscillation, suggests a contribution from extrinsic influences such as feedback. However, a bursting refractory period, presumably membrane-based, could also encourage oscillatory firing. When we investigated the source of synchronization for 60 cell pairs we found only moderate correlation of synchrony with bursts and oscillation. We did, nonetheless, discover a possible functional role for oscillation. In all cases of cross-correlograms that exhibited oscillation, the strength of the synchrony was maintained throughout the stimulation period. When no oscillation was apparent, 75% of the cell pairs showed decay in their synchronization. The synchrony between cells is strongly dependent on similar response onset latencies. We therefore propose that structured input, which yields tight organization of latency, is a more likely candidate for the source of synchronization than oscillation. The reliable synchrony at response onset could be driven by spatial and temporal correlation of the stimulus that is preserved through the earlier stages of the visual system. Oscillation then contributes to maintenance of the synchrony to enhance reliable transmission of the information for higher cognitive processing.

Visual system: how is the retina wired up to the cortex?

A single retinal output neuron transmits to primary visual cortex through multiple pathways with different strengths. A new study in which activity was simultaneously recorded in pairs of retinal and cortical neurons provides evidence that these pathways converge on a single cortical neuron.

Complex receptive fields in primary visual cortex

In the early 1960s, Hubel and Wiesel reported the first physiological description of cells in cat primary visual cortex. They distinguished two main cell types: simple cells and complex cells. Based on their distinct response properties, they suggested that the two cell types could represent two consecutive stages in receptive-field construction. Since the 1960s, new experimental and computational evidence provided serious alternatives to this hierarchical model. Parallel models put forward the idea that both simple and complex receptive fields could be built in parallel by direct geniculate inputs. Recurrent models suggested that simple cells and complex cells may not be different cell types after all. To this day, a consensus among hierarchical, parallel, and recurrent models has been difficult to attain; however, the circuitry used by all models is becoming increasingly similar. The authors review theoretical and experimental evidence for each line of models emphasizing their strengths and weaknesses.

Efficacy of retinal spikes in driving cortical responses

How does a single retinal ganglion cell (RGC) affect the firing of simple cells in the visual cortex? Although much is known of the functional connections between the retina and the lateral geniculate nucleus (LGN) and between LGN and visual cortex, it is hard to infer the effect of disynaptic connections from retina to visual cortex. Most importantly, there is considerable divergence from retina to LGN, so cortical neurons might be influenced by ganglion cells through multiple feedforward pathways. We recorded simultaneously from ganglion cells in the retina and cortical simple cells in the striate cortex with overlapping receptive fields and evaluated disynaptic connections with cross-correlation analysis. In all disynaptically connected pairs, the retinal receptive field center and overlapping cortical subregion always shared the same sign (either both ON or both OFF). Connected pairs were similar in other respects, such as relative position and timing of their receptive fields, and thus obeyed the same rules of connectivity found previously for retinothalamic and thalamocortical connections. We found that a single RGC directly contributed on average to approximately 3% of the activity of its cortical target. The relative timing of pairs of spikes from the retinal cell affected their efficacy in driving the cortical cell. When two retinal spikes were closely spaced (<10 msec), the second spike was several times more likely to drive the cortical target. The relative magnitude of this disynaptic paired spike enhancement was considerably larger than has been found previously for retinogeniculate and geniculocortical connections. The amplified paired spike enhancement from retina to cortex ensures that signal transmission from retina to cortex is particularly effective when the retina fires a series of closely spaced action potentials.

What the brain stem tells the frontal cortex. I. Oculomotor signals sent from superior colliculus to frontal eye field via mediodorsal thalamus

Neuronal processing in cerebral cortex and signal transmission from cortex to brain stem have been studied extensively, but little is known about the numerous feedback pathways that ascend from brain stem to cortex. In this study, we characterized the signals conveyed through an ascending pathway coursing from the superior colliculus (SC) to the frontal eye field (FEF) via mediodorsal thalamus (MD). Using antidromic and orthodromic stimulation, we identified SC source neurons, MD relay neurons, and FEF recipient neurons of the pathway in Macaca mulatta. The monkeys performed oculomotor tasks, including delayed-saccade tasks, that permitted analysis of signals such as visual activity, delay activity, and presaccadic activity. We found that the SC sends all of these signals into the pathway with no output selectivity, i.e., the signals leaving the SC resembled those found generally within the SC. Visual activity arrived in FEF too late to contribute to short-latency visual responses there, and delay activity was largely filtered out in MD. Presaccadic activity, however, seemed critical because it traveled essentially unchanged from SC to FEF. Signal transmission in the pathway was fast ( approximately 2 ms from SC to FEF) and topographically organized (SC neurons drove MD and FEF neurons having similarly eccentric visual and movement fields). Our analysis of identified neurons in one pathway from brain stem to frontal cortex thus demonstrates that multiple signals are sent from SC to FEF with presaccadic activity being prominent. We hypothesize that a major signal conveyed by the pathway is corollary discharge information about the vector of impending saccades.

Retinogeniculate Synaptic Properties Controlling Spike Number and Timing in Relay Neurons

Retinal ganglion cells (RGC) transmit visual signals to thalamocortical relay neurons in the lateral geniculate nucleus via retinogeniculate synapses. Relay neuron spike patterns do not simply reflect those of RGCs, but the mechanisms controlling this transformation are not well understood. We therefore examined synaptic properties controlling the strength and precision of relay neuron firing in mouse (p28–33) brain slices using physiological stimulation patterns and a combination of current clamp and dynamic clamp. In tonic mode (-55 mV), activation of single RGC inputs elicited stereotyped responses in a given neuron. In contrast, responses in different neurons varied from unreliable, to faithfully following, to a gain in the number of spikes. Dynamic clamp experiments indicated these different responses primarily reflected variability in the amplitudes of the N-methyl-d-aspartate (NMDA) and AMPA components. Each of these components played a distinct role in transmission. The AMPA component evoked a single precisely timed, short-latency spike per stimulus, but efficacy decreased during repetitive stimulation due to desensitization and depression. The NMDA component elicited longer-latency spikes and multiple spikes per stimulus and became more effective during repetitive stimuli that led to NMDA current summation. We found that in burst mode (–75 mV), where low-threshold calcium spikes are activated, AMPA and NMDA components and synaptic plasticity influenced spike number, but no combination enabled relay cells to faithfully follow the stimulus. Thus the characteristics of AMPA and NMDA currents, the ratio of these currents and use-dependent plasticity interact to shape how RGC activity is conveyed to relay neurons.

Temporal resolution of ensemble visual motion signals in primate retina

Recent studies have examined the temporal precision of spiking in visual system neurons, but less is known about the time scale that is relevant for behaviorally important visual computations. We examined how spatiotemporal patterns of spikes in ensembles of primate retinal ganglion cells convey information about visual motion to the brain. The direction of motion of a bar was estimated by comparing the timing of responses in ensembles of parasol (magnocellular-projecting) retinal ganglion cells recorded simultaneously, using a cross-correlation approach similar to standard models of motion sensing. To identify the temporal resolution of motion signals, spike trains were low-pass filtered before estimating the direction of motion. The filter time constant that resulted in most accurate motion sensing was in the range of 10-50 msec for a range of stimulus speeds and contrasts and approached a lower limit of approximately 10 msec at high speeds and contrasts. This time constant was, on average, comparable to the length of interspike intervals. These findings suggest that cortical neurons could filter their inputs on a time scale of tens of milliseconds, rather than relying on the precise times of individual input spikes, to sense motion most reliably.

The relay of high-frequency sensory signals in the Whisker-to-barreloid pathway

The present study investigated the operational features of whisker-evoked EPSPs in barreloid cells and the ability of the whisker-to-barreloid pathway to relay high rates of whisker deflection in lightly anesthetized rats. Results show that lemniscal EPSPs are single-fiber events with fast rise times (<500 microsec) that strongly depress at short inter-EPSP intervals. They occur at short latencies (3.84 +/- 0.96 msec) with little jitters (<300 microsec) after electrical stimulation of the whisker follicle. Waveform analysis indicates that one to three lemniscal axons converge on individual barreloid cells to produce EPSPs of similar rise times but different amplitudes. When challenged by high rates of whisker deflection, cells in the whisker-to-barreloid pathway demonstrate a remarkable frequency-following ability. Primary vibrissa afferents could follow in a phase-locked manner trains of sinusoidal deflections at up to 1 kHz. Although trigeminothalamic cells could still faithfully follow deflection rates of 200-300 Hz, the actual frequency-following ability of individual cells depends on the amplitude, velocity, and direction of displacements. The discharges of trigeminothalamic cells induce corresponding phase-locked EPSPs in barreloid cells, which trigger burst discharges at stimulus onset. During the following cycles of the stimulus train, few action potentials ensue because of the strong synaptic depression at lemniscal synapses. It is concluded that the whisker-to-barreloid pathway can relay vibratory inputs with a high degree of temporal precision, but that the relay of this information to the cerebral cortex requires the action of modulators, and possibly phase-locked discharges among an ensemble of relay cells.

Two different types of Y cells in the cat lateral geniculate nucleus

The Y pathway in the cat visual system is traditionally viewed as a single channel that originates in the retina. However, most Y cells from the contralateral retina diverge to innervate two different layers of the lateral geniculate nucleus, suggesting a possible channel split: YC (Y geniculate cell in layer C) and YA (Y geniculate cell in layer A). We tested the functional significance of this anatomical divergence by comparing the response properties of simultaneously recorded YC and YA geniculate cells with overlapping receptive fields. Our results demonstrate that YC and YA cells significantly differ in a large number of temporal and spatial parameters including response latency, response transiency, receptive-field size, and linearity of spatial summation. Furthermore, for some of these parameters, the differences between YC and YA cells are as pronounced as the differences between Y and X cells in layer A. These results along with results from previous studies strongly suggest that Y retinal afferents diverge into two separate channels at the level of the thalamus.

Propagation of correlated activity through multiple stages of a neural circuit

The timing of spikes can carry information, for instance, when the temporal pattern of firing across neurons results in correlated activity. However, in part because central synapses are unreliable, correlated activity has not been observed to propagate through multiple subsequent stages in neural circuits, although such propagation has frequently been used in theoretical models. Using simultaneous single-unit and multiunit recordings from two or three vocal control nuclei of songbirds, measurement of coherency and time delays, and manipulation of neural activity, we provide evidence here for preserved correlation of activity through multiple steps of the neural circuit for song, including a basal ganglia circuit and its target vocal motor pathway. This suggests that these pathways contain highly functionally interconnected neurons and represent a neural architecture that can preserve information about the timing of firing of groups of neurons. Because the interaction of these song pathways is critical to vocal learning, the preserved correlation of activity may be important to the learning and production of sequenced motor acts and could be a general feature of basal ganglia-cortical interaction.

Decoding neuronal spike trains: how important are correlations?

It has been known for >30 years that neuronal spike trains exhibit correlations, that is, the occurrence of a spike at one time is not independent of the occurrence of spikes at other times, both within spike trains from single neurons and across spike trains from multiple neurons. The presence of these correlations has led to the proposal that they might form a key element of the neural code. Specifically, they might act as an extra channel for information, carrying messages about events in the outside world that are not carried by other aspects of the spike trains, such as firing rate. Currently, there is no general consensus about whether this proposal applies to real spike trains in the nervous system. This is largely because it has been hard to separate information carried in correlations from that not carried in correlations. Here we propose a framework for performing this separation. Specifically, we derive an information-theoretic cost function that measures how much harder it is to decode neuronal responses when correlations are ignored than when they are taken into account. This cost function can be readily applied to real neuronal data.

Cooperation between area 17 neuron pairs enhances fine discrimination of orientation

We examined 66 complex cells in area 17 of cats that were paralyzed and anesthetized with propofol and N2O. We studied changes in ensemble responses for small (<10 degrees ) and large (>10 degrees ) differences in orientation. Examination of temporal resolution and discharge history revealed advantages in discrimination from both dependent (e.g., synchronization) and independent (e.g., bursting) interspike interval properties. For 27 pairs of neurons, we found that the average cooperation (the advantage gained from the joint activity) was 57.6% for fine discrimination of orientation but <5% for gross discrimination. Dependency (probabilistic quantification of the interaction between the cells) was measured between 29 pairs of neurons while varying orientation. On average, the dependency tuning for orientation was 35.5% narrower than the average firing rate tuning. The changes in dependency around the peak orientation (at which the firing rate remains relatively constant) lead to substantial cooperation that can improve discrimination in this region. The narrow tuning of dependency and the cooperation provide evidence to support a population-encoding scheme that is based on biologically plausible mechanisms and that could account for hyperacuities.

Position, size and luminosity of phosphenes generated by direct optic nerve stimulation

Pulses of low intensity current, delivered through a cuff electrode chronically implanted around the optic nerve of a blind retinitis pigmentosa patient generate visual sensations. These phosphenes are obtained at lower thresholds for a train of stimuli than for single pulses, which suggests the existence of a spatial and temporal integrating mechanism. The perceptions are much smaller than those predicted from model simulations. A set of equations are derived which show the effect of pulse current, duration, number and frequency on the position, size and, to some extent, luminosity of the resulting phosphenes.

Computer model of passive signal integration based on whole-cell in vitro studies of rat lateral geniculate nucleus

Computer models were used to investigate passive properties of lateral geniculate nucleus thalamocortical cells and thalamic interneurons based on in vitro whole-cell study. Two neurons of each type were characterized physiologically and morphologically. Thalamocortical cells transmitted 37% of steady-state signal orthodromically (distal dendrite to soma) and 93% antidromically (soma to distal dendrite); interneurons transmitted 18% orthodromically and 53% antidromically. Lowering membrane resistance caused a dramatic drop in steady-state signal transmission. Simulation of brief signals such as orthodromically transmitted postsynaptic potentials and antidromically transmitted action potentials showed relatively poor transmission due to the low-pass filtering property of dendrites. This attenuation was particularly pronounced in interneurons. By contrast, bursts of postsynaptic potentials or action potentials were relatively well transmitted as the temporal summation of these recurring signals gave prolonged depolarizations comparable to prolonged current injection. While synaptic clustering, active channels and reduction of membrane resistance by ongoing synaptic activity will have additional profound effects in vivo, the present in vitro modelling suggests that passive signal transmission in neurons will depend on type of signal conveyed, on directionality and on membrane state. This will be particularly important for thalamic interneurons, whose presynaptic dendrites may either work independently or function in concert with each other and with the soma. Our findings suggest that bursts may be particularly well transmitted along dendrites, allowing firing format to alter the functional anatomy of the cell.

Spike timing and visual processing in the retinogeniculocortical pathway

Although the visual response properties of neurons along the retinogeniculocortical pathway have been studied for decades, relatively few studies have examined how individual neurons along the pathway communicate with each other. Recent studies in the cat (Felis domestica) now show that the strength of these connections is very dynamic and spike timing plays an important part in determining whether action potentials will be transferred from pre- to postsynaptic cells. This review explores recent progress in our understanding of what role spike timing has in establishing different patterns of geniculate activity and how these patterns ultimately drive the cortex.

Visual experience before eye-opening and the development of the retinogeniculate pathway

Visual experience before eye-opening is not usually thought to have any developmental significance. Here we show that naturalistic visual stimuli presented through unopened eyelids robustly activate neurons in the ferret dorsal lateral geniculate nucleus. Further, dark-rearing prior to natural eye-opening has striking effects upon geniculate physiology. Receptive field maps after dark-rearing show increased convergence of On- and Off-center responses, and neurons frequently respond to both bright and dark phases of drifting gratings. There is also increased selectivity for the orientation of the gratings. These abnormalities of On-Off segregation can be explained by the finding that the responses of immature On and Off cells to naturalistic stimuli are strongly anticorrelated.

Effects of paired-pulse and repetitive stimulation on neurons in the rat medial geniculate body

Many behaviorally relevant sounds, including language, are composed of brief, rapid, repetitive acoustic features. Recent studies suggest that abnormalities in producing and understanding spoken language are correlated with abnormal neural responsiveness to such auditory stimuli at higher auditory levels [Tallal et al., Science 271 (1996) 81-84; Wright et al., Nature 387 (1997) 176-178; Nagarajan et al., Proc. Natl. Acad. Sci. USA 96 (1999) 6483-6488] and with abnormal anatomical features in the auditory thalamus [Galaburda et al., Proc. Natl. Acad. Sci. USA 91 (1994) 8010-8013]. To begin to understand potential mechanisms for normal and abnormal transfer of sensory information to the cortex, we recorded the intracellular responses of medial geniculate body thalamocortical neurons in a rat brain slice preparation. Inferior colliculus or corticothalamic axons were excited by pairs or trains of electrical stimuli. Neurons receiving only excitatory collicular input had tufted dendritic morphology and displayed strong paired-pulse depression of their large, short-latency excitatory postsynaptic potentials. In contrast, geniculate neurons receiving excitatory and inhibitory collicular inputs could have stellate or tufted morphology and displayed much weaker depression or even paired-pulse facilitation of their smaller, longer-latency excitatory postsynaptic potentials. Depression was not blocked by ionotropic glutamate, GABA(A) or GABA(B) receptor antagonists. Facilitation was unaffected by GABA(A) receptor antagonists but was diminished by N-methyl-D-aspartate (NMDA) receptor blockade. Similar stimulation of the corticothalamic input always elicited paired-pulse facilitation. The NMDA-independent facilitation of the second cortical excitatory postsynaptic potential lasted longer and was more pronounced than that seen for the excitatory collicular inputs. Paired-pulse stimulation of isolated collicular inhibitory postsynaptic potentials generated little change in the second GABA(A) potential amplitude measured from the resting potential, but the GABA(B) amplitude was sensitive to the interstimulus interval. Train stimuli applied to collicular or cortical inputs generated intra-train responses that were often predicted by their paired-pulse behavior. Long-lasting responses following train stimulation of the collicular inputs were uncommon. In contrast, corticothalamic inputs often generated long-lasting depolarizing responses that were dependent on activation of a metabotropic glutamate receptor. Our results demonstrate that during repetitive afferent firing there are input-specific mechanisms controlling synaptic strength and membrane potential over short and long time scales. Furthermore, they suggest that there may be two classes of excitatory collicular input to medial geniculate neurons and a single class of small-terminal corticothalamic inputs, each of which has distinct features.

The lemniscal-cuneate recurrent excitation is suppressed by strychnine and enhanced by GABAA antagonists in the anaesthetized cat

In the somatosensory system, cuneolemniscal (CL) cells fire high frequency doublets of spikes facilitating the transmission of sensory information to diencephalic target cells. We studied how lemniscal feedback affects ascending transmission of cutaneous neurons of the middle cuneate nucleus. Electrical stimulation of the contralateral medial lemniscus and of the skin at sites evoking responses with minimal threshold induced recurrent activation of CL cells at a latency of 1-3.5 ms. The lemniscal feedback activation was suppressed by increasing the stimulating intensity at the same sites, suggesting recurrent-mediated lateral inhibition. The glycine antagonist strychnine blocked the recurrent excitatory responses while GABAA antagonists uncovered those obscured by stronger stimulation. CL cells sharing a common receptive field (RF) potentiate one another by recurrent activation and disinhibition, the disinhibition being produced by serial interactions between glycinergic and GABAergic interneurons. Conversely, CL cells with different RFs inhibit each other through recurrent GABA-mediated inhibition. The lemniscal feedback would thus enhance the surround antagonism of a centre response by increasing the spatial resolution and the transmission of weak signals.

Electrophysiological correlates of synchronous neural activity and attention: a short review

Attentional selection implies preferential treatment of some sensory stimuli over others. This requires differential representation of attended and unattended stimuli. Most previous research has focused on pure rate codes for this representation but recent evidence indicates that a mixed code, involving both mean firing rate and temporal codes, may be employed. Of particular interest is a distinction of attended from unattended stimuli based on synchrony within neural populations. I review electrophysiological evidence at macroscopic, mesoscopic and microscopic spatial scales showing that the degree of synchronous activity varies with the attentional state of the perceiving organism.

Saccadic eye movements modulate visual responses in the lateral geniculate nucleus

We studied the effects of saccadic eye movements on visual signaling in the primate lateral geniculate nucleus (LGN), the earliest stage of central visual processing. Visual responses were probed with spatially uniform flickering stimuli, so that retinal processing was uninfluenced by eye movements. Nonetheless, saccades had diverse effects, altering not only response strength but also the temporal and chromatic properties of the receptive field. Of these changes, the most prominent was a biphasic modulation of response strength, weak suppression followed by strong enhancement. Saccadic modulation was widespread, and affected both of the major processing streams in the LGN. Our results demonstrate that during natural viewing, thalamic response properties can vary dramatically, even over the course of a single fixation.

AMPA and NMDA currents show different short-term depression in the dorsal lateral geniculate nucleus of the rat

Paired-pulse depression was studied at the glutamatergic synapse between retinal afferents and thalamocortical cells in the rat dorsal lateral geniculate nucleus. The main objective of this study was to examine the contributions of the pre- and postsynaptic sites to this depression by comparing AMPA- and NMDA-receptor-mediated responses. Equal depression of the two receptor components would indicate involvement of presynaptic mechanisms, while differences in depression would indicate involvement of postsynaptic mechanisms. Pharmacologically isolated AMPA- and NMDA-receptor-mediated currents were recorded using the whole-cell patch-clamp technique in acute thalamic slices. Both the AMPA and the NMDA components showed pronounced depression when retinal afferents were activated by paired pulses. The depression decayed within 5 s. The AMPA component was more strongly depressed than the NMDA component at paired-pulse intervals ranging from 20 to 200 ms, suggesting the involvement of postsynaptic mechanisms. For intervals of 500 ms and longer, the depression of the two components was identical, suggesting the involvement of purely presynaptic mechanisms. The degree of depression measured without the use of pharmacological tools produced similar results, thus excluding the involvement of presynaptic ionotropic glutamate receptors. Cyclothiazide, a blocker of AMPA-receptor desensitisation, reduced the difference in depression between the two components, suggesting that desensitisation of the AMPA receptors is a postsynaptic mechanism that contributes to the difference in depression between the AMPA and the NMDA components.

The timing of response onset and offset in macaque visual neurons

We used fast, pseudorandom temporal sequences of preferred and antipreferred stimuli to drive neuronal firing rates rapidly between minimal and maximal across the visual system. Stimuli were tailored to the preferences of cells recorded in the lateral geniculate nucleus (magnocellular and parvocellular), primary visual cortex (simple and complex), and the extrastriate motion area MT. We found that cells took longer to turn on (to increase their firing rate) than to turn off (to reduce their rate). The latency difference (onset minus offset) varied from several to tens of milliseconds across cell type and stimulus class and was correlated with spontaneous or driven firing rates for most cell classes. The delay for response onset depended on the nature of the stimulus present before the preferred stimulus appeared, and may result from persistent inhibition caused by antipreferred stimuli or from suppression that followed the offset of the preferred stimulus. The onset delay showed three distinct types of dependence on the temporal sequence of stimuli across classes of cells, implying that suppression may accumulate or wear off with time. Onset latency is generally longer, can be more variable, and has marked stimulus dependence compared with offset latency. This suggests an important role for offset latency in assessing the speed of information transmission in the visual system and raises the possibility that signal offsets provide a timing reference for visual processing. We discuss the origin of the delay in onset latency compared with offset latency and consider how it may limit the utility of certain feedforward circuits.

Contributions of receptor desensitization and saturation to plasticity at the retinogeniculate synapse

The retinogeniculate synapse conveys visual information from the retina to thalamic relay neurons. Here, we examine the mechanisms of short-term plasticity that can influence transmission at this connection in mouse brain slices. Our studies show that synaptic strength is modified by physiological activity patterns due to marked depression at high frequencies. Postsynaptic mechanisms of plasticity make prominent contributions to this synaptic depression. During trains of retinal input stimulation, receptor desensitization attenuates the AMPA EPSC while the NMDA EPSC saturates. This differential plasticity may help explain the distinct roles of these receptors in shaping the relay neuron response to visual stimulation with the AMPA component being important for transient responses, while sustained high frequency responses rely more on the NMDA component.

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.

Correlated neuronal activity and the flow of neural information

For years we have known that cortical neurons collectively have synchronous or oscillatory patterns of activity, the frequencies and temporal dynamics of which are associated with distinct behavioural states. Although the function of these oscillations has remained obscure, recent experimental and theoretical results indicate that correlated fluctuations might be important for cortical processes, such as attention, that control the flow of information in the brain.

An analysis of the effect of retinal ganglion cell impulses upon the firing probability of neurons in the dorsal lateral geniculate nucleus of the cat

This study examines the probabilistic way in which LGN cells produce impulses. Simultaneous extracellular recordings were made from a single lateral geniculate nucleus (LGN) relay cell and the one retinal cell that triggered most of its impulses during vigorous responses. Impulses in the retinal train were classified as 'successful' in triggering an LGN impulse, or 'unsuccessful'. Similarly, the impulses in the LGN train were either 'Triggered' by a successful retinal impulse, or were 'Anonymous'. These impulses delimited various intervals whose distributions were compared to the relevant distribution of all intervals to determine whether short or long intervals tended to dominate in each case. Intervals between unsuccessful and successful impulses tended to be shorter than other retinal intervals, with their probability declining exponentially with duration. These data imply a decaying excitation produced by each impulse, but with a short refractory period following each Triggered impulse. Short intervals between Anonymous impulses were relatively common; Anonymous impulses thus lack the same refractoriness and tend to occur in bursts. The exponential excitation following an unsuccessful retinal impulse also facilitates Anonymous impulses, while Anonymous impulses (during visual stimulation) render the LGN slightly refractory for subsequent retinal impulses.

Membrane potential fluctuations determine the precision of spike timing and synchronous activity: a model study

It is much debated on what time scale information is encoded by neuronal spike activity. With a phenomenological model that transforms time-dependent membrane potential fluctuations into spike trains, we investigate constraints for the timing of spikes and for synchronous activity of neurons with common input. The model of spike generation has a variable threshold that depends on the time elapsed since the previous action potential and on the preceding membrane potential changes. To ensure that the model operates in a biologically meaningful range, the model was adjusted to fit the responses of a fly visual interneuron to motion stimuli. The dependence of spike timing on the membrane potential dynamics was analyzed. Fast membrane potential fluctuations are needed to trigger spikes with a high temporal precision. Slow fluctuations lead to spike activity with a rate about proportional to the membrane potential. Thus, for a given level of stochastic input, the frequency range of membrane potential fluctuations induced by a stimulus determines whether a neuron can use a rate code or a temporal code. The relationship between the steepness of membrane potential fluctuations and the timing of spikes has also implications for synchronous activity in neurons with common input. Fast membrane potential changes must be shared by the neurons to produce synchronous activity.

Synaptic interactions between thalamic inputs to simple cells in cat visual cortex

We performed experiments in the cat geniculocortical pathway, in vivo, to examine how presynaptic spikes interact to influence the firing of postsynaptic targets. In particular, we asked (1) how do multiple spikes from a single presynaptic neuron interact to influence the firing of a postsynaptic target (homosynaptic interactions), (2) how do spikes from two different presynaptic neurons interact (heterosynaptic interactions), and (3) what is the time course of homosynaptic and heterosynaptic interactions? We found that both homosynaptic and heterosynaptic interactions increase the likelihood of driving a postsynaptic spike, although with different time courses. For two spikes traveling down a single geniculate axon, the second spike is more effective than the first for approximately 15 msec. For two spikes on separate axons, the interaction is faster ( approximately 7 msec duration, approximately 2.5 msec time constant). Thus changes in firing rate are perhaps best relayed by homosynaptic interactions, whereas heterosynaptic interactions may help detect coincident spikes from different thalamic inputs.