Context-sensitive synaptic plasticity and temporal-to-spatial transformations in hippocampal slices

Perception of Happening: How the Brain Deals with the No-History Problem

In physics, the temporal dimension has units of infinitesimally brief duration. Given this, how is it possible to perceive things, such as motion, music, and vibrotactile stimulation, that involve extension across many units of time? To address this problem, it is proposed that there is what is termed an "information construct of happening" (ICOH), a simultaneous representation of recent, temporally differentiated perceptual information on the millisecond time scale. The main features of the ICOH are (i) time marking, semantic labeling of all information in the ICOH with ordinal temporal information and distance from what is informationally identified as the present moment, (ii) vector informational features that specify kind, direction, and rate of change for every feature in a percept, and (iii) connectives, information relating vector informational features at adjacent temporal locations in the ICOH. The ICOH integrates products of perceptual processing with recent historical information in sensory memory on the subsecond time scale. Perceptual information about happening in informational sensory memory is encoded in semantic form that preserves connected semantic trails of vector and timing information. The basic properties of the ICOH must be supported by a general and widespread timing mechanism that generates ordinal and interval timing information and it is suggested that state-dependent networks may suffice for that purpose. Happening, therefore, is perceived at a moment and is constituted by an information structure of connected recent historical information.

Neurocomputational Models of Interval and Pattern Timing

Most of the computations and tasks performed by the brain require the ability to tell time, and process and generate temporal patterns. Thus, there is a diverse set of neural mechanisms in place to allow the brain to tell time across a wide range of scales: from interaural delays on the order of microseconds to circadian rhythms and beyond. Temporal processing is most sophisticated on the scale of tens of milliseconds to a few seconds, because it is within this range that the brain must recognize and produce complex temporal patterns-such as those that characterize speech and music. Most models of timing, however, have focused primarily on simple intervals and durations, thus it is not clear whether they will generalize to complex pattern-based temporal tasks. Here, we review neurobiologically based models of timing in the subsecond range, focusing on whether they generalize to tasks that require placing consecutive intervals in the context of an overall pattern, that is, pattern timing.

Spatiotemporal memory is an intrinsic property of networks of dissociated cortical neurons

The ability to process complex spatiotemporal information is a fundamental process underlying the behavior of all higher organisms. However, how the brain processes information in the temporal domain remains incompletely understood. We have explored the spatiotemporal information-processing capability of networks formed from dissociated rat E18 cortical neurons growing in culture. By combining optogenetics with microelectrode array recording, we show that these randomly organized cortical microcircuits are able to process complex spatiotemporal information, allowing the identification of a large number of temporal sequences and classification of musical styles. These experiments uncovered spatiotemporal memory processes lasting several seconds. Neural network simulations indicated that both short-term synaptic plasticity and recurrent connections are required for the emergence of this capability. Interestingly, NMDA receptor function is not a requisite for these short-term spatiotemporal memory processes. Indeed, blocking the NMDA receptor with the antagonist APV significantly improved the temporal processing ability of the networks, by reducing spontaneously occurring network bursts. These highly synchronized events have disastrous effects on spatiotemporal information processing, by transiently erasing short-term memory. These results show that the ability to process and integrate complex spatiotemporal information is an intrinsic property of generic cortical networks that does not require specifically designed circuits.

Diverse thalamocortical short-term plasticity elicited by ongoing stimulation

To produce sensation, neuronal pathways must transmit and process stimulus patterns that unfold over time. This behavior is determined by short-term synaptic plasticity (STP), which shapes the temporal filtering properties of synapses in a pathway. We explored STP variability across thalamocortical (TC) synapses, measuring whole-cell responses to stimulation of TC fibers in layer 4 neurons of mouse barrel cortex in vitro. As expected, STP during stimulation from rest was dominated by depression. However, STP during ongoing stimulation was strikingly diverse across TC connections. Diversity took the form of variable tuning to the latest interstimulus interval: some connections responded weakly to shorter intervals, while other connections were facilitated. These behaviors did not cluster into categories but formed a continuum. Diverse tuning did not require disynaptic inhibition. Hence, monosynaptic excitatory lemniscal TC connections onto layer 4 do not behave uniformly during ongoing stimulation. Each connection responds differentially to particular stimulation intervals, enriching the ability of the pathway to convey complex, temporally fluctuating information.

Timing as an intrinsic property of neural networks: evidence from in vivo and in vitro experiments

The discrimination and production of temporal patterns on the scale of hundreds of milliseconds are critical to sensory and motor processing. Indeed, most complex behaviours, such as speech comprehension and production, would be impossible in the absence of sophisticated timing mechanisms. Despite the importance of timing to human learning and cognition, little is known about the underlying mechanisms, in particular whether timing relies on specialized dedicated circuits and mechanisms or on general and intrinsic properties of neurons and neural circuits. Here, we review experimental data describing timing and interval-selective neurons in vivo and in vitro. We also review theoretical models of timing, focusing primarily on the state-dependent network model, which proposes that timing in the subsecond range relies on the inherent time-dependent properties of neurons and the active neural dynamics within recurrent circuits. Within this framework, time is naturally encoded in populations of neurons whose pattern of activity is dynamically changing in time. Together, we argue that current experimental and theoretical studies provide sufficient evidence to conclude that at least some forms of temporal processing reflect intrinsic computations based on local neural network dynamics.

A novel learning rule for long-term plasticity of short-term synaptic plasticity enhances temporal processing

It is well established that short-term synaptic plasticity (STP) of neocortical synapses is itself plastic - e.g., the induction of LTP and LTD tend to shift STP towards short-term depression and facilitation, respectively. What has not been addressed theoretically or experimentally is whether STP is "learned"; that is, is STP regulated by specific learning rules that are in place to optimize the computations performed at synapses, or, are changes in STP essentially an epiphenomenon of long-term plasticity? Here we propose that STP is governed by specific learning rules that operate independently and in parallel of the associative learning rules governing baseline synaptic strength. We describe a learning rule for STP and, using simulations, demonstrate that it significantly enhances the discrimination of spatiotemporal stimuli. Additionally we generate a set of experimental predictions aimed at testing our hypothesis.

State-dependent computations: spatiotemporal processing in cortical networks

A conspicuous ability of the brain is to seamlessly assimilate and process spatial and temporal features of sensory stimuli. This ability is indispensable for the recognition of natural stimuli. Yet, a general computational framework for processing spatiotemporal stimuli remains elusive. Recent theoretical and experimental work suggests that spatiotemporal processing emerges from the interaction between incoming stimuli and the internal dynamic state of neural networks, including not only their ongoing spiking activity but also their 'hidden' neuronal states, such as short-term synaptic plasticity.

Temporal production and visuospatial processing

Current models of prospective timing hypothesize that estimated duration is influenced either by the attentional load or by the short-term memory requirements of a concurrent nontemporal task. In the present study, we addressed this issue with four dual-task experiments. In Exp. 1, the effect of memory load on both reaction time and temporal production was proportional to the number of items of a visuospatial pattern to hold in memory. In Exps. 2, 3, and 4, a temporal production task was combined with two visual search tasks involving either pre-attentive or attentional processing. Visual tasks interfered with temporal production: produced intervals were lengthened proportionally to the display size. In contrast, reaction times increased with display size only when a serial, effortful search was required. It appears that memory and perceptual set size, rather than nonspecific attentional or short-term memory load, can influence prospective timing.

Experience-dependent plasticity in S1 caused by noncoincident inputs

Prior work has shown that coincident inputs became co-represented in somatic sensory cortex. In this study, the hypothesis that the co-representation of digits required synchronous inputs was tested, and the daily development of two-digit receptive fields was observed with cortical implants. Two adult primates detected temporal differences in tap pairs delivered to two adjacent digits. With stimulus onset asynchronies of > or = 100 ms, representations changed to include two-digit receptive fields across the first 4 wk of training. In addition, receptive fields at sites responsive to the taps enlarged more than twofold, and receptive fields at sites not responsive to the taps had no significant areal change. Further training did not increase the expression of two-digit receptive fields. Cortical responses to the taps were not dependent on the interval length. Stimuli preceding a hit, miss, false positives, and true negatives differed in the ongoing cortical rate from 50 to 100 ms after the stimulus but did not differ in the initial, principal, response to the taps. Response latencies to the emergent responses averaged 4.3 ms longer than old responses, which occurs if plasticity is cortical in origin. New response correlations developed in parallel with the new receptive fields. These data show co-representation can be caused by presentation of stimuli across a longer time window than predicted by spike-timing-dependent plasticity and suggest that increased cortical excitability accompanies new task learning.

Decoding temporal information through slow lateral excitation in the olfactory system of insects

Sensory information is represented in a spatio-temporal code in the antennal lobe, the first processing stage of the olfactory system of insects. We propose a novel mechanism for decoding this information in the next processing stage, the mushroom body. The Kenyon cells in the mushroom body of insects exhibit lateral excitatory connections at their axons. We demonstrate that slow lateral excitation between Kenyon cells allows one to decode sequences of activity in the antennal lobe. We are thus able to clarify the role of the existing connections as well as to demonstrate a novel mechanism for decoding temporal information in neuronal systems. This mechanism complements the variety of existing temporal decoding schemes. It seems that neuronal systems not only have a rich variety of code types but also quite a diversity of algorithms for transforming different codes into each other.

Burst-induced synaptic depression and its modulation contribute to information transfer at Aplysia sensorimotor synapses: empirical and computational analyses

The Aplysia sensorimotor synapse is a key site of plasticity for several simple forms of learning. Plasticity of this synapse has been extensively studied, albeit primarily with individual action potentials elicited at low frequencies. Yet, the mechanosensory neurons fire high-frequency bursts in response to even moderate tactile stimuli delivered to the skin. In the present study, we extend this analysis to show that sensory neurons also fire bursts in the range of 1-60 Hz in response to electrical stimuli similar to those used in behavioral studies of sensitization. Intracellular stimulation of sensory neurons to fire a burst of action potentials at 10 Hz for 1 sec led to significant homosynaptic depression of postsynaptic responses. The depression was transient and fully recovered within 10 min. During the burst, the steady-state depressed phase of the postsynaptic response, which was only 20% of the initial EPSP of the burst, still contributed to firing the motor neuron. To explore the functional contribution of transient homosynaptic depression to the response of the motor neuron, computer simulations of the sensorimotor synapse with and without depression were compared. Depression allowed the motor neuron to produce graded responses over a wide range of presynaptic input strength. In addition, enhancement of synaptic transmission throughout a burst increased motor neuron output substantially more than did preferential enhancement of the initial phase of a burst. Thus, synaptic depression increased the dynamic range of the sensorimotor synapse and can, in principle, have a profound effect on information processing.

Burst-induced synaptic depression and its modulation contribute to information transfer at Aplysia sensorimotor synapses: empirical and computational analyses

The Aplysia sensorimotor synapse is a key site of plasticity for several simple forms of learning. Plasticity of this synapse has been extensively studied, albeit primarily with individual action potentials elicited at low frequencies. Yet, the mechanosensory neurons fire high-frequency bursts in response to even moderate tactile stimuli delivered to the skin. In the present study, we extend this analysis to show that sensory neurons also fire bursts in the range of 1-60 Hz in response to electrical stimuli similar to those used in behavioral studies of sensitization. Intracellular stimulation of sensory neurons to fire a burst of action potentials at 10 Hz for 1 sec led to significant homosynaptic depression of postsynaptic responses. The depression was transient and fully recovered within 10 min. During the burst, the steady-state depressed phase of the postsynaptic response, which was only 20% of the initial EPSP of the burst, still contributed to firing the motor neuron. To explore the functional contribution of transient homosynaptic depression to the response of the motor neuron, computer simulations of the sensorimotor synapse with and without depression were compared. Depression allowed the motor neuron to produce graded responses over a wide range of presynaptic input strength. In addition, enhancement of synaptic transmission throughout a burst increased motor neuron output substantially more than did preferential enhancement of the initial phase of a burst. Thus, synaptic depression increased the dynamic range of the sensorimotor synapse and can, in principle, have a profound effect on information processing.

Excitatory inputs to CA1 interneurons show selective synaptic dynamics

The dynamic properties of synapses between neurons in the hippocampal CA1 area are important for the frequency-dependent signal transfer of the network. We have examined the synaptic dynamics of excitatory inputs to CA1 interneurons and pyramidal cells using whole cell voltage-clamp recordings. The CA1 network was activated using extracellular stimulation electrodes at the Schaffer collaterals (feedforward activation) or at the Alveus (activation of the feedback loop). The dynamic properties of input from the Schaffer collaterals to CA1 interneurons (basket and bistratified cells) were different from the synaptic dynamics of input from the Alveus. Synaptic input from the Schaffer collaterals to CA1 interneurons showed facilitation for most frequencies. After 10 stimuli the synaptic response reached a plateau level that was approximately 150% of the first response in the train. In contrast, the plateau levels of Alveus inputs to interneurons were not different from the first responses for frequencies <or=40 Hz. Paired-pulse facilitation of Schaffer input was stronger than for Alveus input. Cells in stratum oriens with horizontal dendritic trees appeared to be a special group of interneurons because Alveus input to these cells showed strong facilitation with plateau levels of 200% of the first responses. Schaffer input to CA1 basket and bistratified cells showed similar synaptic dynamics compared with Schaffer input to pyramidal cells for frequencies <or=80 Hz. The synaptic dynamics of Schaffer and Alveus input depended only weakly on the stimulus intensity. The difference between the dynamics of Alveus and Schaffer input to CA1 interneurons implies that the relative contribution of feedforward and -back inhibition to network activity depends on the frequency of the input signal at the afferent fibers, adding a level of complexity to transient responses.

Rapid motor adaptations to subliminal frequency shifts during syncopated rhythmic sensorimotor synchronization

The synchronization of rhythmic arm movements to a syncopated metronome cue was studied in a step-change design whereby small tempo shifts were inserted at fixed time points into the metronome frequency. The cueing sequence involved three stimulus types: (1) target contact in synchrony with the metronome beats, (2) syncopated target contact midway in time between audible beats, and (3) syncopated target contact following either a +2% or -2% change in stimulus frequency. Analysis of normalized and aggregated data revealed that (1) during the syncopation condition the response period showed a rapid adaptation to the frequency-incremented stimulus period, (2) response period was less variable during syncopated movement, (3) mean synchronization error and variability, calculated during syncopation relative to the mathematical midpoint of the stimulus cycle, were reduced during syncopated movements, and (4) synchronization error following the frequency increment showed trends to return linearly to pre-increment values which was fully achieved in the -2% change condition only. The results suggest that frequency entrainment to stimulus period was possible during syncopated movement with the response and stimulus onsets 180 degrees out of phase. Most remarkably, 70-80% of the adaptation of the response period to the new stimulus period was immediately attained during the second half cycle of the syncopated movement. Finally, a mathematical model, based on recursion, was introduced that accurately modeled actual data as a function of the previous stimulus and response intervals and a weighted response of period error and synchronization error, which showed dominance of frequency entrainment over phase entrainment during rhythmic synchronization.

Order-sensitive plasticity in adult primary auditory cortex

The neural response to a stimulus presented as part of a rapid sequence is often quite different from the response to the same stimulus presented in isolation. In primary auditory cortex (A1), although the most common effect of preceding stimuli is inhibitory, most neurons can also exhibit response facilitation if the appropriate spectral and temporal separation of sequence elements is presented. In this study, we investigated whether A1 neurons in adult animals can develop context-dependent facilitation to a novel acoustic sequence. After repeatedly pairing electrical stimulation of the basal forebrain with a three-element sequence (high frequency tone--low frequency tone-- noise burst), 25% of A1 neurons exhibited facilitation to the low tone when preceded by the high tone, compared with only 5% in controls. In contrast, there was no increase in the percent of sites that showed facilitation for the reversed tone order (low preceding high). Nearly 60% of sites exhibited a facilitated response to the noise burst when preceded by the two tones. Although facilitation was greatest in response to the paired sequence, facilitation also generalized to related sequences that were either temporally distorted or missing one of the tones. Pairing basal forebrain stimulation with the acoustic sequence also caused a decrease in the time to peak response and an increase in population discharge synchrony, which was not seen after pairing simple tones, tone trains, or broadband stimuli. These results indicate that context-dependent facilitation and response synchronization can be substantially altered in an experience-dependent fashion and provide a potential mechanism for learning spectrotemporal patterns.

Synaptic modification by correlated activity: Hebb's postulate revisited

Correlated spiking of pre- and postsynaptic neurons can result in strengthening or weakening of synapses, depending on the temporal order of spiking. Recent findings indicate that there are narrow and cell type-specific temporal windows for such synaptic modification and that the generally accepted input- (or synapse-) specific rule for modification appears not to be strictly adhered to. Spike timing-dependent modifications, together with selective spread of synaptic changes, provide a set of cellular mechanisms that are likely to be important for the development and functioning of neural networks. When an axon of cell A is near enough to excite cell B or repeatedly or consistently takes part in firing it, some growth or metabolic change takes place in one or both cells such that A's efficiency, as one of the cells firing B, is increased.

Sequence dependence of post-tetanic potentiation after sequential heterosynaptic stimulation in the rat auditory cortex

1. To investigate the mechanisms for the coding stimulus sequence in the auditory cortex (AC), post-tetanic potentiation (PTP) was recorded after sequentially combined heterosynaptic stimulation was applied in rat AC slices. 2. Brief tetanic stimulation (TS) was applied at two sites on AC slices at intervals of 0.5-10 s. PTP of field potentials was induced by the earlier TS, rather than the later TS. PTP was followed by sequence-dependent long-term potentiation (LTP). 3. Using Ca(2+) imaging in the slices loaded with rhod-2, a Ca(2+) indicator, a sequence-dependent distribution of PTP was found in AC slices. 4. The sequence-dependent PTP in excitatory postsynaptic potentials (EPSPs) was observed in supragranular pyramidal neurons. 5. The sequence dependence of PTP was not significantly affected by 1 microM bicuculline, an antagonist of GABA(A) receptors, or 100 microM 2-hydroxysaclofen, an antagonist of GABA(B) receptors. 6. Depolarization and firing recorded in pyramidal neurons during the later TS were less vigorous than when the slices were incubated in the control medium. However, this suppression of the responses during the later TS was not observed in the presence of 50 microM atropine, an antagonist of muscarinic receptors. 7. PTP was induced by the earlier and later TS in the presence of 50 microM atropine, so that the sequence dependence of PTP was abolished. Pirenzepine (50 microM), an antagonist of muscarinic M1 receptors, but not methoctramine (30 microM), an antagonist of M2 receptors, eliminated the sequence dependence of PTP. 8. These findings suggest that the sequence dependence of PTP in AC might have a role in the temporal processing of auditory information on the scale of seconds.

A persistent reduction in short-term facilitation accompanies long-term potentiation in the CA1 area in the intact hippocampus

Exploration of the nature of the relationship between short-term and long-term synaptic plasticity should aid our understanding of their roles in brain function. The effects of inducing long-term potentiation on short-term facilitation at CA1 synapses in the stratum radiatum of the intact hippocampus were examined by recording the slope of the field excitatory postsynaptic potential in both urethane and freely behaving adult rats. Facilitation of the second synaptic response to paired-pulse stimulation (40ms interstimulus interval) was monitored before and after the induction of long-term potentiation by high-frequency stimulation (10 trains of 20 pulses at 200Hz). The tetanus triggered a rapid overall reduction in paired-pulse facilitation that persisted for at least 2h. In the anaesthetized animals a detailed correlation analysis revealed that initial paired-pulse facilitation level correlated strongly with the subsequent reduction in paired-pulse facilitation and the magnitude of long-term potentiation. The reduction in paired-pulse facilitation also correlated with long-term potentiation magnitude. These relationships were not observed in animals with low initial degrees of paired-pulse facilitation. It was concluded that the relative contribution of different expression mechanisms of long-term potentiation varies depending on the initial facilitation characteristics of the synapses. Furthermore, the temporal selectivity and gain control of synapses can be altered persistently in the intact hippocampus. This suggests that there is considerable variation in the fidelity of temporal information storage at different synapses during learning and memory in the CA1 area.

Conscious and preconscious adaptation to rhythmic auditory stimuli: a magnetoencephalographic study of human brain responses

This study was triggered by the experimental evidence that subjects required to tap in synchrony with a heard rhythm spontaneously time their tapping to variations in rhythm frequency even when these variations are so small that they are not consciously detectable. We performed a series of magnetoencephalographic (MEG) measurements, aimed at investigating whether the response of the auditory cortex discriminates randomly administered series of brief tones differing from each other only by their interstimulus intervals (ISI). Moreover, by combining psychophysical measurements, conscious and preconscious adjustments of tapping to rhythm variations were compared with brain cortical responses. The ISIs were varied by 2% or 20% from a "central" value of 500 ms. Subjects always consciously detected the 20% ISI changes and easily adjusted their tapping accordingly, whereas they never consciously detected the 2% ISI changes, even though they always correctly adjusted their tapping to them. Analysis of the auditory evoked fields (AEFs) showed that the intensity of the M100 component decreased with decreasing ISI both for 20% and 2% variations in a statistically significant manner, despite the fact that the 2% variation was not consciously perceived. The M100 behavior indicated that connections between auditory and motor cortexes may exist that are able to use the information on rhythm variations in the stimuli even when these are not consciously identified by the subject. The ability of the auditory cortex to discriminate different time characteristics of the incoming rhythmic stimuli is discussed in this paper in relation to the theories regarding the physiology of time perception and discrimination.

Conceptualizing functional neuroplasticity

There are at least four major forms of functional neuroplasticity that can be studied in humans: homologous area adaptation, cross-modal reassignment, map expansion, and compensatory masquerade. Homologous area adaptation is the assumption of a particular cognitive process by a homologous region in the opposite hemisphere. Cross-modal reassignment occurs when structures previously devoted to processing a particular kind of sensory input now accepts input from a new sensory method. Map expansion is the enlargement of a functional brain region on the basis of performance. Compensatory masquerade is a novel allocation of a particular cognitive process to perform a task. By focusing on these four forms of functional neuroplasticity, several fundamental questions about how functional cooperation between brain regions is achieved can be addressed.

Decoding temporal information: A model based on short-term synaptic plasticity

In the current paper it is proposed that short-term plasticity and dynamic changes in the balance of excitatory-inhibitory interactions may underlie the decoding of temporal information, that is, the generation of temporally selective neurons. Our initial approach was to simulate excitatory-inhibitory disynaptic circuits. Such circuits were composed of a single excitatory and inhibitory neuron and incorporated short-term plasticity of EPSPs and IPSPs and slow IPSPs. We first showed that it is possible to tune cells to respond selectively to different intervals by changing the synaptic weights of different synapses in parallel. In other words, temporal tuning can rely on long-term changes in synaptic strength and does not require changes in the time constants of the temporal properties. When the units studied in disynaptic circuits were incorporated into a larger single-layer network, the units exhibited a broad range of temporal selectivity ranging from no interval tuning to interval-selective tuning. The variability in temporal tuning relied on the variability of synaptic strengths. The network as a whole contained a robust population code for a wide range of intervals. Importantly, the same network was able to discriminate simple temporal sequences. These results argue that neural circuits are intrinsically able to process temporal information on the time scale of tens to hundreds of milliseconds and that specialized mechanisms, such as delay lines or oscillators, may not be necessary.

A model for neural representation of temporal duration

To address how temporal duration is encoded in neural systems, we put forward a simple model for recurrent neural networks. Particular assumptions are only the following two: (1) neuronal bistability and; (2) environmental effects described by a heat bath. The results of Monte Carlo simulation show that population activity triggered at an initial time continues for a prolonged duration, followed by an abrupt self-termination. This time course seems highly suitable for neural representation of temporal duration. The time scale of this prolonged duration is much longer than the time scale of neuronal firing which is of the order of ms. The former time scale implies that of interval timing in cognition and behaviour. Thus, the model provides a possible explanation for a link between these two separated time scales. The Weber law, a hallmark of humans and animals' interval timing, can also be reproduced in our model.

Decoding temporal information: A model based on short-term synaptic plasticity

In the current paper it is proposed that short-term plasticity and dynamic changes in the balance of excitatory-inhibitory interactions may underlie the decoding of temporal information, that is, the generation of temporally selective neurons. Our initial approach was to simulate excitatory-inhibitory disynaptic circuits. Such circuits were composed of a single excitatory and inhibitory neuron and incorporated short-term plasticity of EPSPs and IPSPs and slow IPSPs. We first showed that it is possible to tune cells to respond selectively to different intervals by changing the synaptic weights of different synapses in parallel. In other words, temporal tuning can rely on long-term changes in synaptic strength and does not require changes in the time constants of the temporal properties. When the units studied in disynaptic circuits were incorporated into a larger single-layer network, the units exhibited a broad range of temporal selectivity ranging from no interval tuning to interval-selective tuning. The variability in temporal tuning relied on the variability of synaptic strengths. The network as a whole contained a robust population code for a wide range of intervals. Importantly, the same network was able to discriminate simple temporal sequences. These results argue that neural circuits are intrinsically able to process temporal information on the time scale of tens to hundreds of milliseconds and that specialized mechanisms, such as delay lines or oscillators, may not be necessary.

Distributed synaptic modification in neural networks induced by patterned stimulation

Activity-dependent changes in synaptic efficacy or connectivity are critical for the development, signal processing and learning and memory functions of the nervous system. Repetitive correlated spiking of pre- and postsynaptic neurons can induce a persistent increase or decrease in synaptic strength, depending on the timing of the pre- and postsynaptic excitation. Previous studies on such synaptic modifications have focused on synapses made by the stimulated neuron. Here we examine, in networks of cultured hippocampal neurons, whether and how localized stimulation can modify synapses that are remote from the stimulated neuron. We found that repetitive paired-pulse stimulation of a single neuron for brief periods induces persistent strengthening or weakening of specific polysynaptic pathways in a manner that depends on the interpulse interval. These changes can be accounted for by correlated pre- and postsynaptic excitation at distant synaptic sites, resulting from different transmission delays along separate pathways. Thus, through such a 'delay-line' mechanism, temporal information coded in the timing of individual spikes can be converted into and stored as spatially distributed patterns of persistent synaptic modifications in a neural network.

Distinct functional types of associative long-term potentiation in neocortical and hippocampal pyramidal neurons

The response of a neuron to a time-varying stimulus is influenced by both short- and long-term synaptic plasticity. Both these forms of plasticity produce changes in synaptic efficacy of similar magnitude on very different time scales. A full understanding of the functional role of each form of plasticity relies on understanding how they interact. Here we examine how long-term potentiation (LTP) and short-term plasticity (STP) interact in two different cell types that exhibit NMDA-dependent LTP: neocortical L-II/III and hippocampal CA1 pyramidal cells. STP was examined using both paired pulses and trains of pulses before and after the induction of LTP. In both cell types, the same pairing protocol was used to induce LTP in the presence of an unpaired control pathway. Pairing produced a robust increase in the amplitude of the first EPSP both in the neocortex and hippocampus. However, although in CA1 neurons the same degree of potentiation was maintained throughout the duration of a brief stimulus train, in L-II/III neurons relatively less potentiation was seen in the later EPSPs of the train. Paired-pulse analyses revealed that a uniform potentiation is observed at intervals >100 msec, but at shorter intervals there is a preferential enhancement of the first pulse. Thus, in the cortex LTP may preferentially amplify stimulus onset. These results suggest that there are distinct forms of associative LTP and that the different forms may reflect the underlying computations taking place in different areas.

Distinct functional types of associative long-term potentiation in neocortical and hippocampal pyramidal neurons

The response of a neuron to a time-varying stimulus is influenced by both short- and long-term synaptic plasticity. Both these forms of plasticity produce changes in synaptic efficacy of similar magnitude on very different time scales. A full understanding of the functional role of each form of plasticity relies on understanding how they interact. Here we examine how long-term potentiation (LTP) and short-term plasticity (STP) interact in two different cell types that exhibit NMDA-dependent LTP: neocortical L-II/III and hippocampal CA1 pyramidal cells. STP was examined using both paired pulses and trains of pulses before and after the induction of LTP. In both cell types, the same pairing protocol was used to induce LTP in the presence of an unpaired control pathway. Pairing produced a robust increase in the amplitude of the first EPSP both in the neocortex and hippocampus. However, although in CA1 neurons the same degree of potentiation was maintained throughout the duration of a brief stimulus train, in L-II/III neurons relatively less potentiation was seen in the later EPSPs of the train. Paired-pulse analyses revealed that a uniform potentiation is observed at intervals >100 msec, but at shorter intervals there is a preferential enhancement of the first pulse. Thus, in the cortex LTP may preferentially amplify stimulus onset. These results suggest that there are distinct forms of associative LTP and that the different forms may reflect the underlying computations taking place in different areas.

Distributed representation of spectral and temporal information in rat primary auditory cortex

Modulations of amplitude and frequency are common features of natural sounds, and are prominent in behaviorally important communication sounds. The mammalian auditory cortex is known to contain representations of these important stimulus parameters. This study describes the distributed representations of tone frequency and modulation rate in the rat primary auditory cortex (A1). Detailed maps of auditory cortex responses to single tones and tone trains were constructed from recordings from 50-60 microelectrode penetrations introduced into each hemisphere. Recorded data demonstrated that the cortex uses a distributed coding strategy to represent both spectral and temporal information in the rat, as in other species. Just as spectral information is encoded in the firing patterns of neurons tuned to different frequencies, temporal information appears to be encoded using a set of filters covering a range of behaviorally important repetition rates. Although the average A1 repetition rate transfer function (RRTF) was low-pass with a sharp drop-off in evoked spikes per tone above 9 pulses per second (pps), individual RRTFs exhibited significant structure between 4 and 10 pps, including substantial facilitation or depression to tones presented at specific rates. No organized topography of these temporal filters could be determined.

Plasticity of temporal information processing in the primary auditory cortex

Neurons in the rat primary auditory cortex (A1) generally cannot respond to tone sequences faster than 12 pulses per second (pps). To test whether experience can modify this maximum following rate in adult rats, trains of brief tones with random carrier frequency but fixed repetition rate were paired with electrical stimulation of the nucleus basalis (NB) 300 to 400 times per day for 20-25 days. Pairing NB stimulation with 5-pps stimuli markedly decreased the cortical response to rapidly presented stimuli, whereas pairing with 15-pps stimuli significantly increased the maximum cortical following rate. In contrast, pairing with fixed carrier frequency 15-pps trains did not significantly increase the mean maximum following rate. Thus this protocol elicits extensive cortical remodeling of temporal response properties and demonstrates that simple differences in spectral and temporal features of the sensory input can drive very different cortical reorganizations.

Net interaction between different forms of short-term synaptic plasticity and slow-IPSPs in the hippocampus and auditory cortex

Paired-pulse plasticity is typically used to study the mechanisms underlying synaptic transmission and modulation. An important question relates to whether, under physiological conditions in which various opposing synaptic properties are acting in parallel, the net effect is facilitatory or depressive, that is, whether cells further or closer to threshold. For example, does the net sum of paired-pulse facilitation (PPF) of excitatory postsynaptic potentials (EPSPs), paired-pulse depression (PPD) of inhibitory postsynaptic potentials (IPSPs), and the hyperpolarizing slow IPSP result in depression or facilitation? Here we examine how different time-dependent properties act in parallel and examine the contribution of gamma-aminobutyric acid-B (GABAB) receptors that mediate two opposing processes, the slow IPSP and PPD of the fast IPSP. Using intracellular recordings from rat CA3 hippocampal neurons and L-II/III auditory cortex neurons, we examined the postsynaptic responses to paired-pulse stimulation (with intervals between 50 and 400 ms) of the Schaffer collaterals and white matter, respectively. Changes in the amplitude, time-to-peak (TTP), and slope of each EPSP were analyzed before and after application of the GABAB antagonist CGP-55845. In both CA3 and L-II/III neurons the peak amplitude of the second EPSP was generally depressed (further from threshold) compared with the first at the longer intervals; however, these EPSPs were generally broader and exhibited a longer TTP that could result in facilitation by enhancing temporal summation. At the short intervals CA3 neurons exhibited facilitation of the peak EPSP amplitude in the absence and presence of CGP-55845. In contrast, on average L-II/III cells did not exhibit facilitation at any interval, in the absence or presence of CGP-55845. CGP-55845 generally "erased" short-term plasticity, equalizing the peak amplitude and TTP of the first and second EPSPs at longer intervals in the hippocampus and auditory cortex. These results show that it is necessary to consider all time-dependent properties to determine whether facilitation or depression will dominate under intact pharmacological conditions. Furthermore our results suggest that GABAB-dependent properties may be the major contributor to short-term plasticity on the time scale of a few hundred milliseconds and are consistent with the hypothesis that the balance of different time-dependent processes can modulate the state of networks in a complex manner and could contribute to the generation of temporally sensitive neural responses.

Intrinsic and thalamic excitatory inputs onto songbird LMAN neurons differ in their pharmacological and temporal properties

In passerine songbirds, the lateral portion of the magnocellular nucleus of the anterior neostriatum (LMAN) plays a vital role in song learning, possibly by encoding sensory information and providing sensory feedback to the vocal motor system. Consistent with this, LMAN neurons are auditory, and, as learning progresses, they evolve from a broadly tuned initial state to a state of strong preference for the bird's own song and acute sensitivity to the temporal order of this song. Moreover, normal synaptic activity in LMAN is required during sensory learning for accurate tutor song copying to occur (). To explore cellular and synaptic properties of LMAN that may contribute to this crucial stage of song acquisition, we developed an acute slice preparation of LMAN from zebra finches in the early stages of sensory learning (18-25 days posthatch). We used this preparation to examine intrinsic neuronal properties of LMAN neurons at this stage and to identify two independent excitatory inputs to these neurons and compare each input's pharmacology and short-term synaptic plasticity. LMAN neurons had immature passive membrane properties, well-developed spiking behavior, and received excitatory input from two sources: afferents from the medial portion of the dorsolateral thalamus (DLM), and recurrent axon collaterals from LMAN itself ("intrinsic" input). These two inputs differed in both their pharmacology and temporal properties. Both inputs were glutamatergic, but LMAN responses to intrinsic inputs exhibited a larger N-methyl--aspartate component than responses to DLM inputs. Both inputs elicited temporal summation in response to pairs of stimuli delivered at short intervals, but -2-amino-5-phosphonovalerate (APV) significantly reduced the temporal summation only of the responses to intrinsic inputs. Moreover, responses to DLM inputs showed consistent paired-pulse depression, whereas the responses to intrinsic inputs did not. The differences between these two inputs suggest that intrinsic circuitry plays an important role in transforming DLM input patterns into the appropriate LMAN output patterns, as has been suggested for mammalian thalamocortical networks. Moreover, in LMAN, such interactions may contribute to the profound temporal and spectral selectivity that these neurons will acquire during learning.

Practice-related improvements in somatosensory interval discrimination are temporally specific but generalize across skin location, hemisphere, and modality

This paper concerns the characterization of performance and perceptual learning of somatosensory interval discrimination. The purposes of this study were to define (1) the performance characteristics for interval discrimination in the somatosensory system by naive adult humans, (2) the normal capacities for improvement in somatosensory interval discrimination, and (3) the extent of generalization of interval discrimination learning. In a two-alternative forced choice procedure, subjects were presented with two pairs of vibratory pulses. One pair was separated in time by a fixed base interval; a second pair was separated by a target interval that was always longer than the base interval. Subjects indicated which pair was separated by the target interval. The length of the target interval was varied adaptively to determine discrimination thresholds. After initial determination of naive abilities, subjects were trained for 900 trials per day at base intervals of either 75 or 125 msec for 10-15 d. Significant improvements in thresholds resulted from training. Learning at the trained base interval generalized completely across untrained skin locations on the trained hand and to the corresponding untrained skin location in the contralateral hand. The learning partially generalized to untrained base intervals similar to the trained one, but not to more distant base intervals. Learning with somatosensory stimuli generalized to auditory stimuli presented at comparable base intervals. These results demonstrate temporal specificity in somatosensory interval discrimination learning that generalizes across skin location, hemisphere, and modality.

Practice-related improvements in somatosensory interval discrimination are temporally specific but generalize across skin location, hemisphere, and modality

This paper concerns the characterization of performance and perceptual learning of somatosensory interval discrimination. The purposes of this study were to define (1) the performance characteristics for interval discrimination in the somatosensory system by naive adult humans, (2) the normal capacities for improvement in somatosensory interval discrimination, and (3) the extent of generalization of interval discrimination learning. In a two-alternative forced choice procedure, subjects were presented with two pairs of vibratory pulses. One pair was separated in time by a fixed base interval; a second pair was separated by a target interval that was always longer than the base interval. Subjects indicated which pair was separated by the target interval. The length of the target interval was varied adaptively to determine discrimination thresholds. After initial determination of naive abilities, subjects were trained for 900 trials per day at base intervals of either 75 or 125 msec for 10-15 d. Significant improvements in thresholds resulted from training. Learning at the trained base interval generalized completely across untrained skin locations on the trained hand and to the corresponding untrained skin location in the contralateral hand. The learning partially generalized to untrained base intervals similar to the trained one, but not to more distant base intervals. Learning with somatosensory stimuli generalized to auditory stimuli presented at comparable base intervals. These results demonstrate temporal specificity in somatosensory interval discrimination learning that generalizes across skin location, hemisphere, and modality.