CiteULike: adrien's library [20 articles]

Theta oscillation-coupled dendritic spiking integrates inputs on a long time scale.

Zsófia Huhn — 2005-08-24T22:33:59-00:00

Hippocampus (17 August 2005)

Persistent neural activity lasting for seconds after transient stimulation has been observed in several brain areas. This activity has been taken to be indicative of the integration of inputs on long time scales. Passive membrane properties render neural time constants to be on the order of milliseconds. Intense synaptic bombardment, characteristic of in vivo states, was previously shown to further reduce the time scale of effective integration. We explored how long-term integration in single cells could be supported by dendritic spikes coupled with the theta oscillation, a prominent brain rhythm often observed during working memory tasks. We used a two-compartmental conductance-based model of a hippocampal pyramidal cell to study the interplay of intrinsic dynamics with periodic inputs in the theta frequency band. We show that periodic dendritic spiking integrates inputs by shifting the phase relative to an external oscillation, since spiking frequency is quasi-linearly modulated by current injection. The time-constant of this integration process is practically infinite for input intensities above a threshold (the integration threshold) and can be still several hundred milliseconds long below the integration threshold. The somatic compartment received theta frequency stimulation in antiphase with the dendritic oscillation. Consequently, dendritic spikes could only elicit somatic action potentials when they were sufficiently phase-shifted and thus coincided with somatic depolarization. Somatic depolarization modulated the frequency but not the phase of firing, endowing the cell with the capability to code for two different variables at the same time. Inputs to the dendrite shifted the phase of dendritic spiking, while somatic input was modulating its firing rate. This mechanism resulted in firing patterns that closely matched experimental data from hippocampal place cells of freely behaving rats. We discuss the plausibility of our proposed mechanism and its potential to account for the firing pattern of cells outside the hippocampus during working memory tasks. (c) 2005 Wiley-Liss, Inc.

Hippocampal theta rhythm: a tag for short-term memory.

RP Vertes — 2006-05-09T08:55:06-00:00

Hippocampus, Vol. 15, No. 7. (2005), pp. 923-935.

The theta rhythm is the largest extracellular synchronous signal that can be recorded from the mammalian brain, and has been strongly implicated in mnemonic functions of the hippocampus. We advance the proposal that the theta rhythm represents a "tag" for short-term memory processing in the hippocampus. We propose that the hippocampus receives two main types of input, theta from ascending brainstem-diencephalo-septal systems and "information bearing" mainly from thalamocortical and cortical systems. The temporal convergence of activity of these two systems results in the encoding of information in the hippocampus, primarily reaching it via cortical routes. By analogy to processes associated with long-term potentiation (LTP), we suggest that theta represents a strong depolarizing influence on NMDA receptor-containing cells of the hippocampus. The temporal coupling of a theta-induced depolarization and the release of glutamate to these cells from intra- and extrahippocampal sources activates them. This, in turn, initiates processes leading to a (short-term) strengthening of connections between presynaptic ("information bearing") and postsynaptic neurons of the hippocampus. Theta is selectively present in the rat during active exploratory movements. During exploration, a rat continually gathers and updates information about its environment. If this information is temporally coupled to theta (as with the case of locomotion), it becomes temporarily stored in the hippocampus by mechanisms similar to the early phase of LTP (E-LTP). If the exploratory behavior of the rat goes unreinforced, these relatively short-lasting traces (1-3 h) gradually weaken and eventually fade-to be reupdated. On the other hand, if the explorations of the rat lead to rewards (or punishments), additional modulatory inputs to the hippocampus become activated and convert the short-term, theta-dependent memory, into long-term stores.

Coordinated interactions between hippocampal ripples and cortical spindles during slow-wave sleep.

AG Siapas — 2005-02-18T16:58:59-00:00

Neuron, Vol. 21, No. 5. (November 1998), pp. 1123-1128.

Sleep is characterized by a structured combination of neuronal oscillations. In the hippocampus, slow-wave sleep (SWS) is marked by high-frequency network oscillations (approximately 200 Hz "ripples"), whereas neocortical SWS activity is organized into low-frequency delta (1-4 Hz) and spindle (7-14 Hz) oscillations. While these types of hippocampal and cortical oscillations have been studied extensively in isolation, the relationships between them remain unknown. Here, we demonstrate the existence of temporal correlations between hippocampal ripples and cortical spindles that are also reflected in the correlated activity of single neurons within these brain structures. Spindle-ripple episodes may thus constitute an important mechanism of cortico-hippocampal communication during sleep. This coactivation of hippocampal and neocortical pathways may be important for the process of memory consolidation, during which memories are gradually translated from short-term hippocampal to longer-term neocortical stores.

7-12 Hz cortical oscillations: behavioral context and dynamics of prefrontal neuronal ensembles.

S Sakata — 2006-05-09T08:40:28-00:00

Neuroscience, Vol. 134, No. 4. (2005), pp. 1099-1111.

7-12 Hz Oscillations, characterized by spindle-like high-voltage rhythmic spike components, appear in quiet immobile states of rats. However, it remains unclear what their relationships with preceding behavioral activities are and how prefrontal neuronal dynamics during these oscillations is. In the present study, we first determined the relationship of 7-12 Hz oscillations with the wake-sleep cycle and preceding behavioral activities in several normal rat strains by recording electroencephalograms from the multiple cortical regions. Prolonged awake period transiently enhanced the following appearance of 7-12 Hz oscillations, which were frequently followed by slow-wave sleep. The degree of transient enhancement under the task condition was similar to that by prolonged wakefulness under the no-task condition. In addition, by recording local-field potential and multi-unit activities in the medial prefrontal cortex, we determined the temporal dynamics of prefrontal neuronal activities in relation to 7-12 Hz oscillations. Collective neuronal activities in medial prefrontal cortex were gradually organized into phase-locked patterns and showed highly synchronization during these oscillations. These dynamics were in temporal proximity to those of slow-wave activities (<4 Hz). Since slow-wave activities are thought to synchronize large spatial domains, these results suggest that 7-12 Hz oscillations are involved in the transition from the awake to sleep states by oscillatory entrainment of global cortical networks including the prefrontal neurons.

Hippocampal sharp wave-ripples linked to slow oscillations in rat slow-wave sleep.

Matthias Moelle — 2006-05-09T08:37:38-00:00

J Neurophysiol (12 April 2006)

Slow oscillations originating in the prefrontal neocortex during slow-wave sleep (SWS) group neuronal network activity and thereby presumably support the consolidation of memories. Here, we investigated whether the grouping influence of slow oscillations extends to hippocampal sharp wave-ripple (SPW) activity thought to underlie memory replay processes during SWS. The prefrontal surface EEG and multiunit activity (MUA), along with hippocampal local field potentials (LFP) from CA1 were recorded in rats during sleep. Average spindle and ripple activity and event correlation histograms of SPWs were calculated time-locked to half-waves of slow oscillations. Results confirm decreased prefrontal MUA and spindle activity during EEG slow oscillation negativity and increases of this activity during subsequent positivity. A remarkably close temporal link was revealed between slow oscillations and hippocampal activity, with ripple activity and SPWs being also distinctly decreased during negative half-waves and increased during slow oscillation positivity. Fine~grained analyses of temporal dynamics revealed for the slow oscillation a phase delay of ~90 msec with reference to up and down states of prefrontal MUA, and of only ~60 msec with reference to changes in SPWs, indicating that up and down states in prefrontal MUA precede corresponding changes in hippocampal SPWs by ~30 msec. Results support the notion that the depolarizing surface-positive phase of the slow oscillation and the associated up state of prefrontal excitation promotes via efferent pathways hippocampal SPWs. The preceding disfacilitation of hippocampal events temporally coupled to the negative slow oscillation half-wave appears to serve a synchronizing role in this neocortico-hippocampal interplay.

Why do we sleep?

TJ Sejnowski — 2006-05-09T08:33:48-00:00

Brain Res, Vol. 886, No. 1-2. (15 December 2000), pp. 208-223.

Slow-wave sleep consists in slowly recurring waves that are associated with a large-scale spatio-temporal synchrony across neocortex. These slow-wave complexes alternate with brief episodes of fast oscillations, similar to the sustained fast oscillations that occur during the wake state. We propose that alternating fast and slow waves consolidate information acquired previously during wakefulness. Slow-wave sleep would thus begin with spindle oscillations that open molecular gates to plasticity, then proceed by iteratively 'recalling' and 'storing' information primed in neural assemblies. This scenario provides a biophysical mechanism consistent with the growing evidence that sleep serves to consolidate memories.

Sleep on it: cortical reorganization after-the-fact.

KL Hoffman — 2006-05-03T09:32:57-00:00

Trends Neurosci, Vol. 25, No. 1. (January 2002), pp. 1-2.

Sleep can facilitate memory formation, but its role in cortical plasticity is poorly understood. A recent study found that sleep, following monocular deprivation (MD), facilitated cortical changes in ocular dominance. The magnitude of plasticity was similar to that observed after continued MD, and larger than that seen after sleep deprivation in darkness, suggesting that sleep per se enables mechanisms of cortical plasticity. Experience-dependent plasticity during sleep could be part of a more global process of memory consolidation.

Coordinated reactivation of distributed memory traces in primate neocortex.

KL Hoffman — 2005-02-08T01:44:45-00:00

Science, Vol. 297, No. 5589. (20 September 2002), pp. 2070-2073.

Conversion of new memories into a lasting form may involve the gradual refinement and linking together of neural representations stored widely throughout neocortex. This consolidation process may require coordinated reactivation of distributed components of memory traces while the cortex is "offline," i.e., not engaged in processing external stimuli. Simultaneous neural ensemble recordings from four sites in the macaque neocortex revealed such coordinated reactivation. In motor, somatosensory, and parietal cortex (but not prefrontal cortex), the behaviorally induced correlation structure and temporal patterning of neural ensembles within and between regions were preserved, confirming a major tenet of the trace-reactivation theory of memory consolidation.

Quantitative measures of cluster quality for use in extracellular recordings.

N Schmitzer-Torbert — 2006-04-25T13:01:59-00:00

Neuroscience, Vol. 131, No. 1. (2005), pp. 1-11.

While the use of multi-channel electrodes (stereotrodes and tetrodes) has allowed for the simultaneous recording and identification of many neurons, quantitative measures of the quality of neurons in such recordings are lacking. In multi-channel recordings, each spike waveform is discriminated in a high-dimensional space, making traditional measures of unit quality inapplicable. We describe two measures of unit isolation quality, Lratio and Isolation Distance, and evaluate their performance using simulations and tetrode recordings. Both measures quantified how well separated the spikes of one cluster (putative neuron) were from other spikes recorded simultaneously on the same multi-channel electrode. In simulations and tetrode recordings, both Lratio and Isolation Distance discriminated well- and poorly-separated clusters. In data sets from the rodent hippocampus in which neurons were simultaneously recorded intracellularly and extracellularly, values of Isolation Distance and Lratio were related to the correct identification of spikes.

A role for inhibition in shaping the temporal flow of information in prefrontal cortex

Christos Constantinidis — 2006-04-18T14:36:22-00:00

Nat Neurosci, Vol. 5, No. 2. (February 2002), pp. 175-180.

Ventromedial Prefrontal Cortex Is Obligatory for Consolidation and Reconsolidation of Object Recognition Memory.

Irit Akirav — 2006-04-18T14:06:59-00:00

Cereb Cortex (18 January 2006)

Once consolidated, a long-term memory item could regain susceptibility to consolidation blockers, that is, reconsolidate, upon its reactivation. Both consolidation and reconsolidation require protein synthesis, but it is not yet known how similar these processes are in terms of molecular, cellular, and neural circuit mechanisms. Whereas most previous studies focused on aversive conditioning in the amygdala and the hippocampus, here we examine the role of the ventromedial prefrontal cortex (vmPFC) in consolidation and reconsolidation of object recognition memory. Object recognition memory is the ability to discriminate the familiarity of previously encountered objects. We found that microinfusion of the protein synthesis inhibitor anisomycin or the N-methyl-D-aspartate (NMDA) receptor antagonist D,L-2-amino-5-phosphonovaleric acid (APV) into the vmPFC, immediately after training, resulted in impairment of long-term (24 h) but not short-term (3 h) recognition memory. Similarly, microinfusion of anisomycin or APV into the vmPFC immediately after reactivation of the long-term memory impaired recognition memory 24 h, but not 3 h, post-reactivation. These results indicate that both protein synthesis and NMDA receptors are required for consolidation and reconsolidation of recognition memory in the vmPFC.

Synaptic reentry reinforcement based network model for long-term memory consolidation.

GM Wittenberg — 2006-04-12T15:38:49-00:00

Hippocampus, Vol. 12, No. 5. (2002), pp. 637-647.

The conversion of newly formed declarative memories into long-term memories is known to be dependent on the hippocampus. Recent experiments suggest that memory consolidation requires reactivation of the NMDA receptor in CA1 during the initial week(s) after training. This led to the hypothesis that the repeated post-learning reinforcement of synaptic modifications, termed synaptic reentry reinforcement (SRR), is essential for long-term memory consolidation and storage. Based on experimental observations, we have built a computational model to further illustrate and explore the effect of the SRR process on the formation of long-term memory. We show that SRR is capable of strengthening and maintaining memory traces despite inherent variability in the system due to such processes as the turnover of synaptic receptors and their associated signaling and structural proteins. Furthermore, we demonstrate that new rounds of synaptic modification triggered by memory reactivation, either during conscious recall or sleep, could lead to the selective consolidation of a subset of memory traces. Finally, we show why the SRR process in the hippocampus is required during the initial post-training weeks for synaptic reinforcement based memory consolidation in the cortex.

Dissociation Of working memory from decision making within the human prefrontal cortex.

A Bechara — 2006-03-01T14:48:36-00:00

J Neurosci, Vol. 18, No. 1. (1 January 1998), pp. 428-437.

We tested the hypothesis that cognitive functions related to working memory (assessed with delay tasks) are distinct from those related to decision making (assessed with a gambling task), and that working memory and decision making depend in part on separate anatomical substrates. Normal controls (n = 21), subjects with lesions in the ventromedial (VM) (n = 9) or dorsolateral/high mesial (DL/M) prefrontal cortices (n = 10), performed on (1) modified delay tasks that assess working memory and (2) a gambling task designed to measure decision making. VM subjects with more anterior lesions (n = 4) performed defectively on the gambling but not the delay task. VM subjects with more posterior lesions (n = 5) were impaired on both tasks. Right DL/M subjects were impaired on the delay task but not the gambling task. Left DL/M subjects were not impaired on either task. The findings reveal a cognitive and anatomic double dissociation between deficits in decision making (anterior VM) and working memory (right DL/M). This presents the first direct evidence of such effects in humans using the lesion method and underscores the special importance of the VM prefrontal region in decision making, independent of a direct role in working memory.

Spike count reliability and the Poisson hypothesis.

A Amarasingham — 2006-03-01T14:46:52-00:00

J Neurosci, Vol. 26, No. 3. (18 January 2006), pp. 801-809.

The variability of cortical activity in response to repeated presentations of a stimulus has been an area of controversy in the ongoing debate regarding the evidence for fine temporal structure in nervous system activity. We present a new statistical technique for assessing the significance of observed variability in the neural spike counts with respect to a minimal Poisson hypothesis, which avoids the conventional but troubling assumption that the spiking process is identically distributed across trials. We apply the method to recordings of inferotemporal cortical neurons of primates presented with complex visual stimuli. On this data, the minimal Poisson hypothesis is rejected: the neuronal responses are too reliable to be fit by a typical firing-rate model, even allowing for sudden, time-varying, and trial-dependent rate changes after stimulus onset. The statistical evidence favors a tightly regulated stimulus response in these neurons, close to stimulus onset, although not further away.

Episodic memory, amnesia, and the hippocampal-anterior thalamic axis.

JP Aggleton — 2006-03-01T14:45:49-00:00

Behav Brain Sci, Vol. 22, No. 3. (June 1999)

By utilizing new information from both clinical and experimental (lesion, electrophysiological, and gene-activation) studies with animals, the anatomy underlying anterograde amnesia has been reformulated. The distinction between temporal lobe and diencephalic amnesia is of limited value in that a common feature of anterograde amnesia is damage to part of an "extended hippocampal system" comprising the hippocampus, the fornix, the mamillary bodies, and the anterior thalamic nuclei. This view, which can be traced back to Delay and Brion (1969), differs from other recent models in placing critical importance on the efferents from the hippocampus via the fornix to the diencephalon. These are necessary for the encoding and, hence, the effective subsequent recall of episodic memory. An additional feature of this hippocampal-anterior thalamic axis is the presence of projections back from the diencephalon to the temporal cortex and hippocampus that also support episodic memory. In contrast, this hippocampal system is not required for tests of item recognition that primarily tax familiarity judgements. Familiarity judgements reflect an independent process that depends on a distinct system involving the perirhinal cortex of the temporal lobe and the medial dorsal nucleus of the thalamus. In the large majority of amnesic cases both the hippocampal-anterior thalamic and the perirhinal-medial dorsal thalamic systems are compromised, leading to severe deficits in both recall and recognition.

Evidence that autobiographic memory retrieval does not become independent of the hippocampus: an fMRI study contrasting very recent with remote events.

PV Rekkas — 2006-03-01T14:21:20-00:00

J Cogn Neurosci, Vol. 17, No. 12. (December 2005), pp. 1950-1961.

Traditional consolidation theory, which seeks to explain how new memories are incorporated into the preexisting neural architecture, stipulates that the hippocampus plays a time-limited role in this process. However, although there is abundant research showing that the hippocampus is necessary for the initial (encoding) phase, there is very little experimental evidence with human subjects proving that the structure ceases to play a role in the retrieval of episodic items from memory stores. To test this hypothesis, we investigated recall activation associated with recent memories (2.5 days) versus remote memories (mean of 8 years) using functional magnetic resonance imaging. In accordance with the multiple memory trace theory, recall of consolidated autobiographic information, represented by the remote condition, was not independent of the hippocampus. Both types of memory retrieval produced significant activation in parahippocampal, prefrontal, and midtemporal gyri, the parietal-temporal junction, and a medial region of cortex spanning the posterior cingulate and precuneus gyri. However, where recent events activated bilateral regions of the caudate nucleus, remote events yielded significantly greater activation within the hippocampus proper. The results challenge traditional consolidation theory, which would predict greater hippocampal activity for recent events. Furthermore, they highlight the interplay between multiple memory systems in the brain. We argue that our particular question format, which encouraged depth of recall and did not require a prescan interview, as well as our delineation of the recent and remote time periods, were the determining factors for the observed pattern of hippocampal activation.

A role for the prefrontal cortex in recall of recent and remote memories.

S Blum — 2006-03-01T14:19:43-00:00

Neuroreport, Vol. 17, No. 3. (27 February 2006), pp. 341-344.

Declarative memories are thought to be initially stored in the hippocampus, and then transferred to the neocortex. This is a key feature of the standard model of consolidation and is supported by studies reporting a requirement for activity within the neocortex for recall of remote, but not recent, hippocampal-dependent memories. New evidence from our and other laboratories, however, suggests that, for trace fear conditioning, memories are stored in the rodent medial prefrontal cortex and in the hippocampus from the time of training. Consistent with this, we show that activity in the medial prefrontal cortex is necessary for retrieval of recent and remote memories, suggesting that information stored in this neocortical structure from the time of training is necessary for memory recall.

Differential involvement of the hippocampus, anterior cingulate cortex, and basolateral amygdala in memory for context and footshock

Emily Malin — 2006-03-01T14:17:06-00:00

PNAS, Vol. 103, No. 6. (7 February 2006), pp. 1959-1963.

Extensive evidence from contextual fear conditioning experiments suggests that the hippocampus is involved in processing memory for contextual information. Evidence also suggests that the rostral anterior cingulate cortex (rACC) may be selectively involved in memory for nociceptive stimulation. In contrast, many findings indicate that the basolateral amygdala (BLA) is more broadly involved in modulating the consolidation of different kinds of information. To investigate further the differential involvement of these brain regions in memory consolidation, the present experiments used a modified inhibitory avoidance training procedure that took place on 2 sequential days to separate context training from footshock training. Male Sprague-Dawley rats were implanted with unilateral cannulae aimed at the (i) hippocampus, (ii) rACC, or (iii) BLA, and given infusions of the muscarinic cholinergic agonist oxotremorine (OXO) immediately after either context training (day 1) or footshock training in that context (day 2). OXO enhanced retention when infused into the hippocampus after context, but not footshock, training. Conversely, OXO infusions enhanced memory when administered into the rACC immediately after footshock, but not context, training. Lastly, intra-BLA OXO infusions enhanced retention when administered after either context or footshock training. These findings are consistent with evidence that the hippocampus and rACC play selective roles in memory for specific components of training experiences. Additionally, they provide further evidence that the BLA is more liberally involved in modulating memory consolidation for various aspects of emotionally arousing experiences.

Reversible inactivations of rat medial prefrontal cortex impair the ability to wait for a stimulus.

N S Narayanan — 2006-03-01T12:42:52-00:00

Neuroscience (22 February 2006)

In simple reaction time tasks, lesions of rat dorsomedial prefrontal cortex impair the ability to wait for trigger stimuli and result in increased premature responding. This effect could be due to impairments in attending to trigger stimuli, estimating the timing of trigger stimuli, or inhibitory control of the motor response. Here, we examined these issues by reversibly inactivating dorsomedial prefrontal cortex during simple reaction time tasks with variable or fixed foreperiods. There were three consistent effects of dorsomedial prefrontal cortex inactivation: 1) increased premature responding, 2) increased variability in the timing of premature responses, and 3) speeded response latencies, especially on trials with short foreperiods in tasks with variable foreperiods. We observed these effects independent of differences in foreperiod duration, foreperiod variability, and stimulus probabilities. Therefore, dorsomedial prefrontal cortex appears not to be involved in attending to the trigger stimulus or in time estimation. Instead, we suggest that dorsomedial prefrontal cortex is critical for inhibiting responses before the maximum foreperiod duration, i.e. the "deadline" [Ollman RT, Billington MJ (1972) The deadline model for simple reaction times. Cognit Psychol 3:311-336], after which the rat should respond even if the trigger stimulus has not occurred.

A Large-scale Neurocomputational Model of Task-oriented Behavior Selection and Working Memory in Prefrontal Cortex

George Chadderdon — 2006-02-13T06:06:47-00:00

Journal of Cognitive Neuroscience, Vol. 18, No. 2. (February 2006), pp. 242-257.