Karim Benchenane
email: karim.benchenane@snv.jussieu.fr

Adrien Peyrache

Mehdi Khamassi

Patrick Tierney

Yves Gioanni

Francesco P Battaglia
email: f.p.battaglia@uva.nl

Sidney I Wiener

Coherent theta oscillations and reorganization of spike timing in the hippocampalprefrontal network upon learning

To study the interplay between hippocampus and medial prefrontal cortex (Pfc) and its importance for learning and memory consolidation, we measured, in rats learning new rules on a Y-maze, the coherence in theta oscillations between these two structures. Coherence peaks at the choice point, after acquisition of the task rule. Simultaneously, Pfc pyramidal neurons reorganize their phase, concentrating at hippocampal theta trough, and synchronously active cell assemblies emerge. This highly synchronous state may result from the increased inhibition exerted by interneurons on pyramidal cells, as measured by cross-correlation, and could be modulated by dopamine, because we find similar hippocampal-Pfc theta coherence increases and neuronal phase shifts following local administration of dopamine in Pfc of anesthetized rats.

Pfc cell assemblies emerging during high coherence are preferentially replayed during subsequent sleep, concurrent with hippocampal sharp waves. Thus hippocampal/prefrontal coherence could selectively lead to synchronization of reward predicting activity in prefrontal networks, tagging it for subsequent memory consolidation.

Introduction

Selective communication between brain areas is a key component of the neural processing underlying cognition. This information exchange must adapt to the current situation and brain state, with functional networks continuously forming and disbanding [START_REF] Fell | Human memory formation is accompanied by rhinal-hippocampal coupling and decoupling[END_REF]. This would help filter and select signals to be processed and transmitted to other areas, possibly supported by the emergence of highly synchronized activations of cell groups or cell assemblies representing a highly effective input for afferent structures [START_REF] Hebb | The Organization of Behavior[END_REF][START_REF] Fries | A mechanism for cognitive dynamics: neuronal communication through neuronal coherence[END_REF][START_REF] Fries | The gamma cycle[END_REF]. The resulting dialogue would drive plastic changes in short-and long-range synaptic connections and contribute to create widespread integrated representations of items and experiences, selecting and retaining the information most valuable for adaptive behavior. Such valence and salience information is thought to be transmitted by neuromodulators such as dopamine, which could act as a reward prediction (error) signal [START_REF] Schultz | A neural substrate of prediction and reward[END_REF][START_REF] Dayan | Reward, Motivation, and Reinforcement Learning[END_REF][START_REF] Doya | Modulators of decision making[END_REF].

Brain rhythms are likely to be a fundamental mechanism for modulating, filtering and redirecting information in the nervous system: neural circuits are naturally predisposed to oscillate in a wide range of frequencies [START_REF] Buzsáki | Large-scale recording of neuronal ensembles[END_REF]. Oscillations are a key mechanism of dynamical coupling between brain areas: task-and state-dependent changes are observed in local field potential (LFP) coherence [START_REF] Fell | Human memory formation is accompanied by rhinal-hippocampal coupling and decoupling[END_REF]Varela et al., 2001;Womelsdorf et al., 2007) and in cross-correlated unit activity (Tabuchi et al., 2000;[START_REF] Engel | Dynamic predictions: oscillations and synchrony in top-down processing[END_REF]. From the functional point of view, synchronizing and desynchronizing activity between two brain areas exert dramatic effects on their ability to engage or effectively block out one another"s input (Womelsdorf et al., 2007). Within individual brain areas, oscillations can synchronize neurons, creating coherent cell assemblies [START_REF] Harris | Organization of cell assemblies in the hippocampus[END_REF] and favoring plasticity processes depending on the precise timing of pre-and post-synaptic activity [START_REF] Markram | Regulation of synaptic efficacy by coincidence of postsynaptic APs and EPSPs [see comments[END_REF][START_REF] Bi | Synaptic modifications in cultured hippocampal neurons: dependence on spike timing, synaptic strength, and postsynaptic cell type[END_REF][START_REF] Cassenaer | Hebbian STDP in mushroom bodies facilitates the synchronous flow of olfactory information in locusts[END_REF]. However, little is known about the mechanisms underlying such synchronization-induced modifications of local functional circuitry. Here we will examine learning-related changes in oscillatory coherence between hippocampus and Pfc, and associated changes in Pfc local circuitry, which will be shown to closely resemble the effects of local application of dopamine.

The hippocampus-neocortex network is a primary conduit for cerebral information flow for learning and memory. Anatomically located at the top of the cortical hierarchy [START_REF] Felleman | Distributed hierarchical processing in the primate cerebral cortex[END_REF], the hippocampus (Hpc) was proposed to create "pointer" representations, linking together activity in multiple cortical areas pertaining to the same context and facilitating the formation of permanent cortical episodic memory traces [START_REF] Eichenbaum | A cortical-hippocampal system for declarative memory[END_REF][START_REF] Shastri | Episodic memory and cortico-hippocampal interactions[END_REF][START_REF] Moscovitch | Functional neuroanatomy of remote episodic, semantic and spatial memory: a unified account based on multiple trace theory[END_REF][START_REF] Frankland | The organization of recent and remote memories[END_REF]. Effective communication between the hippocampus and the neocortex involves oscillations: for example, transient synchronization at gamma frequencies between the Hpc and the rhinal cortex accompanies successful memory encoding [START_REF] Fell | Human memory formation is accompanied by rhinal-hippocampal coupling and decoupling[END_REF]. Pfc, which receives monosynaptic hippocampal projections [START_REF] Jay | Distribution of hippocampal CA1 and subicular efferents in the prefrontal cortex of the rat studied by means of anterograde transport of Phaseolus vulgarisleucoagglutinin[END_REF]Thierry et al., 2000), has been shown to be functionally connected to the hippocampus: Hpc theta (5-10Hz) synchronizes medial prefrontal cortex (Pfc) neurons [START_REF] Siapas | Prefrontal phase locking to hippocampal theta oscillations[END_REF] and LFPs [START_REF] Hyman | Medial prefrontal cortex cells show dynamic modulation with the hippocampal theta rhythm dependent on behavior[END_REF] depending on task demands [START_REF] Jones | Phase precession of medial prefrontal cortical activity relative to the hippocampal theta rhythm[END_REF], possibly supporting cortico-hippocampal maintenance of working memory [START_REF] Floresco | Selective Roles for Hippocampal, Prefrontal Cortical, and Ventral Striatal Circuits in Radial-Arm Maze Tasks With or Without a Delay[END_REF].

This study examines changes in synchronization of Hpc and Pfc LFPs and single and ensemble Pfc neuron activity in rats learning and flexibly switching between rules. If oscillatory coherence plays a role in memory encoding and acquisition, it would be expected to vary with learning, and to be associated with the creation of neural substrates of memories, possibly in the form of functionally linked cell assemblies [START_REF] Hebb | The Organization of Behavior[END_REF]. If those cell assemblies carry relevant information, then they should be more readily consolidated into long term memory (Peyrache et al., 2009b). These hypotheses are examined here. However, the mechanism by which these assemblies would be "tagged" at acquisition is not known, and evidence will be put forward for this in the form of coherence-induced and (possibly dopamine influenced) creation of learningrelated, reward-predictive cell assemblies that are tightly synchronized, permitting their stabilization by spike-time dependent plasticity.

Results

Hpc-Pfc theta coherence. We recorded a total of 1475 neurons, in 60 experimental sessions in four rats as they successively learned two reward contingency rules on a Y maze: go to the arm on the right, and then go to the lit arm (selected at random at each trial; Supplementary Figures S1A,S1B). LFPs were recorded in the pyramidal layer of ventral/intermediate hippocampal CA1, while multielectrode single-unit spike activity and LFPs were recorded in medial Pfc (Supplementary Figure S2,S3). Strong 5-10 Hz theta band LFP activity was detectible in both Hpc and Pfc [START_REF] Jones | Phase precession of medial prefrontal cortical activity relative to the hippocampal theta rhythm[END_REF][START_REF] Hyman | Medial prefrontal cortex cells show dynamic modulation with the hippocampal theta rhythm dependent on behavior[END_REF] (Figure 1A,B).

Strong Hpc-Pfc spectral coherence was observed in the theta frequency range (Figure 1A,B).

Hpc-Pfc-θ-coherence fluctuated, with no simple relation with either theta power in the two structures or with rat motion (Figure 1A, Supplementary Figure S4,S5). On a trial-by-trial basis, coherence at the decision point did not correlate with running speed (r=0.04, p > 0.05), a good predictor of Hpc theta amplitude [START_REF] Ekstrom | NMDA receptor antagonism blocks experience-dependent expansion of hippocampal "place fields[END_REF], or with acceleration (r = 0.01, p > 0.05; see Supplementary Figure S5). The phase lag between Hpc and Pfc theta rhythm was 40±4 ms, similar to previous reports [START_REF] Jones | Phase precession of medial prefrontal cortical activity relative to the hippocampal theta rhythm[END_REF] and compatible with Pfc response latencies to stimulation of the Hpc [START_REF] Degenetais | Synaptic influence of hippocampus on pyramidal cells of the rat prefrontal cortex: an in vivo intracellular recording study[END_REF]. Thus, while the mechanism underlying the coherence is not known, it would nonetheless dramatically augment prefrontal receptivity to hippocampal signals.

To control for potential contamination of LFP coherence measures by volume conduction artifacts [START_REF] Sirota | Entrainment of neocortical neurons and gamma oscillations by the hippocampal theta rhythm[END_REF], we compared LFP-LFP coherence with LFP-spikes coherence in the same structures (Womelsdorf et al., 2007) (Fig 1B,C, Supplementary Figure S4). Of the 1373 Pfc neurons with an average firing rate greater than 0.01 Hz, 541 (39%) were significantly modulated by Hpc theta (p < 0.05 Rayleigh test). Coherence of Pfc spikes to Hpc LFP is most marked in the theta range (Figure 1B,C). Moreover, the time courses of coherence of prefrontal LFP and spike trains with hippocampal LFP show peak correlations in the theta frequency range, both for single prefrontal units (Fig 1B,C) and when all theta-modulated cells are considered (Supplementary Figure S4). In the latter case, the correlation reached significance in the theta range only (Supplementary Figure S4B). Thus, local Pfc circuits are likely to be involved in the expression of coherence with Hpc theta both at the LFP and spike train levels.

Hpc-Pfc-θ-coherence increases at the maze choice point, particularly upon learning. Coherence was maximal when the rat was at the fork of the Y-maze (Figure 2A,B). Strikingly, the increase of coherence at the choice point was significantly greater after acquisition of a new rule (Figure 2C-E; "Right" n=123; "Light" n=148, two-way ANOVA, factors: before/after learning (L) and position on maze (P); Right, main effects: L: p<0.01, P p<0.00001, interaction p<0.02; Light, main effects L: p<0.0001, P: p<0.00001, interaction p<0.01). This effect was observed for LFPs recorded in both superficial and deep layers of Pfc (Supplementary Figure S6). Similarly, Hpc-Pfc-θ-coherence at the choice point was higher at a learning stage when the rat reliably complied with the rewarded rule (compared with trials prior to criterion performance; Figure 2F, t-test, p<0.05), as assessed by high values of the "certainty" measure developed by Smith and colleagues (Smith et al., 2004). This was not merely due to the higher incidence of rewards after rule acquisition, since, when considering only trials prior to this, coherence was not significantly different between rewarded and unrewarded trials (Figure 2G, n=271, t-test, p>0.05) across the entire behavioral session. These results, taken together, suggest that Hpc-Pfc-θ-coherence is a predictor of the rat"s performance level: in fact, we show that trials with high coherence had, on average, a high level of performance, whereas in trials with low decision point coherence, rats operated at only slightly better than chance level (Figure 2H).

Increased correlations between Pfc cell pairs during high

Hpc-Pfc-θ-coherence. Because coherence was elevated upon learning and functional connectivity in the Pfc network increases with learning [START_REF] Baeg | Learninginduced enduring changes in functional connectivity among prefrontal cortical neurons[END_REF], we tested for changes in activity correlations in simultaneously recorded prefrontal neurons during high Hpc-Pfc-θ-coherence episodes. During high coherence periods the distribution of correlation coefficients between all simultaneously recorded neurons pairs shifted compared to low coherence periods (Figure 3A, Kolmogorov-Smirnov-test, p<0.05 each day). During high coherence, cell pairs with elevated correlation values appeared (Supplementary Figure S7). This reveals a profound modification in the dynamics of the prefrontal neuronal network when Hpc-Pfc-θ-coherence is elevated.

Preferential participation of Hpc theta modulated Pfc neurons in cell assemblies forming upon

learning. The increase in significant correlations in cell pairs during high (vs. low) Hpc-Pfc-θ coherence periods was restricted to those Pfc neurons that were modulated by Hpc theta (Figure 3B, S6, n=32, two-way ANOVA with factors theta modulation of cells and high/low coherence: effect of interaction p<0.05 and post-hoc t-test, p<0.01). In contrast, pairs of neurons where one or both were not modulated by Hpc theta did not show any significant change in co-activation during high coherence periods (Figure 3B). In the five sessions with new rule acquisition (and at least 20 simultaneously recorded neurons), only after rule acquisition, but not before, were pairs of theta modulated neurons more likely to be significantly correlated than pairs of neurons without theta modulation (Figure 3C, Supplementary Figure S8, n=5 two-way ANOVA with factors high/low coherence and before/after learning and post-hoc t-test, p<0.05). Thus, rule acquisition corresponded to increased co-activation within subpopulations of Hpc modulated Pfc neurons. The increased activity correlations held at two time scales, whether firing rates were binned at 30 ms or at theta cycle (approximately 125 ms) time windows (Supplementary Figure 7). This indicates that, during high coherence periods, theta modulated cell pairs tended to fire together within the same theta cycle, as well as at a shorter time scale.

Pfc cell assemblies are formed during high coherence periods. The network reorganization during high Hpc-Pfc-θ-coherence suggested by the results in Figure 3 may reflect not just coactive pairs of neurons, but rather synchronization of activity in subgroups of neurons, or cell assemblies, as proposed by [START_REF] Hebb | The Organization of Behavior[END_REF] [START_REF] Hebb | The Organization of Behavior[END_REF]. Neuronal groups that co-activate reliably and hence act as cell assemblies can be extracted by principal component analysis (PCA) applied to the matrix of binned simultaneously recorded spike trains [START_REF] Chapin | Principal component analysis of neuronal ensemble activity reveals multidimensional somatosensory representations[END_REF][START_REF] Cassenaer | Hebbian STDP in mushroom bodies facilitates the synchronous flow of olfactory information in locusts[END_REF]Peyrache et al., 2009b;Peyrache et al., 2009a), and the time course of their activation can be calculated from the principal component scores of the instantaneous population vectors (see Methods and (Peyrache et al., 2009b;Peyrache et al., 2009a)).

Because of the increase in pairwise correlations observed during high Hpc-Pfc-θ coherence periods, PCA was applied to 30 ms bins of spike trains of units simultaneously recorded during those episodes only, and the strongest components were retained (see Methods). The signs of the coefficients of each principal component (PC) distinguished two neuronal populations: cells with high weights and same sign coefficients which fire together, and cells with high weights but opposite signs which are anti-correlated (Supplementary Figure S9A). Thus, the projection of the recorded coactivation onto the PC at each time bin describes the temporal evolution of the activation of the neuronal assembly (Peyrache et al., 2009a) (Supplementary Figure S9A,B). CRCA frequency increased in elevated Hpc-Pfc-θ-coherence periods relative to low coherence periods (paired t-test p<10 -200 ; Figure 4B). Similarly to coherence (Figure 2A,B), their spatial distribution presented a distinctive peak at the decision point (Figure 4C).

During high coherence periods Pfc cell assemblies are phase-locked to the trough of hippocampal theta. We analyzed the phase locking of Pfc CRCA activity to the hippocampal theta rhythm. For each recording session, at least one of the principal components identified a Pfc cell assembly significantly modulated by Hpc theta (i.e., with a significant Rayleigh Z-test, p<0.05). Notably, during high coherence periods, the phases of occurrence of these Pfc CRCAs were concentrated at the trough of theta recorded in both Hpc CA1 stratum pyramidale and in Pfc (n= 92, Rayleigh Z-test, p<10 -26 ), that is, the theta phase at which hippocampal CA1 cell assemblies are principally active [START_REF] Harris | Organization of cell assemblies in the hippocampus[END_REF]. On the other hand, during low coherence periods, the theta phase of CRCA occurrence was uniformly distributed (Figure 4D). The same result was obtained when PCA extracted assemblies from spike trains grouped by theta cycle (~125 ms time bins, Supplementary Figure S10). This rules out the possibility that the observed effects could be an artifact due to PCA merely capturing subsets of neurons with similar theta modulation. Moreover, this increased phase locking in high coherence periods was not observed in the general neuronal population. Rather, it was limited to those cell assemblies that occurred more strongly during high coherence periods, because no such effects were observed in PCs with low signal strengths, likely corresponding to "noise" (Peyrache et al., 2009b;Peyrache et al., 2009a) (p>0.05, n=70) or for PCs computed from low coherence periods (p>0.05, n=72; Supplementary Figure S11). Thus, high Hpc-Pfc-θ-coherence signaled a state in which hippocampal and prefrontal assemblies are aligned at the same theta phase, possibly favoring interaction. Here, again, this effect does not depend on the value of the threshold employed to detect CRCA activation (Supplementary Figure S9).

Local circuit dynamical changes possibly underlying assembly formation

To investigate the neurophysiological processes leading to the formation of CRCA, we analyzed the behavior of the cells with the five largest weight coefficients in the PC (positive and negative), that is, the ones most likely to participate in synchronous firing events. These neurons were then subdivided according to their putative cell type on the basis of action potential duration (Bartho et al., 2004) (Supplementary Figure S12).

Interneurons play a specific and crucial role in the generation and maintenance of theta and other brain rhythms [START_REF] Buzsáki | Theta Oscillations in the Hippocampus[END_REF][START_REF] Klausberger | Brain-state-and cell-type-specific firing of hippocampal interneurons in vivo[END_REF]. Furthermore, prefrontal interneurons are directly innervated by hippocampal afferents (Tierney et al., 2004). Thus, we analyzed the theta phase locking behaviors of 234 Pfc theta modulated interneurons and pyramidal cells with high absolute PC weights in the CRCAs. Among neurons forming the cell assemblies, 189 putative pyramidal neurons (81%) and 34 (15%) interneurons were identified. The preferred phases of putative interneurons were concentrated at the peak of theta in periods of both high and low coherence (Figure 5B, n=34, p < 0.005), and showed no coherence-dependent changes in Hpc theta modulation strength kappa (Figure 5D, n=34, paired t-test, p>0.05).

In contrast, in putative pyramidal cells, as LFP Hpc-Pfc-θ coherence increased, the strength of theta modulation also augmented (Figure 5D, n=189, t-test, p<0.05). Moreover, putative pyramidal neurons active in cell assemblies selectively changed their phase preference in a particular fashion: those neurons phase-locked with the Hpc θ peak (at pi) during low coherence shifted their phase preference to 2 pi to match the phase of the CRCA during high coherence.

Indeed, preferred phases of pyramidal neurons and CRCA (Fig. 4D) are not significantly different during high coherence periods, (n= 189, n=92, circular ANOVA, p>0.05). On the other hand, those Pfc neurons whose preferred phase was already at the trough of Hpc θ (2 pi) during low coherence maintained this preferred phase within high-coherence CRCA events (Figure 5A, n=189, n=34, circular ANOVA, p<0.05).

Interestingly, during high coherence, the preferred phases of pyramidal neurons converged on the preferred phase of the CRCA, in contrast with the interneurons" preferred phase (Figure 5C). Note that this is not due to possibly improved reliability of phase measurements resulting from an increase in hippocampal theta power during high coherence periods. Indeed, there is no significant difference in theta power during high and low coherence (n=21, t-test, p>0.05; Figure 7E). Furthermore, Hpc theta power tended to be higher in low coherence periods beyond the fork of the maze, corresponding to when the rat ran faster.

Interneuron-principal neuron inhibitory efficacy is enhanced during high coherence periods.

Behavior can strongly modulate inhibition in the Pfc [START_REF] Constantinidis | Correlated discharges among putative pyramidal neurons and interneurons in the primate prefrontal cortex[END_REF][START_REF] Fujisawa | Behavior-dependent short-term assembly dynamics in the medial prefrontal cortex[END_REF] and, in turn, inhibition can exert a powerful influence on the firing phase of principal neurons relative to neuro-oscillatory activity [START_REF] Chapman | Intrinsic theta-frequency membrane potential oscillations in hippocampal CA1 interneurons of stratum lacunosum-moleculare[END_REF][START_REF] Mann | Role of GABAergic inhibition in hippocampal network oscillations[END_REF]. Since putative principal and interneurons were differentially modulated by theta in high and low coherence periods, we hypothesized that periods of high and low Hpc-Pfc theta coherence would correspond to physiological states characterized by different inhibitory efficacies. Thus, we computed cross-correlograms between interneurons and pyramidal cells with high weights in PCs characterizing the CRCAs.

Our method is derived from Constantinidis and colleagues [START_REF] Constantinidis | Correlated discharges among putative pyramidal neurons and interneurons in the primate prefrontal cortex[END_REF]: for each interneuron within the CRCA, those putative pyramidal cells under its inhibitory influence were identified on the basis of: 1) at least a 50% decrease in the number of spikes in the 5 ms after the interneuron"s spike as compared to the 10 ms period before, and 2) no modification of the firing rate during high versus low coherence periods. Interneurons with small numbers of co-occurring pyramidal cells" spikes were excluded. All cross-correlograms were then normalized and averaged. The procedure was repeated for high and low coherence periods.

Putative interneurons effectively inhibited putative pyramidal cells during high coherence periods, but not during low coherence periods (Figure 5E for a single interneuron-pyramidal pair and Figure 5F for all interneurons analysed, see Methods): during high coherence periods, putative pyramidal cell firing was interrupted for 5-10 ms after putative interneuron spikes, (n=26, paired t-test between high and low coherence in the 0 to 5 ms time window, p<0.01). No deviation from baseline was apparent during low coherence periods. This could have contributed to the principal neurons shifting their preferred phase to the trough of Hpc theta (in phase opposition with interneurons) and hence helped to synchronize the pyramidal neurons to fire in CRCAs. After rule acquisition this inhibition was selective for the decision point relative to other zones in the maze: putative interneurons significantly inhibited putative pyramidal cells in the first 5 ms after interneuronal discharge (p < 0.05). Such inhibition was not significant elsewhere on the maze, or anywhere on the maze before rule acquisition (Fig 5G ; see Supplementary Figure S13 for a cell-by-cell breakdown).

Dynamic modification of the preferred phase of Pfc neurons forming the CRCA. As shown above, a subset of pyramidal neurons composing the CRCA changed their phase preference to match the phase of the CRCA during high coherence. This can best be seen by examining a representative example. In figure 6A, neuron 3 fired at the trough of Hpc theta continuously; on the other hand, neurons 2 and 4 fired at the peak of theta before the period of high coherence.

During, the high coherence period, the firing phase of these two neurons shifted to the trough of theta. Neurons 1 and 5, previously silent, activated with the CRCA. As a result, all five neurons fired synchronously, forming the cell assembly. After the high coherence period, the phase of neuron 4 returned to its previous value at the peak of theta. In this sense, the modification of the phase for the neurons shown in figure 5A can be seen as a dynamical process since this occurs progressively during the high coherence period. In this way phase realignment can effectively lead to synchronously activated cell assemblies.

When the same analysis was made with all pyramidal neurons forming the CRCAs, the phases of the spikes progressively shifted to the trough of theta leading up to activation of the CRCA (Figure 6B). Phase locking of these neurons was increased at the time of the CRCA as shown by the increased value of the concentration parameter κ, as computed by fit with a Von Mises distribution. However average firing rate did not change. This effect is directly related to CRCA since no modification of phase is seen when the reference is chosen at random times during high coherence periods (Supplementary Figure S14).

Local Pfc dopamine application also increases Hpc-Pfc-θ-coherence and reorganize

neuronal θ phase preferences. Since the shifts in spike phases relative to theta and the consequent enhanced synchronization occurred at the decision point after rule acquisition, just as the rat reached an elevated level of certainty about the outcome of the current trial, we hypothesized that these local functional circuitry changes may be triggered by a reward prediction signal. Theoretical and experimental arguments [START_REF] Schultz | A neural substrate of prediction and reward[END_REF][START_REF] Doya | Modulators of decision making[END_REF] suggest that this signal may be carried by a neuromodulator such as dopamine (DA). To test whether DA may induce similar effects on prefrontal neural activity, we recorded prefrontal single units and hippocampal LFPs in anesthetized rats (Figure 7C, Supplementary Figure S15), following locally iontophoretic application of DA in 1 minute pulses. Figure 7A shows examples of spectrograms and a coherogram for Hpc and Pfc local field potentials, showing that, while theta power in the two structures remained constant, coherence increased after DA application (t-test; p < 0.05; Figure 7B). Pyramidal cells and interneurons were classified based on spike width or marker labelling (Tierney et al., 2004). Interestingly, as observed during behavior for high Hpc-Pfc-θ-coherence periods, pyramidal cells changed their theta phase locking, shifting from a preferred phase similar to those of interneurons before DA injection, to a phase opposite to interneuronal firing (Circular ANOVA, p < 0.05; Figure 7D). Phase concentration for putative pyramidal cells increased as well (t-test, p < 0.05; Figure 7F). Similar to behavioral data for high Hpc-Pfc-θ-coherence, interneuron firing did not change theta phase (Figure 7D-F). The average phase change for putative pyramidal cells was significantly larger than that observed for interneurons (t-test, p < 0.01; Figure 7F). Thus, a number of measures show a striking parallel between the effect of local Pfc DA and the high coherence state in the behaving animal (Figure 7E-F). Theta LFP coherence increased while power did not, pyramidal cells shifted and enhanced phase preference, while interneurons kept their average firing preferences unchanged.

Replay of CRCAs was enhanced during hippocampal sharp waves/ripples. The memory consolidation theory holds that hippocampal traces are transmitted to cortical areas to build and reinforce long lasting traces. This would occur during sharp wave ripple activity in slow wave sleep [START_REF] Girardeau | Selective suppression of hippocampal ripples impairs spatial memory[END_REF]. A potential role for the high level of synchronization embodied by CRCAs is to favor synaptic plasticity so that cell assembly activation patterns can be successfully stored and reinstated. If this is the case, then the same cell assemblies would be expected to be more likely to activate spontaneously during the following slow-wave sleep. To test this, in data from slow-wave sleep periods preceding and following the behavioral task, we computed the reactivation strength (Peyrache et al., 2009b) for the principal components corresponding to CRCAs and we computed event-triggered averages for the resulting time series, centered on sharp wave/ripple complexes (SWR). During sleep after the task, but not before, there was a significant increase in reactivation strength peaking 30-60 ms after SWRs (p < 0.01; Figure 8). This was not the case for cell assemblies derived for low-coherence periods.

Discussion

The principal findings here are 1) Hpc-Pfc theta coherence increases as the rat must determine the probability of future reward at the choice point in the maze, in particular after rule acquisition, when reward can be reliably predicted, 2) during elevated Hpc-Pfc theta coherence and upon new learning Hpc theta-modulated Pfc cell assemblies form, and 3) local circuit properties reorganize in a manner which could underlie the formation of synchronized cell assemblies. Furthermore we observed similar coherence increases and spike phase shifts following dopamine application, suggesting a role for this neuromodulator, which is implicated in signaling reward expectation [START_REF] Schultz | A neural substrate of prediction and reward[END_REF]. Activity synchronization may then lead to synaptic plasticity and the consolidation of assembly-related memory traces, as suggested by the enhanced replay of high-coherence related assemblies during post-training sleep.

Hpc-Pfc-θ-coherence increases further and cell assemblies form at the decision point after rule acquisition

Theta band coherence between Hpc and Pfc has been shown previously [START_REF] Siapas | Prefrontal phase locking to hippocampal theta oscillations[END_REF][START_REF] Hyman | Medial prefrontal cortex cells show dynamic modulation with the hippocampal theta rhythm dependent on behavior[END_REF] and linked with working memory [START_REF] Jones | Phase precession of medial prefrontal cortical activity relative to the hippocampal theta rhythm[END_REF]. The present data demonstrate that Hpc-Pfc-θ-coherence augments at a critical part of the maze, where the rat has to commit to a potentially costly decision, based on a prediction of which arm is most likely to be rewarded. A reliable prediction is possible only when the rat has acquired the current task rule: indeed, coherence is maximally elevated only after the animal has reached criterion performance levels.

The increases in LFP coherence at the choice point and upon learning are accompanied by profound modifications of the dynamics of Pfc neural assemblies and local circuit dynamics.

First, during high coherence, preferred firing phases shift, leading to Pfc cell assemblies formation and firing in tight synchrony. These assemblies fire preferentially at the trough of Hpc theta, and would hence be temporally aligned with hippocampal assemblies [START_REF] Harris | Organization of cell assemblies in the hippocampus[END_REF].

This would enhance the impact of hippocampal inputs relative to those coming from other areas, influencing the formation of cell assemblies related to rule learning. After rule acquisition, this could also have the effect of reinforcing this recently established network by increasing the probability that, within an assembly, the pre-and the post-synaptic spikes of two connected cells fall within a time interval suitable for triggering spike-timing dependent plasticity (STDP) [START_REF] Markram | Regulation of synaptic efficacy by coincidence of postsynaptic APs and EPSPs [see comments[END_REF][START_REF] Bi | Synaptic modifications in cultured hippocampal neurons: dependence on spike timing, synaptic strength, and postsynaptic cell type[END_REF][START_REF] Buzsáki | Large-scale recording of neuronal ensembles[END_REF][START_REF] Cassenaer | Hebbian STDP in mushroom bodies facilitates the synchronous flow of olfactory information in locusts[END_REF]. Thus, this would facilitate consequent memory trace storage during sleep when these experiencerelated cell assemblies appearing during high Hpc-Pfc-θ-coherence are specifically replayed in transient bursts in concert with hippocampal sharp waves (Figure 8; also see (Peyrache et al., 2009b)).

Progressive change of phase of pyramidal neurons leading up to synchronization in cell assemblies.

The phase of Pfc principal neurons relative to Hpc theta changes progressively during high coherence to form synchronously firing cell assemblies. Experience-related modification of preferred phase relative to theta has been previously shown in hippocampus during REM sleep [START_REF] Poe | Experience-dependent phase-reversal of hippocampal neuron firing during REM sleep[END_REF], as well as during active behavior [START_REF] Hyman | Medial prefrontal cortex cells show dynamic modulation with the hippocampal theta rhythm dependent on behavior[END_REF], and negative phase shifts (phase precession) towards the trough of Hpc theta have been seen in some Pfc cells [START_REF] Jones | Phase precession of medial prefrontal cortical activity relative to the hippocampal theta rhythm[END_REF]. Here the Pfc neurons taking part in CRCAs displayed a phase advance instead of precession (Figure 6). However, it is important to note that this pertains only to transient synchronization of small subsets of cells, which would not necessarily be detected by the population-wide, time-averaged analyses usually applied to study phase precession. In any case, both mechanisms would have the same effect, i.e., convergence of the firing phases towards the trough of Hpc theta.

Increased local inhibition during high Hpc-Pfc theta coherence.

During high coherence, putative prefrontal interneurons exercise more effective inhibitory control of principal cells. The importance of interneurons for generating and maintaining hippocampal theta oscillations is well documented [START_REF] Buzsáki | Theta Oscillations in the Hippocampus[END_REF]. Some interneurons -e.g. in the hippocampus [START_REF] Chapman | Intrinsic theta-frequency membrane potential oscillations in hippocampal CA1 interneurons of stratum lacunosum-moleculare[END_REF] -intrinsically oscillate at the theta rhythm, and interneurons are likely to support theta oscillations in the prefrontal cortex as well [START_REF] Blatow | A novel network of multipolar bursting interneurons generates theta frequency oscillations in neocortex[END_REF]. Pfc interneurons receive direct connections from the hippocampus (Tierney et al., 2004) and thus it is not surprising that we observed that prefrontal interneurons consistently remain phase locked to hippocampal theta while putative pyramidal cells only followed theta during high Hpc-Pfc LFP coherence. Furthermore, interneurons can modify their activity in relation to external stimuli or to behavioral changes [START_REF] Constantinidis | Correlated discharges among putative pyramidal neurons and interneurons in the primate prefrontal cortex[END_REF][START_REF] Monier | Orientation and direction selectivity of synaptic inputs in visual cortical neurons: a diversity of combinations produces spike tuning[END_REF][START_REF] Fujisawa | Behavior-dependent short-term assembly dynamics in the medial prefrontal cortex[END_REF], and such changes may lead to the appearance and dissolution of cell assemblies [START_REF] Geisler | Hippocampal place cell assemblies are speed-controlled oscillators[END_REF]. Since interneuronal activity remains in phase with the peak of Hpc theta, an increase in inhibitory efficacy would constrain the pyramidal cells to fire primarily at a later phase (i.e. the trough) of the cycle, hence enforcing synchrony [START_REF] Chapman | Intrinsic theta-frequency membrane potential oscillations in hippocampal CA1 interneurons of stratum lacunosum-moleculare[END_REF]. Therefore, the Pfc interneurons modulated by hippocampal theta could act as a "latent theta oscillator", which is selectively enabled to control principal cell dynamics when behaviorally relevant information is processed, leading to the formation of cell assemblies.

Learning related changes in Hpc-Pfc coherence, inhibitory efficacy and theta phase locking in

Pfc resemble effects of dopamine application

At the choice point and after learning, Hpc-Pfc coherence and inhibitory efficacy increase, neurons become synchronized to the Hpc theta trough and cell assemblies are observed. The possible physiological mechanisms of the variations in synaptic efficacy enabling these events remain unknown, although neuromodulatory systems are presumed to play a role. Signals related to changes in reward anticipation (which would be expected as the rat chooses a goal arm) are broadcast in the brain via neuromodulatory signals such as dopamine [START_REF] Schultz | A neural substrate of prediction and reward[END_REF].

In our behavioral task, the choice point is likely to trigger reward expectation-dependent processes, possibly conveyed by dopamine. In a similar, multiple T-maze task, the ventral striatum displays a transient representation of expected reward (van der Meer and Redish, 2009) at the choice point. Accordingly, dopamine efflux in the Pfc has been shown to increase during the acquisition of a rule (van der Meulen et al., 2007). The increase in Hpc-Pfc theta coherence at the choice point would have the impact of heightening Pfc receptivity to Hpc signals as the rat evaluates the possible outcome of the two alternative choices. In fact, the hippocampus [START_REF] Johnson | Neural ensembles in CA3 transiently encode paths forward of the animal at a decision point[END_REF] generates representations of the two possible choice arms at the maze fork.

We speculate that the Pfc, in concert with the striatum, participates in "parsing" these anticipated representations in terms of the predicted outcome of each choice. Reward expectation signals in the Pfc would be instrumental for this.

At the cellular level, activation of the D2 receptor can modulate the inhibitory effect of GABA-ergic interneurons on NMDA excitation of pyramidal cells (Tseng and O'Donnell, 2004). Moreover, dopamine in Pfc increases the reliability of interneurons" spike timing in response to hippocampal stimulation (Tierney et al., 2008), and in vitro it increases IPSP amplitude and reduces spatiotemporal spread of stimulation-evoked activity [START_REF] Bandyopadhyay | Dopaminergic modulation of local network activity in rat prefrontal cortex[END_REF]. Finally, co-application of dopamine and NMDA in slices was shown to induce neuronal synchronization nested within peaks in theta and gamma oscillations, a phenomenon that required activation of the dopamine D1 receptor [START_REF] Gireesh | Neuronal avalanches organize as nested theta-and beta/gamma-oscillations during development of cortical layer 2/3[END_REF]. Dopamine has powerful effects on synaptic plasticity as well: D1 agonists enhance LTP on hippocampal-prefrontal synapses [START_REF] Gurden | Essential role of D1 but not D2 receptors in the NMDA receptor-dependent long-term potentiation at hippocampal-prefrontal cortex synapses in vivo[END_REF]. In the hippocampus, dopamine affects STDP as well, by increasing induction sensitivity and expanding the temporal window for potentiation between pre-and post-synaptic spikes to at least 45 ms (Zhang et al., 2009). This enlarged window is compatible with the levels of temporal precision of our theta-synchronized assemblies in Pfc.

In anesthetized animals, we show that Pfc dopamine applications provoke the same pattern of changes observed in the maze when a reward prediction was required for making a choice. LFP coherence between Hpc and Pfc theta increased, while theta power in the two structures was virtually unchanged. Pfc pyramidal cells shifted their theta phase preference to the Pfc theta trough under dopamine, while interneurons" preferred phase remained constant (Figure 7). This striking parallel between anesthetized and behaving preparations is consistent with a role for phasic dopamine release in Pfc cell assembly formation at the decision point with learning.

Taken together, these data suggest a possible mechanism linking together mechanisms at the network and cellular levels for reward signals to construct and "tag" activity configurations for long-term encoding: dopamine would increase assembly synchronization, making learning related assemblies to emerge, and then, because of the tight synchronization to be stabilized via STDP. Such mechanisms also may be applicable to understand DA mechanisms for learning in striatum. Dopamine is not the only neuromodulator that might be involved in these processes.

For example, in vitro acetylcholine both increases intrinsic theta oscillations in interneuronal networks [START_REF] Blatow | A novel network of multipolar bursting interneurons generates theta frequency oscillations in neocortex[END_REF] and, through the nicotinic receptor, increases GABAergic inhibition of pyramidal cells [START_REF] Couey | Distributed network actions by nicotine increase the threshold for spike-timing-dependent plasticity in prefrontal cortex[END_REF].

These findings, taken together, point to a possible mechanism wherein coherence would help determine which information is stored in long-term memory. Thus, the hippocampal/neocortical axis, deeply implicated in memory consolidation [START_REF] Marr | A theory for cerebral neocortex[END_REF][START_REF] Buzsaki | Two-stage model of memory trace formation: a role for "noisy" brain states[END_REF][START_REF] Mcclelland | Why there are complementary learning systems in the hippocampus and neocortex: insights from the successes and failures of connectionist models of learning and memory[END_REF], may also be responsible during encoding, for the selection of the relevant information to be retained, possibly under neuromodulatory control.

Subjects

Four pigmented Long-Evans male rats (René Janvier, Le Genest-St-Isle, France) weighing 250-300g at arrival, were kept on a 12h on/12h off light schedule (lights on at 8am) in an approved animal facility. Pre-training and experiments were performed during the lights-on period. Rats were permitted to habituate to the colony room for at least two weeks before the beginning of pre-training; in that period they were regularly handled by the experimenters. Rats were housed in pairs until the beginning of pre-training, then singly housed for the duration of the experiment. From the start of the pre-training, rats were on a mild food-deprivation regime (14g rat chow daily). All experiments were in accord with institutional (CNRS Comité Opérationnel pour l"Ethique dans les Sciences de la Vie), international (NIH guidelines) standards and legal regulations (Certicat no. 7186, Ministère de l"Agriculture et de la Pêche) regarding the use and care of animals.

Surgical procedures

For at least a week before surgery, rats were habituated to running on the maze where the behavioral task took place by allowing them to forage on the maze for 5-25 minutes daily, with reward available at the end of the arms.

For surgery, rats were anesthetized with intra-muscular Xylazine (Rompun 0.1 ml), followed by intra-peritoneal pentobarbital (35 mg/kg) with supplements of the latter as necessary. An assembly containing 6-8 independently drivable tetrodes was implanted on the skull above the right medial prefrontal cortex (AP 3.5-5 mm, ML 0.5-2 mm). Each tetrode was inserted in 30 gauge hypodermic tubing, with the tubes soldered together in two parallel adjacent rows.

Tetrodes [START_REF] Gray | Tetrodes markedly improve the reliability and yield of multiple single-unit isolation from multi-unit recordings in cat striate cortex[END_REF] were twisted bundles of 13 μm diameter polyimide-coated nichrome wire (Kanthal, Palm Coast, FL). Individual microdrives allowed independent adjustment of tetrode depths. After dura retraction, the cannulae assembly was implanted parallel and adjacent to the sagittal sinus, so that they targeted, respectively, the superficial and deep layers of the medial bank of the cortex. A separate micro-drive containing three tetrodes was targeted to mid-ventral hippocampal CA1 (AP -5.0 mm, ML 5.0 mm). Each of these tetrodes was electrically connected in a single-electrode configuration (all channels shorted together) and used for local field potential (LFP) recordings. For all LFP recordings, a screw implanted in the occipital bone above the cerebellum was used as a reference. After surgery, rats were allowed to recover for at least 2 weeks, while the tetrodes were gradually lowered to reach the prefrontal cortical prelimbic area and the CA1 pyramidal layer, respectively.

Behavioral protocols

Rats were trained in a rule-learning task involving attentional set shifts on a Y-maze (see supplemental information for a description of the apparatus). Such extradimensional set shift tasks require an intact medial prefrontal cortex (Pfc) in rats [START_REF] Birrell | Medial frontal cortex mediates perceptual attentional set shifting in the rat[END_REF][START_REF] Delatour | Functional role of rat prelimbic-infralimbic cortices in spatial memory: evidence for their involvement in attention and behavioural flexibility[END_REF]. This parallels the involvement of the human prefrontal cortex in the Wisconsin Card Sorting Task, which inspired the present experimental design. As explained in detail below, at each trial, the rat had to choose one of two arms, the rewarded arm being either spatially guided to the left or the right or cue-guided to the illuminated or the dark arm. As soon as the rat acquired a rule, the reward contingency rule was changed, without any further signal, to a rule in the other stimulus dimension (i.e., from spatially-guided to cue-guided, and vice versa). Because we were interested in the neural correlates of the early phases of task learning, rats were not exposed to the task until the first electrophysiological recording session.

Each recording session consisted of a 20-30 minutes sleep, or rest, epoch in which the rat was allowed to remain undisturbed in a padded flowerpot placed on the central platform of the maze. This was followed by an "awake" epoch, in which the rat performed the behavioral task (described below) for 20-40 minutes, followed by a second sleep or rest epoch of 20-30 minutes.

The first recording sessions corresponded to the first time rats encountered reward contingencies. Rats started each trial in the same "departure" arm with the central barrier in place. One of the two other (goal) arms was illuminated at random (pseudo-random schedule: runs of more than 4 consecutive trials with the same illuminated arm were avoided as were extended runs of alternation). After that, the barrier to the central platform was removed, allowing the rat to access the goal arms. Only one of the goal arms was rewarded according to one of four rules. Two rules were spatially guided (always go to the right arm, or to the left arm), the other two were cue guided (go to the illuminated arm, or to the dark arm). The current rule was not signaled in any way, so that the animal had to determine the rule by trial and error.

Once the rat reached a criterion of 10 consecutive correct trials, or only one error out of 12 trials, the rule was changed with no further cue warning to the rat. Rule changes were extradimensional, that is, from a spatially-guided rule to a cue-guided rule, and vice versa. The software used for analysing behavioral performance (certainty measure) is a modified version of software downloaded from Anne Smith's homepage (http://neurostat.mgh.harvard.edu/BehavioralLearning/Matlabcode) (Smith et al., 2004).

Histology

At the end of the experiments, small electrolytic lesions were made by passing a small cathodal DC current (20 μA, 10 s) through each recording tetrode to mark the location of its tip.

The rats were then deeply anesthetized with pentobarbital. Intracardial perfusion with saline was followed by 10% formalin saline. Histological sections were stained with cresyl violet.

Electrophysiological procedures

Signals from all electrodes were fed into a unity-gain headstage pre-amplifier (HS-54;

Neuralynx, Bozeman, MT) and then, through a tether cable, to programmable amplifiers set at 2000x (Lynx-8, Neuralynx, Bozeman, MT). Signals for single unit recordings were bandpass filtered between 600 and 6000 Hz, signals for LFP recordings were filtered between 0.1 and 475

Hz (Pfc) and 1 and 475 Hz (Hpc). Data were digitized and stored on hard disk by the Power 1401 (CED, Cambridge, UK) acquisition system, controlled by Spike2 software (also by CED).

Single unit data were sampled at 25 kHz, and 1.3 ms samples were timestamped and stored for all the channels in a tetrode whenever any of the 4 channels exceeded a pre-set threshold. Local field potentials were sampled and stored at 2 kHz.

Data Preprocessing

From spike waveforms, the energy (or squared averaged amplitude) and the first three principal components of the energy-normalized waveforms were computed for the 4 tetrode channels, generating a 12-dimensional vector describing each spike. Those vectors were input to the KlustaKwik [START_REF] Harris | Accuracy of tetrode spike separation as determined by simultaneous intracellular and extracellular measurements[END_REF] clustering program. The resulting classification was manually refined using the Klusters interface [START_REF] Hazan | Klusters, NeuroScope, NDManager: a free software suite for neurophysiological data processing and visualization[END_REF].

Recording sessions

58 recording sessions were analyzed (Rat12 (12 days), Rat15 (19 days), Rat18 (14 days), Rat 19 (13 days)). A total of 1475 of putative single units were identified. All 4 rats learned the right and light rule. Because not all animals learned the remaining two rules, and insufficient data were available, these were excluded from the analyses.

Coherence analyses

Coherence and spectrograms were analyzed with multi-taper Fourier analysis [START_REF] Mitra | Analysis of dynamic brain imaging data[END_REF], making use of the Chronux toolbox (http://www.chronux.org), and with customwritten, MATLAB-based programs (MathWorks, Natick, MA). Coherence analyses concerned periods starting when the rat returned to the extremity of the start arm until it reached the reward site. The threshold for coherence to be considered as "high" was determined with the theoretical bound provided by the Chronux toolbox. A coherence of about 0.7 reached the statistical criterion (p<0.05) and was then used as the threshold in the whole data set. Furthermore, since "low coherence" epochs were usually longer than "high coherence" ones, all analyses which aimed at comparing results between the two (correlation coefficient distributions, principal cells inhibition, etc.) were systematically controlled: statistics were carried out on randomly selected epochs from low coherence periods whose size (e.g., number of bins) were the same as the duration of high coherence epochs. Thus, the presented results were all independent from possible size effects.

Theta modulation analyses

The spike phase distribution relative to the theta oscillation is hypothesized to follow a Von Mises distribution. κ (concentration factor, a measure of phase dispersion) and φ pref (preferred phase) [START_REF] Fisher | Statistical analysis of circular data[END_REF]. Since the phase of theta depends on the recording site in Hpc, only recordings from electrodes in sessions where ripple oscillations were observed in the sleep periods before and after the task were included (ensuring recording in the CA1 pyramidal layer).

Principal Component Analysis

The method was based on that of (Peyrache et al., 2009b;Peyrache et al., 2009a) (Figure S5). 1)

Spike trains from ensembles of simultaneously recorded Pfc cells were obtained from high coherence periods.

2) The spike trains were binned in 30 ms time windows (corresponding to the time window for Hpc cell assemblies [START_REF] Harris | Organization of cell assemblies in the hippocampus[END_REF]) or by theta cycle periods and then ztransformed, so that each binned spike train had a zero mean and unit variance.

3) The correlation matrix of the z-transformed binned spike trains was computed (an N cells x N cells matrix). 4) The principal components of the matrix were computed: the matrix is diagonalized, and the eigenvectors are sorted according to their associated eigenvalues. The eigenvectors corresponding to the three largest eigenvalues were termed "signal" components, while the others were considered "non-signal", or noise. 5) Spike trains from ensembles of simultaneously recorded Pfc cells were obtained from the whole session. 6) The spike trains were binned in 30 ms time windows and then z-transformed, leading to population vector of activity at any given time. 7) From each principal component, the dot product of the population vector with the projector of a principal component and then with the transposed population vector at any given time is computed, yielding the time course for the activation of that component referred as to "subnetwork coactivation measure". Values that exceeded a threshold were taken to signify CRCA occurrence. For most analyses, we chose a threshold of 3, however different values of the threshold yielded similar results (Figure S5E).

Pfc recordings following local dopamine infusions

The detailed protocol was described before (Tierney et al., 2008). Briefly, male Sprague-Dawley rats (Charles River, L"Arbresle, France) weighing 280-350 g were placed in a stereotaxic apparatus (Unimécanique, Asnières, France) after anesthesia induction with a 400mg/kg intraperitoneal injection of chloral hydrate. Anesthesia maintenance was ensured by intraperitoneal infusion of chloral hydrate with a peristaltic pump set at 60 mg/kg/h turned on 1 h after induction. Proper depth of anesthesia was assessed regularly by testing the limb withdrawal reflex and monitoring of cortical activity for signs of arousal. Glass pipettes (20-30 MΩ) recorded extracellular unit activity in the PL/MO areas of the PFC (AP: + 2.8-4 mm from bregma; ML: 0.4-1 mm; depth: 2.5-4 mm from the cortical surface. For the iontophoresis experiments, 5-barrel glass pipettes (Harvard Apparatus, Kent, UK) were used (10-40 MΩ).

Under microscopic control the glass recording pipette was glued 15-30 microns below the tip of the 5-barrel iontophoresis pipette. Five high impedance current control units (Bionic Instruments, Bris-sur-Forges, France) were used to deliver currents for the iontophoresis experiments. Retention currents were set at 8-10 nA. Ejection currents were adjusted for each cell. In brief, DA was initially iontophoresed with a current of 10 nA and increased in increments of 10-20 nA until an effect was observed.

Action potential duration was measured on the 2nd phase of the spike and the criteria for identification as an interneuron was that the action potential be inferior to 0.6 ms, as in Tierney and collaborators (Tierney et al., 2004). Coherence, preferred phase change and theta modulation strength were compared between the five minutes before and after dopamine injection, but only when spontaneous theta activity was detected before the dopamine injection.

Data from coherence and theta power were then averaged over all dopamine injections. All spikes before and after dopamine injection were pooled for the calculation of the preferred phase. Because DA has been shown to decrease neuronal firing rate in Pfc, spike analysis was performed when firing rate under dopamine injection remained at level over one half of that of the pre-dopamine injection period.

Smith, A.C., Frank, L.M., Wirth, S., Yanike, M., Hu, D., Kubota, Y., Graybiel, A.M., Suzuki, W.A., and Brown, E.N. (2004). Dynamic analysis of learning in behavioral experiments. J

Neurosci 24, 447-461.

Tabuchi, E.T., Mulder, A.B., and Wiener, S.I. (2000). Position and behavioral modulation of synchronization of hippocampal and accumbens neuronal discharges in freely moving rats.

Hippocampus 10, 717-728.

Thierry, A.M., Gioanni, Y., Degenetais, E., and Glowinski, J. (2000). Hippocampo-prefrontal cortex pathway: anatomical and electrophysiological characteristics. Hippocampus 10, 411-419. Tierney, P.L., Degenetais, E., Thierry, A.M., Glowinski, J., and Gioanni, Y. (2004). Influence of the hippocampus on interneurons of the rat prefrontal cortex. Eur J Neurosci 20, 514-524. ). This moderate correlation could be due to the increased likelihood of registering larger coherence readouts when the signal to noise ratio for these phase is higher. In fact when we eliminate the data points with very weak theta (theta/delta ratio < 2), the correlations are markedly smaller (with Pfc power: r = 0.15, p = 0.03, with Hpc power: r = 0.04, n.s.). Cell assemblies were obtained from the 3 principal components with the highest eigenvalues (which explains much of the variance of the correlation matrix, see Peyrache et al., 2009a;Peyrache et al., 2009b). This analysis was performed during the high coherence periods The same analyses were performed with principal components corresponding to 'noise' or 'non-signal' (i.e. low PC eigenvalues, see Peyrache et al., 2009a;Peyrache et al., 2009b) related to high coherence periods (top row), and for cell assemblies with high eigenvalues but calculated from low coherence periods (bottom row), as a control. In each case, all preferred phases were uniformly distributed throughout the theta cycle (n=70 for noise PC, and n=72

for cell assemblies related to low coherence periods, Rayleigh Z-test, p>0.05), with no modification during high versus low coherence periods (circular ANOVA, p>0.05).

Figure S12. Identification of putative pyramidal cells and interneurons

Distribution of spike widths for all Pfc neurons recorded. According to previous studies This is the same analysis as in Fig. 6B with the same number of CRCA activations used as reference, but chosen randomly during high coherence periods. Note that the phases of the neuronal population remained fixed at the trough of theta (the preferred phase of pyramidal neurons; see Fig. 5A) without modification of phase, suggesting that the modification of phase seen before CRCA activation is specific to cell assembly formation rather than simply a general phase procession/precession process as previously described in Pfc (Jones et al., 2005a).

Figure

  Figure 4A shows examples of time courses of PC activations (see Methods), showing high,

Figure 1 .

 1 Figure 1. Characterization of Hpc-Pfc theta coherence

Figure 2 .

 2 Figure 2. Coherence increases upon learning at the choice point

Figure 3 .

 3 Figure 3. Modification of Pfc functional circuitry during high coherence periods

Figure 4 .

 4 Figure 4. Identification of theta coherence-related cell assemblies (CRCA)

Figure 5 .

 5 Figure 5. Pfc interneurons and the formation of cell assemblies

Figure 6 .

 6 Figure 6. Formation of cell assemblies during high theta coherence periods

Figure 7 .

 7 Figure 7. Local dopamine injection increases coherence and shifts Pfc spike phase relative to

E)

  From left to right: coherence before and after rule acquisition, theta power in Pfc and Hpc between high and low coherence, change of preferred phase between high and low coherence for interneurons and pyramidal cells, strength of Hpc theta modulation in Pfc pyramidal (n=6) cells or interneurons (n=9) with high PC weights during high and low coherence periods (t-test, * -p<0.05). (w.r.t.: with respect to; A.U.: arbitrary units). F) From left to right: coherence, theta power in Pfc and Hpc, preferred phase changes for interneurons and pyramidal cells, strength of Hpc theta modulation in Pfc pyramidal (n=6) cells or interneurons (n=9), before and after dopamine injection (t-test, * p<0.05). Note that dopamine induced significant changes in coherence, preferred phase changes and Hpc theta modulation.

Figure 8 .

 8 Figure 8. Reactivation of CRCA during hippocampal sharp wave/ripples (SPW-Rs)

  Figure1

Figure S4 .

 S4 Figure 2

Figure S5 .

 S5 Figure S5. Comparison of speed and acceleration with coherence over the trials

Figure S6 .Figure S7 .Figure S8 .

 S6S7S8 Figure S6. Comparison of learning related coherence increases in different Pfc layers.

Figure S10 .

 S10 Figure S10. Control analysis of theta modulation of coherence related-cell assemblies

Figure S11 .

 S11 Figure S11. Control analyses for theta modulation of cell assemblies.

(

  leading to CRCA), or during low coherence periods. We then examined the modulation by Hpc theta during high or low coherence periods. Results in Fig 4D show that the preferred phase of CRCA tends to occur at the trough of Pfc and Hpc theta during high coherence periods while it is uniformly distributed during low coherence periods.

(

  Bartho et al., 2004), putative interneurons and pyramidal cells in Pfc can be distinguished according to spike width. Cells with spike widths inferior to 0.3 ms were classified as putative interneurons (blue zone) whereas those superior to 0.35 ms (vertical bars) were classified as putative pyramidal cells (pink zone).

Figure S13 .

 S13 Figure S13. Detailed analysis of inhibition at the decision point as a function of the learning

Figure S14 .

 S14 Figure S14. Control analysis for the phase modification of putative pyramidal neurons.

Figure S15 .

 S15 Figure S15. Identification of putative Pfc pyramidal cells and interneurons in acute

Acknowledgements.

We thank Prof. J.-M. Deniau, Drs. A.-M. Thierry and M.B. Zugaro for interesting discussions, S. Doutremer for histology, Prof. D. Hopkins and N. Quenech"du for the anatomical reconstructions, V. Douchamps for help with the experiments, and Prof. A. Berthoz for support throughout the project. Supported by Fondation Fyssen (FPB), Fondation pour la Recherche Medicale (AP), EC contracts FP6-IST 027819 (ICEA), FP6-IST-027140 (BACS), and FP6-IST-027017 (NeuroProbes).

Experimental procedures

Supplementary Figures.

Figure S1. Description of the behavioral task

A) In the Y maze, the liquid rewards were first at the right arm independent of which arm was lit. (Left and right arms were lit in a pseudo-random sequence). In the subsequent cueguided task the reward was at the lit arm, independent of whether it was at the left or right position.

B) Average number of days necessary for acquisition for these two rules (Right and Light, n=4; mean±SEM).

Figure S2. Histology

Analyses of Cresyl violet stained 60 µm sections showing an electrolytic lesion at the recording site in the Hpc (A) (scale bar=1 mm) for one rat, and tracks of recording tetrodes in Pfc for three of the four rats in this study (B-D). Histology from one rat didn't allow the tetrodes' tracks reconstruction.

Figure S9. Criteria for selecting neurons composing cell assemblies

A) Raster display of 31 simultaneously recorded neurons ordered from top to bottom according to their weights in the first principal component in this session. Below) Temporal evolution of subnetwork coactivation (black trace) superimposed on the concurrent coherence spectrogram (ordinate is frequency in Hz, abscissa is time in ms). Note that the peak in the subnetwork coactivation measure corresponds to synchronous firing of neurons with high absolute weights in the PC (red star). Note the looser synchronization of the cell assembly when Hpc-Pfc theta coherence is reduced (black star above). Such events correspond only to small bumps in the temporal evolution of subnetwork coactivation trace.

The signs of the PC weights distinguish two neuronal populations: cells with same signed PC weights fire together (orange shaded zone), whereas cells with opposite signed coefficients are anti-correlated (blue shaded zone). Cells with the highest PC weights (in absolute value) were those that made the strongest contribution to the subnetwork activation measure.

Mathematically, peaks of the subnetwork activation measure could be due to alternate cofiring of the two groups (with respectively high positive and negative PC weights). However, practically, peaks of the subnetwork reactivation measure are always due to the co-firing of the same group, the second remaining silent at those times. Since, the sign obtained with the PCA is undetermined, the PC weight of co-firing neurons was arbitrarily chosen as positive.

Visual inspection of rasters of the whole data set reveals that in every case, at least five neurons fired together during peaks of the subnetwork activation measure. The cells with the five highest weights in the PC were thus considered as composing the CRCA. In Figure 5,