Time as the fourth dimension in the hippocampus.

Experiences of animal and human beings are structured by the continuity of space and time coupled with the uni-directionality of time. In addition to its pivotal position in spatial processing and navigation, the hippocampal system also plays a central, multiform role in several types of temporal processing. These include timing and sequence learning, at scales ranging from meso-scales of seconds to macro-scales of minutes, hours, days and beyond, encompassing the classical functions of short term memory, working memory, long term memory, and episodic memories (comprised of information about when, what, and where). This review article highlights the principal findings and behavioral contexts of experiments in rats showing: 1) timing: tracking time during delays by hippocampal 'time cells' and during free behavior by hippocampal-afferent lateral entorhinal cortex ramping cells; 2) 'online' sequence processing: activity coding sequences of events during active behavior; 3) 'off-line' sequence replay: during quiescence or sleep, orderly reactivation of neuronal assemblies coding awake sequences. Studies in humans show neurophysiological correlates of episodic memory comparable to awake replay. Neural mechanisms are discussed, including ion channel properties, plateau and ramping potentials, oscillations of excitation and inhibition of population activity, bursts of high amplitude discharges (sharp wave ripples), as well as short and long term synaptic modifications among and within cell assemblies. Specifically conceived neural network models will suggest processes supporting the emergence of scalar properties (Weber's law), and include different classes of feedforward and recurrent network models, with intrinsic hippocampal coding for 'transitions' (sequencing of events or places).

Merging information in the entorhinal cortex: what can we learn from robotics experiments and modeling?

Place recognition is a complex process involving idiothetic and allothetic information. In mammals, evidence suggests that visual information stemming from the temporal and parietal cortical areas ('what' and 'where' information) is merged at the level of the entorhinal cortex (EC) to build a compact code of a place. Local views extracted from specific feature points can provide information important for view cells (in primates) and place cells (in rodents) even when the environment changes dramatically. Robotics experiments using conjunctive cells merging 'what' and 'where' information related to different local views show their important role for obtaining place cells with strong generalization capabilities. This convergence of information may also explain the formation of grid cells in the medial EC if we suppose that: (1) path integration information is computed outside the EC, (2) this information is compressed at the level of the EC owing to projection (which follows a modulo principle) of cortical activities associated with discretized vector fields representing angles and/or path integration, and (3) conjunctive cells merge the projections of different modalities to build grid cell activities. Applying modulo projection to visual information allows an interesting compression of information and could explain more recent results on grid cells related to visual exploration. In conclusion, the EC could be dedicated to the build-up of a robust yet compact code of cortical activity whereas the hippocampus proper recognizes these complex codes and learns to predict the transition from one state to another.

A test of the time estimation hypothesis of place cell goal-related activity.

Place cells are hippocampal pyramidal neurons that discharge strongly in relation to the rat's location in the environment. We recently reported that many place cells recorded from rats performing place or cue navigation tasks also discharged when they were at the goal location rather than in the primary firing field. Furthermore, subtle differences in discharge timing were found in the two navigation tasks, with activity occurring later in the place task compared to the cue task. Here we tested the possibility that such delayed firing in the place task may reflect the differential involvement of time estimation, which would allow the rat to predict forthcoming reward delivery. More specifically, we reasoned that failure to obtain a reward after a fixed 2s delay in the place task reliably reflected the rat's misplacement relative to the correct location, thus making time a valuable cue to help the rat perform the task. To test this hypothesis, well-trained rats were run on a partial extinction procedure in place and cue navigation tasks so that no feed-back signal was provided about their actual accuracy during extinction periods. Although the time estimation hypothesis predicts that only in the place task will the rat make correction movements at the end of goal periods during extinction, we found that such movements occurred in all rats, indicating correct time estimation in both place and cue tasks. We briefly discuss the results in the light of current computational theories of hippocampal function.

From reflex to planning: Multimodal versatile complex systems in biorobotics.

As models of living beings acting in a real world biorobots undergo an accelerated "philogenic" complexification. The first efficient robots performed simple animal behaviours (e.g., those of ants, crickets) and later on isolated elementary behaviours of complex beings. The increasing complexity of the tasks robots are dedicated to is matched by an increasing complexity and versatility of the architectures now supporting conditioning or even elementary planning.