Theta-modulated place-by-direction cells in the hippocampal formation in the rat

Lateralized functional components of spatial cognition in the avian hippocampal formation: evidence from single-unit recordings in freely moving homing pigeons

Previous research has revealed that the functional components of spatial cognition are lateralized in the forebrain of birds, including the hippocampal formation (HF). To investigate how HF cells in the left and right avian brain may differentially participate in representations of space, we recorded single-units from the HF of homing pigeons as they ran a plus maze for food. The rate maps of left HF cells often displayed elongated regions of increased activity in the center of the maze and along the maze corridors, whereas right HF cells tended to display patches at the ends of maze arms at/near goal locations. Left HF cells displayed a higher degree of spatial-specificity compared with right HF cells, including higher patch-specificity, higher reliability, and a higher incidence of location-correlated activity. Analysis of speed-correlated and trajectory-dependent activity also revealed significant HF-lateralized differences. Right HF cells tended to display significant negative correlations between spike rate and speed, although speed-dependent rate maps indicate that this relationship did not explain their space-specific activity. Left HF cells displayed a significantly higher incidence of trajectory-dependent space-specific activity than was observed in the right HF, suggesting that left HF cells may participate in navigating among goal locations. Differences in the correlates of left and right pigeon HF cells are consistent with unilateral HF-lesion data suggesting that the functional components of spatial cognition are lateralized in the avian brain, and furthermore, provide a basis for hypotheses regarding how the left and right HF support different aspects of spatial cognition.

Tracking Route Progression in the Posterior Parietal Cortex

Quick and efficient traversal of learned routes is critical to the survival of many animals. Routes can be defined by both the ordering of navigational epochs, such as continued forward motion or execution of a turn, and the distances separating them. The neural substrates conferring the ability to fluidly traverse complex routes are not well understood, but likely entail interactions between frontal, parietal, and rhinal cortices and the hippocampus. This paper demonstrates that posterior parietal cortical neurons map both individual and multiple navigational epochs with respect to their order in a route. In direct contrast to spatial firing patterns of hippocampal neurons, parietal neurons discharged in a place- and direction-independent fashion. Parietal route maps were scalable and versatile in that they were independent of the size and spatial configuration of navigational epochs. The results provide a framework in which to consider parietal function in spatial cognition.

Neuronal information coding by oscillation phase prediction error: Implications for consciousness and control of voluntary function

Neuronal oscillatory firing patterns are widely thought to serve as coincidence detectors, so as to synchronize information processing across brain regions. I hypothesize here that, instead, oscillatory function can be better understood by reversing conventional models and regarding oscillations slower than 30 Hz as background activity occurring in the absence of conscious or voluntary function. Action potentials occurring out of phase with the local population oscillation then emerge as information carriers, according to a prediction error mechanism that was first described in the drug abuse literature. Shannon information calculations show that coding by out-of-phase action potentials is far more efficient than are conventional synchronization models, and a simple calculation of the degree to which an action potential is out of phase correctly predicts the amount of information content. This model appears to account for a wide range of existing experimental observations of visual attention, voluntary motor function, and movement disorders. It also suggests an intuitively simple way of understanding consciousness, based upon a "self-observing" feedback mechanism of neocortical neurons firing out of phase. The hypothesis also suggests that the function of slow-wave sleep may be to re-entrain desynchronized oscillations.

Path integration and lesions within the head direction cell circuit: Comparison between the roles of the anterodorsal thalamus and dorsal tegmental nucleus

Experiments were designed to determine whether 2 regions of the head direction cell circuit, the anterodorsal thalamic nucleus (ADN) and the dorsal tegmental nucleus (DTN), contribute to navigation. Rats were trained to perform a food-carrying task with and without blindfolds prior to receiving sham lesions or bilateral lesions of the ADN or DTN. ADN-lesioned rats were mildly impaired in both versions of the task. DTN-lesioned rats, however, were severely impaired and showed reduced heading accuracy in both task versions. These findings suggest that although both the DTN and ADN contribute to navigation based on path integration and landmarks, disruption of the head direction cell circuit at the level of the DTN has a significantly greater effect on spatial behavior than lesions of the ADN.

Dual phase and rate coding in hippocampal place cells: theoretical significance and relationship to entorhinal grid cells

We review the ideas and data behind the hypothesis that hippocampal pyramidal cells encode information by their phase of firing relative to the theta rhythm of the EEG. Particular focus is given to the further hypothesis that variations in firing rate can encode information independently from that encoded by firing phase. We discuss possible explanation of the phase-precession effect in terms of interference between two independent oscillatory influences on the pyramidal cell membrane potential, and the extent to which firing phase reflects internal dynamics or external (environmental) variables. Finally, we propose a model of the firing of the recently discovered "grid cells" in entorhinal cortex as part of a path-integration system, in combination with place cells and head-direction cells.

Place cells, spatial maps and the population code for memory

The study of population dynamics in hippocampal place cells has emerged as one of the most powerful tools for understanding the encoding, storage and retrieval of declarative memory. Recent work has laid out the contours of an attractor-based hippocampal population code for memory in recurrent circuits of the hippocampus. The code is based on inputs from a topographically organized, path-integration-dependent spatial map that lies upstream in the medial entorhinal cortex. The recurrent networks of the hippocampal formation enable these spatial inputs to be synthesized with nonspatial event-related information.

What are the memory sources of dreaming?

Investigators since Freud have appreciated that memories of the people, places, activities and emotions of daily life are reflected in dreams but are typically so fragmented that their predictability is nil. The mechanisms that translate such memories into dream images remain largely unknown. New research targeting relationships between dreaming, memory and the hippocampus is producing a new theory to explain how, why and when we dream of waking life events.

Spatial memory in the rat requires the dorsolateral band of the entorhinal cortex

The extensive connections of the entorhinal cortex with the hippocampus and the neocortex point to this region as a major interface in the hippocampal-neocortical interactions underlying memory. We asked whether hippocampal-dependent recall of spatial memory depends on the entorhinal cortex, and, if so, which parts are critical. After training in a Morris water maze, rats received fiber-sparing lesions in the dorsolateral band of the entorhinal cortex, which mediates much of the visuospatial input to the dorsal hippocampus. These lesions entirely disrupted retention and retarded new learning. Spatial memory was spared by lesions in the ventromedial band, which connects primarily with ventral hippocampus, but these lesions reduced defensive behavior on an elevated plus maze, mirroring the effects of damage to ventral hippocampus. The results suggest that the functional differences between dorsal and ventral hippocampus reflect their connectivity with modules of the entorhinal cortex that are differently linked to the rest of the cortex.

The Effects of Hippocampal Lesions on Response, Direction, and Place Learning in Rats

Rats with hippocampal or sham lesions were trained to find food on a T maze located at 2 positions. Response rats were required to make a right or left turn. Direction rats were required to go in a consistent direction (east or west). Place rats were required to go to a consistent location, relative to room cues. One place group had distinguishable start points at the 2 maze positions, whereas another place group had start points facing the same side of the room. Controls took longer to solve a place problem than the response and direction problems when the start points were not distinguishable. Rats with hippocampal lesions were not different than controls on the response problem but were impaired on the direction and place problems.

Characterizing multiple independent behavioral correlates of cell firing in freely moving animals

The heterogeneous sampling of behavioral states by freely moving animals hinders our ability to relate neuronal firing rates to behavioral variables by introducing dependencies between them. We specifically consider the animal's location and orientation, although our analyses may generalize to other behavioral variables, such as speed of movement. A maximum-likelihood approach is presented for producing estimates of the separate histograms relating firing rate to multiple independent causes. Examples show that the method can be used to avoid the artifactual behavioral correlates of place and head direction-cell firing produced by standard analyses; to characterize the independent influences of both location and orientation in a third cell type (Cacucci et al., 2004); and to demonstrate the location-independence of the directional firing of head-direction cells.