Spatial maps in the hippocampus: phenomena and mechanisms

Summary

Date: 
May 20, 2014 - 1:00pm
Location: 
NW 243
About the Speaker
Name: 
Albert Lee (Janelia Farm)

The hippocampus is crucial for the formation of new long term memories of facts and events in humans as well as for spatial learning and memory in rodents.  Extracellular recordings from the rodent hippocampus have revealed that a map consisting of neurons called place cells - each with spatially-tuned spiking - rapidly appears whenever the animal explores a new environment.  Furthermore, more than half of all neurons that are of the same type as place cells, e.g. CA1 pyramidal cells, fire few or no spikes across the environment and are called silent cells.  Models have generally assumed that synaptic inputs from upstream neurons, many of which are themselves spatially tuned, summate to determine both place cell firing and the silence of silent cells.  Over the past few years, we and our colleagues have developed methods to record intracellularly in animals as they freely explore a maze environment, allowing more direct measurement of the inputs and cellular properties of these neurons than was possible before.  We have found that cellular properties appear to play a surprisingly strong role in determining which neurons will be place and silent cells in a novel environment.  In particular, place cells appear to be more excitable than silent cells, and silent cells appear to receive no spatially-tuned inputs.  Moreover, these findings are causally related, as we have found that experimentally increasing the excitability of silent cells reveals spatially-tuned input that was previously hidden from the soma, converting them into place cells.  In addition to revealing the importance of cellular mechanisms for neuronal integration in general, these results have unexpected implications for spatial learning in rodents as well as the formation of declarative memories in humans.  We have been pursuing several of these directions with additional intracellular and extracellular recording experiments, and the results we will describe have provided both new phenomena as well as new questions.