Connectionists: from hippocampal grid cells to place cells

[Stephen Grossberg]

The following article is now available at 
http://www.cns.bu.edu/Profiles/Grossberg:

Gorchetchnikov, A. and Grossberg, S. (2006). Space, time, and 
learning in the hippocampus:
How fine spatial and temporal scales are expanded into population 
codes for behavioral control.
Neural Networks, in press.

ABSTRACT:
The hippocampus participates in multiple functions, including spatial 
navigation, adaptive timing, and declarative (notably, episodic) 
memory. How does it carry out these particular functions?  The 
present article proposes that hippocampal spatial and temporal 
processing are carried out by parallel circuits within entorhinal 
cortex, dentate gyrus, and CA3 that are variations of the same 
circuit design. In particular, interactions between these brain 
regions transform fine spatial and temporal scales into
population codes that are capable of representing the much larger 
spatial and temporal scales that are needed to control adaptive 
behaviors. Previous models of adaptively timed learning propose how a 
spectrum of cells tuned to brief but different delays are combined 
and modulated by learning to create
a population code for controlling goal-oriented behaviors that span 
hundreds of milliseconds or even seconds. Here it is proposed how 
projections from entorhinal grid cells can undergo a similar learning 
process to create hippocampal place cells that can cover a space of 
many meters that are needed to control navigational behaviors. The 
suggested homology between spatial and temporal processing may 
clarify how spatial and temporal information may be integrated into 
an episodic memory. The model proposes how a path integration process 
activates a spatial map of grid cells. Path integration has a limited 
spatial capacity, and must be reset periodically, leading to the 
observed grid cell periodicity. Integration-to-map transformations 
have been proposed to exist in other brain systems. These include 
cortical mechanisms for numerical representation in the parietal 
cortex. As in the grid-to-place cell spatial expansion, the analog 
representation of number is extended by additional mechanisms to 
represent
much larger numbers. The model also suggests how visual landmarks may 
influence grid cell activities via feedback projections from 
hippocampal place cells to the entorhinal cortex.

Keywords: spatial navigation, adaptively timed learning, grid cells, 
place cells, entorhinal cortex, dentate gyrus, hippocampus, CA3.