Role of oscillations and plasticity in generating a temporal code
from a rate code
Mayank R. Mehta and Matthew A. Wilson
Center for Learning and Memory,
Department of Brain and Cognitive Sciences,
Massachusetts Institute of Technology
Recent studies have shown that the hippocampal rate code for space is
dynamic, such that size, specificity and shape of place fields change
dramatically with experience (Mehta et al., 1997, 2000). As a result, the
place fields become negatively skewed, such that the firing rate is low as
a rat enters the place field but high at the end. It was shown that these
results could arise due to spike timing dependent plasticity (STDP) of the
feed-forward synapses from CA3 to CA1.
In addition to this asymmetric rate modulation, hippocampal neuronal
firing is also modulated by an ~8 Hz theta rhythm. Further, the phase of
the theta rhythm at which a neuron fires precesses to smaller values as
the rat advances through a place field. This phase precession (O'Keefe and
Recce, 1993) thus results in a unique phase, or spike time with respect to
the local field potential, for each spatial location. The origin of this
temporal code and its relationship to the rate code are unclear.
We show three novel results that address these issues. First, the mean
phase of spikes at a given location, i.e. the temporal code, is highly
correlated with the mean firing rate of the neuron at that location, i.e.
the rate code. Second, the phase precession is nonlinear such that the
high phase (360-180 degrees) is more correlated with position than the low
phase (180-0 degrees). Third, hippocampal inhibitory neurons also show
phase precession, such that the firing rate is correlated with the phase.
These results are explained using a simple feed-forward model. This
suggests that a robust temporal code can arise from an interaction between
oscillations, delayed inhibition, and an asymmetric rate code that arises
from temporally asymmetric, NMDA dependent LTP.
Similar phenomena have been observed in the directional, spatio-temporal
receptive fields in V1 (Movshon et al., 1978; Reid et al., 1987), and our
model suggests that V1 direction selectivity could arise due to STDP