papers on phase coding

[Ole Jensen]

Dear colleagues,
I would like to draw your attention to two papers on phase coding and
information transfer between rhythmically coupled networks. The papers
area available in PDF at http://boojum.hut.fi/~ojensen/ or contact me for
hard copies.
Ole Jensen

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Jensen. O. (in press) Information transfer between rhythmically coupled
networks: reading the hippocampal phase code. Neural Computation

Brain Research Unit, Low Temperature Laboratory, Helsinki University of
Technology, P.O. Box 2200, FIN-02015 Espoo, Finland

There are numerous reports on rhythmic coupling between separate brain
networks. It has been proposed that this rhythmic coupling indicates
exchange of information. So far, few computational models have been
proposed which explore this principle and its potential computational
benefits. Recent results on hippocampal place cells of the rat provide new
insight: it has been shown that information about space is encoded by the
firing of place cells with respect to the phase of the ongoing theta
rhythm. This principle is termed phase coding and suggests that upcoming
locations (predicted by the hippocampus) are encoded by cells firing late
in the theta cycle, whereas current location is encoded by early firing at
the theta phase. A network reading the hippocampal output must inevitably
also receive an oscillatory theta input in order to decipher the phase
coded firing patterns.  In this work I propose a simple physiologically
plausible mechanism implemented as an oscillatory network which can decode
the hippocampal output.  By changing only the phase of the theta input to
the decoder, qualitatively different information is transferred: the theta
phase determines whether representations of current or upcoming locations
are read by the decoder. The proposed mechanism provides a computational
principle for information transfer between oscillatory networks and might
generalize to brain networks beyond the hippocampal region. 

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Jensen O. and J.E. Lisman (2000) Position reconstruction from an ensemble
of hippocampal place cells: contribution of theta phase coding. Journal of
Neurophysiology 83:2602-2609

Department of Biology, Volen Center for Complex Systems, Brandeis
University, Waltham, Massachusetts 02454 

Previous analysis of the firing of individual rat hippocampal place cells
has shown that their firing rate increases when they enter a place field
and that their phase of firing relative to the ongoing theta oscillation
(7-12 Hz) varies systematically as the rat traverses the place field, a
phenomenon termed the theta phase precession. To study the relative
contribution of phased-coded and rate-coded information, we reconstructed
the animal's position on a linear track using spikes recorded
simultaneously from 38 hippocampal neurons. Two previous studies of this
kind found no evidence that phase information substantially improves
reconstruction accuracy. We have found that reconstruction is improved
provided epochs with large, systematic errors are first excluded. With
this condition, use of both phase and rate information improves the
reconstruction accuracy by >43% as compared with the use of rate
information alone. Furthermore, it becomes possible to predict the rat's
position on a 204-cm track with very high accuracy (error of <3 cm). The
best reconstructions were obtained with more than three phase divisions
per theta cycle. These results strengthen the hypothesis that information
in rat hippocampal place cells is encoded by the phase of theta at which
cells fire.

==============================================================================
 Ole Jensen, Ph.D.
 Helsinki University of Technology     
 Low Temperature Laboratory         
 Otakaari 3A                       
 P.O. Box 2200                          
 FIN-02015 HUT                          
 Finland                                
                                        
 Office : (+358) 9 4512951         
 Mobile : (+358) 405049936               
 Fax    : (+358) 9 4512969

 e-mail : ojensen at neuro.hut.fi             
 URL    : http://boojum.hut.fi/~ojensen/