INFORMATION CODING IN THE RODENT PREFRONTAL CORTEX: II. ENSEMBLE ACTIVITY
IN ORBITOFRONTAL CORTEX.
Schoenbaum, Geoffrey and Howard Eichenbaum.
Department of Psychology, University of North Carolina at Chapel Hill,
Chapel Hill, NC 27599-3270, Center for Behavioral Neuroscience, State
University of New York at Stony Brook, Stony Brook, NY 11794-2575.
APStracts 2:0095N, 1995.
SUMMARY AND CONCLUSIONS
1) Neural activity was recorded from the orbitofrontal cortex (OF) of rats
performing an 8 odor discrimination task that included predictable
associations between particular odor pairs. A modified linear discriminant
analysis was employed to characterize the population response on each trial of
the task as a point in an N dimensional activity space with the firing rate of
each cell in the population represented on one of the N dimensions. The
ability of the ensemble to discriminate among conditions of a variable was
reflected in the tendency of population responses to cluster together in this
activity space for repetitions of a given condition. We assessed coding of
several variables describing the period of odor-sampling, focusing on aspects
of current, past, and future events reflected in single neuron firing
patterns, in ensembles composed of 22 to 138 cells active during the period
when the rats sampled the discriminative stimulus on each trial. 2) OF
ensembles performed well at discriminating variables with relevance to task
demands represented in single neuron firing patterns, specifically the
physical attributes and assigned reward contingency of the current odor as
well as the expectation of reward on the following trial that could be
inferred from the predictable associations between particular pairs of odors.
OF ensembles were able to correctly identify the identity and assigned reward
contingency of the current odor on up to 52% (chance=12.5%) and 99%
(chance=50%) of all trials respectively such that the observed behavioral
performance required a population of 5,364 odor-responsive cells in the case
of odor identity and only 40 cells in the case of valence. Expectations
regarding upcoming rewards based on both assigned response contingency and
associations between particular pairs of odors were correctly classified on up
to 67% (chance=20%) of all trials such that the observed level of behavioral
performance required a population of 3,169 cells. 3) Other information
represented in the single neuron firing patterns, such as the identity and
reward contingency of the preceding odor and specific odor-odor associations,
was poorly encoded by OF ensembles. Thus neural ensembles in OF may represent
only some of the information reflected in single neuron activity. Stable
coding of only the most useful and relevant information by the ensemble might
emerge from the tuning properties of single neurons under the influence of the
task at hand, producing in the well-trained animal the observed pattern of
broad and diverse coding by single neurons and selective, task-relevant coding
by neural ensembles in OF.
Received 24 October 1994; accepted in final form 24 March 1995.
APS Manuscript Number J660-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 25 April 1995.