Associative Fear Learning Enhances Sparse Network Coding in Primary Sensory Cortex

Amos Gdalyahu, Elaine Tring, Pierre-Olivier Polack, Robin Gruver, Peyman Golshani,Michael S. Fanselow,Alcino J. Silva, and Joshua T. Trachtenberg

University of California Los Angeles

Neuron 75, 121–132, July 12, 2012

SPARSE CODING

Background

Redundancy: the total number of spikes elicited by a sensory stimulus exceeds the number needed for sensory perception.

‘‘Sparse coding’’: while individual neurons can fire at high instantaneousfrequencies, particularly in primary sensory cortices, the maximum sustainable average firing rate has been estimated to be between 1 and 4 Hz.

The cortex can optimize the fraction of neurons responding when the stimulus is presented (sparse population coding) and/or can optimize how frequently a single neuron responds when the stimulus is presented n times (lifetime sparseness, or fidelity)

Developing a variant of fear conditioning in freely exploring mice in which whisker stimulation (conditional stimulus [CS]) was either paired or explicitly unpaired with foot shock.

Examining how learning the association between the CS and the shockaffected subsequent encoding of the CS using in vivo calciumimaging. Examining whether the results were specific to associative learning

What is the relationship between cortical sparsification and associative learning?

Three sets of experiments

PART I The Model of Associative Fear Learning

Results

Mice Learn to Associate Passive Whisker Stimulation with Shock

PART II Does the learned CS-US association affect subsequent encoding of the CS in primary sensory cortex?

Associative Fear Learning Enhances Sparse Population Coding

Average magnitude of spontaneous activity defines response threshold

Fidelity of spontaneous activity defines response threshold

Associative Fear Learning Increases Response Strengthwithout Altering Response Fidelity

Conclusion 1 : fear learning reduces the fraction of neurons responding to the CS, while increasing the strength of responsive neurons. The net effect is an enhancement of sparse population coding with a moderate decrease in total activity.

PART III Are the effects observed after associative fear conditioning general to learning per se, or specific to associative fear learning?

Nonassociative Training Reduces Response Strengthand Enhances Response Fidelity, but Does Not AffectSparse Population Coding

Conclusion 2: exposure to a nonreinforced stimulus has no effect on population sparsification, but does enhance response fidelity at the expense of response strength.

Thank you