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This Article Full Text Full Text (PDF) All Versions of this Article: 102/5/2771    most recent 00636.2009v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Bale, M. R. Articles by Petersen, R. S. PubMed PubMed Citation Articles by Bale, M. R. Articles by Petersen, R. S.

RESEARCH-ARTICLE

Transformation in the Neural Code for Whisker Deflection Direction Along the Lemniscal Pathway

Faculty of Life Sciences, University of Manchester, Manchester, United Kingdom

Submitted 21 July 2009; accepted in final form 5 September 2009

ABSTRACT

A prominent characteristic of neurons in the whisker system is their selectivity to the direction in which a whisker is deflected. The aim of this study was to determine how information about whisker direction is encoded at successive levels of the lemniscal pathway. We made extracellular recordings under identical conditions from the trigeminal ganglion, ventro-posterior medial thalamus (VPM), and barrel cortex while varying the direction of whisker deflection. We found a marked increase in the variability of single unit responses along the pathway. To study the consequences of this for information processing, we quantified the responses using mutual information. VPM units conveyed 48% of the mutual information conveyed by ganglion units, and cortical units conveyed 12%. The fraction of neuronal bandwidth used for transmitting direction information decreased from 40% in the ganglion to 24% in VPM and 5% in barrel cortex. To test whether, in cortex, population coding might compensate for this information loss, we made simultaneous recordings. We found that cortical neuron pairs conveyed 2.1 times the mutual information conveyed by single neurons. Overall, these findings indicate a marked transformation from a subcortical neural code based on small numbers of reliable neurons to a cortical code based on populations of unreliable neurons. However, the basic form of the neural code in ganglion, thalamus, and cortex was similar—at each stage, the first poststimulus spike carried the majority of the information.