Proceedings of The Physiological Society
University of Manchester (2010) Proc Physiol Soc 19, PC238
Transformation in the Neural Code for Whisker Deflection Direction
M. R. Bale1, R. S. Petersen1
1. Faculty of Life Sciences, University of Manchester, Manchester, United Kingdom.
Changes in neural circuitry are often associated with dramatic changes in neural codes. In the rat whisker system, there is a massive increase in neuron number with each whisker being innervated by ~150 primary afferents, represented by ~250 neurons in a thalamic barreloid and ~10,000 neurons in barrel cortex. A prominent characteristic of neurons in this sensory pathway is a decrease in tuning to the direction of whisker deflection. However, the reliability of neural responses to this parameter have not been characterised. We made extracellular single-unit recordings from the trigeminal ganglion, the ventroposterior medial (VPM) nucleus of the thalamus and barrel cortex in urethane anaesthetised Wistar rats (1.5 g/kg, ip). Whiskers were deflected in over 400 randomly selected directions using a custom-made 2D piezoelectric actuator and neural responses were analysed using information theoretic methods. We found a dramatic transformation in the neural code regarding whisker deflection direction. VPM neurons conveyed only 48% of the mutual information about whisker direction than trigeminal ganglion neurons, cortical neurons just 12%. Pairs of simultaneously recorded cortical neurons conveyed 2.1 times the mutual information than their constituent single units suggesting a change in strategy to population coding at the level of barrel cortex. Despite this transformation, at all levels of the pathway the first spike was critical in that it conveyed the majority of the mutual information.
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