Proceedings of The Physiological Society
Cardiff University (2009) Proc Physiol Soc 17, PC36
Transformation in the neural code for whisker deflections from receptors to cortex
M. Bale1, R. S. Petersen1
1. Faculty of Life Sciences, The University of Manchester, Manchester, United Kingdom.
A common principle for the organization of sensory systems is a massive expansion in neuronal numbers from the periphery to the cerebral cortex. Neural codes at successive stages of the sensory pathway thus operate under markedly different anatomical constraints. We have investigated how the neural representation of whisker stimulation compares at successive stages of the lemniscal whisker pathway. Although changes in tuning are well documented, changes in reliability have only been investigated by a few studies. We have investigated the variability in neural responses at different levels of the whisker lemniscal pathway by studying both deflections of whiskers in different directions and dynamic whisker stimulation using white noise. We made extracellular single unit recordings under identical experimental conditions in the trigeminal ganglion, ventroposterior medial (VPM) nucleus of the thalamus and barrel cortex of urethanised rats (1.5 g/kg, ip). We found a dramatic decrease in both reliability and the amount of mutual information that single units convey about whisker direction deflection across the pathway. VPM units conveyed 48% of the mutual information conveyed by ganglion units, cortical units 12%. Simultaneously recorded cortical pairs conveyed 2.1 times the information than their constituent single units suggesting cortex may compensate for information loss by population coding. Yet despite the tranformation in the code the first post-stimulus spike at each level transmitted the majority of the information. Transformations in the coding of dynamic whisker stimulation included a decrease in firing rate, a sharp increase in adaptation at the level of barrel cortex and a decrease in spike timing precision along the pathway. The key result was a decrease in the reliability of white noise representation. In sum, these results indicate a shift in coding strategy along the whisker pathway that is optimised for fast and reliable sensory guided behaviour.
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