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1 Faculty of Life Sciences, University of Manchester, UK, Manchester, United Kingdom
2 Instituto de Neurociencias de Alicante UMH-CSIC, Alicante, Spain
3 Alicante, Spain; Instituto de Neurociencias de Alicante UMH-CSIC, Alicante, Spain
4 Faculty of Life Sciences, University of Manchester, UK, Manchester, United Kingdom; Manchester, United Kingdom
* To whom correspondence should be addressed. E-mail: r.petersen{at}manchester.ac.uk.
Rats discriminate texture by whisking their vibrissae across the surfaces of objects. This process induces corresponding vibrissa vibrations, which must be accurately represented by neurons in the somatosensory pathway. In this study, we investigated the neural code for vibrissa motion in the ventro-posterior medial (VPm) nucleus of the thalamus by single-unit recording. We found that neurons conveyed a great deal of information (up to 77.9 bits/s) about vibrissa dynamics. The key was precise spike timing, which typically varied by less than a millisecond from trial to trial. The neural code was sparse, the average spike being remarkably informative (5.8 bits/spike). This implies that as few as 4 VPm spikes, coding independent information, might reliably differentiate between 106 textures. To probe the mechanism of information transmission, we compared the role of time-varying firing rate to that of temporally correlated spike patterns in two ways. 93.9% of the information encoded by a neuron could be accounted for by a hypothetical neuron with the same time-dependent firing rate but no correlations between spikes. Moreover, at least 93.4% of the information in the spike trains could be decoded even if temporal correlations were ignored. Taken together, these results suggest that the essence of the VPm code for vibrissa motion is firing rate modulation on a sub-millisecond time-scale. The significance of such a code may be that it enables a small number of neurons, firing only few spikes, to convey distinctions between very many different textures to the barrel cortex.
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