Abstract
Neural information processing involves multistagetransmission mechanisms in which neurons act as stochastic communicationchannels. We set forth an information theoretical measure to quantifyneurotransmission reliability while taking into full account the metricalproperties of spike trains. We stress the importance of considering theproperties of actual neural decoders ‚Äîas opposed to ideal observers as inShannon theory‚Äî when studying neurotransmission. We derive a novel spike trainmetrics based on a parametric non-linear distance inspired by the probabilisticbehaviour of the Spike-Response Model (SRM).  We apply the proposed informationtheoretical analysis to study neurotransmission along the ascendingsomatosensory pathway. We focus on the encoding/decoding of primary tactilesignals (fingertip mechanoreceptor responses) recorded via humanmicroneurography experiments. We show that first-spike latencies ofmechanoreceptor responses contain enough information to perform perfectdiscrimination of 81 distinct stimuli within 40 ms of the first afferent spike.We also demonstrate that the relative spike times of entire primary spiketrains permit to go beyond stimulus discrimination and encode isometricrepresentations of the stimulus space, a likely basis for generalisation inhaptic perception.  Then, we consider the propagation of mechanoreceptorsignals along peripheral nerve fibres up to 2nd order cuneate neurons (CNs). Wehypothesise that CNs mediate optimal decoding/re-encoding of primary hapticinformation. We model CN as SRM neurons and tune the model parameters accordingto in vivo intracellular CN recordings from non-anaesthetised but decerebratedcats. We modulate the efficacy of mechanoreceptor-CN synapses byspike-timing-dependent plasticity. Our simulations of the integration of thesensory quanta in single CN neurons predict that optimal information transferoccurs when only as few as 1-3 \% of CN synapses are non-silent. Thisprediction is verified by in vivo recordings in cats showing that inputs fromjust a few of the primary afferents have a high synaptic weight, whereas thevast majority have very low or undetectable synaptic efficacy.