Information theoretic analysis of pulmonary stretch receptor spike trains

Primary afferent neurons transduce physical, continuous stimuli into discre te spike trains. Investigators have long been interested in interpreting th e meaning of the number or pattern of action potentials in attempts to deco de the spike train back into stimulus parameters. Pulmonary stretch recepto rs (PSRs) are visceral mechanoreceptors that respond to deformation of the lungs and pulmonary tree. They provide the brain stem with feedback that is used by cardiorespiratory control circuits. In anesthetized, paralyzed, ar tificially ventilated rabbits, we recorded the action potential trains of i ndividual PSRs while continuously manipulating ventilator rate and volume. We describe an information theoretic-based analytical method for evaluating continuous stimulus and spike train data that is of general applicability to any continuous, dynamic system. After adjusting spike times for conducti on velocity, we used a sliding window to discretize the stimulus (average t racheal pressure) and response (number of spikes), and constructed co-occur rence matrices. We systematically varied the number of categories into whic h the stimulus and response were evenly divided at 26 different sliding win dow widths (5, 10, 20, 30,..., 230, 240, 250 ms). Using the probability dis tributions defined by the co-occurrence matrices, we estimated associated s timulus, response, joint, and conditional entropies, from which we calculat ed information transmitted as a fraction of the maximum possible, as well a s encoding and decoding efficiencies. We found that, in general, informatio n increases rapidly as the sliding window width increases from 5 to similar to 50 ms and then saturates as observation time increases. In addition, th e information measures suggest that individual PSRs transmit more "when" th an "what" type of information about the stimulus, based on the finding that the maximum information at a given window width was obtained when the stim ulus was divided into just a few (usually <6) categories. Our results indic ate that PSRs provide quite reliable information about tracheal pressure, w ith each PSR conveying about 31% of the maximum possible information about the dynamic stimulus, given our analytical parameters. When the stimulus an d response are divided into more categories, slightly less information is t ransmitted, and this quantity also saturates as a function of observation t ime. We consider and discuss the importance of information contained in win dow widths on the time scales of an excitatory postsynaptic potential and H ering-Breuer reflex central delay.