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.