Evaluation of Postspike Changes in Neuronal Excitability by Comparing Usual and Shuffled Autocorrelation Functions
Responses of single neurons in the dorsal (cochlear) medullary nucleus and midbrain auditorycenter (torus semicircularis) of grass frog (Rana temporaria) to tonal signals amplitudemodulatedby repeating segments of low-frequency noise were studied. The autocorrelation function of thisresponse to whole presentation and also shuffled autocorrelation function were derived. In thelatter, the impulse response to each segment was correlated with responses to all other segmentsexcluding its own. After the required normalization, this function differs from the initial autocor-relation in absence of postspike changes in excitability. The dependence of the ratio of these twofunctions on the delay directly demonstrates the time course of the postspike change in excitabilityof the investigated cell. The majority of the second-order neurons, located in the dorsal nucleusof the medulla oblongata were characterized only with the brief intervals of absolute and relativerefractoriness. However, even in this nucleus, cells with marked facilitation of excitability immediatelyafter the refractory period were seen. In the torus semicircularis, neurons with a rather complexdynamics of the postspike excitability were found. For these cells the comparatively long postspikedecrease in excitability was usually interrupted by intervals in which the sensitivity of the neuronwas significantly higher than normal. Our data demonstrate that spike generation has a markedeffect on the subsequent activity in brainstem auditory units. This effect could play an importantrole in the formation of the temporary pattern of neuronal responses to auditory signals.