Sensory information handling is an essentially nonstationary process even under a periodic stimulation. We show how the time evolution of ridges in the wavelet spectrum of spike trains can be used for quantification of the dynamical stability of the neuronal responses to a stimulus. We employ this method to study neuronal responses in trigeminal nuclei of the rat provoked by tactile whisker stimulation. Neurons from principalis (Pr5) and interpolaris (Sp5i) show the maximal stability at the intermediate (50？ms) stimulus duration, whereas Sp5o cells “prefer” shorter (10？ms) stimulation. We also show that neurons in all three nuclei can perform as stimulus frequency filters. The response stability of about 33% of cells exhibits low-pass frequency dynamics. About 57% of cells have band-pass dynamics with the optimal frequency at 5？Hz for Pr5 and Sp5i, and 4？Hz for Sp5o, and the remaining 10% show no prominent dependence on the stimulus frequency. This suggests that the neural coding scheme in trigeminal nuclei is not fixed, but instead it adapts to the stimulus characteristics. 1. Introduction The rodent vibrissae system is one of the most used experimental models for the study of the sensory information handling. Rats actively use their whiskers to detect and localize objects and also to discriminate surface textures. By sweeping whiskers at rates of 5–20？Hz, they can localize objects within a few whisking cycles or even with a single vibrissa [1]. Thus relatively short temporal, but not spatial mechanical information, dominates in the object detection. Similarly, the texture mechanical coding, attributed to the whisker resonance (i.e., the vibration amplitude across the whisker array codifies the texture [2]), occurs in awake rats and shapes natural whisker vibration. However, it seems that textures are not encoded by the differential resonance. Instead, slip-stick events contribute to a kinetic signature for texture in the whisker system, which suggests the presence of a temporal structure in neural spike trains under these experimental conditions [3]. Thus the efficacy of the sensory information transmission and processing in the time domain resides in the possibility of multiple cells to generate coherent responses to a stimulus, which constitutes the neural code. Though there was a long discussion about what type of the neural code is employed by each individual neuron or neuron group, the growing experimental evidence shows that a same neuron may use different coding schemes (see reviews in [4, 5]). The temporal correlation of multiple cell
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