Coding of deflection velocity and amplitude by whisker primary afferent neurons: implications for higher level processing

Within the rat whisker-to-barrel pathway, local circuits in cortical layer IV are more sensitive to the initial timing of deflection-evoked thalamic r esponses than to the total number of spikes comprising them. Because thalam ic response timing better reflects whisker deflection velocity than amplitu de, cortical neurons are more responsive to the former than the latter. The aim of this study is to determine how deflection velocity and amplitude ma y be encoded by the primary afferent neurons innervating the vibrissae. Res ponses of 81 extracellularly recorded trigeminal ganglion neurons (60 slowl y and 21 rapidly adapting) were studied using controlled whisker stimuli id entical to those used previously to investigate the velocity and amplitude sensitivities of thalamic and cortical neurons. For either slowly (SA) or r apidly adapting (RA) neurons, velocity is reflected by both response magnit ude, measured as the total number of evoked spikes/stimulus, and initial fi ring rate, measured as the number of spikes discharged during the first 2 m s of the response. Deflection amplitude, on the other hand, is represented only by the SA population in their response magnitudes. Thus, in both popul ations initial firing rates unambiguously reflect deflection velocity. Toge ther with previous findings, results demonstrate that information about def lection velocity is preserved throughout the whisker-to-barrel pathway by c entral circuits sensitive to initial response timing.