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

Abstract

Within the rat whisker-to-barrel pathway, local circuits in cortical layer IV are more sensitive to the initial timing of deflection-evoked thalamic responses than to the total number of spikes comprising them. Because thalamic response timing better reflects whisker deflection velocity than amplitude, cortical neurons are more responsive to the former than the latter. The aim of this study is to determine how deflection velocity and amplitude may be encoded by the primary afferent neurons innervating the vibrissae. Responses of 81 extracellularly recorded trigeminal ganglion neurons (60 slowly and 21 rapidly adapting) were studied using controlled whisker stimuli identical to those used previously to investigate the velocity and amplitude sensitivities of thalamic and cortical neurons. For either slowly (SA) or rapidly adapting (RA) neurons, velocity is reflected by both response magnitude, measured as the total number of evoked spikes/stimulus, and initial firing rate, measured as the number of spikes discharged during the first 2 ms of the response. Deflection amplitude, on the other hand, is represented only by the SA population in their response magnitudes. Thus, in both populations initial firing rates unambiguously reflect deflection velocity. Together with previous findings, results demonstrate that information about deflection velocity is preserved throughout the whisker-to-barrel pathway by central circuits sensitive to initial response timing.