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Mechanosensory neurons are often classified as either rapidly adapting or slowly adapting. We examined response decay (adaptation) during constant deflection of the vibrissae with quantitative, repeatable, ad hoc measures. We found that first-order vibrissa-activated neurons of the fifth ganglion exhibit a variety of adaptation rates that appear to be distributed continuously between the rapidly and slowly adapting extremes. Also, adaptation rate is influenced markedly by stimulus magnitude. We found no evidence for a dichotomy within the more slowly adapting neurons on the basis of discharge regularity. Threshold tuning curves were used to evaluate vibration sensitivity. Both the best frequencies and 1:1 discharge thresholds for sinusoidal stimulation ranged over two orders of magnitude and were continuously distributed. First-order vibrissa-activated afferents exhibit a broad variety of response patterns to constant-velocity stimulation. The pattern of discharge varied both as a function of time during constant-velocity (ramp) deflection and as a function of stimulus velocity. Although information about the parameters of a stimulus may be conveyed by any of several features of the response pattern, it appears that few if any neurons function as "pure" encoders of any particular stimulus parameter. We examined quantitatively the relationship between discharge rate and both velocity and amplitude of vibrissa deflection with the aid of a computer-based curve-fitting procedure. We found that about half the observed rate-level functions were best described by a power function; the remainder were best fit by a logarithmic function. The parameters of the best-fitting functions varied widely and continuously, emphasizing further the diversity of coding properties of the rat's vibrissa afferents. Rate-level curves for stimulus magnitude generally exhibited saturation; some were nonmonotonic. None were described adequately by either a logarithmic function or a power function.

We examined stimulus-response relationships of vibrissa-activated mechanosensory neurons of the rat's fifth (trigeminal) ganglion. Single-unit activity was recorded with tungsten microelectrodes. The vibrissae were deflected with a variety of parametrically controlled stimulus waveforms. We found that the receptive field of each vibrissa-activated neuron consisted of a single vibrissa. Few, if any, unambiguous examples of spontaneous activity were observed in these neurons. Even if true spontaneous activity was present, its observed incidence was low, as were the measured discharge rates. Thresholds of individual neurons were usually quite discrete; often a 1-2% increase in pulse magnitude (angular displacement) above a level to which the neuron did not respond caused it to discharge on every trial. The distribution of thresholds for the sample was continuous with a median of about 1 degree and a range of over three orders of magnitude. The most sensitive neurons responded to deflections of less than 0.1 degrees. Many neurons responded to a single suprathreshold pulse with more than one spike. We found no consistent relationships among the thresholds of the additional evoked discharges of an individual neuron other than that the total number of evoked spikes either increased or stayed the same, but never decreased, as stimulus magnitude increased. About one-third of the neurons examined had velocity thresholds below 3 degrees/sec. Above that value, thresholds were distributed continuously throughout a range of over three orders of magnitude. The median velocity threshold was about 100 degrees/sec. The broad and continuous distributions of both magnitude and velocity thresholds suggest that a population of vibrissa-activated neurons can code stimulus strength smoothly and continuously over a wide range, even though individual neurons may be poorly suited to do so.