Estimating auditory neuronal dynamic range using a fitted function

To obtain the dynamic range of an auditory afferent, the neuron's firing rate is plotted versus stimulus level, and the dynamic range is taken as the difference between the threshold for evoked firing, and the level at which firing rate saturates. Those dynamic range endpoints are typically defined in terms of the neuron's spontaneous firing rate and its maximum firing rate, according to a plurality of schemes, each of which depends on user-chosen sets of numerical criteria. The dynamic ranges predicted by some of these schemes are compared for the first time, and the resulting estimates can differ by a factor of 2. A step can be taken towards standardizing the measurement of neuronal dynamic range, if dynamic range is incorporated into a rate-level function as a parameter. To build this function, it is first assumed that the neuron's rate-level response reaches half its maximum at a level half-way between the threshold and the level at saturation, i.e. at threshold plus half the dynamic range. Then the firing rates at threshold and at threshold plus dynamic range are defined according to the most popular of the endpoint schemes. The resulting equation produces credible estimates of neuronal properties when fitted, and correctly predicts the behavior of the slope of the empirical rate-level plot [McGee, 1983. M.S. thesis, Creighton University; Ohlemiller et al., 1991. J. Acoust. Soc. Am. 90, 274-287]. Thus, despite not being deterministic, the new equation has remarkable predictive power. When two of the rate-level functions are added and weighted, the resulting equation fits sloping-saturating data better than any functions presently employed.