(A) Cells were characterized by their activity during each of the response epochs: *spontaneous*, *early*, *late*, and *off*, each 50 ms long. *Spontaneous* epochs cover spontaneous activity before the stimulus, *early* and *late* epochs cover first and second half of the stimulus duration (100 ms), respectively, and *off* epochs cover 50-ms period after stimulus termination. In frequency space, individual trials were grouped into one-octave-wide bins, and averaged to provide a firing rate value for each octave bin. This figure shows a spike raster plot for an example neuron (with a sustained excitatory response), where each row represents a single trial, and each dot marks the occurrence of a spike. Shown are responses to 1–40 kHz tones (60 dB SPL, left ordinate.) Individual trials were grouped into five spontaneous, and 15 evoked response bins (right ordinate.) Note that the top quarter of an octave is not included in any of the bins.

(B and C) Firing rates of most neurons were low and followed a lognormal distribution.

(B) Frequency histogram of nonzero spontaneous firing rates in individual octave bins (*n* = 567 octave bins, from 145 neurons). Each neuron contributed a maximum of four or five data points (because each neuron had four or five octave bins per epoch). The filled arrow shows the position of the median spontaneous firing rate, and the open arrow shows the position of the mean spontaneous firing rate.

(C) The distribution of spontaneous firing rates (dots) was fit with a lognormal distribution (gray line), the mean and variance of which were given by the mean and variance of the original firing rate distribution (see ). The lognormal distribution appears as a normal distribution on a (semi-) logarithmic scale. The error bars show 95% confidence intervals determined by bootstrapping.

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