Figure 1.

Two example auditory neurons that show sensitivity (left) and invariance (right) to stimulus intensity. A, The song waveform. B, Stacked spike time rasters aligned in time with increasing amplitudes lower in the panel. A 6 dB increase in stimulus amplitude corresponds to a doubling of the amplitude. Each stimulus was presented randomly interleaved, and the responses to successive stimuli are separated by a horizontal line. C, Overlay of the peristimulus time histograms for the lowest (in red) and highest (in black) stimulus amplitudes. D, Plot of mean spike rate shows a significant increase in spike rate over the range of stimulus amplitudes (p < 0.001, 1-way repeated-measures ANOVA; error bars indicate SD of the mean) for neuron 1 and less (although still significant; p < 0.001, 1-way repeated-measures ANOVA; error bars indicate SD of the mean) variation in spike rates for neuron 2.