FIGURE 5. Categorical responses were evoked by changes in specific features of individual notes in the syllable.

(a-c) A subset of 10 categorically responsive HVC_X neurons (2 birds) was further tested to probe the acoustic stimulus features that directed categorical responsiveness. (a) In all cells, computer-generated duplicates of natural replacement notes were sufficient to evoke a categorical response (p = 0.47; Wilcoxon signed rank test), establishing the validity of further testing using synthetic stimuli. Synthetic stimuli in which note duration was varied while either (b) the rate of frequency modulation (FM; p = 0.13) or (c) the frequency bandwidth (BW; p = 0.43; Wilcoxon signed rank test) was held constant evoked responses like those evoked by natural replacement notes, implicating note duration as the salient song feature driving categorical responses of HVC_X neurons.