This suggests that birds can use the multiple acoustic cues available to them for the task, but they are best when all cues are available. This is surprising, because phase changes take place over milliseconds, whereas envelope changes take place over much slower timescales. When only phase information is present, birds perform almost as well as they do on natural motifs. When spectral structure is removed, but the amplitude envelope remains intact, birds show a decrease in performance but are still able to discriminate most syllable reversals. Both the waveform of the motif and the spectrogram are shown.īirds are best at temporal reversals of syllables when all acoustic cues are present, in the natural song motif (shown in blue). In each case, the last syllable G is reversed. Examples of a syllable reversal in a natural motif and both synthetic motifs are shown below. Thus, there was an upward sweep of phase across frequencies that became a downward sweep when the harmonic was temporally reversed. However, there were phase changes across frequencies such that the starting phase of each harmonic increased. The amplitude envelope and spectrum remained the same across time. A second type of synthetic motif (Schroeder motif) consisted of Schroeder harmonic waveforms in which only fine structure changed when the waveform was temporally reversed (i.e., no pitch or short-term loudness cues). This type of motif removes any spectral structure of individual syllables, and the main cue that remains is amplitude envelope (short-term loudness cues). One type of synthetic motif (noise motif) consisted of the motif amplitude envelope filled with random noise. Syllable reversals were tested in natural motifs, as well as synthetic motifs in which certain acoustic cues were removed. Below is a video showing the experimental setup and a bird completing this task. By recording which keys the bird pecked during the changed motifs, we could determine which changes the bird could detect, how quickly he could detect these changes, and which changes the bird seemed unable to detect. If he did not detect a change, then he was to peck the left LED. If the bird detected a change from the original motif, he was trained to peck the right (green) report LED in order to receive a food reward. At random times, a target stimulus (the changed motif) was played to the bird instead of the original motif. The original song motif was played as a repeating background, and the bird had to peck the left (red) observation LED as this motif is being played. The testing setup consists of a cage with two LEDs (red and green) placed on the wall and a speaker overhead through which sounds are played to the bird. Birds were tested on discrimination tasks, in which they had to discriminate between two sounds: the original song motif and a song motif with a temporally reversed syllable.
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