Spectrally specific temporal analyses of spike-train responses to complex sounds: A unifying framework
Fig 11
The harmonicgram can be used to quantify the coding of time-varying stimulus features at superior spectrotemporal resolution compared to the spectrogram.
Harmonicgrams were constructed using ϕ(t) for the same two AN-fiber pools described in Fig 10. PSTH bin width = 50 μs. A 9-Hz bandwidth was employed to low-pass filter the demodulated signal for each harmonic. The data were collected from one chinchilla in response to the speech stimulus, s3. Stimulus intensity = 65 dB SPL. A 500-ms segment corresponding to the voiced phrase “amle” was considered. (A, B) Spectrograms constructed from the average ϕ(t) for the low-CF pool (A) and from the medium-CF pool (B). (C, D) Average harmonicgrams for the same set of fibers as in A and B, respectively. Warm (cool) colors represent regions of high (low) power. The first-formant contour (F1 in A and B, F1/F0 in C and D) is highlighted in purple. The second-formant contour (F2 in A and B, F2/F0 in C and D) is highlighted in red. Trajectories of the fundamental frequency (black in A and B, right y-axis) and the formants were obtained using Praat [67]. (E, F) Harmonicgram power near the first formant (purple) and the second formant (red) for the low-CF pool (E) and the medium-CF pool (F). Harmonicgram power for each formant at any given time (t) was computed by summing the power in the three closest F0 harmonics adjacent to the normalized formant contour [e.g., F1(t)/F0(t)] at that time. The noise floor (NF) for power was estimated as the sum of power for the 29th, 30th, and 31st harmonics of F0 because the frequencies corresponding to these harmonics were well above the CFs of both fiber pools. These time-varying harmonicgram power metrics are spectrally specific to F0 harmonics and are computed with high temporal sampling rate (same as the original signal). This spectrotemporal resolution is much better than the spectrotemporal resolution that can be obtained using spectrograms.