Figure 2
(Experiment 1) Photodiode recordings of uniform sinusoid and AM signals revealed in the frequency domain (using FFT) and two-dimensional frequency spectrum (using HHSA, see Supplementary Material and methods). (a) The photodiode response to 14 Hz sinusoidal signal. (b) The FFT was applied to 2-second photodiode recordings of sinusoidal flicker showing the strong amplitude spectrum at 14 Hz. (c) The HHS showed the fundamental 14 Hz carrier frequency and shows no envelope modulations. (d) The waveform of 2 Hz envelope modulating 14 Hz AM stimulus captured by the photodiode. (e) The FFT analysis revealed 13 and 15 Hz peaks. However, the Fourier spectrum did not show the amplitude modulation frequency. (f) The HHSA provided separately the amplitude spectrum of a 2 Hz fam modulating 14 Hz fc and 14 Hz carrier in two-dimensional frequency spectra. In subfigures (c,f) the x-axis denotes the fast-changing carrier intra-mode frequency variations (fc), and the y-axis denotes the slow-changing envelope inter-mode frequency variations (fam). At 0.5 Hz y-axis, this x-axis is the summed amplitude of carrier frequencies over time. The frequency axes represent dyadic frequency, in which the edges of dyadic frequency bins are defined by the formula 2n (where n = −1, 0, 1, 2, 3, 4, 5).
