Examples of fractal waveforms (blue) and the related pitch sequences (red, rounded to the nearest integer) based on inverse Fourier transforms of f^–n power spectra, with exponent n = 0 (top), n = 0.9 (middle), n = 1.5 (bottom). Equitempered pitch (ten-note octave, ranging over two octaves, resulting in 21 possible pitches, with ordinal indices 0 to 21 corresponding to 300–1,200 Hz) is denoted on the y-axis, time (in seconds) on the x-axis. Entropy is largest for the top pitch sequence and decreases as exponent n increases.

Depicted are three consecutive trials with pairs of pitch sequences drawn from entropy levels 5, 1, and 2. Coloured boxes indicate the presentation of a pitch sequence, black boxes indicate the acquisition of a scan volume, and the white gaps between scans denote silent periods. Identical colours within one trial indicate that the two pitch sequences of a pair are identical (trial 1), whereas slightly different hues indicate trials where the second pitch sequence was different from the first one, but drawn from the same entropy level (trials 2 and 3). There were three possible delays within and between trials: for example, trial 1 has a within-trial delay of two scans before the presentation of the second pitch sequences, and a between-trial delay of three scans before the beginning of the subsequent trial. Trial 2 has within-trial and between-trial delays of one scan, etc. Visual cues as depicted above the schematic of the design were presented to guide participants through the experiment. Participants received immediate feedback (correct/incorrect) after giving their response.

Entropy increase (p < 0.001, uncorrected) (red) and [sound–silence] (p < 0.05, FWE corrected) (blue) contrasts superimposed on a horizontal section (z = −10) of participants' normalised average structural scan that covers IC and MGB (note that z coordinates vary slightly between maxima and arrows, therefore they are only indicative of the exact location). Normalised mean percent BOLD signal change (±SEM) at the local maxima in left and right IC (bottom) and MGB (top) is plotted for the five levels of exponent n.

Areas showing an increase in BOLD signal (p < 0.001, uncorrected for multiple comparisons across the brain) as a function of increasing entropy (red) and areas that responded to sound in general ([sound–silence] contrast, p < 0.05, FWE corrected) (blue) rendered on a tilted (pitch: −0.4) axial section of participants' normalised average structural scan. Normalised mean percent BOLD signal change (±SEM) at the local maxima in the left and right PT (bottom) and HG (top) is plotted for the six levels of exponent n.

Areas showing an increase in BOLD signal (p < 0.005, uncorrected for multiple comparisons across the brain) as a function of increasing entropy (red) and areas that responded to sound in general ([sound–silence] contrast, blue) rendered on a tilted (pitch: −0.5) axial section of participants' normalised average structural scan. Normalised mean percent BOLD signal change (±SEM) at the local maxima in the left and right PT (bottom) and central HG (cHG, top) is plotted for the five levels of exponent n. See Figure S1 for corresponding plots of BOLD signal in medial and lateral HG.

Areas that show stronger activation (p < 0.05, FWE corrected) for retrieval and comparison than encoding, rendered on coronal (y = 22 and y = −48, top left and right, respectively), and sagittal (x = 6, bottom left) sections of participants' normalised average structural scan. See also Table S1 for exact MNI coordinates.