(A) Top: linear IPD curve (constant ITD) with ITDg = 5 ms (group ITD, see ‘Materials and methods’). Bottom: If the IPD is nonlinear (gray curve), then it can be locally approximated with an affine function (black plain and dotted line). This introduces a non-zero IDI (Interaural Diffraction Index, see ‘Materials and methods’). (B) an amplitude modulated tone (top panel, black: envelope, gray: signal) models the left monaural signal. The right signal is passed through model head-related transfer functions (HRTFs) with either ITDg = 5 ms, and IDI = 0 cycles (middle panel) or ITDg = 5 ms and IDI = 0.5 cycles (bottom panel). The right signal (blue: signal, green: envelope) is delayed by the amount of the ITDg, while the fine structure of the signal undergoes an additional phase shift equal to the IDI (see text). (C) interaural cross-correlation functions in the two cases IDI = 0 (top) and IDI = 0.5 (bottom). The envelope peak (green segment) is unaffected by the IDI, while the fine structure peak (blue segment) is shifted by an amount (in phase) equal to the IDI. (D) ITDp (phase ITD, see ‘Materials and methods’) and ITDg for one position of the cat HRTFs (top panel), IDI for the same position (bottom panel) as a function of frequency. (E) Top: Distribution of ITDp (dashed line) and ITDg (solid line) in the cat over all positions and frequencies. Bottom: Distribution of IDI.