Figure 1.
Models predicting the distribution of best ITDs.
(A) Illustration of an ITD from a sound source (B) Illustration a sound source near the interaural axis (horizontal dotted line) having maximum ITD. In A–B red and blue lines are the shortest paths from the sound source (black dot) to the ears. (C) A illustration of a model rate-vs-IPD function and best IPD. IPD is ITD as a proportion of the period of the sound frequency. (D) Optimal-coding model: complex frequency dependent distribution. (E) Jeffress model: homogeneous distribution or unimodal distribution of best IPDs at all frequencies, largely within the physiological range. (F) Two-channel model, bimodal at all frequencies. In Figure (D–F), solid black line, maximum IPD, white line, limit of IPD sensitivity, color, percentage of neurons at a given frequency with that best IPD. The ordinate is the sound frequency normalized with respect to the reciprocal of the maximum ITD (i.e. sound frequency as a proportion of 1/maximum ITD).
Figure 2.
Best delay distributions for small and intermediate-sized mammals.
(A) Predicted distributions of best IPDs by the optimal-coding model for the kangaroo rat (max. ITD, ∼105 µs, highest frequency with ITD sensitivity ∼1500 Hz). IPD is ITD is a proportion of the period of the sound frequency on the ordinate. Black line, maximum IPD that limits the physiological range, white line, limit of IPD-sensitivity, color, number of best IPDs in an IPD bin for a given frequency band. The predictions uses the same size frequency and IPD bins as the corresponding data in Figure 2B, and each frequency band has been scaled to have the same maximum as the corresponding frequency band in the data. (B) Best ITD data from the Crow et al. [18] kangaroo rat SOC study (59 data points successfully extracted), converted from ITD to IPD, mirrored, and then re-plotted as a 2D histogram using 300-Hz frequency bins and 0.05 cycle IPD bins. Format a Figure 2A. (C) Best IPD data from Figure 2B collapsed over frequency, solid blue bars are mirrored data, empty light blue bars unmirrored data. Inset figure plots data from the kangaroo rat inferior colliculus (IC) in the midbrain [25], showing number of neurons with a given characteristic delay (similar to best ITD). The abscissa on the inset figure spans −7.5 ms to 7.5 ms, with an ordinate spanning 0 to 14 neurons. (D) Predicted distributions of best IPDs by the optimal-coding mode for the Mongolian gerbil (max. ITD, ∼120 µs, highest ITD-sensitive frequency ∼1500 Hz). Format as Figure 2A. (E) Best ITD data from Pecka et al. [28] Mongolian gerbil medial superior olive study (38/40 data points successfully extracted), converted from ITD to IPD, mirrored, and re-plotted as a 2D histogram using 300 Hz frequency bins and 0.05 cycle IPD bins. Format as Figure 2A. (F) Data in Figure 2E collapsed over frequency. Format as Figure 2C. (G) Predicted distributions of best IPDs by the optimal-coding model for the guinea pig (max. ITD, 245–330 µs, highest frequency with ITD sensitivity ∼1800 Hz). Format as Figure 2A. (H) 260 best IPDs from the guinea pig inferior colliculus mirrored and re-plotted as a 2D histogram using 250 Hz frequency bins and 0.05 cycle IPD bins. Format as Figure 2A. (I) Predicted distributions of best IPDs by the optimal-coding model for the cat (max. ITD, 250–325 µs, highest frequency with ITD sensitivity ∼2000 Hz). Format as Figure 2A. (J) Combined best IPD data from the Hancock and Delgutte [19] and the Joris et al. [37] cat inferior colliculus studies (86/107 and 193/219 data points successfully extracted respectively) converted from ITD to IPD, mirrored, and re-plotted as a 2D histogram using 250 Hz frequency bins and 0.05 cycle IPD bins. Format as Figure 2A.
Figure 3.
Best delay distributions for large primates.
(A) The predicted distributions of best IPDs by the optimal-coding model for the human (max. ITD, ∼690 µs, highest frequency with ITD-sensitivity ∼1500 Hz). Color, number of best IPDs in an IPD bin for a given frequency band (200 best IPDs per band modelled), bin sizes as Figure 1D, otherwise format as Figure 2A. (B) The predicted distributions of best IPDs by the optimal-coding model for the macaque (max. ITD, 470–575 µs, max. ITD-sensitive frequency ∼2000 Hz). Format as Figure 2A. (C) Distribution of 1280 best IPDs, recorded from macaque auditory cortex, mirrored and then plotted as a 2D histogram using 200-Hz frequency bins and 0.05 cycle IPD bins. Format as Figure 2A.