Listeners weighting of cues for lateral angle: The duplex theory of sound localization revisited E. A. MacPherson & J. C. Middlebrooks (2002) HST. 723 Theme 3 March 30 th, 2009

Duplex Theory and Goals Duplex Theory (by Lord Rayleight 1907): Primary cue to lateral localization of low frequency tones is interaural time difference (ITD), while for higher frequency tones primary cue is interaural level difference (ILD). Reasons for revisiting the theory: 1. Listeners are sensitive to ITD in high-frequency complex sounds 2. ITD dominates listeners' judgment of location of broadband sound sources 3. Spectral-shape cues, provide essential cues to the vertical and front/back localization; they also vary with azimuth of the source, how would they contribute to lateral localization?

Bias Weights Goal: derive dimensionless weights that relate bias in response to imposed bias 1. Measure correspondence between lateral angle and physical cue. ITD: μs to degrees ILD: dB to degrees 2. Use (μs to degrees) and (dB to degrees ) correspondence to convert any shift in angular response to quantity expressed in units of manipulated cue.

Bias Weights Steps: 1.For a given angle natural ITD (or ILD) is determined. 2.Bias is imposed on top of natural ITD (or ILD) [Imposed Bias]. 3.Response angle is measured and is converted to corresponding ITD in μs (or ILD in dB) [Response or Observed Bias] 4.Response bias is plotted against Imposed bias and the slope of linear fit to the data is the listeners weighting for given cue

Experiment 1: Weighting of ITD and ILD cues in low- pass, high- pass and wideband noise Low – pass (0.5 – 2 kHz ): ITD weights > 0.8 for 8 out of 10 listeners & are significantly higher than ILD weights for all listeners. Wideband (0.5 – 16 kHz): both ITD and ILD have moderate to large weights, although ITD weighted more strongly than ILD for all listeners. High – pass ( kHz): ILD weights significantly more than ITD for all except 1 subject. Note: difference in weight given to ITD cue => subjects use onset and ongoing envelop information for ITD cues and vary in ability to do so

Experiment 2: Weighting of onset and ongoing envelop bias ITD cues in high- pass noise Onset/offset duration: 1 or 25 ms Envelop: flat or amplitude modulated. Weakening onset ITD cues (1ms vs 50 ms onset) Flat envelop: lengthening the onset had small impact on listeners with already small ITD, for subject S92 with large ITD, there was slight decrease. (C1 vs C2) AM envelop: lengthening the onset had little effect for all listeners. (C2 vs C3) Strengthening ongoing ITD cue (flat vs modulated amplitude) For both onset durations, addition of envelop modulation resulted in increase of weight of ITD cue (and slight decrease of ILD cue) [for 4 out 5 subjects]

Experiment 3: Weighting of onset interaural level spectrum (ISL) cue The stimulus was manipulated so that ITD corresponded to one location, but ILS corresponded to another one. Low weight given to the ILS cue in low-pass condition and higher weight in high-pass condition. Overall, ILS is ineffective cue for lateral angle. ILS bias weight deg/deg: linear fit to observed lateral angle response bias vs. imposed ILS bias measured in degrees. ILS bias weight dB/dB: gross ILD overall interaural energy difference integrated across the frequencies of the stimulus. (linear fit to observed vs. imposed bias) ILD bias weight dB/dB : from Experiment 1

Experiment 4: Weighting of onset near-ear spectral cue Monaural spectral cues in lateral localization are of interest. The stimulus was manipulated so that ITD and ILD corresponded to one location, but near ear directional transfer function (DTF) corresponded to another location. Weights for wideband and high-pass conditions were obtained and weights for all were <0.1, and in most cases not significant from 0. Monaural spectral cues play negligible role in lateral localization.

Summary & Conclusions Experiment 1: Results are in agreement with previous studies (and Duplex theory) and show that ITD is the dominant lateral angle cue for stimuli with low frequency components, while ILD is dominant lateral angle cue for stimuli with high frequency components. For wideband stimuli both cues have substantial weight. Note: this as been observed in anechoic conditions and might not apply in reverberant environments. Experiment 2: Both onset and ongoing envelop cues play role in sensitivity of some listeners to high frequency ITD. Highest bias cues are obtained when listeners can process ongoing envelop ITD cues, under such circumstances onset cue is of reduced importance. Experiment 3: ILS is ineffective cue for lateral angle. Experiment 4: Monaural spectral cues have little to no influence on perceived lateral angel.