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A physiologically based model of interaural time difference discrimination
Kenneth E Hancock1, Bertrand Delgutte
1Eaton-Peabody Laboratory, Massachusetts Eye and Ear Infirmary, Boston, Massachusetts 02114, USA. keh@epl.meei.harvard.edu
Insights
Neural pooling across auditory neurons refines sound localization. A new model explains how pooling information from inferior colliculus cells improves interaural time difference (ITD) discrimination, especially for broadband noise.
Area of Science:
- Auditory Neuroscience
- Computational Neuroscience
- Acoustic Signal Processing
Background:
- Interaural time difference (ITD) is a crucial cue for low-frequency sound localization.
- Cells in the inferior colliculus (IC) represent ITD with specific best delays (BD).
- Previous models suggested single-cell ITD information was sufficient for midline localization.
Purpose of the Study:
- To develop a neural pooling model for interaural time difference (ITD) discrimination.
- To account for the degradation of ITD acuity with increasing ITD in human listeners.
- To investigate the role of neuronal pooling in auditory spatial processing.
Main Methods:
- A cross-correlation model was used to fit rate-versus-ITD curves of IC cells in anesthetized cats.
- Parameters from single-cell models constrained a population model of ITD discrimination.
- The model predicted ITD acuity for broadband noise and 500 Hz tones.
Main Results:
- The neural pooling model accurately predicted human ITD acuity for broadband noise as a function of ITD.
- Pooling responses across best frequency (BF) was essential for model accuracy.
- Internal delay systems alone were insufficient to predict ITD acuity for pure tones.
Conclusions:
- Neuronal pooling across auditory neurons is critical for accurate ITD discrimination.
- The model provides a framework for understanding how the brain processes spatial auditory information.
- Observed systematic variation of BD with BF in cats generalizes findings across species.
Abstract:
Interaural time difference (ITD) is a cue to the location of sounds containing low frequencies and is represented in the inferior colliculus (IC) by cells that respond maximally at a particular best delay (BD). Previous studies have demonstrated that single ITD-sensitive cells contain sufficient information in their discharge patterns to account for ITD acuity on the midline (ITD = 0). If ITD discrimination were based on the activity of the most sensitive cell available ("lower envelope hypothesis"), then ITD acuity should be relatively constant as a function of ITD. In response to broadband noise, however, the ITD acuity of human listeners degrades as ITD increases. To account for these results, we hypothesize that pooling of information across neurons is an essential component of ITD discrimination. This report describes a neural pooling model of ITD discrimination based on the response properties of ITD-sensitive cells in the IC of anesthetized cats. Rate versus ITD curves were fit with a cross-correlation model of ITD sensitivity, and the parameters were used to constrain a population model of ITD discrimination. The model accurately predicts ITD acuity as a function of ITD for broadband noise stimuli when responses are pooled across best frequency (BF). Furthermore, ITD tuning based solely on a system of internal delays is not sufficient to predict ITD acuity in response to 500 Hz tones, suggesting that acuity is likely refined by additional mechanisms. The physiological data confirms evidence from the guinea pig that BD varies systematically with BF, generalizing the observation across species.
