Perception and coding of high-frequency spectral notches: potential implications for sound localization

Area of Science:

• Neuroscience
• Auditory Perception
• Acoustics

Background:

• The human pinna creates high-frequency spectral notches (5-10 kHz) aiding vertical sound localization.
• A prevailing theory suggests these notches are encoded by the firing rate profiles of auditory nerve (AN) fibers.

Purpose of the Study:

• To challenge the rate-profile encoding hypothesis for spectral notches.
• To investigate the role of temporal coding in the AN for spectral notch discrimination.
• To explore the neural basis of inter-subject variability in spectral notch detection.

Main Methods:

• Review of existing human psychoacoustical data and computational models.
• New recordings from guinea pig AN fibers.
• "Ideal observer" analyses of neural responses to noises with and without spectral notches.

Main Results:

• Psychoacoustical evidence and models dispute the rate-profile encoding theory.
• AN recordings and ideal observer analyses suggest temporal discharge patterns are key for notch discrimination.
• Simulations indicate spike timing in the 4-7 kHz range is important, while ideal observer analysis points to sampling rates with 4-9 ms bins.
• Sensitivity to notches is higher in low- and medium-spontaneous rate AN fibers compared to high-spontaneous rate fibers.

Conclusions:

• The neural code for high-frequency spectral notches likely relies on temporal patterns in AN activity.
• Variability in vertical sound localization may stem from individual differences in the number of functional low- and medium-spontaneous rate AN fibers.