Ten Microseconds Between the Ears: How the Brain Computes Sound Location
When a sound originates from one side, the acoustic wave reaches one ear a fraction of a millisecond before the other. The human brainstem detects arrival disparities as small as 10 microseconds — an interval 100 times shorter than the duration of a single action potential, corresponding to an acoustic path in air of just 3.4 millimetres.
1. Headphone Psychoacoustic Test: Pure Temporal Delay
Listen with stereo headphones. Both audio channels play at identical volumes: the perception of left-to-right movement inside your skull is generated entirely by microsecond timing offsets between your eardrums.
2. Jeffress Delay Lines in the Medial Superior Olive (MSO)
Lloyd Jeffress's (1948) model explains how the brain converts time disparities into spatial maps: axons from the ventral cochlear nucleus act as biological delay lines. MSO neurons fire maximally only when action potentials from both ears arrive synchronously at the cell soma.
When a sound originates on the left (e.g. at -30°, corresponding to a 261 µs lead), the left ear pulse starts earlier and must travel along a longer axonal segment. The right ear pulse starts later but travels a short segment, meeting simultaneously at the tuned target detector neuron.
3. Geometric Calculator: The Woodworth Spherical Model
Compute your personalized Interaural Time Difference (ITD) and spatial angular resolution based on head dimensions using the Woodworth-Schlosberg formula: Δt = (r / c) · (θ + sin θ).
- Equivalent Head Radius (r) 8.91 cm
- Maximum Delay at 90° in Air 668 µs
- Delay at Selected Azimuth (ITD) 266 µs
- Extra Acoustic Path Distance 91.2 mm
- Midline Angular Resolution (at 10 µs) 1.10°
- Max Delay Underwater (at 1,480 m/s) 155 µs
4. Four Stages of Binaural Processing
How the auditory pathway achieves sub-millisecond temporal precision exceeding the duration of nerve impulses.
| Anatomical Stage | Key Structure | Physiological Mechanism | Temporal Resolution |
|---|---|---|---|
| 1. Inner Ear | Hair cells in the Organ of Corti | Phase locking to the mechanical acoustic wave cycle for frequencies below 1,500 Hz. | 0.2–0.5 ms |
| 2. Cochlear Nucleus | Endbulbs of Held | Giant synapses with large glutamate release and ultrafast AMPA receptor kinetics. | < 0.2 ms |
| 3. Medial Superior Olive | Bipolar MSO neurons | Jeffress delay lines, coincidence detection, and Kv1.1 channels suppressing repetitive firing. | 10–20 µs |
| 4. Lateral Superior Olive | LSO and MNTB neurons | Interaural Level Difference (ILD) processing for frequencies above 3,000 Hz via cross-glycinergic inhibition. | 0.5–1.0 dB |
Scientific Method & References
Lord Rayleigh's (1907) Duplex Theory establishes that horizontal sound localization relies on two separate cues: at low frequencies (below 1,500 Hz), where the acoustic wavelength (λ > 23 cm) exceeds the head diameter, the brain relies on Interaural Time Differences (ITD). At high frequencies (above 3,000 Hz, λ < 11 cm), the head casts an acoustic shadow of up to 20 dB, enabling Interaural Level Differences (ILD).
In water, the speed of sound rises to approximately 1,480 metres per second (4.3 times faster than in air). As a result, the maximum interaural delay collapses to under 160 microseconds, while direct skull bone conduction bypasses middle ear isolation, eliminating directional hearing in submerged humans.
- Jeffress, L. A. (1948). A place theory of sound localization. Journal of Comparative and Physiological Psychology, 41(1), 35–39.
- Rayleigh, Lord (1907). On our perception of sound direction. Philosophical Magazine, 13(74), 214–232.
- Woodworth, R. S., & Schlosberg, H. (1954). Experimental Psychology. Holt, Rinehart and Winston, New York.
- Yin, T. C., & Chan, J. C. (1990). Interaural time sensitivity in medial superior olive of cat. Journal of Neurophysiology, 64(2), 465–488.
- Carr, C. E., & Konishi, M. (1990). A circuit for detecting interaural time differences in the brain stem of the barn owl. Journal of Neuroscience, 10(10), 3227–3246.
- Grothe, B., Pecka, M., & McAlpine, D. (2010). Mechanisms of sound localization in mammals. Physiological Reviews, 90(3), 983–1012.