Auditory perception transforms pressure waves traveling through the air into meaningful experiences — music, speech, environmental sounds, and the spatial layout of the auditory world. While vision has historically dominated perception research, auditory perception involves equally sophisticated computational challenges and relies on specialized neural machinery optimized for analyzing temporal patterns at extraordinary precision.
From Sound Waves to Neural Signals
Sound enters the ear canal as variations in air pressure and is transduced by the cochlea, a spiral-shaped organ in the inner ear. The basilar membrane within the cochlea performs a frequency analysis: high frequencies maximally displace the base, and low frequencies maximally displace the apex. This tonotopic organization — frequency mapped to place — is preserved throughout the auditory pathway from cochlea to auditory cortex.
Position along the basilar membrane is approximately logarithmic with frequency, matching the perceptual scale of pitch.
Inner hair cells transduce basilar membrane vibration into neural impulses. The auditory nerve carries this information to the cochlear nucleus, then through the superior olivary complex, inferior colliculus, and medial geniculate nucleus of the thalamus before reaching the primary auditory cortex (A1) in the superior temporal gyrus.
Pitch Perception
Pitch — the perceptual correlate of sound frequency — is determined by both place information (which region of the basilar membrane is activated) and temporal information (the timing pattern of neural firing). Place coding dominates for high frequencies, while temporal coding (phase-locking of neural firing to the stimulus waveform) is more important for low frequencies. The missing fundamental phenomenon — hearing a pitch corresponding to a fundamental frequency even when it is absent from the stimulus — demonstrates that the brain computes pitch from the harmonic structure of a sound rather than from any single frequency component.
Sound Localization
The auditory system localizes sounds using binaural cues. Interaural time differences (ITDs) — the difference in arrival time at the two ears — provide information about horizontal position for low frequencies. Interaural level differences (ILDs) — the difference in intensity at the two ears due to head shadow — are more useful for high frequencies. Neurons in the superior olivary complex are specialized for computing these binaural differences. Elevation and front-back distinctions rely on spectral cues created by the filtering properties of the pinna (outer ear).
In reverberant environments, sound reaches the ears via multiple paths (direct and reflected). The precedence effect ensures that the auditory system localizes sound based on the first-arriving wavefront rather than later reflections. For delays up to about 5-10 ms, the reflected sound is perceptually fused with the direct sound. This mechanism is essential for accurate localization in rooms and explains why we are not confused by the complex pattern of reflections in everyday listening.
Auditory Cortex
The primary auditory cortex (A1) is organized tonotopically, with neighboring neurons responding to similar frequencies. Beyond A1, belt and parabelt areas process increasingly complex auditory features. Analogous to the visual system's "what" and "where" streams, auditory processing may divide into a ventral stream for sound identification and a dorsal stream for sound localization, though this division is less firmly established than in vision.
Temporal Processing
The auditory system has exceptional temporal resolution, capable of detecting gaps of only 2-3 milliseconds. This precision is essential for speech perception (distinguishing "ba" from "pa" depends on voice onset time differences of tens of milliseconds) and for music perception (rhythm and meter depend on precise temporal patterns). The superior temporal resolution of audition compared to vision reflects the physical nature of sound as a temporal signal.