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Sound waves are transduced in the cochlea by hair cells arranged tonotopically — different frequencies activate different locations (high frequencies at the base, low at the apex). The auditory nerve projects to the cochlear nucleus in the brainstem, then through the inferior colliculus and medial geniculate nucleus of the thalamus to primary auditory cortex (A1) in the temporal lobe. Auditory processing is bilateral — each cortex receives input from both ears — which explains why unilateral cortical damage rarely causes complete deafness in one ear. Higher auditory areas extract speech, music, and spatial location.
The tonotopic map analogy (cochlea as a piano keyboard with place coding frequency) makes the organizing principle memorable. Contrasting auditory tonotopy with visual retinotopy reveals the shared principle of topographic cortical maps across sensory modalities.
From your study of sensory pathways, you know the general architecture: a sensory organ transduces physical energy into neural signals, which relay through subcortical structures before reaching the cortex for higher processing. The auditory pathway follows this template but has an unusually rich set of subcortical processing stations — more than any other sensory modality — each of which contributes something specific to how the brain ultimately makes sense of sound.
Transduction happens in the cochlea, the fluid-filled, spiral-shaped structure of the inner ear. The cochlea's genius is its physical design: it is effectively a frequency analyzer built out of anatomy. The basilar membrane running through the cochlea varies in width and stiffness along its length — stiff and narrow at the base, wide and flexible at the apex. When a sound wave enters, it creates a traveling wave along this membrane, and the location of maximum displacement depends on frequency: high frequencies peak near the base, low frequencies near the apex. Hair cells sitting on the basilar membrane at each location fire in response to their local peak. This spatial arrangement — where physical location encodes frequency — is called tonotopy, and it is the cochlea's primary output code. The auditory system encodes "what frequency?" as "where is the active cell?"
The auditory nerve carries these signals to the cochlear nucleus in the brainstem, which is the first stage of central processing. From there, projections travel bilaterally — crossing to the opposite side — through the superior olivary complex (critical for computing sound localization from tiny timing differences between the two ears), then up to the inferior colliculus in the midbrain, the medial geniculate nucleus (MGN) of the thalamus, and finally to primary auditory cortex (A1) in the superior temporal lobe. Each of these stations does real computational work: the superior olivary complex extracts spatial information, the inferior colliculus integrates timing cues, and A1 maintains a tonotopic map that mirrors the cochlea.
The bilateral routing is the key structural feature that distinguishes auditory from visual processing. In vision, each eye's signal stays largely ipsilateral through early processing, so a lesion to left visual cortex causes right visual field blindness. In audition, the crossing happens early and extensively — by the time signals reach A1, each cortex is receiving input from both ears. This is why unilateral damage to auditory cortex causes difficulty with sound localization and subtle perceptual deficits rather than deafness in one ear. Beyond A1, auditory processing splits into two streams: a ventral "what" pathway that identifies sounds (voices, words, music) and a dorsal "where" pathway that tracks their spatial location — a functional organization that parallels the ventral and dorsal visual streams you will encounter in higher cortical processing.