Overview
This lecture covers the organization and function of the auditory cortex, the processing pathways for sound in the brain, and the consequences of damage to these structures.
Auditory Cortex Structure & Organization
- The cochlea sends tonotopically organized (pitch-mapped) information through the auditory nerve.
- Auditory signals pass from the vestibulocochlear nerve into the cochlear nuclei in the brainstem.
- The information travels through several brainstem nuclei, including the inferior colliculus.
- The signal then goes via the brachium of the inferior colliculus to the medial geniculate nucleus (MGN) of the thalamus.
- From the MGN, auditory information reaches the primary auditory cortex, located on the transverse temporal gyrus inside the Sylvian fissure (lateral sulcus).
Functional Mapping of the Auditory Cortex
- Primary auditory cortex can be mapped using functional MRI, showing regions sensitive to high, medium, and low pitches.
- Vocalization-sensitive areas are mapped (blue), and speech-sensitive areas are found more laterally (pink).
- Processing of complex sounds (e.g., speech) expands laterally into higher auditory association areas.
Effects of Damage to Auditory Areas
- Lesion to one primary auditory cortex leads to difficulty localizing sound and subtle hearing loss, but not total deafness.
- Bilateral damage to both hemispheres' auditory cortex eliminates conscious sound perception, but acoustic reflexes may remain due to intact brainstem.
- Damage to auditory association cortex (superior temporal gyrus/sulcus) can cause central hearing loss—sound is detected but difficult to interpret.
- Left hemisphere lesions (e.g., Wernicke's area) impair language comprehension; sound is heard but words are not understood.
Key Terms & Definitions
- Tonotopic organization — Spatial arrangement of where sounds of different pitch are processed in the brain.
- Primary auditory cortex — The main brain area for processing basic auditory information, located on the transverse temporal gyrus.
- Medial geniculate nucleus (MGN) — Thalamic relay for auditory information.
- Acoustic reflexes — Involuntary muscle responses to sudden sounds, mediated by brainstem.
- Central hearing loss — Hearing loss due to brain (not ear) damage, affecting interpretation of sounds.
- Wernicke’s area — Region in the left hemisphere responsible for language comprehension.
Action Items / Next Steps
- Review anatomical pathways of the auditory system, especially the route from cochlea to auditory cortex.
- Study the effects of cortical vs. subcortical lesions on hearing and language comprehension.