NOTES ON BASIC HEARING NEUROSCIENCE
Order of Neural Processing:
1) Organ of Corti
2) Cochlear Nucleus
3) Superior Olivary Complex
4) Inferior Colliculus
5) Medial Geniculate Body
6) Auditory Cortex
- Organ of Corti
- 95% of nerve connections are afferent (they send messages to the brain)
- 95% of these connect to inner hair cells (1 to 20 nerve fibers for each hair cell)
- 5% are efferent (they receive feedback from the brain)
- Mostly to outer hair cells
- Neural Coding
- Place Principle
- Location on basilar membrane correspnds to frequency
- Temporal Code
- Hair cells are phase locked to audio input (Below about 4kHz)
- A single nerve may not be able to catch every cycle, but there are several nerves that work together
- Rate Code
- Encoding of Intensity Information
- Small changes in intensity results in large changes in rate
- For full dynamic range, several neural fibers of varying thresholds are used
- Cochlear Nucleus
- Distributes audio to various parts of the brain and performs basic processing
- Anterior Ventral Cochlear Nucleus
- Sends info to Superior Olivary Complex for binaural processing
- Posterior Ventral Cochlear Nucleus
- Poorly understood, but it sends info to the Inferior Colliculus
- Dorsal Cochlear Nucleus
- Carries info to opposite side of brain
- This pathway also includes many efferent nerves
- Superior Olivary Complex
- Processes interaural time delays (ITD) and interaural level differences (ILD)
- Wired to separate sounds according to left/right origin rather than left/right ear
- Medial Superior Olive (Deals with low frequencies - ITD)
- Lateral Superior Olive (Deals with high frequencies - ILD)
- Each neuron receives input from both ears
- ipsilateral (near) ear creates excitatory input
- contralateral (far) ear creates inhibitory input
- Inferior Colliculus
- Thought to provide a spatial map of the auditory environment
- Sensitive to changes in spectrum like amplitude or frequency modulation
- May be responsible for virtual pitch
- Switchboard for startle reflex, ocular reflexes
- Affects attention and learning
- Medial Geniculate Body
- Part of thalamus
- Ventral MGB
- relays frequency, intensity, binaural info to auditory cortex (10% monaural, 90% binaural)
- Medial MGB
- detects relative intensity and duration of sounds
- Dorsal MGB
- responses are non-specific (some cells respond to complex stimuli, other respond to other senses)
- Auditory Cortex
- Divided into 6 layers
- Areas have been found which relate to virtual pitch perception, timbre discrimination, spatial localization, and even noise filtering
- Inputs
- Medial Geniculate Body
- AI (contralateral)
- AII
- Other cortical areas
- Outputs
- Sensory Association Areas
- -Parietal Lobe
- -Temporal Lobe
- Speech Areas
- -Broca's Area
- -Wernicke's Area
- Medial Geniculate Body
- Inferior Colliculus
| |
|