Auditory cortex Flashcards
CENTRAL AUDITORY PATHWAYS
- First central synapse is in the cochlear nuclei of the medulla
- Complex synaptic network in brainstem
- Projection to primary auditory cortex is via thalamus (medial geniculate nucleus (MGN) also called medial geniculate body (MGB) or complex
Explain the coding of sound location
- Organization of brainstem auditory nuclei suggests complex processing
- Ascending projections of the auditory brainstem (above cochlear nuclei) show high degree of bilateral connectivity
- Best understood function is sound localization
Spatial localization of sound source depends on….
- Interaural time differences (sound arrives to 1 ear before the other; timing difference in activation of auditory nerve afferent)
- Interaural intensity differences
- Requires comparison of sounds from the two ears
Sound source localization in the superior olivary nuclear complex: time differences
- Medial superior olive (MSO) receives inputs from left and right
- AV cochlear nucleus
- Longest interaural time differences ~ 700 μs
- Psychophysics indicates differences can be detected as short as 10 μs
- Time differences used for computing location for sound frequencies up to 3 kHz
Detection of ITDs requires…?
=precise spike timing
- AN afferents can phase lock in low-mid frequency range
- Class of cells in anteroventral cochlear nucleus (bushy cell) show even stronger phase locking
- AVCN projects to MSO, potentially providing precise timing information
Sound source localization in the superior olivary nuclear complex: intensity
- Lateral superior olive (LSO) receives excitatory inputs from ipsilateral AV cochlear nucleus
- Receives inhibitory inputs from contralateral AV cochlear nucleus (indirectly, via MNTB)
- Above 3 kHz sound is increasingly absorbed by head
- Creates acoustic shadow on opposite side (i.e., lower intensity)
- LSO neurons will receive maximal excitation and minimal inhibition from sounds on the same side (and the reverse for sounds on the opposite)
Mammalian auditory cortex
- Location of auditory cortex in ferret, cat, and macaque
- Two or three primary (core) fields can be distinguished based on direct connections from MGB
- Second order (belt) areas
- Interact widely with temporal and prefrontal areas (recognition and action planning)
Dorsal and ventral auditory processing streams
- Anatomical connectivity studies suggest parallel processing streams
- Dorsal: core, belt, parabelt, parietal and prefrontal areas
- Ventral: core, belt, parabelt, temporal and prefrontal areas
- ‘What’ and ‘where’ functions analogous to visual system?
Location of human auditory cortex
- Auditory cortex occupies dorsal and lateral surface of superior temporal gyrus
- Primary auditory cortex is in the area of Heschl’s gyrus on the superior temporal plane, buried in the Sylvian fissure
Call selectivity of neurons in marmoset auditory cortex
- Characteristic marmoset twitter call
- Certain neurons in A1 found to respond to playback of natural call, but not when speeded up, slowed down or played backwards
- Not simply responding to components at preferred frequency?
Neurons in marmoset, but not cat, auditory cortex are selective for marmoset twitter calls
- Mean spike rate per natural call plotted against mean rate per reversed call
- Neurons in cat A1 may also respond, but do not show selectivity for natural over time- reversed calls
- However, ferrets trained on marmoset calls (e.g., reward on natural not reversed) still did not show selectivity between calls
- Assembly to acoustic features of stimulus?
Ferrets trained to marmoset calls
- The two responses are not significantly different when counting spikes per trial over a relatively long time window
- However sampling spikes over shorter time windows shows the spiking patterns to the two calls are distinguishable
Precise delineation of human auditory cortex is still not clear
- Recent fMRI work has identified two fields in primary auditory cortex
- Probably homologous to A1 and R in non-human primates
- Possible to show further tonotopic fields but no clear agreement over layout and borders
- Not clear what dimension beyond tonotopy may be mapped within a field
Species-typical vocalizations have meaning
- Vervet monkey alarm calls: different calls denote different predators
- Playback of recorded calls elicited behavior according to predator class
What v where processing in auditory cortex
- Seven call types presented in 7 positions
- compared sensitivity to call type with sensitivity to call location
