The document summarizes the auditory pathway from the cochlea to the auditory cortex. It describes how sound is processed at each stage, including the cochlear nuclei, superior olivary complex, lateral lemniscus, inferior colliculus, medial geniculate body, and primary auditory cortex. It also discusses how the medial superior olive and lateral superior olive encode sound localization based on timing and intensity differences between the two ears. The auditory cortex shows asymmetry with language processing lateralized to the left hemisphere in most individuals.

1. Central auditory processing
Domina Petric, MD

2. Auditory pathway
• Action potentials are generated in the pheripheral processes of the spiral
ganglion neurons.
• Input travels the cochlear nerve and enters the brainstem in the lateral
aspect of the junction between the pons and the medulla: the cochlear
nuclei.
• The cochlear nuclei have the important function of distributing the auditory
information to additional brainstem centers, for example nucleus of the
lateral lemniscus.
• Nucleus of the lateral lemniscus gets input from the contralateral cochlear
nucleus.

3. Auditory pathway
• Nuclei of the superior olivary complex recieve inputs from bilateral
cochlear nuclei: comparison of signals that are derived from two ears.
• So the nucleus of the lateral lemniscus is concerned with monaural
signals (signals derived from contralateral ear).
• Inferior colliculus in the midbrain (tectum) is very important integrator
of ascending auditory information.
• Inferior colliculus seems to be the first place in the auditory pathway
where a complete map of the auditory world is computed (azimuths,
elevations).

4. Auditory pathway
Next station is medial geniculate complex of the thalamus.
From the medial geniculate complex inputs travel up to the primary auditory
complex in the superior plane of the superior temporal gyrus, in the lateral fissure.
Lateral fissure is between the temporal lobe and operculum (formed by parietal and
temporal lobe).

5. http://www.cochlea.eu
Superior olivary complex:
binaural signals.
Nucleus of the lateral lemniscus:
monaural signals.
Inferior colliculus: integrator of
ascending auditory information.

6. Sound localisation
• Medial superior olive: neurons become precisely tuned to the timing
difference between the encoding of sound in one ear and the other ear.
• Medial superior olive recieves inputs from both ears (both cochleas).
• For neurons in medial superior olive to fire there must be the simultaneous
arrival of input from the left and right ear.
• Neurons are organised in the next pattern: when there is right-sided shift of
the sound localisation (sound source is closer to the right ear), neuronal
pathway from the right cochlea to the medial superior olive is longer than
the neuronal pathway from the left cochlea so the inputs come from both
ears at the same time.

7. Medial superior olive
A B C D E
Neurons: A to E
Left ear
cochlea
Right ear
cochlea
Source of sound is closer to the right ear.
Neuron A is the target.
Neuronal pathway from the left ear cochlea to neuron A
is shorter than from the right ear cochlea.
Neuron A gets inputs
from both cochleas
at the same time and
fires the action potential.
If the sound source was closer to the left ear, target neuron would be E neuron.
The neuronal pathway from the right ear cochlea would be shorter.

8. Lateral superior olive
• Encodes the information about the location of the sound source based on
interaural intensity differences.
• Secondary nucleus is the source of inhibitory input to the lateral superior
olive.
• Secondary nucleus is called medial nucleus of the trapezoid body.
• Inputs from the cochlea cause the excitatory response of the lateral superior
olive cells.
• There are collaterals from the cochlear axons that project across the midline
and go to the contralateral medial nucleus of the trapezoid body.

9. Lateral superior olive
• Neurons in the medial nucleus of the trapezoid body grow short connections
to the other lateral superior olive and inhibits neurons in it.
• For example, there is sound source on the left side.
• Left lateral superior olive is excited whereas the right lateral superior olive is
inhibited so the intensitiy of the sound from left sided sound source will be
much more intensive in the left ear.
• The intensitiy differences arise because of the acoustic shadow cast by the
head.
• When the sound source is in the middle (between the eyes) there is no
intensity differencies.

10. Left ear
cochlea
Left
lateral
superior
olive
Left medial
nucleus of the
trapezoid body
Right medial
nucleus of the
trapezoid body
Right
lateral
superior
olive
Collateral axons cross the midline and send inputs
to the contralateral medial nucleus of the trapezoid body
(in this case right).
Axons then send inhibitory
input to the right lateral
superior olive.
When the sound source is on the left,
there will be excitatory response in the
left lateral superior olive: greater intensity
of the sound in the left ear.
There will be inhibitory
response in the right
lateral superior olive:
lower intensity of the
sound in the right ear.

11. Auditory cortex (A1)
Primary auditory cortex is a core region surrounded with higher order auditory
areas.
Heschl´s gyri are transverse gyri of the primary auditory cortex.
More anterior aspects of A1 encode low frequencies (from the apex of the cochlea)
and posterior aspects of A1 encode high frequencies (from the base of the cochlea).

12. Wikipedia.org
Frequencies from lower to
higher go in antero-posterior
direction.
Low frequencies are encoded
in the anterior portion of A1
and high frequencies in the
posterior portion of A1.
Low frequencies

13. Asymmetry of the auditory cortex
• Wernicke´s area is in the posterior aspect of the superior temporal gyrus and
serves for human speech understanding.
• Functional Wernicke´s area might extend much more including the inferior
parietal lobule and to the anterior direction (anterior pole of the temporal
lobe): there are several important nodes for each of different languages that
person knows.
• Functional Wernicke´s area is present in the left hemisphere for most of the
people.
• Contralateral (non-dominant) Wernicke´s area (usually on the right side) has
also important functions like PROSODY.

14. Asymmetry of the auditory cortex
• Prosody refers to the emotional content of speech.
• Planum temporale (the superior aspect of the posterior
temporal lobe) is much larger in the left hemisphere for most
of the people.
• Left lateral fissure is longer and straighter than on the right
side.
• People with perfect pitch ability have the larger asymmetry
of the planum temporale.

15. Asymmetry of the auditory cortex
While listening to the human speech, greater volume of the left
auditory belt is activated compared to the right.
While listening to the environmental sounds, there is virtually
no asymmetry in the activation of the two hemispheres.
While listening to the music, right hemispheric activation is
significantly greater than the left one.

16. Literature
https://www.coursera.org/learn/medical-
neuroscience: Leonard E. White, PhD, Duke
University
http://www.cochlea.eu