Central auditory processing Domina Petric, MD

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.

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).

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).

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

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.

Sound wave travels a longer distance to the left ear. Source of sound is closer to the right ear. Sound localisation 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. Sound wave travels a longer distance to the left ear. Left ear cochlea Neuron A is the target. Neuronal pathway from the left ear cochlea to neuron A is shorter than from the right ear cochlea. Medial superior olive A B C D E Neuron A gets inputs from both cochleas at the same time and fires the action potential. Right ear cochlea Neurons: A to E

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.

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.

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

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).

Frequencies from lower to higher go in antero-posterior direction. 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

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.

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.

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.

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