Neuroscience/Objectives/Lecture 34

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Auditory system II

Understand the brainstem mechanisms for sound localization.

In humans, sound localization depends upon interaural time, intensity (level), and phase differences. Sound localization in animals with small heads does not depend upon interaural time and phase differences because these differences are so small.

Interaural time differences are processed by the medial superior olivary nucleus (MSO; homologous to the nucleus laminaris in birds). The MSO is predominantly composed of neurons with low characteristic frequencies. These neurons receive bilateral input from the cochlear nuclei, with fibers from the contralateral cochlear nuclei decussating in the trapezoid body. Neurons of the MSO behave as coincidence detectors, requiring simultaneous bilateral input in order to fire action potentials. The axons extending from the ipsilateral and contralateral cochlear nuclei to the MSO are of varying lengths: the shortest ipsilateral axons synapse most anteriorly, while the shortest contralateral axons synapse on neurons in the posterior MSO. Because of the anatomy and physiology of the MSO and its cochlear afferents, sounds most strongly activate the MSO contralateral to their source. Additionally, greater time differences are processed by the neurons most anterior in the MSO.

Interaural intensity differences are processed by the lateral superior olivary nucleus (LSO). LSO neurons receive bilateral afferents from second-order neurons in the ventral cochlear nuclei. These projections are organized tonotopically, with dorsal LSO regions receiving low frequency, and ventral LSO regions receiving high frequency afferents. Input from the ipsilateral cochlear nuclei are excitatory, while neurons of the contralateral cochlear nuclei reach the LSO via inhibitory interneurons in themedial nucleus of the trapezoid body (MNTB). Sound from one side of the body activates the ipsilateral LSO and inhibits the contralateral LSO.

Identify the organization of the nuclei and axonal pathways by which auditory information reaches the thalamus and cortex.

Fibers from the LSO decussate and travel to the contralateral inferior colliculus (IC) and thence (without decussating) to the medial geniculate body of the thalamus via the branchium of the IC. The superior colliculi also receive some afferents from the superior olivary complex and are the first structures to begin processing binaural input. (Birds have a nulceus mesencephalicus lateralis dorsalis (MLD) that is homologous to the IC.)

The ventral division of the medial geniculate projects to the primary auditory cortex, while the dorsal division projects to secondary auditory cortex and association cortices.

Identify the location of the primary and secondary auditory cortices.

• Primary auditory cortex = Heschl's gyrus = BA 41
• Secondary auditory cortex = BA 42
• Association areas = BA 22 (includes Wernicke's area)

Describe the organization of the auditory cortex and the response properties of auditory cortical neurons.

The primary auditory cortex is tonotopically organized, with low frequency sounds most anterior and high frequency sounds most posterior. This can be related to the tonotopic map of the cochlea: the cochlear apex, which is tuned for low frequency sounds, corresponds to the anterior portion of primary auditory cortex; the cochlear base, which is tuned for high frequency sounds, corresponds to the posterior primary auditory cortex.

During the generation of thalamocortical projections, ephrin A establishes the tonotopy of the primary auditory cortex. Broadly, it contains two cell types: EE and EI cells (the first letter indicates what effect contralateral afferents have on the cell; the second letter indicates the effect of ipsilateral afferents). EE and EI cells are also found in the inferior colliculus.

Surrounding the primary auditory cortex are the belt areas of the secondary auditory cortex. Their borders are determined based on the reversal of the orientation of the cochleotopic axis. Neurons of the secondary auditory cortex respond to noise bands and complex sounds, and project to higher auditory association areas (including parabelt areas) in the temporal lobe (eg, Wernicke's area) and prefrontal cortex (eg, Broca's area). Similar to visual processing, auditory information may be processed via two separate streams that allow for independent sound localization and identification.

The response of cortical neurons to auditory input allows the perception of complex sound. Cortical neurons have highly tuned receptive fields, much more tuned so than the cochlear hair cells. Some neurons respond only to combinations of two or more frequencies. Cortical neurons also respond differentially to sound intensity. Some neurons have monotonic rate-level functions and increase their firing rate as sound intensity increases. Others are non-monotonic and fire most rapidly in response to a particular sound intensity. The response of cortical neurons to frequency is independent of sound intensity: increasing the intensity does not change the neurons' tuning curves. However, neurons' response to frequency and intensity can both be modulated by attention.

Understand the relationship between the auditory cortex and the language areas in the brain.

Human language depends upon Wernicke's area (BA 22), the angular gyrus (BA 39), and Broca's area (BA 44,45). Right-handed individuals typically have a dominant speech processing center in the left hemisphere, with an enlarged planum temporale containing secondary auditory and Wernicke's areas.

Speech contains acoustic patterns: constant frequencies, frequency modulations, and noise bands (or bursts), the latter not having a clear frequency structure. Phonemes are simple sounds made of constant frequencies (vowels) and frequency modulation patterns. Phonemes are combined to form words.

Speech can be analyzed with power spectrums and spectrograms. A power spectrum plots sound intensity versus frequency and indicates which frequencies are present in a sound. A spectrogram plots sound frequency versus time, giving an indication of how frequency changes over time (frequency modulation). Natural sounds are made of simple sounds and combinations of simple and complex sounds.

As discussed earlier, cortical neurons respond to sounds in combinations of time and frequency. ERP, PET, and fMRI studies have identified tonotopic maps in primates and humans.

The secondary auditory cortex (ie, belt areas) process the properties of speech in distinct pathways. The anterolateral belt area is is most active in object recognition and speech decoding, while the caudolateral area is most important for sound localization:

Describe music and how it is processed by the auditory system.

Like speech, music is made of sounds, whose frequency and intensity must be deciphered and assigned meaning. We perceive music as melody and rhythm. It is processed on the right side of the brain.

Define amusia, William's syndrome, tinnitis, and cochlear implant.

Amusia

Inability to appreciate music. May result from genetic abnormalities or specific brain lesions.

William's syndrome

Despite a limited IQ (eg, due to hydrocephalus), individuals may be especially gifted to reproduce melodies and sing in many different languages.

Tinnitus

Ringing of the ears due to hair cell death. Following injury to hair cells, there is reorganization of the auditory cortex, with surrounding areas growing into the areas associated with damaged hair cells. These grown-in cortical neurons respond to a greater range of stimuli, resulting in a tonic ringing sound in the ears.

Cochlear implant

The cochlear implant is a surgical intervention used in the therapy of deaf individuals. Deafness results from the death of hair cells, but in deaf individuals the remaining auditory apparatus (including spiral ganglion afferents to the hair cells, apparently) is intact. The cochlear implant bypasses the hair cells, directly stimulating the afferents of the spiral ganglion, which projects to the cochlear nuclei via CN VIII. The cochlear implant operates using a sound processor connected to a microelectrode array that is implanted directly into the cochlea. The processor decodes natural sounds into digital signals that are transmitted to the microelectrode array. Sounds of different frequencies will differentially activate regions of the microelectrode array in a tonotopically organized way; this is analogous to the tonotopic map of the cochlea of individuals with intact hearing. High frequencies would map to the base of the microelectrode array and low frequencies to the apex.