AUDITORY II: CENTRAL MECHANISMS

I. The Central Auditory Pathway:  an Overview

1. Auditory Brain Regions
• Medulla:  Cochlear Nucleus, the Dorsal and Ventral Stria, Superior Olivary Complex, and projections of the Lateral Lemniscus.
• Pons:  Nucleus of the Lateral Lemniscus.
• Midbrain:  Inferior Colliculus.
• Thalamus:  Medial Geniculate.
• Cortical Radiations.
• Primary and Secondary Auditory Cortex:  Superior Temporal Gyrus.
• From the hair cell to layer four of the Auditory Cortex there are a minimum of five synaptic junctions and a maximum of seven.
• Why does this system need so many synaptic relays?  Time is a critical dimension that is analyzed by this system, and the synapses may spread time out along the axis of the brainstem and thus assist in this analytic process.
2. Tonotopic Organization
• Recall the cochlea was organized from base to apex in a high to low frequency direction.  The translation from frequency to a spatial domain is called tonotopic organization.
• The spatial mapping of a sensory sheet onto the nervous system is a fundamental property of the nervous system.
• A topographic representation of frequency is seen at all levels of the auditory pathway.
• The mapping is always orderly from low to high frequencies.
• A narrow band of frequencies is often mapped as a sheet of neurons; layers of sheets are tonotopically organized.
3. Sound Localization Cues
• Sound travels at 344 meters/second in air.
• This is the propagation velocity and is independent of frequency.
• There are two cues for the localization of sound; the time of arrival difference between the two ears; the intensity difference between the two ears.
• If a sound were directly in front of the left ear the interaural time difference (ITD) between the left and right ear would be 600 microseconds; as the sound moved around the head in a horizontal plane, the ITD would be 0 microseconds when the source was directly in front of the nose.
• Differences in ITD can be used to derive the location of a sound source in space.
• The ITD associated with each location in the horizontal plane is the same regardless of the frequency at that location.
• The interaural intensity difference (IID) is more complicated.
• At low frequencies where the wavelength of sound is long, the sound wave diffracts about the head and sound levels at the left and right ear are the same regardless of location.
• Above 2.0 kHz sound begins to reflect off the head because the wavelength is small with regard to head size; hence the sound is more intense in the ipsilateral than the contralateral ear.
• The IID is ineffective at low frequencies and we rely on the ITD.
• At high frequencies both cues are used.
• The detection of ITD is about 10 microseconds while IID is about 2 dB.
4. The Cochlear Nucleus
• There are three divisions: DCN, PVCN, and AVCN
• Each incoming auditory nerve fiber trifurcates and sends an axon to each subdivision.
• The CN receives only monaural inputs, no binaural processing is found here.  Six or seven different PST histograms have been identified in the CN and they have been associated with specific types of cells in the nucleus.
• Some of these histograms are quite complex indicating neural interactions.
• The DCN is a layered structure and interneurons are found here; convergent from different auditory nerve inputs are found on the same DCN cell.
• Complex tuning curves are found for the first time in the DCN.
• These tuning curves exhibit inhibitory sidebands.
• The presence of an acoustic "center/surround" organization serves to enhance the contrast across the boundary of the tuning curve.
• Interneurons are not found in the PVCN or AVCN and the tuning curves of these cells appear to be "primary like".
• Rate intensity functions from the DCN also show a non-monotonic characteristic indicating interneuron activity.
5. The Superior Olivary Complex
• The SOC contains about 8 nuclei, both sensory and motor relays.
• Only two are of interest to us; the Lateral and Medial Superior Olivary nuclei.
• These are the first locations in the auditory path to receive inputs from both the left and right ears.
• The LSO is sensitive to intensity differences between the two ears.
• The MSO is sensitive to the time difference between the two ears.
• Each cell in the right LSO receives an input from the right AVCN and from the left AVCN via the medial nucleus of the trapezoid body; the neuron from the MTB to the LSO is in effect an inhibitory interneuron.
• Thus, ipsilateral input to the LSO cell is excitatory while contralateral input is inhibitory.
• The LSO cell is called an EI cell; if the ipsilateral excitatory input is stronger (because the stimulus at the ipsi ear is more intense) than the contralateral inhibitory input, then the LSO cell will be activated.
• The degree of activation depends on the intensity difference between the two ears.
• The ipsi- and contralateral inputs to the MSO cell are both excitatory.
• The MSO cell is often referred to as an EE cell.
• Output from the MSO cell will only occur when both inputs are simultaneous.
• The axon from left or right AVCN to the MSO cell has different lengths because this nucleus is located off the midline; this difference in axon length acts as a delay line.
• The delay line offsets the time of arrival difference at the two ears to produce a simultaneous input to the MSO cell.
• Both the LSO and MSO are tonotopically organized.
• As you would expect the LSO cells code for higher frequencies while the MSO cells are predominantly tune to the low frequencies.
6. Inferior Colliculus
• The DCN projects to the contralateral IC directly or via the nucleus of the lateral lemniscus
• The MSO and LSO project fibers to both the ipsi- and contralateral IC.
• Thus, the IC is a cite for convergence of information processed for time and intensity differences at the two ears and for complex tuning curves containing inhibitory sidebands; in essence, edge detection meets localization!
• The main area for afferent processing is the central nucleus of the IC.
• Here the cells are organized in layers called sheets; the sheets are the frequency or tonotopic dimension of the nucleus.
• Within each sheet there appears to be a segregation of the EE and EI inputs as they converge on IC cells along with the input from the DCN.
• The IC is also implicated in many forms of auditory reflexes and in particular the auditory startle response.
7. Medial Geniculate
• This is the thalamic auditory relay.
• Fibers project upward to the thalamus from the IC, and downward as descending efferent control fibers from the cortex.
• Output fibers ascend the cortical radiations and project into layer four of the primary and secondary auditory cortex; efferent fibers leave the MG and project to more peripheral nuclei and provide control over ascending information.
• Only two cell types are found in MG; "Principle" cells and the "Golgi" type 2 cells.
• Tuning curves are generally complex with two or more tip regions.
• Rate-intensity functions are most often non-monotonic.
• A unique property of many cells is hypersensitivity to interaural intensity differences; a 2dB difference may produce a change in firing rate equal to 80% of the cell's dynamic range.
8. Auditory Cortex
• There are three auditory regions: one primary (AI) and two secondary (AII and EP).
• Tonotopic organization found in all regions.
• In cat the frequency organization runs in dorsal to ventral iso-frequency bands; the bands are further organized from low to high frequency in a rostral to caudal plane.
• All of the coding properties seen in lower brain areas can be found in the cortex.
• Cortical columns are related to EE and EI cells.
• Cortical cells appear particularly sensitive to complex sounds and rhythmic patterns of sound presentation.
• Cortical cells sensitive to the directional motion of a sound stimulus have been identified; motion detection appears to be a unique property of the cortex.
• Cortical tuning curves may have multiple CFs indicating convergence of peripheral inputs.
• Animals can hear at normal threshold levels and can discriminate successfully between tone intensities and frequencies after ablation of the auditory cortex.
• The general importance of the cortex seems to be its role in processing complex sounds.