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The auditory thalamus (or medial geniculate body - MGB) is a structure composed of a number of nuclei containing neurons that pass the auditory signal up the auditory pathway to the primary auditory cortex. The MGB plays a major part in sound processing, with a particular role in frequency, intensity and temporal integration.
Functional anatomy of the medial geniculate body
Ascending fibers from the inferior colliculus (IC) project to the ipsilateral MGB (in red). Neurons from both MGBs also receive input from the contralateral IC due to the commissure between the two colliculi. This organisation means that most MGB neurons are responsive to binaural signals.
Projections IC - MGB - Auditory Cortex
The mammalian MGB can be subdivided into three main regions:
Dorsal (1a), medial (1b) and ventral (1c). The dorsal and medial regions receive connections from the dorsal and external nucei of the IC (2) and the ventral nucleus receives connections from the central nucleus of the IC (2).
Reminder: the IC receives fibers from the cochlear nuclei (3, in green), the superior olivary complex and the lateral lemniscus. Both IC are linked by a commissure (x2), meaning that each neuron projecting to the MGB sends binaural information.
- Neurons in the ventral region (1b) are involved in frequency analysis, whilst at the same time integrating the intensity and latency of the auditory signal. Their complex electrophysiological activity is similar to that of IC neurons. This region is tonotopically organised, and certain neurons response to a specific frequency, but only in a particular intensity range. These neurons project mainly to layer IV of the primary auditory cortex.
- The dorsal region (1a) has no layers or tonotopic organisation; here, neurons do not have characteristic frequencies, and they project to the secondary auditory cortex (layer IV). Most of these neurons have a multimodal role: they respond to stimuli from different sensory modalities, and have a role in sensory integration.