The magnocellular division of the human medial geniculate nucleus preferentially responds to auditory transients

The medial geniculate nucleus (MGN), the auditory relay in the thalamus, is composed of three anatomical subdivisions: the ventral, dorsal and medial or magnocellular division. The functional differences among these nuclei have not been studied in humans, and in particular, the function of the magnocellular division is poorly understood in mammals in general. We anatomically segmented the MGN using proton-density-weighted magnetic resonance images (MRI) and measured the functional responses of the MGN to sustained and transient sounds, using functional MRI (fMRI). We observed that voxels in the ventromedial portion of the MGN, corresponding to the magnocellular division, exhibited a strong preference to transient sounds, whereas the remainder of the MGN showed no preference between sustained and transient sounds. We concluded that the magnocellular neurons in the MGN parallel the magnocellular neurons in its visual counterpart, the lateral geniculate nucleus (LGN), and constitute an information stream specialized for encoding stimuli dynamics. Significance statementThe medial geniculate nucleus is the auditory relay in the thalamus. It is composed of three anatomical divisions, of which the function of the magnocellular division is poorly understood. We show, using functional magnetic resonance imaging (fMRI) in humans, that the magnocellular neurons are strongly activated by transient auditory stimuli, similar to the magnocellular neurons in the lateral geniculate nucleus, the visual thalamic relay, which are rapidly adapting and specialized to encode visual transients. These results confirm that the auditory system represents stimuli using parallel information streams, employing similar encoding strategies as in other sensory modalities.