The thalamic mediodorsal nucleus receives input from thalamic and cortical regions related to vision

The mediodorsal nucleus of the thalamus (MD) so far has been regarded as being not closely connected to visual regions. Based on the method of retrograde transport of horseradish peroxidase, direct efferents to the cat's mediodorsal nucleus were demonstrated from two visual regions: from the ventral lateral geniculate nucleus and from parts of the visually responsive cortex. These projections were obtained following injections which covered most of MD, but also following injections which were restricted to medial or lateral parts and to the anterior two-thirds or merely to the center of MD. Projections from the ventral lateral geniculate nucleus arose mainly from its caudal portion; projections from the visually responsive cortex originated predominantly in area 20a, but labeled cells were also occasionally detected in areas 18 and 19.