Unit Discharges in the Mediodorsal Nucleus of the Squirrel Monkey Evoked by Electrical Stimulation of the Olfactory Bulb

The thalamic mediodorsal nucleus (MD) of the squirrel monkey was explored with microelectrodes for units responsive to electrical stimulation of the olfactory bulb. All responsive units were localized throughout the magnocellular subdivision of MD, most (88%) ipsilateral to the stimulated bulb. Other parts of MD were unresponsive. Latencies to the first poststimulus spike ranged from 4 to 50 msec with a median value of 14 msec. In preparations with both frontal poles ablated eliminating the possibility of stimulus spread to orbitofrontal cortical neurons and subsequent anti- or orthodromic activation of Mc cells, the shortest response latency was 6 msec. Thus relatively direct and extensive connections exist between the olfactory bulb and one cytoarchitectonic subdivision of this prominent ‘association’ nucleus in a microsmatic primate.