Extent of the ipsilateral representation in the ventral posterior medial nucleus of the monkey thalamus.

Single and multiunit mapping was used to determine the extent of the representation of ipsilateral structures in the ventral posterior medial (VPM) nucleus of the thalamus in cynomolgus monkeys. The extent of the VPM occupied by terminations of afferent fibers arising in the ipsilateral principal trigeminal nucleus was also determined by anterograde transport of horseradish peroxidase. Both methods indicate that most of the medial half of VPM is occupied by the ipsilateral representation. This is much larger than previously suspected. Units in the medial half of VPM have small, well localized receptive fields on the ipsilateral side of the lower lip, tongue and palate, in the ipsilateral cheek pouch and on the ipsilateral teeth. The representation is largest for the ipsilateral side of the tongue and the cheek pouch. Most units in the lateral half of VPM have small, contralateral receptive fields. Few units in VPM have bilateral receptive fields. VPM is clearly distinguishable by cytochrome oxidase (CO) staining. Anteroposteriorly elongated, CO-positive aggregations correspond to elongated aggregations of units with the same or closely similar receptive fields, especially in the medial, ipsilateral representation.