SOME THALAMIC AND CORTICAL MECHANISMS OF 



TASTE 



Robert M. Benjamin 



Laboratory of Neurophysiology and Department of Physiology, 

 University of Wisconsin Medical School, Madison, Wisconsin 



The first and major part of this paper will describe the thalamic taste 

 systems of the rat and squirrel monkey. Later, there will be some specula- 

 tions about the cortex. 



The research on the thalamus utilized two electrophysiological tech- 

 niques. The first mapped the thalamic projections of three tongue nerves, 

 the chorda tympani and the lingual branch of the IXth, both known to 

 contain taste fibers, and the lingual branch of the Vth which is presumably 

 devoid of taste. The nerves were stimulated electrically while the thalamus 

 was probed with a large microelectrode. The slow components of the 

 evoked response were attenuated by filters leaving a multi-unit burst of 

 spike discharges as the criterion response. Such maps do not give any 

 information about the functional characteristics of the system or for that 

 matter, cannot even identify the modalities involved, but they do provide 

 an indispensable guide for the second stage, microelectrode recording 

 from single units using adequate stimulation. The single unit technique 

 provides the only definitive localization and one form of information about 

 the functional characteristics of the system. 



Figure 1 shows the thalamic projection of the three tongue nerves in the 

 rat (Emmers, Benjamin and Blomquist, 1962). The electrode tracks are 

 reconstructed on six standard diagrams, most anterior at the top, most 

 posterior at the bottom. The broad terminal bars on the tracks mark the 

 extent of response, which in these experiments was a multi-unit discharge 

 recorded with a large microelectrode. The oval-shaped, morphologically 

 distinct subnucleus of the ventralis is outlined in the middle 4 diagrams. 



A comparison of these three maps reveals one important relationship. 

 The projection of the lingual nerve, presumably tasteless, is located more 

 laterally in the subnucleus than the projection of the two taste nerves. 

 They fill the most medial part. This spatial separation of nerve responses 

 suggested the possibility that there might also be a spatial separation of 

 modalities. Taste might project exclusively to this medial tip and have its 

 own nuclear territory independent of touch, temperature and other modali- 



309 



22 



