THALAMIC VPM NUCLEUS IN THE BEHAVING MONKEY .3. EFFECTS OF REVERSIBLEINACTIVATION BY LIDOCAINE ON THERMAL AND MECHANICAL DISCRIMINATION

1. The present study evaluates the necessity of the ventroposterior me dial thalamic nucleus (VPM) for discrimination of the intensity of nox ious heating, innocuous cooling, and innocuous tactile (airpuff) stimu lation of the maxillary skin. 2. Two rhesus monkeys were trained to de tect small differences (< 1.0-degrees-C) in the intensity of noxious h eat (near 46-degrees-C) and innocuous cold (near 30-degrees-C) as well as differences in the force of an airpuff applied to the skin over th e maxilla. As a control the monkeys also detected small differences in the intensity of a white light. Lidocaine hydrochloride (2%) was micr oinjected into regions of thalamus where single-unit recordings had id entified neuronal responses to the noxious heating and/or cooling stim uli. The effectiveness of the anesthetic blockade was monitored by mul tiunit recordings using microelectrodes positioned 1-3 mm from the ori fice of the injection cannula. The monkey's ability to detect near-thr eshold changes in stimulus intensity was compared before and after eac h injection. 3. During six experimental sessions, single injections of 1-4 mul lidocaine near the dorsomedial border of VPM did not signific antly alter the monkey's ability to detect small changes in the intens ity of noxious heat, cool, airpuff, or visual stimuli despite neurophy siological evidence that spontaneous neuronal activity was blocked wit hin parts of VPM. 4. During three experiments, dual simultaneous micro injections of lidocaine (delivered through 2 microcannulae separated b y approximately 1 mm) resulted in profound deficits in noxious heat di scrimination, with lesser deficits in cool and airpuff discrimination; visual discrimination was never altered. Monitoring of adjacent micro electrodes revealed that although activity ventral to the injection si tes was blocked, activity in medial thalamic nuclei, implicated in noc iceptive processing, was probably not altered by these injections. 5. These data suggest that VPM is important for the perception of noxious and innocuous thermal stimuli as well as for the perception of tactil e stimuli. However, considering the ineffectiveness of small single mi croinjections of lidocaine, it appears that some critical proportion o f VPM must be inactivated to disrupt thermal or tactile discrimination , possibly because of overlapping receptive field properties of neuron s in different areas of the nucleus.