Thermal distribution over the cortical surface of the rat brain under direct electrical stimulation

The dynamics of thermal diffusion from the dorsal brain surface were studied in white rats through the intact skull during acute experiments on immobilized or Nembutal anesthetized white rats using a new thermovisualization technique involving direct electrical stimulation of the cortex. Local cortical heating of a 1–4 mm site in the vicinity of the electrodes set in within 160 msec after presentation of a single stimulus, reaching a maximum of 0.2°C by the 2nd-5th sec and slowly decaying to the initial level by the 2nd-3rd min. At the same time, but at a somewhat slower rate, the symmetrically opposite local site in the other hemisphere heated up, followed by a small area in the ipsilateral motor cortex and a number of other zones. Depending on the dose administered, Nembutal inhibits and decelerates the development of these effects and localizes them, raises the minimum threshold of their occurrence, and prolongs their closed state. As anesthesia deepened, the primary heat focus near the stimulating electrodes persisted the longest. The mechanisms underlying local cortical thermal responses and the time parameters of these are discussed and compared with those of the dynamics of standard electrographic effects.