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Freezing has many advantages over other destructive methods in stereotactic surgery, because freezing lesion has flexibility, reproducibility and predictablity with a sharp spherical delimination, and inhibitory area around it in the C.N.S. The purpose of this experiments as to investigate those charactaristics of the freezing lesions and reversibility of the cooling area. Apparatus used for making the freezing lesions was Cooper's cryosurgery unit, which utilizes liquid nitrogen as a refrigeration sourse. The cryoprobe with a 3 mm caliber was placed upon the suprasylvian gyrus of the cat, and was inserted to the required depth as well. Recording electrode and microthermisters are placed at the point with various distances from the tip of probe. The size of freezing lesions was chiefly depended on the freezing temperature, because it grew gradually larger but became constant if the freezing time was over 3 minutes at each temperature down to -100°C. There was no difference of the size of lesion between the surface and the depth. After the cessation of freezing procedure the size of the lesion was assured again by accounting the melting time on time-temperature record of the unit. The freezing point of the cat brain was about -2°C and supercooling, which means freezing of tissue by the temperature of -2°C or below without ice formation, was also observed. The temperature fall gradient was steep in the cooling area. The transcallosal response (TCR) was used as a method for indication of the cortical activities in the cooling area. The contralateral cortex was stimulated with bipolar, silver, ball-tipped electrodes. The recording electrode was concetric bipolar with diameters of 0.1 mm-0.2 mm. The TCR showed a biphasic wave with positive and negative portion. Its average latency was a few milliseconds, and the duration of the response was 30-45 msec. In general, the negative portion of the wave was the more unstable to the deflection of the temperature. As the temperature on the recording point fell down, negative portion decreased its amplitude and increased its latency and duration gradually. The positive portion decreased the amplitude with a constant latency. The response was completely abolished, when the temperature of the recording point reached to 15°C from the body tempera-

15. Physical and Physiological Studies on Localized Freezing in the Cat Brain