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unf~ and bipolar electrode arrangement, and electrode size or area. Since the parameters are interrelated in their effectiveness, one can draw from this study differing parametric points 4 for adequate stimuli that may be suitable for different experimental purposes; perhaps even more importantly, one finds that certain parametric regions may not be suitable at all for certain purposes. In addition, the influence of some aspects of the alerting procedures, and their correlated statesbf EEG-alpha rhythm, on cerebral and peripheral sensory thresholds was analyzed. METHODSSubjects and general procedure. The subjects for this study were selected from among cooperative patients who were to undergo neurosurgical treatment for dyskinesias, mainly parkinsonism. Pneumoencephalography and trephination is carried out in a first stage under general anesthesia; the actual therapy is carried out at another time with the patient unanesthetized (13, 26) in order to permit appropriate activation and inactivation tests to be conducted on the responses of the target tissues (2). It was during these second stage operations that the present studies were carried out. The use of the Leksell stereotaxic instrument permits flexibility in choosing the cortical site of entry of the therapy electrode. A surprisingly high percentage of the patients was able to tolerate well the 30-to 90-min. period of cortical testing. A study period was terminated when the patient demonstrated reluctance or an unreliability of responses which often appeared in association with fatigue. The total number of such study sessions in the operating room which were successful in part or whole in contributing to this study was 99, carried out on 92 patients. Since each testing period could ordinarily contribute data significant to only one or a few of the points under investigation, the total number of subjects involved in attempting to establish individual relationships is in some instances not as large as one would like.The general procedure during those second-stage operations which were used for investigative purposes was as follows. With the stereotaxic instrument in place and the available area of cerebral cortex exposed, the primary somatosensory area on the flat region of the postcentral gyrus was localized with an exploring stimulating electrode (usually with 0.5-msec. square pulses, alternating in polarity (1), 60 pulses/sec. trains of less than 1 sec. duration). Assurance that the area under investigation was in fact the postcentral gyrus was obtained in almost all cases by eliciting typical pyramidal-type motor responses (3) from the cortical gyrus just anterior to the sulcus bordering the gyrus in question. The cortical area of representation of sensation usually involved some portion of the contralateral hand or, less commonly, of the wrist or forearm. The appropriate stimulating electrode was then placed on that cortical point requiring the minimum current for eliciting this conscious sensory response. There was no particular locus on the postcentral ...

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