Cortical Activation in Conscious and Unconscious Experience

CORTICAL ACTIVATION IN CONSCIOUS AND UNCONSCIOUS EXPERIENCE BENJAMIN LIBET* It has become generally accepted that a large, perhaps even a major part ofour mental activities can take place without our being consciously aware ofthem. Though apparently unconscious, they are nevertheless a part of significant mental experience since there is evidence that such activities can participate in later mental and behavioral manifestations—cognitive, affective , or conative. It is also generally assumed that specific temporo-spatial configurations ofcerebral activity will be found to correspond to specific mental states or experiences. However, it is easier and at present more meaningful to look for the differences between conscious and unconscious experiences rather than to attempt a more complete specification ofthe cerebral processes of each mental state. Some clues to such differences stem from our investigations of those physiological parameters which are uniquely involved in eliciting conscious sensory experience by electrical stimulation of the human cerebral cortex [i, 2]. Cortical processes are studied which are accessible to measurement in awake human subjects during neurosurgical procedures. Conditions of cortical activation which are at the threshold level for eliciting a conscious sensory experience are compared to conditions of activation which are just below this threshold. ("Threshold" is considered here in the broad sense of adequacy in all parameters, not merely in intensity.) Since states ofcortical activity which arejust below the threshold for conscious sensory experience may represent some form * Department of Physiology, University of California School of Medicine, San Francisco, and Mount Zion Neurological Institute, San Francisco. The unpublished experimental work included here was carried out in the Mount Zion Neurological Institute, San Francisco, in collaboration with Drs. W. W. Alberts, Bertram Feinstein, and Grant Levin, Mr. E. W. Wright, Jr., and Mrs. L. D. Delatore. It was partially supported by U.S. Public Health Service Grant NB 05061 from the National Institute ofNeurological Diseases and Blindness. 77 ofunconscious mental experiences, the differences between conditions at and below these thresholds may have a bearing on our general problem. Before going on with the findings and hypotheses, I shall attempt to reformulate, out ofthe background ofvarious notions that have been held by others, an appropriate usage ofthe terms "conscious" and "unconscious experience." (Incidentally, treatingthese mentalphenomenaasexperiences gives them a more operational meaning than using the nouns "consciousness " and "the unconscious" would [3]). I begin with the premise that the subjective or introspective experience ofawareness ofsomething is the primary criterion ofconscious experience. It would, then, be some clear indication ofsuch an experience ofawareness by the subject to an external observer that is operative in any investigation or analysis of conscious experience. "Awareness," as a subjective experience, is a "primitive (undefined ) term" [4], with phenomenological rather than behavioral meaning . Although it is experienced and known privately by each individual, we are confident that others have experienced it too, and that they know what we mean by our questions about it [3, 5]. There is, however, the experimental problem of the suitability of the voluntary expression employed by the subject to indicate or report his conscious experience, which will be considered further below. In addition, recallability, or at least recognizability, of the subjective experience is required in order for some report to be made. But this must be anessentialrequirement inany case ifanexperience isto supply content to the kind ofsubjective awareness which has continuity. There may well be an immediate but ephemeral kind ofexperience ofawareness which is not retained for recall at conscious levels ofexperience. Ifsuch experiences exist, however, their content would have direct significance only in later unconscious mental processes, although, like other unconscious experiences , they might play an indirect role in later conscious ones. Dreaming and daydreaming, on the other hand, introduce borderline cases. Most of the dream experiences during natural sleep can hardly be recalled unless the individual is awakened within minutes oftheir occurrence [6]. The recall ofexperiences ofawareness in the normal waking state is not subject to such limitations in any obligatory way. Nevertheless, investigation of these experiences is simplified by eliciting a report or indication ofthem from the subject shortly after their occurrence. The degree ofrecallability can, ofcourse, be altered by some subsequent events or in certain psycho78 Benjamin Libet · Cortical Activation Perspectives in Biology and Medicine · Autumn...