Benjamin Libet

SUBJECTIVE REFERRAL OF THE TIMING FOR A CONSCIOUS SENSORY EXPERIENCE

PREVIOUS studies had indicated that there is a substantial delay, up to about 0.5 s, before activity at cerebral levels achieves ‘neuronal adequacy’ for eliciting a conscious somatosensory experience (Libet, Alberts, Wright, Delattre, Levin and Feinstein, 1964; Libet, 1966). The delay appeared necessary not only with stimulation of medial lemniscus, ventrobasal thalamus, or postcentral cortex, but even when the stimulus was a single electrical pulse at the skin (Libet, Alberts, Wright, and Feinstein, 1967, 1972; Libet, 1973). The present investigation began with an experimental test of whether there is in fact also a subjective delay in the conscious experience for a peripheral sensory stimulus. That is, is there a delay in the subjective timing of the experience that would correspond to the presumed delay in achieving the neuronal state that ‘produces’ the experience? The results of that test led to a modified hypothesis ; this postulates (a) the existence of a subjective referral of the timing for a sensory experience, and (b) a role for the specific (lemniscal) projection system in mediating such a subjective referral of timing. Experimental tests of the new proposal were carried out and are reported here.

Electrical Stimulation of Cortex in Human Subjects, and Conscious Sensory Aspects

Most problems in neurophysiology can be attacked more fruitfully in animals other thar, man, for the obvious reasons of controllability of conditions and of our moral restraints on the experimental procedures which are tolerable for human studies. But if one wants to investigate cerebral mechanisms underlying subjective experience (of sensation, in the present context), it should also be obvious that recourse must be had to human subjects for primary validation of the subjective phenomenon under study. Direct approaches to the brain of waking subjects are of course limited by compatibility with therapeutic procedures and by the patient’s condition and informed consent. Electrical stimulation of (and, more recently, recording from) the cerebral cortex and deeper structures has provided one approach which, when suitably utilized, makes possible informative studies with no irreversible effects on the subject. The problems susceptible to investigation can be much broader than the initial classical one of the topographical relations of cortical sites to the body sites of the subjectively referred sensations, and some of these problems will be considered in this article.

The Experimental Evidence for Subjective Referral of a Sensory Experience Backwards in Time: Reply to P. S. Churchland

Our experimental investigations of cerebral neuronal activities that might uniquely be part of the processes that elicit a conscious sensory experience, produced evidence for the hypothesis that two remarkable temporal factors govern this kind of mind-brain relationship (see Libet 1973, 1978a; Libet et al. 1979). (1) There is a substantial delay before cerebral activities, initiated by a sensory stimulus, achieve “neuronal adequacy” for eliciting any resulting conscious sensory experience. For stimuli close to the threshold level for sensation the delay would average approximately 500 msec; for stronger stimuli this delay could be reduced, possibly to as little as 100 msec. The response in question is the introspective awareness of a localized somatic sensation elicited by the stimulus, as reported by the subject. It is in the category of a subjective “raw feel” (e.g., Feigl 1960), as distinct from behavioral responses to a stimulus that could reflect a form of detection that may be unaccompanied by a conscious, subjective experience (Libet 1965). (2) After neuronal adequacy is achieved, the subjective timing of the experience is (automatically) referred backwards in time, utilizing a “timing signal” in the form of the initial response of cerebral cortex to the sensory stimulus. This initial response is represented by the primary evoked potential, an electrophysiological response recordable at the primary sensory cortical area that receives the earliest (as well as the most localized) neural message, within 10 to 20 msec after the peripheral sensory nerve fibers are excited by the stimulus. The experience would thus be “antedated”, and its timing would appear to the subject to occur without the actual substantial delay required before neuronal adequacy for eliciting the experience is achieved.

Suppression of an Epileptiform Type of Electrocortical Activity in the Rat by Stimulation in the Vicinity of Locus Coeruleus

Stimulation in the locus coeruleus, or in the vicinity of this nucleus or of its ascending tracts, could markedly suppress the appearance of epileptiform‐like ECoG bursts. The latter were i induced in rats by a subconvulsive dose of pentylenetetrazol. Electrode sites were identified histologically. A unilateral stimulus suppressed bursts bilaterally. An individual burst already in progress could be aborted, stopping within less than 0.5‐1 sec after onset of a stimulus train. The antiepileptiform actions occurred with no evidence of any desynchronizing effect of the stimulus on the resting ECoG; they appear to be different in sites of origin and nature from those reported for stimulation of the reticular activating system. It is proposed that stimulation of the ascending noradrenergic system in the brain stem may limit the development and spread of hyperexcitatory, epileptiform states.

Is inactivation of potassium conductance involved in slow postsynaptic excitation of sympathetic ganglion cells? Effects of nicotine☆

Abstract In untreated or curarized B cells of frog ganglia, the effects of conditioning polarization on the S-EPSP are dual and opposite in the early vs. later phases of the S-EPSP. An actual “reversal” of polarity of S-EPSP with conditioning hyperpolarization was seen only in nicotinized B cells, but even this cannot be regarded as a true reversal of a PSP; only the initial phase is reversed and it has a reduced latency. Nicotine depresses the normal S-EPSP in rabbit ganglia; the residual muscarinic depolarizing response has different characteristics, which resemble the response in nicotinized frog ganglia. On the basis of all available evidence it is proposed: (1) that inactivation of g K (potassium conductance may contribute to the early phase of the orthodromic response in nicotinized and/or somewhat depolarized ganglion cells, though little or none to that of non-nicotinized cells at normal membrane potentials; and (2) that the normal S-EPSP of mammalian ganglion cells as well as the slower-developing orthodromic depolarizing response (“actual” S-EPSP) in frog cells, is generated by a mechanism that does not involve inactivation of g K .

The Timing of Mental Events: Libet's Experimental Findings and Their Implications☆

The major findings by Libet et al. are briefly summarized. The criticisms and alternative proposals by Trevena and Miller, Pockett, and Gomes (this issue) are analyzed and found to be largely unwarranted.

Stimulation of Locus Coeruleus in Man

Stimulating electrodes were chronically implanted unilaterally (in 1975–1977) in the vicinity of the locus coeruleus (LC) in three patients, one with cerebral palsy-spastic quadriplegia, two with epil

Brain Stimulation and the Threshold of Conscious Experience

The subject of this paper stems out of a more general question which asks, what is the spatiotemporal configuration of neuronal activity which effectively elicits or is at least uniquely correlated with a conscious awareness of something? Now, to attempt a study of whole configurations of neuronal activities is obviously much too difficult and complicated. One would like to delimit severely the experimental approach to this question, and this we have done in two ways.

Generation of Adrenergic and Cholinergic Potentials in Sympathetic Ganglion Cells

Norepinephrine elicited a hyperpolarizing response, and acetylcholine (during nicotinic blockade) elicited a depolarizing one. Both responses showed no increase in membrane conductance. The norepinephrine response was suppressed by initial depolarization; the acetylcholine response (frog cells); by hyperpolarization. These neurotransmitters apparently can activate electrogenic mechanisms which do not involve movement of ions down their electrochemical gradients.

Sleep disturbance produced by electrical stimulation of the locus coeruleus in a human subject.

A 25-year-old man with a chronically implanted stimulating electrode placed in the region of the locus coeruleus (LC) was monitored for 5 nights in a sleep laboratory to study the role of the LC in sleep. Sleep patterns were compared between the 2 nights in which the stimulation was applied periodically every 90 min and the 2 nights in which no stimulation was applied. In contrast to the normal sleep patterns that occurred during the 2 nonstimulation nights, electrical stimulation of the LC produced a profound disruption of sleep and significant reductions in the total amounts of NREM sleep, REM sleep, REM sleep as a percent of total sleep (NREM + REM sleep), and total sleep. Results suggest that the LC has a role in maintaining normal sleep patterns.