Brain stimulation and conscious experience

Libet discovered that a substantial duration (> 0.5-1.0 s) of direct electrical stimulation of the surface of the somatosensory cortex at threshold currents is required before human subjects can report that a conscious somatosensory experience had occurred. Using a reaction time method we confirm that a similarly long stimulation duration at threshold currents is required for activation of elementary visual experiences (phosphenes) in human subjects following stimulation of the surface of the striate cortex. However, the reaction times for the subject to respond to the cessation of the visual experience after the end of electrical stimulation could be as brief as 225-242 ms. We also carried out extensive studies in cats under a variety of anesthetic conditions using the same electrodes and parameters of stimulation employed in the human studies to study the patterns of neuronal activity beneath the stimulating surface electrode. Whereas sufficiently strong currents can activate neurons within milliseconds, stimulating currents close to threshold activate sustained neural activity only after at least 350-500 ms. When currents are close to threshold, some neurons are inhibited for several hundreds of millisecond before the balance between inhibition and excitation shifts towards excitation. These results suggest that the prolonged latencies, i.e., latencies beyond 200-250 ms, for the emergence of conscious experience following direct cortical stimulation result from a delay in the sustained activation of underlying cortical neurons at threshold currents rather than being due to any unusually long duration in central processing time. Intracellular records from cortical neurological cells during repetitive electrical stimulation of the surface of the feline striate cortex demonstrate that such stimulation induces a profound depolarizing shift in membrane potential that may persist after each stimulus train. Such a depolarization is evidence that extracellular K+ concentrations have increased during electrical stimulation. Such an increase in extracellular K+ progressively increases cortical excitability until the threshold for sustained activation of cortical neurons is reached and then exceeded. Consequently, the long latency for threshold activation of cortical neurons depends upon a dynamically increasing cortical facilatory process that begins hundreds of milliseconds before there is sustained activation of such neurons. In some cases, this facilatory process must overcome an initial stimulus-induced inhibition before neuronal firing commences.     

The Timing of Conscious Experience: A Critical Review and Reinterpretation of Libet''s Research

An extended examination of Libet's works led to a comprehensive reinterpretation of his results. According to this reinterpretation, the Minimum Train Duration of electrical brain stimulation should be considered as the time needed to create a brain stimulus efficient for producing conscious sensation and not as a basis for inferring the latency for conscious sensation of peripheral origin. Latency for conscious sensation with brain stimulation may occurafterthe Minimum Train Duration. Backward masking with cortical stimuli suggests a 125–300 ms minimum value for the latency for conscious sensation of threshold skin stimuli. Backward enhancement is not suitable for inferring this latency. For determining temporal relations between stimuli that correspond to subjects' reports, theendof cerebral Minimum Train Duration should be used as reference, rather than its onset. Results of coupling peripheral and cortical stimuli are explained by a latency after the cortical Minimum Train Duration, having roughly the same duration as the latency for supraliminal skin stimuli. Results of coupling peripheral stimuli and stimuli to medial lemniscus (LM) are explained by a shorter LM latency and/or a longer peripheral latency. This interpretation suggests a 230 ms minimum value for the latency for conscious sensation of somatosensory near-threshold stimuli. The backward referral hypothesis, as formulated by Libet, should not be retained. Long readiness potentials preceding spontaneous conscious or nonconscious movements suggest that both kinds of movement are nonconsciously initiated. The validity of Libet's measures of W and M moments (Libet et al., 1983a) is questionable due to problems involving latencies, training, and introspective distinction of W and M. Veto of intended actions may be initially nonconscious but dependent on conscious awareness.     

简论里贝特的意识理论

里贝特是人类意识和自由意志的实验研究领域的一个卓越的、先驱性的神经科学家。里贝特的意识研究工作涉及如下四个方面：（1）关于意识研究的认识论原则;（2）对意识现象本性的界定;（3）意识机制的时控理论;（4）对自由意志的阐释和有意识的心智场理论。里贝特的意识研究独树一帜,其时控理论具有坚实可信的实验证据,它从时间维度揭示了有意识的主观体验以及无意识的心智功能与神经活动之间的时间机制。     

Cortical movement preparation before and after a conscious decision to move

The idea that our conscious decisions determine our actions has been challenged by a report suggesting that the brain starts to prepare for a movement before the person concerned has consciously decided to move (Libet, Gleason, Wright, & Pearl, 1983). Libet et al. claimed that their results show that our actions are not consciously initiated. The current article describes two experiments in which we attempted to replicate Libet et al.'s comparison of participants' movement-related brain activity with the reported times of their decisions to move and also the reported times of their decisions of which hand to move. We also looked at the distribution of participants' reports over time to evaluate an alternative explanation of Libet et al.'s (1983) results. Although the Readiness Potential was usually present before all of the decisions to move, consistent with the findings of Keller and Heckhausen (1990) and Libet et al. (1983), we found that many reported decision times were before the onset of the Lateralized Readiness Potential, which measures hand-specific movement preparation. The latter finding is consistent with the conclusion that the LRP always started after the conscious decision to move. We conclude that even though activity related to movement anticipation may be present before a conscious decision to move, the cortical preparation necessary for the movement to happen immediately may not start until after the conscious decision to move.     

Brain preparation before a voluntary action: Evidence against unconscious movement initiation

Benjamin Libet has argued that electrophysiological signs of cortical movement preparation are present before people report having made a conscious decision to move, and that these signs constitute evidence that voluntary movements are initiated unconsciously. This controversial conclusion depends critically on the assumption that the electrophysiological signs recorded by Libet, Gleason, Wright, and Pearl (1983) are associated only with preparation for movement. We tested that assumption by comparing the electrophysiological signs before a decision to move with signs present before a decision not to move. There was no evidence of stronger electrophysiological signs before a decision to move than before a decision not to move, so these signs clearly are not specific to movement preparation. We conclude that Libet’s results do not provide evidence that voluntary movements are initiated unconsciously.     

Conscious intention and human action: Review of the rise and fall of the readiness potential and Libet’s clock

Is consciousness—the subjective awareness of the sensations, perceptions, beliefs, desires, and intentions of mental life—a genuine cause of human action or a mere impotent epiphenomenon accompanying the brain’s physical activity but utterly incapable of making anything actually happen? This article will review the history and current status of experiments and commentary related to Libet’s influential paper (Brain 106:623–664, 1983) whose conclusion “that cerebral initiation even of a spontaneous voluntary act …can and usually does begin unconsciously” has had a huge effect on debate about the efficacy of conscious intentions. Early (up to 2008) and more recent (2008 on) experiments replicating and criticizing Libet’s conclusions and especially his methods will be discussed, focusing especially on recent observations that the readiness potential (RP) may only be an “artifact of averaging” and that, when intention is measured using “tone probes,” the onset of intention is found much earlier and often before the onset of the RP. Based on these findings, Libet’s methodology was flawed and his results are no longer valid reasons for rejecting Fodor’s “good old commonsense belief/desire psychology” that “my wanting is causally responsible for my reaching.”.     

Libet’s intention reports are invalid: A replication of Dominik et al. (2017)

In the “Libet experiment” the onset of movement-related brain activity preceded the reported time of the conscious intention to move, suggesting that conscious intention may not play a role in initiating voluntary movements (Libet, Gleason, Wright, & Pearl, 1983). Dominik et al. (2017) provided evidence that the intention reports employed in the Libet experiment, which Libet et al. (1983) found to precede movement reports, are invalid. In the study by Dominik et al., intention reports preceded movement reports only when participants had prior experience making movement reports. Individuals without such experience reported intention around the same time as movement. These findings suggest that Libet’s intention reports do not reflect experiences of intention, but, rather, inferences based on prior experience with movement reports. Our study replicated the core findings of Dominik et al. We argue that Libet’s intention reports are invalid and explore the phenomenology of intention in the Libet experiment.     

Transcranial direct current stimulation and visual perception

Membrane potentials and spike sequences represent the basic modes of cerebral information processing. Both can be externally modulated in humans by quite specific techniques: transcranial direct current stimulation (tDCS) and repetitive transcranial magnetic stimulation (rTMS). These methods induce reversible circumscribed cortical excitability changes, either excitatory or inhibitory, outlasting stimulation in time. Experimental pharmacological interventions may selectively enhance the duration of the aftereffects. Whereas rTMS induces externally triggered changes in the neuronal spiking pattern and interrupts or excites neuronal firing in a spatially and temporally restricted fashion, tDCS modulates the spontaneous firing rates of neurons by changing resting-membrane potential. The easiest and most common way of evaluating the cortical excitability changes is by applying TMS to the motor cortex, since it allows reproducible quantification through the motor-evoked potential. Threshold determinations at the visual cortex or psychophysical methods usually require repeated and longer measurements and thus more time for each data set. Here, results derived from the use of tDCS in visual perception, including contrast as well as motion detection and visuo-motor coordination and learning, are summarised. It is demonstrated that visual functions can be transiently altered by tDCS, as has been shown for the motor cortex previously. Up- and down-regulation of different cortical areas by tDCS is likely to open a new branch in the field of visual psychophysics.     

Hypnotizing Libet: Readiness potentials with non-conscious volition

The readiness potential (RP) is one of the most controversial topics in neuroscience and philosophy due to its perceived relevance to the role of conscious willing in action. Libet and colleagues reported that RP onset precedes both volitional movement and conscious awareness of willing that movement, suggesting that the experience of conscious will may not cause volitional movement (Libet, Gleason, Wright, & Pearl, 1983). Rather, they suggested that the RP indexes unconscious processes that may actually cause both volitional movement and the accompanying conscious feeling of will (Libet et al., 1983; pg. 640). Here, we demonstrate that volitional movement can occur without an accompanying feeling of will. We additionally show that the neural processes indexed by RPs are insufficient to cause the experience of conscious willing. Specifically, RPs still occur when subjects make self-timed, endogenously-initiated movements due to a post-hypnotic suggestion, without a conscious feeling of having willed those movements.     

Libet's Timing of Mental Events: Commentary on the Commentaries

This issue of Consciousness and Cognition presents four target articles and eight commentaries on the target articles. The present article presents comments on those commentaries, grouped into backward referral and volition categories. Regarding backward referral: (1) I disagree with my fellow commentators and take the unpopular position of defending Libet's notion of backward referral. (2) I join my fellow commentators in critiquing Libet's notion of a 500-ms delay. (3) I examine several of the hypotheses suggested by other commentators for why cortical and lateral meniscus stimulation give very different timing results. I suggest a simple experiment to help discriminate among the hypotheses. (4) I comment on why temporal reordering is more likely to occur late rather than early in mental processing. Regarding Libet's volition experiments, I ask what is the root of the controversy, given the general agreement on the data. I agree with a commentator that Libet's chronotheology rather than his chronoscience is the cause of much of the controversy. Rather than joining others in criticizing Libet for his chronotheology I point out that he is making a respectable philosophical (or theological) point regarding nondeterministic Free Will, but one that is easily misunderstood. I discuss two ways by which Libet's viewpoint can be brought into mainstream science.     

Effects of intralaminar thalamic stimulation on language functions

Fifteen neurosurgical subjects, who were undergoing thalamic chronic electrode implants as a treatment for dyskinesia and chronic pain, were evaluated on a series of neurolinguistic functions to determine if the stimulation of the centromedianum nucleus of the thalamus affected language and cognitive processing. Analysis of the data revealed that the stimulation of the left centromedianum nucleus had resulted in a notable facilitatory influence on the processing of language functions; this facilitatory effect was measured in terms of the number of errors produced and the patients' response latency time. The facilitatory effect was also noted to have extended to other language functions. The nature of the facilitation from the stimulation of the left centromedianum nucleus, an extension of the brainstem reticular formation, on language functions is discussed.     

In support of Pockett's critique of Libet's studies of the time course of consciousness

Susan Pockett presents sound arguments supporting her reinterpretations of data that Libet and co-workers used to support a number of intriguing and influential conclusions regarding the microgenesis and timing of (a) conscious sensory experience and (b) volitionally controlled motor responses. The following analysis, extending and elaborating some of her main arguments, proposes that Libet's experimental methodologies and rationales, and thus also his interpretation of data, are flawed and that neglect or ignorance of methodological and empirical constraints well known to sensory psychologists risks drawing premature or faulty conclusions about the timing of conscious experience.     

The timing of brain events: reply to the "Special Section" in this journal of September 2004, edited by Susan Pockett

In this "Reply" paper, the arguments and experimental findings by Pockett, Pollen, and Haggard et al. are analyzed. It had been shown () that a 0.5s duration of repetitive activations of sensory cortex is required to produce a threshold of sensation. The view that this is due to a facilitatory buildup in excitatory state to finally elicit neuronal firing is shown to be incompatible with several lines of evidence. Objections to the phenomenon of subjective referral backwards in time (for the delayed sensation) are also untenable. report that a self-initiated act can, under hypnotic suggestion, appear to the subject to be "involuntary." The act under hypnosis is better viewed as one initiated unconsciously, not as an act of conscious will.     

Libet's temporal anomalies: a reassessment of the data

Benjamin Libet compared the perceived time of direct brain stimulation to the perceived time of skin stimulation. His results are among the most controversial experiments at the interface between psychology and philosophy. The new element that I bring to this discussion is a reanalysis of Libet's raw data. Libet's original data were difficult to interpret because of the manner in which they were presented in tables. Plotting the data as psychometric functions shows that the observers have great uncertainty about the relative timing of events, as seen the shallow psychometric slopes. A second indication of uncertainty comes from Libet's use of three response categories, A first; B first; and A and B simultaneous. The large number of "perceptually simultaneous" responses provides a further measure of the difficulty of the judgment. There are thus a very broad range of stimulus delays in which the subject is unable to make an accurate ordering response. These points provide evidence that there is no compelling reason to invent exotic or ad hoc mechanisms to account for Libet's data since the uncertainty window is large enough to allow simple mechanism such as memory shifts. Libet argued that his data provide evidence for a backward referral in time. I argue that even though Libet's own data are weak, there are good arguments for a backward referral mechanism to help the subject make sense out of the tangled chaos of asynchronous information associated with experienced events.     

Flicker-induced color and form: interdependencies and relation to stimulation frequency and phase

Our understanding of human visual perception generally rests on the assumption that conscious visual states represent the interaction of spatial structures in the environment and our nervous system. This assumption is questioned by circumstances where conscious visual states can be triggered by external stimulation which is not primarily spatially defined. Here, subjective colors and forms are evoked by flickering light while the precise nature of those experiences varies over flicker frequency and phase. What's more, the occurrence of one subjective experience appears to be associated with the occurrence of others. While these data indicate that conscious visual experience may be evoked directly by particular variations in the flow of spatially unstructured light over time, it must be assumed that the systems responsible are essentially temporal in character and capable of representing a variety of visual forms and colors, coded in different frequencies or at different phases of the same processing rhythm.     

Pain complaint and muscle soreness associated with high-voltage electrical stimulation: effect of ramp time

The purpose of this study was to determine the effects of ramp time on subjects' perception of discomfort/pain and muscle soreness associated with high-voltage electrical stimulation. 31 female students were randomly assigned to three treatment groups, each of which was assigned a different ramp time (1.0, 3.5, and 5.0 sec.). The subjects' wrist and finger flexors were stimulated with ElectroStim 180-2 unit using the bipolar electrode arrangement. During the stimulation, the subjects rated their experience of discomfort/pain at the threshold of sensory stimulation, the threshold of motor stimulation, and maximum tolerance of painful stimulation. The session was terminated after 10 tetanic isometric contractions. The subjects were instructed to rate their soreness, 24- and 36-hr. poststimulation using a 10-point ratio scale. A significant F ratio was noted for discomfort/pain perceptual levels but not for ramp time. Scheffé's post hoc analysis showed that the discomfort increased as the current intensity was increased. The F ratio for ramp time and time frame (24- and 36-hr. poststimulation) was not statistically significant for the ratings of muscle soreness. The findings suggest that the patients's complaint of pain and muscle soreness associated with high-voltage electrical stimulation is not affected by the rate of rise of current.     

Biases in the subjective timing of perceptual events: Libet et al. (1983) revisited

We report two experiments in which participants had to judge the time of occurrence of a stimulus relative to a clock. The experiments were based on the control condition used by Libet, Gleason, Wright, and Pearl [Libet, B., Gleason, C. A., Wright, E. W., & Pearl, D. K. (1983). Time of conscious intention to act in relation to onset of cerebral activities (readiness-potential): The unconscious initiation of a freely voluntary act. Brain 106, 623-642] to correct for any bias in the estimation of the time at which an endogenous event, the conscious intention to perform a movement, occurred. Participants' responses were affected systematically by the sensory modality of the stimulus and by the speed of the clock. Such findings demonstrate the variability in judging the time at which an exogenous event occurs and, by extension, suggest that such variability may also apply to the judging the time of occurrence of endogenous events. The reliability of participants' estimations of when they formed the conscious intention to perform a movement in Libet et al.'s (1983) study is therefore questionable.     

Aggressive behavior induced by electrical stimulation in the midbrain central gray of male rats

Electrical stimulation via electrodes implanted in the lateral hypothalamus may induce intraspecific aggressive behavior. Small electrolytic lesions placed via these electrodes resulted in a five– to tenfold increase in the current threshold for aggression. Degenerating fibers were stained by means of the Fink-Heimer method and could be followed caudally to the dorsal midbrain central gray and to the mammillary bodies. A few axons could be traced rostrally to the medial septum. Aggression could be induced from 10 of 112 electrodes implanted in the central gray; the other electrodes elicited either locomotion, vocalization, jump, or “alarm-like reactions.” The morphology of the induced aggression was similar to the morphology of the hypothalamically induced aggression, though it was often accompanied with motor disturbances and was less intense. Hypothalamic stimulation was combined with simultaneous central gray stimulation in rats with electrodes both in the hypothalamus and in the central gray. Hypothalamic thresholds for aggression could be lowered by this stimulation of the central gray, even when no aggressive responses were observed during central gray stimulation alone. This suggests that, although aggression is not manifest, electrical stimulation may activate neural tissue involved in aggressive behavior. It is concluded that in rats central gray and hypothalamus are part of the same neural network mediating intraspecific aggression.     

经颅电刺激与视功能调控

经颅电刺激(Transcranial Electrical Stimulation, TES)通过电极将特定模式的低强度电流作用于大脑头皮以调控皮层活动, 是一种非侵入、无创的神经刺激方法。根据刺激电流的模式的不同, TES分为经颅直流电刺激(tDCS), 经颅交流电刺激(tACS)和经颅随机电刺激(tRNS)。TES能对视功能诸如光幻视阈值、视野、对比敏感度、视知觉运动等进行一定程度上的调控, 并且能够与传统的视觉知觉学习训练相结合以调控视觉功能。对于不同的视觉功能, 不同的TES参数和模式的调控效果有所不同。     

Electrical stimulation of the pedunculopontine tegmental nucleus in freely moving awake rats: time- and site-specific effects on two-way active avoidance conditioning

The pedunculopontine tegmental nucleus (PPTg) is involved in the regulation of thalamocortical transmission and of several functions related to ventral and dorsal striatal circuits. Stimulation of the PPTg in anesthetized animals increases cortical arousal, cortical acetylcholine release, bursting activity of mesopontine dopaminergic cells, and striatal dopamine release. It was hypothetized that PPTg stimulation could improve learning by enhancing cortical arousal and optimizing the activity of striatal circuits. We tested whether electrical stimulation (ES) of the PPTg, applied to freely-moving awake rats previously implanted with a chronic electrode, would improve the acquisition and/or the retention of two-way active avoidance conditioning, and whether this effect would depend on the specific PPTg region stimulated (anterior vs posterior) and on the time of ES: just before (pre-training) or after (post-training) each of three training sessions. The treatment consisted of 20 min of ES (0.2 ms pulses at 100 Hz; current intensity: 40-80 microA). The results showed that (1) this stimulation did not induce either any signs of distress nor abnormal behaviors, apart from some motor stereotyped behaviors that disappeared when current intensity was lowered; (2) pre-training ES applied to the anterior PPTg improved the acquisition of two-way active avoidance, (3) no learning improvement was found after either post-training ES of the anterior PPTg, or pre- and post-training ES of the posterior PPTg. The results give support to a role of PPTg in learning-related processes, and point to the existence of functional PPTg regions.