The timing of conscious experience: a critical review and reinterpretation of Libet's research

Libet's legacy: A primer to the neuroscience of volition

The neuroscience of volition is an emerging subfield of the brain sciences, with hundreds of papers on the role of consciousness in action formation published each year. This makes the state-of-the-art in the discipline poorly accessible to newcomers and difficult to follow even for experts in the field. Here we provide a comprehensive summary of research in this field since its inception that will be useful to both groups. We also discuss important ideas that have received little coverage in the literature so far. We systematically reviewed a set of 2220 publications, with detailed consideration of almost 500 of the most relevant papers. We provide a thorough introduction to the seminal work of Benjamin Libet from the 1960s to 1980s. We also discuss common criticisms of Libet's method, including temporal introspection, the interpretation of the assumed physiological correlates of volition, and various conceptual issues. We conclude with recent advances and potential future directions in the field, highlighting modern methodological approaches to volition, as well as important recent findings.

(Non)-experiencing the intention to move: On the comparisons between the Readiness Potential onset and Libet's W-time

A seminal study of Libet et al. (1983) provided a popular approach to compare introspective timing of movement execution (the M-time) and the intention to move (the W-time) with respect to the onset of the readiness potential (RP). The difference between the W-time and the RP onsets contributed significantly to the current free-will discussion, insofar as it has been repeatedly shown that the RP onset unequivocally precedes the W-time. However, the interpretations of Libet's paradigm continuously attract criticism, questioning the use of both the W-time and the RP onset as indicators of the motor intention. In the current study, we further probe whether the W-time is rather an intention-unrelated product of the participant's inference than an unambiguous temporal marker of the intention to move. Using behavioral reports and concurrent multichannel EEG, we investigated the relation between the W-time and M-time introspective reports in two groups of participants who started an experiment with the series of different reports. Congruently with previous studies, we have shown that the W-time is affected by the experimental procedures: participants who had prior experience reporting the M-time provided significantly earlier W-time. However, contrary to previous papers, we revealed that even naive participants do introspectively differentiate the W-time and the M-time, which suggests that the W-time might actually reflect the intention to move, at least, to some extent. We therefore suggest that training-based modulation of the W-time values may explain this finding. Moreover, we further confirm the absence of a direct link between the RP onset and the W-time by showing no covariation between them in both experimental groups. In turn, our findings question the overall interpretation of the comparison between these two time points. Overall, our study further emphasizes the ambiguity of Libet's paradigm, and suggests that the relatedness of both the RP and the W-time to the movement initiation processes should not be assumed as priori.

Time marking in perception

Several authors have proposed that perceptual information carries labels that identify temporal features, including time of occurrence, ordinal temporal relations, and brief durations. These labels serve to locate and organise perceptual objects, features, and events in time. In some proposals time marking has local, specific functions such as synchronisation of different features in perceptual processing. In other proposals time marking has general significance and is responsible for rendering perceptual experience temporally coherent, just as various forms of spatial information render the visual environment spatially coherent. These proposals, which all concern time marking on the millisecond time scale, are reviewed. It is concluded that time marking is vital to the construction of a multisensory perceptual world in which things are orderly with respect to both space and time, but that much more research is needed to ascertain its functions in perception and its neurophysiological foundations.

It's time for attentional control: Temporal expectation in the attentional blink

The attentional blink (AB) reveals a limitation in conscious processing of sequential targets. Although it is widely held that the AB derives from a structural bottleneck of central capacity, how the central processing is constrained is still unclear. As the AB reflects the dilemma of deploying attentional resources in the time dimension, research on temporal allocation provides an important avenue for understanding the mechanism. Here we reviewed studies regarding the role of temporal expectation in modulating the AB performance primarily based on two temporal processing strategies: interval-based and rhythm-based timings. We showed that both temporal expectations can help to organize limited resources among multiple attentional episodes, thereby mitigating the AB effect. As it turns out, scrutinizing on the AB from a temporal perspective is a promising way to comprehend the mechanisms behind the AB and conscious cognition. We also highlighted some unresolved issues and discussed potential directions for future research.

Manipulations of Libet clock parameters affect intention timing awareness

W judgments are a widely used intention timing awareness estimate. These judgments are typically obtained by using the classic Libet-style paradigm whereby participants are asked to estimate the time they become aware of their intention to act by using the location of a rotating object on a clock face. There is an inconsistency in the Libet clock parameters used in previous studies, and it is unclear whether this variability impacts W judgments and other outcome measures, with implications for the construct validity of this measure and the generalisability of results across studies. Here, we present a four-experiment study that systematically manipulated the Libet clock speed, number of clock markings, length of the clock hand and type of clock radius in order to examine whether these parameter manipulations affect intention timing awareness estimates. Our results demonstrate W judgments can be significantly influenced by the clock speed and number of clock markings. The meaning and implications of these results are discussed.

Hypnosis and Hypnotic ability between old beliefs and new evidences: An epistemological reflection

The history of hypnosis has been marked by its effectiveness paralleled by prejudicial refusal, due to its ostensible incompatibility with the ruling Weltbild (picture of the world). Its interpretation has been mainly based on concepts like suggestibility, dissociation, hallucination, impairment of sense of agency, and free will. Nevertheless, little evidence and agreement has been reached so far on the nature of hypnosis and hypnotic ability, an uncertainty enhanced by the wide range of meanings and ambiguities of the used terms. This article analyzes the main epistemological implications involved in the topic.

Differentiating the reported time of intent and action on the basis of temporal binding behaviors and confidence ratings

The reported time of intent (W) and the reported time of action (M) have been used as indices of consciousness during simple voluntary actions. However, it is unclear whether W is exclusively inferred from M. Past studies have suggested that W is inferred from M by demonstrating that W varies when judged in conjunction with M. The current study offers counterevidence by showing that W is independent of M under some circumstances related to temporal binding. Participants performed a voluntary keypress that elicited a tone (briefly delayed at 5 and 60 ms). Subsequently, they reported W or M and indicated the confidence of their report. Binding strength was measured as the extent to which the W and M reports gravitated toward the time of the tone. Moreover, the binding strength was evaluated in conjunction with time course and knowledge to assess whether the strength increases due to repeated exposure or weakens if informed of the tone delay manipulation, respectively. We observed that the binding strength associated with W increased over time, and being informed of the tone manipulation did not affect W's binding behaviors. In contrast, M's binding behaviors did not change over time but being informed of the tone manipulation may release M from binding. The corresponding confidence ratings associated with W were uniform whereas those associated with M fluctuated over time. Collectively, the results suggest that binding behaviors associated with W and M differ, and that W is not simply derived from M.

A meta-analysis of Libet-style experiments

In the seminal Libet experiment (Libet et al., 1983), unconscious brain activity preceded the self-reported, conscious intention to move. This was repeatedly interpreted as challenging the view that (conscious) mental states cause behavior and, prominently, as challenging the existence of free will. Extensive discussions in philosophy, psychology, neuroscience, and jurisprudence followed, but further empirical findings were heterogeneous. However, a quantitative review of the literature summarizing the evidence of Libet-style experiments is lacking. The present meta-analysis fills this gap. The results revealed a temporal pattern that is largely consistent with the one found by Libet and colleagues. Remarkably, there were only k = 6 studies for the time difference between unconscious brain activity and the conscious intention to move - the most crucial time difference regarding implications about conscious causation and free will. Additionally, there was a high degree of uncertainty associated with this meta-analytic effect. We conclude that some of Libet et al.'s findings appear more fragile than anticipated in light of the substantial scientific work that built on them.

Free will and neurosurgical resections of the supplementary motor area: a critical review

BACKGROUND

Research suggests that unconscious activity in the supplementary motor area (SMA) precedes not only certain simple motor actions but also the point at which we become aware of our intention to perform such actions. The extent to which these findings have implications for our understanding of the concepts of free will and personal responsibility has been subject of intense debate during the latest four decades.

METHODS

This research is discussed in relation to effects of neurosurgical removal of the SMA in a narrative review.

RESULTS

Removal of the SMA typically causes a transient inability to perform non-stimulus-driven, voluntary actions. This condition, known as the SMA syndrome, does not appear to be associated with a loss of sense of volition but with a profound disruption of executive function/cognitive control.

CONCLUSIONS

The role of the SMA may be to serve as a gateway between the corticospinal tract and systems for executive function. Such systems are typically seen as tools for conscious decisions. What is known about effects of SMA resections would thus seem to suggest a view that is compatible with concepts of personal responsibility. However, the philosophical question whether free will exists cannot be definitely resolved on the basis of these observations.

The involuntary nature of binge drinking: goal directedness and awareness of intention

Binge drinking represents a public health issue and is a known risk factor in the development of alcohol use disorders. Previous studies have shown behavioural as well as neuroanatomical alterations associated with binge drinking. Here, we address the question of the automaticity or involuntary nature of the behaviour by assessing goal‐directed behaviour and intentionality. In this study, we used a computational two‐step task, designed to discern between model‐based/goal‐directed and model‐free/habitual behaviours, and the classic Libet clock task, to study intention awareness, in a sample of 31 severe binge drinkers (BD) and 35 matched healthy volunteers. We observed that BD had impaired goal‐directed behaviour in the two‐step task compared with healthy volunteers. In the Libet clock task, BD showed delayed intention awareness. Further, we demonstrated that alcohol use severity, as reflected by the alcohol use disorders identification test, correlated with decreased conscious awareness of volitional intention in BD, although it was unrelated to performance on the two‐step task. However, the time elapsed since the last drinking binge influenced the model‐free scores, with BD showing less habitual behaviour after longer abstinence. Our findings suggest that the implementation of goal‐directed strategies and the awareness of volitional intention are affected in current heavy alcohol users. However, the modulation of these impairments by alcohol use severity and abstinence suggests a state effect of alcohol use in these measures and that top‐down volitional control might be ameliorated with alcohol use cessation.

Intentions matter: Avoidance intentions regulate anxiety via outcome expectancy

Intentions prospectively correlate with behaviour (e.g., Ajzen, 1991) but there is little research on whether they play a role in regulating emotion. Two experiments investigated whether avoidance intentions, mediated by expectancy, play a role in reducing anxiety. In Experiment 1, participants performed either an instrumental avoidance response that cancelled the shock signalled by a Pavlovian stimulus; a control response that had no effect on shock; or no response. Prior to this, they indicated their intention to respond or not respond on a form. Both shock expectancy and skin conductance level (SCL) were significantly lower on trials in which an avoidance response was intended compared to not intended, relative to control trials. Experiment 2 replicated these results when intention was retrospectively reported at the end of each trial, arguing against the possibility that the behavioural action of recording intentions in Experiment 1 became directly associated with the shock or no shock outcome. The results indicate that avoidance intentions reduce anxiety through the mediating effect of expectancy, suggesting that avoidance intentions may play an important role in shaping avoidance behaviours for people with anxiety.

Anticipatory Vibrotactile Cueing Facilitates Grip Force Adjustment during Perturbative Loading

Grip force applied to an object held between the thumb and index finger is automatically and unconsciously adjusted upon perception of an external disturbance to the object. Typically, this adjustment occurs within approximately 100 ms. Here, we investigated the effect of anticipatory vibrotactile cues prior to a perturbative force, which the central nervous system may use for rapid grip re-stabilization. We asked participants to grip and hold an instrumented, actuated handle between the thumb and index finger. Under computer control, the handle could suddenly be pulled away from a static grip and could independently provide vibration to the gripping fingers. The mean latency of corrective motor action was 139 ms. When vibrotactile stimulation was applied 50 ms before application of tractive force, the latency was reduced to 117 ms, whereas the mean latency of the conscious response to vibrotactile stimuli alone was 229 ms. This suggests that vibrotactile stimulation can influence reflex-like actions. We also examined the effects of anticipatory cues using a set of perturbative loads with different rising rates. As expected, facilitation of grip force adjustment was observed for moderate loads. In contrast, anticipatory cues had an insignificant effect on rapid loads that evoked an adjustment within 60-80 ms, which approaches the minimum latency of human grip adjustment. Understanding the facilitative effects of anticipatory cues on human reactive grip can aid the development of human-machine interfaces to enhance human behavior.

Happiness in action: the impact of positive affect on the time of the conscious intention to act

The temporal relationship between our conscious intentions to act and the action itself has been widely investigated. Previous research consistently shows that the motor intention enters awareness a few 100 ms before movement onset. As research in other domains has shown that most behavior is affected by the emotional state people are in, it is remarkable that the role of emotional states on intention awareness has never been investigated. Here we tested the hypothesis that positive and negative affects have opposite effects on the temporal relationship between the conscious intention to act and the action itself. A mood induction procedure that combined guided imagery and music listening was employed to induce positive, negative, or neutral affective states. After each mood induction session, participants were asked to execute voluntary self-paced movements and to report when they formed the intention to act. Exposure to pleasant material, as compared to exposure to unpleasant material, enhanced positive affect and dampened negative affect. Importantly, in the positive affect condition participants reported their intention to act earlier in time with respect to action onset, as compared to when they were in the negative or in the neutral affect conditions. Conversely the reported time of the intention to act when participants experienced negative affect did not differ significantly from the neutral condition. These findings suggest that the temporal relationship between the conscious intention to act and the action itself is malleable to changes in affective states and may indicate that positive affect enhances intentional awareness.

Preconscious Prediction of a Driver's Decision Using Intracranial Recordings

While driving, we make numerous conscious decisions such as route and turn direction selection. Although drivers are held responsible, the neural processes that govern such decisions are not clear. We recorded intracranial EEG signals from six patients engaged in a computer-based driving simulator. Patients decided which way to turn (left/right) and subsequently reported the time of the decision. We show that power modulations of gamma band oscillations (30–100 Hz) preceding the reported time of decision (up to 5.5 sec) allow prediction of decision content with high accuracy (up to 82.4%) on a trial-by-trial basis, irrespective of subsequent motor output. Moreover, these modulations exhibited a spatiotemporal gradient, differentiating left/right decisions earliest in premotor cortices and later in more anterior and lateral regions. Our results suggest a preconscious role for the premotor cortices in early stages of decision-making, which permits foreseeing and perhaps modifying the content of real-life human choices before they are consciously made.

The neuroscience of free will: implications for psychiatry

Belief in free will has been a mainstay in philosophy throughout history, grounded in large part in our intuitive sense that we consciously control our actions and could have done otherwise. However, psychology and psychiatry have long sought to uncover mechanistic explanations for human behavior that challenge the notion of free will. In recent years, neuroscientific discoveries have produced a model of volitional behavior that is at odds with the notion of contra-causal free will and our sense of conscious agency. Volitional behavior instead appears to have antecedents in unconscious brain activity that is localizable to specific neuroanatomical structures. Updating notions of free will in favor of a continuous model of volitional self-control provides a useful paradigm to conceptualize and study some forms of psychopathology such as addiction and impulse control disorders. Similarly, thinking of specific symptoms of schizophrenia as disorders of agency may help to elucidate mechanisms of psychosis. Beyond clinical understanding and etiological research, a neuroscientific model of volitional behavior has the potential to modernize forensic notions of responsibility and criminal punishment in order to inform public policy. Ultimately, moving away from the language of free will towards the language of volitional control may result in an enhanced understanding of the very nature of ourselves.

The Time of Perception and the Other Way Around

The world we perceive is delayed in relation to its flowing content, as well as the outcome of our actions on the world in relation to the moment we decide to act. This mosaic of different latencies permeating both perception and action has to be taken into account critically in order for us to cope with the temporal challenges constantly imposed by the environment. Fundamental notions, such as the sense of agency and causality, depend on the temporal relationship of events occurring in well-defined windows of time. Here, we offer a broad, yet abridged, historical view of some thought-provoking issues concerning the time of perception and action. From the pioneering work of Wundt, Titchener, and Libet to recent findings and ideas related to the employment of visual illusions as psychophysical probes (such as the flash-lag effect), we have tried to expose some problems inherent to the act of measuring the time of both perception and action, and devise possible solutions as well.

Active sensing of target location encoded by cortical microstimulation

Cortical microstimulation has been proposed as a method to deliver sensory percepts to circumvent damaged sensory receptors or pathways. However, much of perception involves the active movement of sensory organs and the integration of information across sensory and motor modalities. The efficacy of cortical microstimulation in such an active sensing paradigm has not been demonstrated. We report a novel behavioral paradigm which delivers microstimulation in real-time based on a rat's movements and show that rats can perform sensorimotor integration with electrically delivered stimuli. Using a real-time whisker tracking system, we delivered microstimulation in barrel cortex of actively whisking rats when their whisker crossed a particular spatial location which defined the target. Rats learned to integrate microstimulation cues with their knowledge of whisker position to infer target location along the rostro-caudal axis in less than 200 ms. In a separate experiment, we found that rats trained to respond to cortical microstimulation responded similarly to whisker deflections while ignoring auditory distracters, suggesting that barrel cortex stimulation may be perceptually similar to somatosensory stimuli. This ability to deliver sensory percepts using cortical microstimulation in an active sensing system might have significant implications for the development of sensorimotor neuroprostheses.

Preparing to move and deciding not to move

A commentary is given on Trevena and Miller (2010). The comparability of their experimental task and of the potential they recorded with those used and recorded by Libet, Gleason, Wright, and Pearl (1983a) is questioned. An interpretation is given for the similarity of event-related potentials recorded when subjects decided to move and when they decided not to move.

The neural correlate of consciousness

I propose that we are only aware of changes in our underlying cognition. This hypothesis is based on four lines of evidence. (1) Without changes in visual input (including fixational eye movements), static images fade from awareness. (2) Consciousness appears to be continuous, but is actually broken up into discrete cycles of cognition. Without continuity, conscious awareness disintegrates into a series of isolated cycles. The simplest mechanism for creating continuity is to track the changes between the cycles. (3) While these conscious vectors are putative, they have a clear source: the dorsolateral prefrontal cortex (DLPFC). The DLPFC is active during awareness of changes, and this awareness is disrupted by repetitive transcranial magnetic stimulation. (4) When the DLPFC and the orbital and inferior parietal cortices are deactivated during dreaming, conscious awareness is absent even though the rest of the brain is active. Moreover, Lau and Passingham showed that activation of the DLPFC, but no other brain region, correlates with awareness. In summary, if the DLPFC and conscious vectors are the neural correlate of consciousness, then we are only aware of changes in our underlying cognition. The glue that holds conscious awareness together is conscious awareness.

The rotating spot method of timing subjective events

The rotating spot method of timing subjective events involves the subject's watching a rotating spot on a computer and reporting the position of the spot at the instant when the subjective event of interest occurs. We conducted an experiment to investigate factors that may impact on the results produced by this method, using the subject's perception of when they made a simple finger movement as the subjective event to be timed. Seven aspects of the rotating spot method were investigated, using a factorial experiment. Four of these aspects altered the physical characteristics of the computer generated spot or clock face and the remaining three altered the instructions given to the participant. We found compelling evidence that one factor, whether the subject was instructed to report the instant when the finger movement was initiated or the instant when it was completed, resulted in a systematic shift in the response. Evidence that three other factors affect the observed variability in the response was also found. In addition, we observed that there are substantial systematic differences in the responses made by different subjects. We discuss the implications of our findings and make recommendations about the optimal way of conducting future experiments using the rotating spot method. Our overall conclusion is that our results strongly validate the rotating spot method of timing at least the studied variety of subjective event.

Free will, the self, and the brain

The free will problem is defined and three solutions are discussed: no-freedom theory, libertarianism, and compatibilism. Strict determinism is often assumed in arguing for libertarianism or no-freedom theory. It assumes that the history of the universe is fixed, but modern physics admits a certain degree of randomness in the determination of events. However, this is not enough for a compatibilist position-which is favored here-since freedom is not randomness. It is the I that chooses what to do. It is argued that the core of the free will problem is what this I is. A materialist view is favored: The I is an activity of the brain. In addition to absence of external and internal compulsion, freedom involves absence of causal sufficiency of influences acting on the I. A more elaborate compatibilist view is proposed, according to which causal determination is complete when we add events occurring in the I (of which the subject is not conscious). Contrary to what several authors have argued, the onset of the readiness potential before the decision to act is no problem here. The experience of agency is incomplete and fallible, rather than illusory. Some consequences of different views about freedom for the ascription of responsibility are discussed.

Manipulating the Experienced Onset of Intention after Action Execution

Using transcranial magnetic stimulation (TMS), we have tested the time needed for the perceived onset of spontaneous motor intention to be fully determined. We found that TMS applied over the presupplementary motor area after the execution of a simple spontaneous action shifted the perceived onset of the motor intention backward in time, and shifted the perceived time of action execution forward in time. The size of the effect was similar regardless of whether TMS was applied immediately after the action or 200 msec after. The results of three control studies suggest that this effect is time-limited, specific to modality, and also specific to the anatomical site of stimulation. We conclude that the perceived onset of intention depends, at least in part, on neural activity that takes place after the execution of action. A model, which is based on the mechanism of cue integration under the presence of noise, is offered to explain the results. The implications for the conscious control of spontaneous actions are discussed.

On measuring the perceived onsets of spontaneous actions

We investigated the neural mechanisms underlying the timing procedure that was devised by Libet et al. (1983) to measure the onset of conscious motor intentions in spontaneous actions. We previously showed that, when participants were required to estimate the onset of their intentions using this procedure, the activity in the presupplementary motor area (pre-SMA) was enhanced. Here, we show that when participants were required to estimate the onset of their motor executions (instead of their intentions), the activity in the cingulate motor area was enhanced. Across participants, the degree of this neural enhancement was correlated with the degree of perceptual bias: the higher the degree of enhancement, the earlier the perception. Analysis of data from a previous experiment suggests that the same principle holds true for the relationship between the perceived onset of intentions and the activity in the pre-SMA. We therefore argue that the timing method of Libet et al. (1983) is problematic, because the measuring process affects the neural representations of action and thus also the perceived onsets that the method is designed to measure.

Cortical oscillatory changes occurring during somatosensory and thermal stimulation

Brief somatosensory stimuli are followed by amplitude decreases (event-related desynchronization, ERD) of the 10 and 20 Hz oscillations over the bilateral primary sensorimotor cortices, and by post-stimulus synchronization (event-related synchronization, ERS) of the 20 Hz oscillations in the contralateral primary sensorimotor cortex and in the supplementary motor area (SMA). The 10 and 20 Hz ERD differentiate weak and strong somatosensory stimuli but not fine intensity gradations, and the ipsilateral ERD is especially sensitive to habituation. Stimulus anticipation, motor imagery, action viewing as well as voluntary movements modulate the stimulus-related changes of cortical oscillations. Noxious laser stimuli, selectively activating Adelta and/or C fibers, and innocuous warm and cold stimuli are associated with 10 and 20 Hz ERD but not with the post-stimulus 20 Hz ERS suggesting that the post-stimulus ERS is only related to neuronal transmission in the lemniscal system. It is proposed that phase-unlocked cortical oscillations modulate the preparedness of a particular sensory channel for upcoming somatosensory processing.

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.

Perceptual acceleration of objects in stream: evidence from flash-lag displays

An object in continuous motion is perceived ahead of the briefly flashed object, although the two images are physically aligned (Nijhawan, 1994), the phenomenon called flash-lag effect. Flash-lag effects have been found also with other continuously changing features such as color, pattern entropy, and brightness (Sheth, Nijhawan, & Shimojo, 2000) as well as with streamed pre- and post-target input without any change of the feature values of streaming items in feature space (Bachmann & Põder, 2001a. 2001b). We interpret all instances of the flash-lag as a consequence of a more fundamental property of conscious perception in general: acceleration of the speed with which samples of perceptual information become represented in explicit format immediately after the stimulation onset. Decreased visual latency of the samples of stimulus information from the streamed input leads to the relative perceptual lag for the separately flashed stimulus because it is not preceded by adjacent sensory input that would have accelerated its perception. Experimental support for the notion of perceptual acceleration is reviewed.

Effects of nonconscious perception on motor response

The present study reviews the literature on the empirical evidence for the dissociation between perception and action. We first review several key studies on brain-damaged patients, such as those suffering from blindsight and visual/tactile agnosia, and on experimental findings examining pointing movements in normal people in response to a nonconsciously perceived stimulus. We then describe three experiments we conducted using simple reaction time (RT) tasks with backward masking, in which the first (weak) and second (strong) electric stimuli were consecutively presented with a 40-ms interstimulus interval (ISI). First, we compared simple RTs for three stimulus conditions: weak alone, strong alone, and double, i.e., weak plus strong (Experiment 1); then, we manipulated the intensity of the first stimulus from the threshold (T) to 1.2T and 2T, with the second stimulus at 4T (Experiment 2); finally, we tested three different ISIs (20, 40, and 60 ms) with the stimulus intensities at 1.2T and 4T for the first and second stimuli (Experiment 3). These experiments showed that simple RTs were shorter for the double condition than for the strong-alone condition, indicating that motor processes under the double condition may be triggered by sensory inputs arising from the first stimulus. Our results also showed that the first stimulus was perceived without conscious awareness. These findings suggested that motor processes may be dissociated from conscious perceptual processes and that these two processes may not take place in a series but, rather, in parallel. We discussed the likely mechanisms underlying nonconscious perception and motor response to a nonconsciously perceived stimulus.

Time course and magnitude of movement-related gating of tactile detection in humans. III. Effect of motor tasks

This study investigated the relative importance of central and peripheral signals for movement-related gating by comparing the time course and magnitude of movement-related decreases in tactile detection during a reference motor task, active isotonic digit 2 (D2) abduction, with that seen during three test tasks: a comparison with active isometric D2 abduction (movement vs. no movement) evaluated the contribution of peripheral reafference generated by the movement to gating; a comparison with passive D2 abduction (motor command vs. no motor command; movement generated by an external agent) allowed us to evaluate the contribution of the central motor command to tactile gating; and finally, the inclusion of an active "no apparatus," or freehand, D2 abduction task allowed us to evaluate the potential contribution of incidental peripheral reafference generated by the position detecting apparatus to the results (apparatus vs. no apparatus). Weak electrical stimuli (2-ms pulse; intensity, 90% detected at rest) were applied to D2 at different delays before and after movement onset or electromyographic (EMG) activity onset. Significant time-dependent movement-related decreases in detection were obtained with all tasks. When the results obtained during the active isotonic movement task were compared with those obtained in the three test tasks, no significant differences in the functions describing detection performance over time were seen. The results obtained with the isometric D2 abduction task show that actual movement of a body part is not necessary to diminish detection of tactile stimuli in a manner similar to the decrease produced by isotonic, active movement. In the passive test task, the peak decrease in detection clearly preceded the onset of passive movement (by 38 ms) despite the lack of a motor command and, presumably, no movement-related peripheral reafference. A slightly but not significantly earlier decrease was obtained with active movement (49 ms before movement onset). Expectation of movement likely did not contribute to the results because stimulus detection during sham passive movement trials (subjects expected but did not receive a passive movement) was not different from performance at rest (no movement). The results obtained with passive movement are best explained by invoking backward masking of the test stimuli by movement-related reafference and demonstrate that movement-related reafference is sufficient to produce decreases in detection with a time course and amplitude not significantly different from that produced by active movement.

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.

The interpretation of Libet's results on the timing of conscious events: a commentary

A commentary on articles by Klein, Pockett, and Trevena and Miller, in this issue, is given. Average shift in the point of subjective equality (PSE), calculated by Klein on Libet's data, and corresponding change in mean shift, calculated by Libet et al. (1983), may be "corrected," taking as a reference point the end of the minimum train duration. Values obtained, if significant, indicate a latency for conscious sensation of the skin stimulus of at least 230 ms. Pockett's main conclusions are favored, but her explanation of peripheral-lemniscal couplings is found to be unconvincing. Trevena and Miller's article unsuccessfully tries to rescue a dualist interactionist view. Libet's method of timing intentions is thoroughly criticized.

Problems in the timing of conscious experience

Libet's (2000) arguments in defense of his interpretation of his experimental results are insufficient. The claims of my critical review (Gomes, 1998) do not suffer with his new statements.

On subjective back-referral and how long it takes to become conscious of a stimulus: a reinterpretation of Libet's data

The original data reported by Benjamin Libet and colleagues are reinterpreted, taking into account the facilitation which is experimentally demonstrated in the first of their series of articles. It is shown that the original data equally well or better support a quite different set of conclusions from those drawn by Libet. The new conclusions are that it takes only 80 ms (rather than 500 ms) for stimuli to come to consciousness and that "subjective back-referral of sensations in time" to the time of the stimulus does not occur (contrary to Libet's original interpretation of his results).

Libet's research on the timing of conscious intention to act: a commentary

S. Pockett (Consciousness and Cognition, this issue) and G. Gomes (Consciousness and Cognition, this issue) discuss a possible bias in the method by which Libet's subjects estimated the time at which they became aware of their intent to move their hands. The bias, caused by sensory delay processing the clock information, would be sufficient to alter Trevena and Miller's (Consciousness and Cognition, this issue) conclusions regarding the timing of the lateralized readiness potential. I show that the flash-lag effect would compensate for that bias. In the last part of my commentary I note that the other target articles do not examine the most interesting aspect of Libet's unfashionable views on free will. I point out that Libet's views are less strange than they at first appear to be.

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.