Using a Veto paradigm to investigate the decision models in explaining Libet-style experiments.

The question of whether free will exists or not has intrigued philosophers for centuries. About 40 years ago, cognitive neuroscientists such as Benjamin Libet have joined the discussion by demonstrating that an ERP component, the readiness potential (RP), precedes the reported time of decision to act by a few hundred milliseconds. Libet et al. (1983) argued that our brains unconsciously prepare the movement before we experience any conscious intention, which led some free will skeptics (e.g., Ebert & Wegner, 2011) to argue that free will does not exist. While Libet's interpretation of his findings initiated an intense philosophical debate, alternative interpretations have been put forward more recently (Bode et al., 2014; Brass et al., 2019; Schurger et al., 2012; 2021). Integration to bound models (ITB) of Libet-style experiments suggest that we accumulate information until an intention threshold is reached, which triggers our experience of intention and execution of voluntary behaviors. The RP, from this perspective reflects the decision process itself rather than the consequence of an unconscious decision. To determine if the ITB model better predicts behavioral patterns in Libet-style experiments, we added a whether-component to the classical Libet task (the Veto Libet task) and compared the behavioral measures in the Veto Libet task with the Classical Libet task. We hypothesized that the signal accumulation in the Veto Libet task would be less steep than in the Classical Libet task, resulting in longer wait times and earlier self-reported intentions to act (i.e., the W). The result in general supported our hypotheses. In addition, these behavioral differences between the Classical Libet task and the Veto Libet task established valuable behavioral correlates for future investigations into the vetoing phenomenon. Finally, this study was also the first application of the Libet task in an online setting, and the behavioral parameters were highly comparable to the previous offline studies, further supporting the possibility of using the online platform to study arbitrary decision-making.

A spatial-attentional mechanism underlies action-related distortions of time judgment.

Temporal binding has been understood as an illusion in timing judgment. When an action triggers an outcome (e.g. a sound) after a brief delay, the action is reported to occur later than if the outcome does not occur, and the outcome is reported to occur earlier than a similar outcome not caused by an action. We show here that an attention mechanism underlies the seeming illusion of timing judgment. In one method, participants watch a rotating clock hand and report event times by noting the clock hand position when the event occurs. We find that visual spatial attention is critically involved in shaping event time reports made in this way. This occurs because action and outcome events result in shifts of attention around the clock rim, thereby biasing the perceived location of the clock hand. Using a probe detection task to measure attention, we show a difference in the distribution of visual spatial attention between a single-event condition (sound only or action only) and a two-event agency condition (action plus sound). Participants accordingly report the timing of the same event (the sound or the action) differently in the two conditions: spatial attentional shifts masquerading as temporal binding. Furthermore, computational modeling based on the attention measure can reproduce the temporal binding effect. Studies that use time judgment as an implicit marker of voluntary agency should first discount the artefactual changes in event timing reports that actually reflect differences in spatial attention. The study also has important implications for related results in mental chronometry obtained with the clock-like method since Wundt, as attention may well be a critical confounding factor in the interpretation of these studies.

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.

The place of Free Will: the freedom of the prisoner.

Debates about the concept of Free Will date back to ancient times. About 40 years ago, Benjamin Libet designed an experiment showing that the conscious intention to move is preceded by a specific pattern of brain activation. His finding suggested that unconscious processes determine our decisions. Libet-style experiments have continued to dominate the debate about Free Will, pushing some authors to argue that the existence of Free Will is a mere illusion. We believe that this dispute is because we often measure Free Will using arbitrary human decisions rather than deliberate actions. After reviewing the definition of Free Will and the related literature, we conclude that the scientific evidence does not disprove the existence of Free Will. However, our will encounters several constraints and limitations that should be considered when evaluating our deeds' personal responsibility.

Clarifying the nature of stochastic fluctuations and accumulation processes in spontaneous movements.

Experiments on choice-predictive brain signals have played an important role in the debate on free will. In a seminal study, Benjamin Libet and colleagues found that a negative-going EEG signal, the readiness potential (RP), can be observed over motor-related brain regions even hundreds of ms before the time of the conscious decision to move. If the early onset of the readiness potential is taken as an indicator of the "brain's decision to move" this could mean that this decision is made early, by unconscious brain activity, rather than later, at the time when the subject believes to have decided. However, an alternative kind of interpretation, involving ongoing stochastic fluctuations, has recently been brought to light. This stochastic decision model (SDM) takes its inspiration from leaky accumulator models of perceptual decision making. It suggests that the RP originates from an accumulation of ongoing stochastic fluctuations. In this view, the decision happens only at a much later stage when an accumulated noisy signal (plus imperative) reaches a threshold. Here, we clarify a number of confusions regarding both the evidence for the stochastic decision model as well as the interpretation that it offers. We will explore several points that we feel are in need of clarification: (a) the empirical evidence for the role of stochastic fluctuations is so far only indirect; (b) the interpretation of animal studies is unclear; (c) a model that is deterministic during the accumulation stage can explain the data in a similar way; (d) the primary focus in the literature has been on the role of random fluctuations whereas the deterministic aspects of the model have been largely ignored; (e) contrary to the original interpretation, the deterministic component of the model is quantitatively the dominant input into the accumulator; and finally (f) there is confusion regarding the role of "imperative" in the SDM and its link to "evidence" in perceptual decision making. Our aim is not to rehabilitate the role of the RP in the free will debate. Rather we aim to address some confusions regarding the evidence for accumulators playing a role in these preparatory brain processes.

Physiological and introspective antecedents of tics and movements in adults with tic disorders.

OBJECTIVE: To investigate the subjective phenomenon and the neural underpinnings of tics compared with voluntary movements in patients with tic disorders. METHODS: We recorded electroencephalographic and electromyographic data while subjects completed a Libet clock paradigm. Patients and healthy volunteers reported the times of W (willing to move) and M (movement occurrence) while performing voluntary movements. This was repeated only for the patients for the tics. RESULTS: In the patients, W and M times preceding voluntary movements and tics did not significantly differ from voluntary movements of healthy volunteers. The Bereitschaftspotentials in the patients were similar to healthy volunteers. Tics were only assessable for 7 patients due to artifacts. Two subjects did not show Bereitschaftspotentials, and they reported the lowest levels of tic voluntariness. 5 subjects did not show beta band event-related desynchronization before tics. CONCLUSIONS: For patients, the sense of volition for tics is similar to that of their voluntary movements which is similar to normal. Patients showed dissociations between the Bereitschaftspotential and beta desynchronization for tics, with 5/7 showing normal Bereitschaftspotentials and 2/7 showing desynchronization. The absence of desynchronization may suggest attempts to suppress tics. SIGNIFICANCE: This physiology shows a difference for most tics compared with normal movements.

Stimulus-dependent deliberation process in left- and right-handers obtained via current source density analysis.

The aim of the present study was to compare the activity patterns of young, healthy right- (RH, n = 25) and left-handed (LH, n = 20) subjects in high-density electroencephalograpic (EEG) recordings during a deliberation task. The deliberation task consisted of pressing one of two keys depending on a color-word Stroop task (Stroop, 1935) presented on a computer screen. Depending on the color shown and the meaning of the color word, participants responded with the index finger of the dominant or non-dominant hand. This leads to different activities in the hemispheres depending on the acting hand and on subject's handedness. Presenting the word "black" in black color, subjects were not to press any key (no-go-trial). Prior to this, subjects were tested for simple motor tasks, during which they were informed about the motor action to be performed. The temporal activity patterns obtained from RH and LH were very similar in shape and constituent components. The comparison of the three types of trials lead to the assumption that the deliberation process is based on a two-step decision: The first decision was characterized by the choice between move (match-trials, mismatch-trials) or not to move (no-go-trials). The second decision resulted in the final judgment of which index finger has to be used. The latter decision, in particular, can be tracked via the local spread of activity over the scalp. Our hypothesis is based on a comparison of activities and locations of RH and LH and yields some insights about processing a two-step decision in a deliberation task.

Implicit mechanisms of intention.

High-level cortical regions encode motor decisions before or even absent awareness, suggesting that neural processes predetermine behavior before conscious choice. Such early neural encoding challenges popular conceptions of human agency. It also raises fundamental questions for brain-machine interfaces (BMIs) that traditionally assume that neural activity reflects the user's conscious intentions. Here, we study the timing of human posterior parietal cortex single-neuron activity recorded from implanted microelectrode arrays relative to the explicit urge to initiate movement. Participants were free to choose when to move, whether to move, and what to move, and they retrospectively reported the time they felt the urge to move. We replicate prior studies by showing that posterior parietal cortex (PPC) neural activity sharply rises hundreds of milliseconds before the reported urge. However, we find that this "preconscious" activity is part of a dynamic neural population response that initiates much earlier, when the participant first chooses to perform the task. Together with details of neural timing, our results suggest that PPC encodes an internal model of the motor planning network that transforms high-level task objectives into appropriate motor behavior. These new data challenge traditional interpretations of early neural activity and offer a more holistic perspective on the interplay between choice, behavior, and their neural underpinnings. Our results have important implications for translating BMIs into more complex real-world environments. We find that early neural dynamics are sufficient to drive BMI movements before the participant intends to initiate movement. Appropriate algorithms ensure that BMI movements align with the subject's awareness of choice.

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.

Diverging implicit measurement of sense of agency using interval estimation and Libet clock.

Besides relying on explicit agency ratings, research concerning the sense of agency seeks to employ implicit measures. One such measure is intentional binding, i.e., the shortening of the perceived temporal interval between an action and its consequence. Multiple paradigms have been used to measure intentional binding. Our aim was to compare two common paradigms within the same participants: interval estimation and the Libet clock paradigm. As both are supposed to measure intentional binding, we hypothesized the two measures correlated positively. In two experiments, we used Bayes factors as a stopping criterion for data collection in a correlational study design resulting in valid samples of 46 and 36 healthy adult students. Despite good reliability, Bayes factors showed moderate evidence for intentional binding from the two paradigms to be uncorrelated. Implications for future research of sense of agency and the necessity for nomological validity of its measures are discussed.

Impulsivity traits and awareness of motor intention in Parkinson's disease: a proof-of-concept study.

INTRODUCTION: In patients with Parkinson's disease (PD), impulsivity is still a matter of investigation. It has been hypothesized that impulsive personality traits may favour impulse control disorder (ICD) onset during dopaminergic therapy. In healthy subjects, a relationship between the awareness of motor intention and impulsive personality traits assessed by the Barratt impulsivity scale (BIS-11) has been reported. The aim of this study was to evaluate the relationship between the awareness of voluntary action and impulsivity traits in PD. METHODS: Twenty-eight PD patients (stages I-III on the Hoehn and Yahr scale) underwent an impulsivity trait assessment by the BIS-11 scale and a task based on the Libet's clock. Participants were requested to perform a self-initiated movement and report the time they first feel their intention to move (W-judgement) or the time of the actual movement (M-judgement). RESULTS: In patients with higher BIS-11 scores, the time lag between the W-judgement and the actual movement was significantly lower than in patients with lower BIS-11. No difference emerged in the M-judgement. CONCLUSION: Data suggest that also in PD patients, the impulsive personality trait is related to a "delayed" awareness of motor intention and therefore to a shorter interval to allow a conscious "veto" of the impending action. Characterization of the temporal profile of awareness of motor intention could prove useful in identifying PD patients at risk of developing ICDs during dopaminergic treatment.

Studying sense of agency online: Can intentional binding be observed in uncontrolled online settings?

Intentional binding is often used as an implicit index of the sense of agency. However, intentional binding research has primarily been conducted in controlled lab environments. During the COVID-19 global pandemic, there has been a shift to implementing studies using online platforms and it is an open question whether the intentional binding effect can be found using an online experimental set-up and participant sample. Here, we address this question by asking online participants to complete the Libet clock version of the intentional binding task, which we make freely available to researchers as a jsPsych (De Leeuw, 2014) plugin. Intentional binding was observed in the form of later keypress estimates and earlier auditory tone estimates, when the auditory tone followed the keypress. These findings confirm that intentional binding can be assessed in online contexts. We discuss these findings in relation to the broader intentional binding literature.

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.".

Inferior parietal lobule involved in representation of "what" in a delayed-action Libet task.

Intentional motor action is typically characterized by the decision about the timing, and the selection of the action variant, known as the "what" component. We compared free action selection with instructed action, where the movement type was externally cued, in order to investigate the action selection and action representation in a Libet's task. Temporal and spatial locus of these processes was examined using the combination of high-density electroencephalography, topographic analysis of variance, and source reconstruction. Instructed action, engaging representation of the response movement, was associated with distinct negativity at the parietal and centro-parietal channels starting around 750 ms before the movement, which has a source particularly in the bilateral inferior parietal lobule. This suggests that in delayed-action tasks, the process of action representation in the inferior parietal lobule may play an important part in the larger parieto-frontal activity responsible for movement selection.

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.

Mind and soul? Two notions in the light of contemporary philosophy.

From the perspective of the Aristotelian notion of 'Form', the author explores the history of the concepts of mind and soul focusing on their ontologized version, as entertained by conventional science. He concludes that current neuroscience lacks the conceptual wherewithal required to deal with the meaning of mind and soul and with agential consequences such as free will and moral decision making. [GEB].

A reduction in the implicit sense of agency during adolescence compared to childhood and adulthood.

Sense of agency(SoA), the fundamental feeling of control over our actions and their consequences, may show key developmental changes during adolescence. We examined SoA in childhood (9-10), mid-adolescence (13-14), late-adolescence (18-20) and adulthood (25-28) using two tasks (Libet Clock and Stream of Letters). SoA was implicitly indexed by intentional binding that reflects the agency effect on action-outcome temporal association. We found age effects on the sub-processes in both tasks. In the Libet Clock task, where performance was more reliable, we observed a U-shaped developmental trajectory of intentional binding suggesting an adolescent-specific reduction in the experience of control. This study provides evidence for the developmental effects on the implicit agency experience and suggests adolescence as a critical period. We discuss the possible implications of these findings.

When reflex reactions oppose voluntary commands: The StartReact effect on eye opening.

A startling auditory stimulus (SAS) induces a reflex response involving, among other reactions, a strong contraction of the orbicularis oculi muscle (OOc) and subsequent eye closure. A SAS also induces the StartReact effect, a significant shortening of reaction time in subjects ready for task execution. We examined the obvious conflict appearing when a StartReact paradigm requires participants with eyes closed to open their eyes to look for a visual target. We recorded OOc EMG activity and eyelid movements in healthy volunteers who were instructed to open their eyes at perception of a somatosensory imperative stimulus (IS) and locate the position of a Libet's clock's hand shown on a computer screen at 80 cm distance. In 6 out of 20 trials, we delivered a SAS simultaneously with the IS. The main outcome measures were reaction time at onset of eyelid movement and the time gap (TG) separating subjective assessment of the clock's hand position from real IS issuing. Control experiments included reaction time to eye closing and target location with eyes open to the same IS. Reaction time was significantly faster in SAS than in noSAS trials and slower for eye opening than for eye closing in both conditions. In the eye-opening task, TG was significantly shorter in SAS with respect to noSAS trials, despite the presence of the SAS-related burst in the OOc before EMG cessation. Our results indicate that the StartReact effect speeds up eye opening and location of a target in the visual field despite the startle reaction opposing the task.

Timing of the Sense of Volition in Patients With Schizophrenia.

Schizophrenic patients often do not have the sense that they direct their own movements or author their own thoughts (passivity phenomena). As willing must precede movement to be causal and thus generate the sense of agency, it is possible that the timing between the senses of willing and movement is shortened in schizophrenia. We tested the subjective perception of this time interval in patients with schizophrenia using a method based on Libet's paradigm, in which subjects specify a time W - the time of willing a movement - and a time M - the time that movement occurred. Patients with schizophrenia and healthy volunteers made voluntary movements at times of their own choice while looking at a fast-rotating clock on a computer screen and reported when their movements were willed and made. We recorded surface electromyography to determine the time of actual movement, and electroencephalography to record brain potentials associated with movement. Results showed a significantly reduced interval between the reported M and W in patients with respect to the healthy volunteers (p < 0.05). Specifically, patients did not report a significant difference in the timing of W at 19 ms prior to movement onset and M at 7.4 ms prior to movement onset (p > 0.05), while the control group experienced a time W at 100 ms prior to movement onset and this differed significantly from their time M at 19 ms prior to movement onset (p < 0.01). These results suggest that patients with schizophrenia do have an altered timing of awareness of action - or an impaired judgment of the sequence of events - and that this might be etiologic in the development of the abnormal sense of agency.

Methodological Factors Involved in the Study of Temporal Binding Using the Open Source Software Labclock Web.

Temporal binding occurs when an action and an outcome that follows it after a short period of time are judged as occurring closer to each other in time than they actually are. This effect has often been studied using Libet's clock methodology. Garaizar et al. (2016) presented Labclock Web, a free HTML5 open source software that allows researchers to conduct temporal binding and other experiments using Libet's clock through the Internet. The purpose of the three experiments presented here was to test how certain methodological modifications in the Labclock Web task could impact the temporal binding effect. In comparison with the original study, we aimed to: (a) reduce the interval between action and outcome in the delayed condition to 100 ms, instead of 500, (b) present the two types of trials, immediate and delayed, in two separate consecutive blocks, instead of intermixed, (c) use a visual, rather than auditory, outcome following the action, and (d) reduce the number of trials. In addition to its potential theoretical implications, the results confirm that Labclock Web is a useful and reliable tool for conducting temporal binding experiments and that it is well suited to measure temporal binding effects in a broad range of situations.