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Analysis of a Choice-Reaction Task Yields a New Interpretation of Libet's Experiments ⁎ Christoph S

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Analysis of a Choice-Reaction Task Yields a New Interpretation of Libet's Experiments ⁎ Christoph S Available online at www.sciencedirect.com International Journal of Psychophysiology 67 (2008) 151–157 www.elsevier.com/locate/ijpsycho Fast track report Analysis of a choice-reaction task yields a new interpretation of Libet's experiments ⁎ Christoph S. Herrmann a,b,e, , Michael Pauen b,d,f, Byoung-Kyong Min a, Niko A. Busch a, Jochem W. Rieger c a Otto-von-Guericke-University, Dept. for Biological Psychology, Magdeburg, Germany b Center for Behavioural Brain Sciences, Magdeburg, Germany c Otto-von-Guericke-University, Dept., for Neurology II, Magdeburg, Germany d Humboldt-University, Institute for Philosophy, Berlin, Germany e Max Planck Institute for Human Cognitive and Brain Science, Leipzig, Germany f Berlin School of Mind and Brain, Germany Received 23 March 2007; received in revised form 11 October 2007; accepted 15 October 2007 Available online 22 November 2007 Abstract Benjamin Libet has demonstrated that the readiness potential precedes the time at which participants consciously decide to perform an intentional motor act, and suggested that free will is an illusion. We performed an experiment where participants observed a stimulus on a computer monitor and were instructed to press one of two buttons, depending on the presented stimulus. We found neural activity preceding the motor response, similar to Libet's experiments. However, this activity was already present prior to stimulus presentation, and thus before participants could decide which button to press. Therefore, we argue that this activity does not specifically determine behaviour. Instead, it may reflect a general expectation. This interpretation would not interfere with the notion of free will. © 2007 Elsevier B.V. All rights reserved. Keywords: Bereitschaftsfeld; Bereitschaftspotential; EEG; MEG; Voluntary action 1. Introduction determined, since it is known that this activity precedes motor acts. Libet's results showed that RPs preceded W by 350 ms. The actual In several experiments, Benjamin Libet and his colleagues motor act followed W after about 200 ms (Libet, 1985). Libet investigated the sequence of events between the neural preparation concluded that W and, consequently, the motor act were initiated of a motor act, the conscious intention to act, and the actual motor unconsciously ‘by the brain’, before the participant was aware of act (Libet et al., 1982, 1983). In the critical conditions, the his or her wish to move. Accordingly, it was concluded that the participants were instructed to flex their hand whenever they participant's conscious will plays no causal role in the initiation of wished to do so, and to determine the time of the related conscious the motor act (Libet et al., 1982; Libet, 1985). This interpretation act of will (W) from a fast rotating dot on a clockface. At the same was provocative and elicited an ongoing debate (summarized in time, the emergence of the so called ‘Bereitschaftspotential’ Libet, 1993). The increasing interest in neurophysiological results (readiness potential, RP; Kornhuber and Deecke, 1965)was and their interpretations in philosophy and popular science and the normative conclusions from Libet's results drawn by neurophy- siologists (Libet, 1993) demonstrate that these results may have a strong impact on human self-understanding. Therefore, Libet's ⁎ Corresponding author. Otto-von-Guericke-University of Magdeburg, PO experiments and their premises should be examined carefully. box 4120 39016 Magdeburg, Germany. Tel.: +49 391 67 18477; fax: +49 391 67 12222. Aside from the questionable generalisation of a millisecond E-mail address: [email protected] time scale to long term behavioural planning, Libet's conclu- (C.S. Herrmann). sions critically depend on several assumptions. One of them is 0167-8760/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ijpsycho.2007.10.013 152 C.S. Herrmann et al. / International Journal of Psychophysiology 67 (2008) 151–157 the idea that the RP determines the specific voluntary movement, movement. Accordingly, 40% of the W-times reported by and not just indicates a general preparation to move. However, as Trevena and Miller (2002) were later than the movements. Bridgeman (1985) pointed out, the participants' movements in Two conclusions can be drawn from these observations: First, Libet's experiments were requested by the experimenter. The W-judgements are problematic in time-critical tasks. Second, the participants were only ‘free’ to decide when to perform the specificity of the causal role of the RP (and of the LRP) with respect requested movement within a limited time range of several to W and the subsequent movement is still unclear. Although seconds, and only once per trial. This raises the question of the results of Haggard and Eimer (1999) and Trevena and Miller whether Libet in fact investigated decisions, given that there (2002) raise doubts whether the RP really determines W and were no alternatives to choose between. The decisive point is the movement, they provide no conclusive evidence with respect that, if the instructions determined what the participants were to to this problem. A critical test for the specificity of the causal role of do, it is less clear whether the RP really determines what the the RP that is not affected by problems concerning W-judgements, participants will do. Maybe it leaves room for different actions, would be to elicit an RP at a time when it is still uncertain which including, say, a movement of the other hand. This offers motor act has to be performed. If different movements can be participants the ability to do otherwise, which is often regarded performed after RP onset, this would undermine the assumption as one of the most important components of free action. that the RP determines a specific movement. This problem is also discussed by Haggard and Eimer We used a choice-reaction paradigm in which participants were (1999). In one of their experiments, participants could choose instructed to press a button with either their right or left hand after between a movement of the left or the right hand. The authors they had perceived the appropriate visual stimulus. Participants in recorded the lateralized RP (LRP; Eimer, 1998) in addition to our experiment did not make a free choice, because we were only the RP. While the RP is symmetric and represents the early part interested in the role of one constituent of voluntary acts, namely of the motor activity preceding movements, the LRP represents the RP. The advantage of a choice-reaction paradigm is that the a later phase and is lateralised to the hemisphere contralateral to presentation of the stimulus provides a clear terminus ante, with the side of movement. Haggard and Eimer (1999) analysed respect to the participant's knowledge of the response that has to covariations between the time of W and the onset times of the be performed, thus evading all the difficulties with W-judgements. RP/LRP. It turned out that the onset time of the RP did not covary with the W-time, while the onset time of the LRP did. 2. Materials and methods Based on the assumption that causally linked processes should covary over time, the authors argued that only the LRP, but not Sixteen subjects participated in the study. Six of them had to the RP, may be the unconscious cause of the conscious act of be rejected due to excessive artefacts or inappropriate will. However, there are several reasons why Haggard & behavioural performance. Data of ten participants with a Eimer's results are inconclusive as far as the causal role of the mean age of 21.7 years (ranging from 18 to 25 years, 6 female) RP is concerned. First, a causal connection between the RP and were analyzed. Participants gave written informed consent prior W cannot be ruled out completely, because the authors admit to the start of the experiment. All participants had normal or that they could not determine a clear baseline for the RP within corrected to normal vision and had no recorded history of the epoch they investigated. Note that, according to their own neurological or psychiatric disorders. The study protocol standards, such a covariation would provide evidence for a conformed with the guidelines of the ethics committee of the causal connection between the RP and W. Second, the causal Max-Planck-Institute for Human Cognitive and Brain Science connection between LRP and W is questionable. Two of the (Leipzig, Germany) and the Declaration of Helsinki. eight participants in the early W condition (participants 5 and 8 The present investigation is a reanalysis of the data reported in Table 2 in Haggard and Eimer, 1999) reported W-times in Herrmann and Mecklinger (2000). The stimulus material before LRP onset. So, although there is still a statistically consisted of two Kanizsa figures, i.e. a Kanizsa square and a significant covariance between the LRP and W, this observation Kanizsa triangle, and two non-Kanizsa figures, i.e. a non- raises doubts concerning a causal interpretation of this covari- Kanizsa square and a non-Kanizsa triangle (cf. Herrmann and ation. In addition, the experimental setup does not provide Mecklinger, 2000, for details). Stimuli were presented for conclusive evidence concerning the time of the decision be- 700 ms with randomized interstimulus-intervals ranging from tween the two options. It may well be that participants made 1000 to 1500 ms. Figures were displayed in black along with a their choice between left and right-hand movements shortly black central fixation cross on white background. Stimuli after completing the previous trial and long before W. In this subtended a visual angle of 4.28°, including inducer disks. We case, the W-judgement would reflect only the moment when the used a liquid crystal display (LCD) monitor whithout raster predetermined action is triggered. Moreover, W-judgements scan.
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