On Libet et al.: 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

Conscious intention and human action: Review of the rise and fall of the readiness potential and Libet’s clock. Edward J. Neafsey. Consciousness and Cognition, Volume 94, September 2021, 103171. https://doi.org/10.1016/j.concog.2021.103171

Abstract: 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 Based on these findings, Libet’s methodology was flawed and his results are no longer valid reasons for rejecting Fodor’s “good old commonsense belief/desire psychology” that “my wanting is causally responsible for my reaching.”.

Keywords: Readiness potentialBereitschaftspotentialIntentionDecisionFree willHard problemConsciousnessLibetKornhuberNeuroscienceEpiphenomenon

4. Discussion

4.1. Intention Before RP: Has the Ghost Returned?

If intentions precede the RP, does that mean that the “ghost in the machine” (Ryle, 1949) has returned and intentions are present without any brain activity? No. Even if the RP is plausibly only an artifact of averaging and even if tone probes have shown the onset of intentions takes place well before the onset of any RPs, that does not mean that nothing is going on in the brain when these intentions begin. The UCLA neurosurgeon Itzhak Fried and his coworkers recorded neuronal activity from depth electrodes implanted into the medial frontal lobe (SMA, pre-SMA, and ACC (anterior cingulate cortex)) during performance of the Libet clock task in “12 subjects with pharmacologically intractable epilepsy to localize the focus of seizure onset” (Fried, Mukamel, & Kreiman, 2011). Each depth electrode included nine microwires capable of recording single and multi-unit neuronal activity, and 760 units (254 single units and 496 multiunits) were recorded in the SMA, pre-SMA, and ACC of the 12 patients. As seen in Fig. 5A, they found “progressive neuronal recruitment over $~$1500 ms before subjects report making the decision to move …[with a] progressive increase or decrease in neuronal firing rate, particularly in the supplementary motor area (SMA), as the reported time of decision was approached.” Much of this early neuronal activity took place in the 1500 ms preceding movement, but there were a number of neurons whose firing rates changed even earlier. And, as illustrated in Fig. 5B, in experiments in monkeys done in the lab of Mark Churchland by Lara, Cunningham, and Churchland (Jul. 2018) SMA neurons showed “preparatory and movement-related activity that covaried with reach direction,” in marked contrast to the human early RP’s lack of any movement specificity. So there is early, movement-specific neuronal activity during these early intentions.

Fig. 5

Fried, like Libet, found the W time was only about 0.2 s before the movement, but, as shown above in the studies from the labs of Matsuhashi and Hallett and Verbaarschot, W time utterly fails to capture when intention actually begins. Related to such early intentions, Miller and Schwarz (2014) comment that “At the start of each trial, it seems plausible that participants would already have a weak yet conscious urge to move within the next few seconds, simply because they know that their task is to make such movements. …In this scenario, the fact that brain activity appears to emerge before the conscious decision—i.e., before W—is merely an artifact of the experimenter’s requirement that the observer impose an arbitrary criterion for making a binary judgment about an inherently gradual process that underlies decision making.” In other words, the mere presence of the subjects in the experimental situation indicates that some form of intention is already and always present; the actual intention to “move now” (Searle’s (1980) “intention in action”) arises from and “is caused by this [earlier, pre-existing] prior intention” (Searle, 1980) to move sometime (but not now).8 In a sense, given the Libet-type experimental situation, it is not possible for the subject to be in an “intentionless” state from which a new, fully-formed intention arises, as when Athena was born as a fully formed adult from the head of Zeus. This makes it impossible, in principle, to even address the question of timing of mental and physical processes in a Libet-type experiment. And, as shown by tone probes, even the final intention to move now is not instantaneous or abrupt.

Lastly, as noted by Miller and Schwarz (2014), this same fact about intention being always and already present in experimental subjects also applies to the fMRI study by Soon, Brass, Heinze, and Haynes (2008) who found very early prefrontal activity as long as 10 s before the conscious decision times of their subjects. This early activity “predicted” whether the left or right index finger would be moved (57% accurate vs. 50% for chance)9 but this activity so long before movement likely had just as much to do with the intention NOT to move my left (or right) finger now that must also be present, especially in the frontal areas where brain damage leads to loss of frontal cortical inhibitory control over behavior (Malmo, 1942, Pribram et al., 1964). Or, as suggested by Koenig-Robert and Pearson (2019), the early activity could be viewed “not in terms of unconscious decision processes …but rather by a process in which a decision (which could be conscious) is informed.” Guggisberg and Guggisberg (2013) expressed a similar view that “intention consciousness does not appear instantaneously but builds up progressively …[and] early neural markers of decision outcome are not unconscious but simply reflect conscious goal evaluation stages which are not final yet and therefore not reported with the clock method.”

5. Conclusions

5.1. The RP Is Not What It Seemed To Be

The “paradigm” (Kuhn, 1970). that the early RP indicates brain activity preparing for movement was and is beset by several important “anomalies” The first and perhaps most important anomaly is the RP’s dependence on averaging EEG potentials whose noise, when averaged, can reproduce the RP’s waveform. This was the point of attack for Eccles (1985), Ringo (1985), Stamm (1985), Schurger et al. (2012), Schmidt et al. (2016), and Maoz et al. (2019). The second anomaly is that an RP is also seen before involuntary or unconscious movements (Keller & Heckhausen, 1990) and even before decisions that involve no movement at all (Alexander et al., 2016). The third anomaly is the early RP’s lack of movement specificity, since very similar RPs occur before completely different movements, such as right hand vs. left hand (Haggard and Eimer, 1999, Herrmann et al., 2008). Related to this is that the RPs do not differ before movements with completely different motives and intentions, as seen in the RPs in the Free Wally and Object Tasks (Verbaarschot et al., 2019). The fourth anomaly is the onset time of the RP, which, for the exact same movement, has an almost perfect linear relationship to the interval between movements (Verleger et al., 2016). And the last anomaly is the absence of the RP before deliberate choice movements (Maoz et al., 2019). All of these facts argue against the early RP having anything to do with preparation for a specific movement or the voluntary intention to move and make any comparison of RP onset times and W times pointless. Whether the RP starts before (Libet) or after (tone probes) intention means nothing because the RP’s relation to upcoming movement is an illusion.

5.2. Intentions Begin Much Earlier Than Libet’s W Times

The results from the labs of Matsuhashi and Hallett (2008) and Verbaarschot et al., 2016, Verbaarschot et al., 2019 using tone probes to measure intention clearly suggest that intention is not an all-or-none phenomenon but a gradual process that begins much earlier than estimated by Libet’s W time and in many cases before the onset of the RP. But is Libet’s clock time W intention the same as the intention detected by tone probes? Matsuhashi and Hallett (2008) told their subjects to make the movement “as soon as you think about the next movement,” to ignore the tone if they are “not thinking about the next movement,” and to stop the movement “if you hear the tone after you have started thinking about the next movement or making the movement.” So these instructions clearly identify intention to move with “thinking about the next movement.” In the two studies from Verbaarschot’s lab, the instructions explicitly said to “veto their act if they were intending to act at the time they heard the beep” so “intention to move” very clearly meant “intending to act at the time.” Both labs had similar results, with tone probe intentions beginning early and even sometimes before the onset of the RP, so the small differences in the language of the instructions given to the subjects (“thinking about the next movement” vs. “intending to act at the time”) do not seem significant and are both roughly equivalent to the variety of terms Libet’s study used for reporting the time of “conscious awareness of ‘wanting’ to perform a given self-initiated movement,” which was “also described as an “urge’ or ‘intention’ or ‘decision’ to move” (Libet et al., 1983). So it would seem that the subjects in the different labs had the same concept of “intention” and that tone probes were a more sensitive way to measure the presence of intention, forcing subjects to attend to even the slightest inkling or trace of intention.