Confusions regarding stochastic fluctuations and accumulators 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 already a few hundred ms before a participant makes a conscious decision to move. If the onset of the readiness potential is taken as an indicator of the "brain's decision to move" this could mean that this decision to move is made early, by unconscious brain activity, rather than later, at the time when the subject believes to be deciding. However, an alternative interpretation has recently been discussed, the stochastic decision model (SDM), that takes its inspiration from models of perceptual decision making. It suggests that the RP originates from an accumulation of stochastic internal fluctuations. In this view the decision happens only at a much later stage when an accumulated noise signal reaches a threshold. Here we address a number of confusions regarding both the evidence for the stochastic decision model as well as its interpretation. We will show: (a) that the evidence for the role of stochastic fluctuations is highly indirect; (b) that there is little direct support for the SDM from animal studies; (c) that deterministic (non-random) processes can explain the data in a similar way; (d) that the relative components of the model have been mischaracterized leading to an over-emphasis on the role of random fluctuations and an under-emphasis of deterministic aspects of the model; (e) that there is confusion regarding the role of "urgency" and "evidence" in the SDM and its link to perceptual decision making; (f) that the question whether the decision happens early or late depends on the nature of the noise fluctuations, specifically, whether they reflect "absolute" or "epistemic" randomness; (g) finally, that the model does not explain the temporal relationship between conscious decision and neural decision. Our aim is not to rehabilitate the role of RPs in the free will debate. Rather we aim to address some confusions and premature conclusions regarding the evidence for accumulators playing a role in these preparatory brain processes.