Wednesday, January 23, 2013
An accumulator model for spontaneous neural activity prior to self-initiated movement
In this previous post there was a false link; this is the correct one. Following are some excerpts from the article. My summary: In Libet’s experiments the readiness potential (RP) was assumed to be an unconscious neural precursor to the conscious decision to move. Whereas in this experiment the RP is present whether or not a neural decision to move arises. The RP’s “causal role is incidental” to the movement. The actual neural decision to move, that is when the RP crosses a certain threshold, just happens to “coincide in time with average subjective estimates of the time of awareness of intention to move.” Now what does that tell us about free will?
“Our account departs from the prevailing assumptions about the nature of the RP and thus suggests that some very basic questions be revisited from a different perspective.
“It is widely assumed that the neural decision to move coincides with the onset of the RP (which, given its slow nonlinear character, is difficult to pinpoint) (11). Our model challenges that assumption by suggesting that the “neural decision to move now” might come very late in the time course of the RP.
“Thus, according to our model, uncued movements in a task like Libet’s tend to be preceded by a gradual increase in neural activity [measured at the scalp (8, 9) or the single-neuron level (16)] whose causal role is incidental—not directed (consciously orotherwise) at producing a movement.
“Given that such spontaneous fluctuations are always present (55), even when we are not even thinking about moving, is it reasonable to conclude that the brain “decided” to move 2 s before the threshold crossing? We suggest reserving the term “decision” to the commitment to move achieved once neural activity (spontaneous or goal directed) crosses a specific threshold. Libet et al.’s (9) findings were surprising because they suggested that the neural decision to move happens well before we are aware of the urge to move, by 1/2 s or more. According to our model, this conclusion is unfounded. The reason we do not experience the urge to move as having happened earlier than about 200 ms before movement onset is simply because, at that time, the neural decision to move (crossing the decision threshold) has not yet been made. A very similar fluctuation in neuronal firing could equally well, at some other time, have not preceded a movement.
“Finally, although our model is silent with respect to the urge to move and its temporal relation to motor decisions, it helps dissolve another puzzling question that seemed to arise from Libet’s paradigm. Libet himself found that subjects were able to estimate the time of a tactile sensory decision in relation to a quickly rotating clock dial with only about 50 ms of error on average (9). Why then should there be such a long and variable gap between the time of a motor decision and the subjective estimate of the time of the motor decision, whereas no such gap exists for sensory decisions? In fact, this question arises only when we assume that the motor decision coincides in time with the onset of the RP. We have argued that this need not be the case and that the neural decision to move may come much closer in time to the movement itself (e.g., −150 ms). We propose that the neural decision to move coincides in time with average subjective estimates
of the time of awareness of intention to move (9, 11) and that the brain produces a reasonably accurate estimate of the time of its movement-causing decision events.”