Tuesday, February 16, 2016

A 50 year misunderstanding of how we decide to initiate action - our intuition is valid

A commonly accepted assumption (that underlies all of the essays at dericbownds.net and many mindblog posts) is that a line of experiments starting in the early 1980s with Benjamin Libet’s work has demonstrated that our brains initiate an action 300 sec or more before our conscious ‘urge’ to move. It is as if we are ‘late’ to action because separate pathways are initiating the actual movement and our delayed ‘intention’ to move. This counterintuitive paradox has generated vigorous controversy for many years, debate of its implications regarding ‘free will’, etc.

Schurger et al. point out that the ‘readiness potential’ (RP, a slow build-up of scalp electrical potential preceding the onset of subjectively spontaneous voluntary movements ) discovered 50 years ago was presumed to reflect the consequence of decision process in the brain (‘the electro-physiological sign of planning, preparation, and initiation of volitional acts’). A new generation of experiments is now suggesting that brain activity preceding spontaneous voluntary movements (SVMs) "may reflect the ebb and flow of background neuronal noise, rather than the outcome of a specific neural event corresponding to a ‘decision’ to initiate movement... [Several studies] have converged in showing that bounded-integration processes, which involve the accumulation of noisy evidence until a decision threshold is reached, offer a coherent and plausible explanation for the apparent pre-movement build-up of neuronal activity."
SVMs rely on the same neural decision mechanism used for perceptual decisions – integration to bound – except that in this case there is no specific external sensory evidence to integrate. In particular, when actions are initiated spontaneously rather than in response to a sensory cue, the process of integration to bound is dominated by ongoing stochastic fluctuations in neural activity that influence the precise moment at which the decision threshold is reached. In this context, time locking to movement onset means time locking to crests in these temporally autocorrelated background fluctuations, which results in the appearance of a slow, nonlinear build-up in the average. This in turn gives the natural but erroneous impression of a goal-directed brain process corresponding to the ‘cerebral initiation of a spontaneous voluntary act’
The Philosophical Implications:
Many have found Libet et al.’s results so striking because they appear to clash with our commonsense view of action initiation. However, the novel interpretation of the RP that the stochastic decision model provides actually suggests a close correspondence between the two. When one forms an intention to act, one is significantly disposed to act but not yet fully committed. The commitment comes when one finally decides to act. The stochastic decision model reveals a remarkably similar picture on the neuronal level, with the decision to act being a threshold-crossing neural event that is preceded by a neural tendency toward this event.
In addition, dropping the problematic theoretical assumption that a decision to act cannot occur without being conscious also helps to dispel the apparent air of ‘paradox’ surrounding these findings. As with other types of mental occurrence, the decision to initiate an action can occur before one is aware of it. So we can identify the neural initiating event with a decision that we may become aware of only a brief instant later. All this leaves our commonsense picture largely intact.
Finally, distinguishing between the decision to act and the earlier forming of an intention fits well with the distinction between a prediction and a forecast. If our concern is merely forecasting, what is relevant is the less-committed event of an intention's forming, which we identify with the neural tendency. If our concern is prediction, we should focus on the later event of deciding, which we identify with the crossing of the threshold.
Added Note: Those interested in this vein of work might check Schultze-Kraft et al.:
In humans, spontaneous movements are often preceded by early brain signals. One such signal is the readiness potential (RP) that gradually arises within the last second preceding a movement. An important question is whether people are able to cancel movements after the elicitation of such RPs, and if so until which point in time. Here, subjects played a game where they tried to press a button to earn points in a challenge with a brain–computer interface (BCI) that had been trained to detect their RPs in real time and to emit stop signals. Our data suggest that subjects can still veto a movement even after the onset of the RP. Cancellation of movements was possible if stop signals occurred earlier than 200 ms before movement onset, thus constituting a point of no return.

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