Affordance competition

Since the Libet experiment (and many following it), it has been difficult to assume that conscious intention causes motor action as the action decision is chronologically ahead of the conscious feeling of intention. But there was some who thought that the vetoing of a decision was still possibly caused by conscious intention – the ‘free-wont’ idea. A recent paper (citation below) looks at the mechanism of switching motor plans and makes ‘free-wont’ less attractive.

(The) “affordance competition” hypothesis stands in contrast to the classical serial model, in which decisions are made in higher cognitive centers and the resulting choice passed down to the sensorimotor system for execution. Instead, it suggests that decisions are determined when a competition between actions is resolved within the sensorimotor system – e.g., for reaching, within the fronto-parietal cortex and associated corticostriatal loops. This means that although the biases that influence the decision may come from many sources, including the activity of higher cognitive regions, it is in the sensorimotor system that the final decision is taken. … the same “forward models” useful for predicting the consequences of motor commands may also play a role in selecting the actions themselves by biasing activity in sensorimotor cortices. … The affordance competition hypothesis predicts that if we present a monkey with multiple reaching options associated with different rewards, neural activity in Pmd (dorsal premotor cortex) will be modulated by the relative value of those rewards. … However, the hypothesis also makes a complementary prediction: that the same cells involved in selecting the initial action will continue to be involved in adjusting and even switching between actions during overt behavior. In other words, if the environment changes and old opportunities are lost or new ones become available, the same integrated selection and sensorimotor guidance system should reflect the switch of the plan.

 

The question investigated was whether the very same cells that steer the initial decision will continue with their activities if an animal changes its mind and guide the new action.

It is plausible that once a decision is made and an action is launched toward a given target, the decision-related cells fall silent while a separate circuit becomes responsible for guiding movement toward the selected target. The results presented here suggest that this is not the case. We found that the very same PMd (dorsal premotor cortex) cells previously shown to reflect relative value during a delay period continue to update their activity to reflect when the monkey changes its plan during situations in which a previously selected action becomes unavailable. This argues against the distinction between regions responsible for choosing an action and those responsible for its guidance through on-line feedback, and in favor of the hypothesis that decisions emerge through a competition within the same circuit that guides movement execution. … This suggests a view whereby sensory information continuously flows into the motor system, as opposed to a view of separate computational stages involved in canceling one motor program and computing a new one.

 

Here is the abstract which outlines the method as well as the results:

Previous studies have shown that neural activity in primate dorsal premotor cortex (PMd) can simultaneously represent multiple potential movement plans, and that activity related to these movement options is modulated by their relative subjective desirability. These findings support the hypothesis that decisions about actions are made through a competition within the same circuits that guide the actions themselves. This hypothesis further predicts that the very same cells that guide initial decisions will continue to update their activities if an animal changes its mind. For example, if a previously selected movement option suddenly becomes unavailable, the correction will be performed by the same cells that selected the initial movement, as opposed to some different group of cells responsible for online guidance. We tested this prediction by recording neural activity in the PMd of a monkey performing an instructed-delay reach selection task. In the task, two targets were simultaneously presented and their border styles indicated whether each would be worth 1, 2, or 3 juice drops. In a random subset of trials (FREE), the monkey was allowed a choice while in the remaining trials (FORCED) one of the targets disappeared at the time of the GO signal. In FORCED-LOW trials the monkey was forced to move to the less valuable target and started moving either toward the new target (Direct) or toward the target that vanished and then curved to reach the remaining one (Curved). Prior to the GO signal, PMd activity clearly reflected the monkey’s subjective preference, predicting his choices in FREE trials even with equally valued options. In FORCED-LOW trials, PMd activity reflected the switch of the monkey’s plan as early as 100 ms after the GO signal, well before movement onset (MO). This confirms that the activity is not related to feedback from the movement itself, and suggests that PMd continues to participate in action selection even when the animal changes its mind on-line. These findings werereproduced by a computational model suggesting that switches between action plans can be explained by the same competition process responsible for initial decisions.

 

This doesn’t rule out ‘free-wont’ but it certainly weakens it.

 

ResearchBlogging.org

Pastor-Bernier A, Trembley E, & Cisek P (2012). Dorsal premotor cortex is involved in switching motor plans Frontiers in Neuroengineering, 5

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