Integration-to-bound decision model

Neuroskeptic has a posting (here) with the title ‘Do Rats have Free Will?’ It is a review of a paper by Murakami and others - abstract is below.

The paper supports the integration-to-bound model of decision making. A population of secondary motor cortex neurons ramp up their output to a constant threshold. Crossing the threshold triggers the motor action. The researchers found a second group of neurons that appeared to establish the rate of rise of the integrating neurons and therefore the time that elapses before the threshold is reached. This fits the model. But what does it say about free will?

The abstract does not mention free will but Neuroskeptic does. It is fortunate that he has talked with the group and shared it in his post. He points out the similarity between the intergration signal and the readiness potential that Libet and others found preceded an action and preceded conscious awareness of a decision to act. He quotes Murakami: “activity preceding bound crossing, either input or accumulated activity, could be said to participate causally in the timing of an action, but does not uniquely specify it. The integration-to-bound theory implies that no decision has been made until the bound has been reached… as at any moment up to bound crossing, the arrival of opposing inputs may avert an action.” Neuroskeptic comments that the readiness potential may be a contributor to a decision rather than the consequence of a decision. And again quotes Murakami: “Crossing the threshold from unawareness to awareness [could be] a reflection of bound crossing [in the integrator]…In this way, the integration-to-bound theory may help to resolve the contradiction between the subjective report of free will and the requirement for causal antecedents to non-capricious, willed actions.…our results provide a starting point for investigating mechanisms underlying concepts such as self, will and intention to act, which might be conserved among mammalian species.”

Although their results do give confirmation to the integration-to-bound theory, I do not think they say much about free will. First, I cannot see how they have any information on when the rats are consciously aware of whatever they may be aware of in a decision. Second, if another signal is controlling the rate of integration, when was it set on course and what are the signals that might control it? This is a long way from an understanding of how decisions are made and whether consciousness is involved.

Abstract of paper (Murakami M, Vicente MI, Costa GM, & Mainen ZF (2014). Neural antecedents of self-initiated actions in secondary motor cortex. Nature neuroscience, 17 (11), 1574-82 PMID: 25262496):

The neural origins of spontaneous or self-initiated actions are not well understood and their interpretation is controversial. To address these issues, we used a task in which rats decide when to abort waiting for a delayed tone. We recorded neurons in the secondary motor cortex (M2) and interpreted our findings in light of an integration-to-bound decision model. A first population of M2 neurons ramped to a constant threshold at rates proportional to waiting time, strongly resembling integrator output. A second population, which we propose provide input to the integrator, fired in sequences and showed trial-to-trial rate fluctuations correlated with waiting times. An integration model fit to these data also quantitatively predicted the observed inter-neuronal correlations. Together, these results reinforce the generality of the integration-to-bound model of decision-making. These models identify the initial intention to act as the moment of threshold crossing while explaining how antecedent subthreshold neural activity can influence an action without implying a decision.


Leave a Reply

Your email address will not be published. Required fields are marked *