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Neural antecedents of self-initiated actions in secondary motor cortex
Masayoshi Murakami, M Ins Vicente, Gil M Costa & Zachary F Mainen
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.
npg 201 4 Nature America, Inc. All rights reserved.
Decision-making involves the selection of goals or actions, but it also requires determination of the timing of action. When there is a stimulus or cue to react to, the problem of when to act is constrained by sensory and motor requirements and may involve balancing of speed versus accuracy and other facets of sensorimotor coordination, which have been intensively studied1. In reaction time tasks, decisions are associated with trial-to-trial variations in response time. These fluctuations have been theorized to arise from the integration of noisy sensory signals by a neural integrator, whose crossing of activation threshold triggers an action24. In support of this theory, neurons in the primate frontal cortex, parietal cortex and superior colliculus show gradual increases in neural activity after presentation of visual stimuli, reaching a constant activity threshold at which a saccadic eye movement is generated; reaction times are correlated with rates of such ramping activity57.
In the absence of immediate sensory stimuli, the timing of more spontaneous actions is also important. Action initiation may be affected by internal state (for example, urgency signals) but, as with reaction...