Abstract
The roles of prefrontal and anterior cingulate cortices have been widely studied, yet little is known on how they interact to enable complex cognitive abilities. We investigated this issue in a complex yet well-defined symbolic paradigm: algebraic problem solving. In our experimental problems, the demands for retrieving arithmetic facts and maintaining intermediate problem representations were manipulated separately. An analysis of functional brain images acquired while participants were solving the problems confirmed that prefrontal regions were affected by the retrieval of arithmetic facts, but only scarcely by the need to manipulate intermediate forms of the equations, hinting at a specific role in memory retrieval. Hemodynamic activity in the dorsal cingulate, on the contrary, increased monotonically as more information processing steps had to be taken, independent of their nature. This pattern was essentially mimicked in the caudate nucleus, suggesting a related functional role in the control of cognitive actions. We also implemented a computational model within the Adaptive Control of Thought-Rational (ACT-R) cognitive architecture, which was able to reproduce both the behavioral data and the time course of the hemodynamic activity in a number of relevant regions of interest. Therefore, imaging results and computer simulation provide evidence that symbolic cognition can be explained by the functional interaction of medial structures supporting control and serial execution, and prefrontal cortices engaged in the on-line retrieval of specific relevant information.
Highlights
Humans are set apart from other species by their ability to perform sequences of arbitrary operations over complex representations
Our view is that the control of behavior is mainly due to the activity of medial frontal and subcortical structures, whereas prefrontal and parietal cortices are mostly engaged in maintaining specific types of task information
We found responses of identical magnitude in the motor region and the characteristic sensitivity of the lateral PFC to arithmetic retrievals. With their involvement in cognitive control, monotonically larger responses were found in the dACC and the caudate nucleus
Summary
Humans are set apart from other species by their ability to perform sequences of arbitrary operations over complex representations. Our view is that the control of behavior is mainly due to the activity of medial frontal and subcortical structures (and, in particular, the basal ganglia and ACC), whereas prefrontal and parietal cortices are mostly engaged in maintaining specific types of task information. The current task relies on two kinds of representations: (a) arithmetic knowledge retrieved during numeric calculations; and (b) intermediate states of the problem that must be maintained and updated during the solution process, and which we assumed to be represented as imagery of how the equations would be written after each operation. Activity in both ACC and the caudate nucleus should exhibit a monotonic increase in activation from the simplest to the most difficult condition The reason for this is that both retrieving arithmetic knowledge and maintaining intermediate problem states increase the amount of control states required and the amount of information transferred during cognitive operations.
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