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The multiple-demand (MD) system of the primate brain: mental programs for intelligent behaviour

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A common or multiple-demand (MD) pattern of frontal and parietal activity is associated with diverse cognitive demands, and with standard tests of fluid intelligence. In intelligent behaviour, goals are achieved by assembling a series of sub-tasks, creating structured mental programs. Single cell and functional magnetic resonance imaging (fMRI) data indicate a key role for MD cortex in defining and controlling the parts of such programs, with focus on the specific content of a current cognitive operation, rapid reorganization as mental focus is changed, and robust separation of successive task steps. Resembling the structured problem-solving of symbolic artificial intelligence, the mental programs of MD cortex appear central to intelligent thought and action.

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Multiple-demand activity in the human brain

Over the past 20 years, functional neuroimaging has identified many individual associations between specific cognitive functions and specific regions of the human cerebral cortex. In parallel, there has been a more unexpected discovery – a common pattern of activity that is a salient part of the brain's response to many different kinds of cognitive challenge. Illustrated in Figure 1a, this multiple-demand (MD) pattern 1, 2 extends over a specific set of regions in prefrontal and parietal

Neurophysiology of mental programs

Each step of a mental program defines a new cognitive epoch, with different operations, goals, and task-relevant information. In everyday behaviour, such epochs follow in rapid succession, requiring frequent changes of processing context. Both single unit and functional magnetic resonance imaging (fMRI) data show features of MD activity well matched to the construction of focused cognitive epochs, and rapid transition from one to the next.

An example is shown in Figure 2 24, 25, 26, 27. In these

MD activity in new task assembly

As a new task is learned, its components must be identified, separated and assembled. The ability to learn and use the multiple rules of novel, complex behaviour is strongly correlated with fluid intelligence 51, 52. In extreme cases, failure is manifest in ‘goal neglect’ [51]; although the subject might correctly describe the different task components, one component is lost in actual behaviour. Similar mismatch between knowledge and behaviour has long been described in frontal lobe patients 20

Conclusions and open questions

The importance of frontal lobe processes in complex, sequential activity has long been recognized 20, 21. Symbolic artificial intelligence has shown how complex, goal-directed behaviour is achieved through sequential mental programming, or dividing complex problems into component sub-problems. With the ascendancy of parallel, neural network models, interest in sequential mental programming has been somewhat in abeyance. Much work has addressed isolated acts of cognitive control, such as

Acknowledgements

This work was funded by Medical Research Council (UK) intramural program U.1055.01.001.00001.01. Russell Thompson provided expert assistance with figures, in particular data preparation for Figure 3.

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