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Wednesday, September 29, 2010

The Prefrontal Cortex Is Holistic

The question of whether the brain is "modular" - whether different parts do different things - has been a neuroscientific talking point since the days of the phrenologists.

They were the guys who believed that, not only were there modules, but that you could tell how big they were by measuring the shape of someone's skull, and so learn about their personality.

Phrenology made modules unfashionable for a while, but today they're back, and most of fMRI consists in trying to find areas of the brain that do different stuff, but in a new paper Wilson et al argue against taking modularism too far: Functional localization within the prefrontal cortex: missing the forest for the trees?

Their focus is the prefrontal cortex (PFC), a large chunk of the front of the brain which is bigger in humans than in any other species. The PFC is routinely subdivided into segments, each with (presumably) a different function. So we have the "emotional" vmPFC, the "memory" dlPFC, the "pleasure" OFC, etc.

Wilson et al don't dispute that there are some variations in function between different bits of the PFC, but they say that in all the excitement over localization, we may have overlooked the role of the PFC as a whole.

They discuss evidence from monkeys with PFC damage (or lesions which disconnect it from the rest of the brain). Damage to the entire PFC, they say, leaves monkeys completely unable to perform tasks which require storing concepts over time. For example, they can't learn that whenever they see, say, a red button, they ought to press it to get food. But if part of the PFC is intact, and it doesn't matter which part, monkeys can do this with only minor problems.

However, the PFC isn't required for all tasks. If the task only involves information which is all presented at once, the lesioned monkeys are OK. So they could learn, given a big panel covered in red buttons, to push the buttons to get food, because the buttons are all there simultaneously.
Hence the data from these tasks are congruent with the notion that [the PFC] is only crucial in memory during tasks requiring the processing of temporally complex events. This can be defined as an event to be learned about, in which information that is crucial to that learning is presented at more than one point in time, or that can only be interpreted with respect to a preceding event.
They say that evidence from human neuroimaging studies supports this view.
A meta-analysis has shown consistent recruitment of the same network of regions in the PFC across a range of cognitive demands. The authors argue that this supports specialization of function within the PFC, but of an unexpected nature, namely ‘a specific frontal-lobe network that is consistently recruited for solution of diverse cognitive problems’. The idea that large and different regions of the PFC are recruited by any task at hand supports our argument that the function of the PFC as a whole exceeds the sum of the functions of its subcomponents.
This all has echoes of Karl Lashley, an early neuroscientist (died 1958) who proposed the theory of "mass action" - that the whole cortex contributes to behaviour, rather than each part doing different things ("modularism").

Jerry Fodor, whose classic book The Modularity of Mind (1983) helped to rehabilitate modularism from its reputation as "phrenological", was also an advocate of this view - within limits.

Fodor argued that some brain systems, like vision, hearing and language, were cortical modules, but that above this, there was a non-modular system which was the basis for thought, intelligence and decision making. If I remember correctly, he didn't explicitly say that the prefrontal cortex was this system, but I'm sure he'd have no objections to Wilson et al's account.

ResearchBlogging.orgWilson CR, Gaffan D, Browning PG, & Baxter MG (2010). Functional localization within the prefrontal cortex: missing the forest for the trees? Trends in neurosciences PMID: 20864190

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