Authors:

BOR, D., and OWEN, A.M.

Reference:

Year of publication:

2004

CBU number:

5981

Abstract:

Most neuroimaging studies investigating the lateral prefrontal cortex (LPFC) report an association of LPFC activity with task demand. However, two recent studies provide important counter-examples. Using fMRI, in both the spatial and verbal domains, sequences of stimuli were presented for subjects to retain during a short interval and then retrieve. Unbeknownst to the subjects, stimuli were either randomly arranged or structured (forming symmetries and regular shapes for the spatial task and mathematical patterns for the verbal task). While subjects performed the structured tasks better by reorganising or "chunking" them into more efficient forms, LPFC activity was greater for the structured, compared with the random sequences. However, although these two experiments involved mathematical chunking, in daily life mnemonic content is at least as common a form of chunking. The current study sought to investigate the level of cortical overlap for mnemonic and mathematical chunking. During the week prior to fMRI scanning, normal subjects spent approximately 4 hours learning 20 sets of 4-digit numbers, so that very high competency was reached. While being scanned, subjects carried out 4 conditions. Three conditions shared the common format of presenting subjects visually with 4x2-digit numbers in sequence, requiring subjects to retain the 4 items over a short delay, and then make a verbal response demonstrating accurate retrieval of the sequence. One of these three conditions presented an entirely random sequence; a second involved sequences with a mathematical pattern, while a third incorporated numbers learnt during the prior behavioural training. A fourth condition sought to control for simple retrieval processes, outside of the context of a digit-span task. Differential cortical activity was found for the different forms of chunking processes employed in the same digit-span task.