Authors:

BOR, D., CUMMING, N. & OWEN, A.M.

Reference:

Year of publication:

2003

CBU number:

5696

Abstract:

It is often proposed that lateral prefrontal cortex (LPFC) is important in organization and control of working memory (WM) contents. In some cases, effective reorganization can decrease task difficulty, implying a dissociation between LPFC activity and WM demand. Previous research using a visuo-spatial WM task investigated the improvement of performance that occurs when materials can be recoded into higher level chunks. Structured sequences of locations, encouraging chunking, were compared with unstructured sequences. Though structured sequences were easier to remember, fMRI showed increased activation of LPFC for these sequences, in particular during encoding. Although further behavioural evidence supported the claim that this dissociation between PFC activity and WM demand was due to chunking the structured sequences, it was unclear whether this result was specific to the visuo-spatial domain. This study extends this novel result by using auditory-verbal material and a different form of structure to be chunked. In the fMRI scanner, normal subjects heard a sequence of 8 single digits. Following a pause of 6-10 seconds, subjects attempted to repeat the sequence vocally. Sequences were either mathematically structured (e.g. 2-4-6-8-9-7-5-3) or unstructured (e.g. 2-8-5-1-7-3-9-4). Subjects were not informed that there were different types of sequences. During encoding events, easier structured sequences produced significantly greater activation in the LPFC bilaterally, compared to the harder unstructured sequences. These results further demonstrate that cognitively less demanding tasks may elicit greater activation in LPFC. In addition, such a result is not tied to any one type of modality or sequence structure. Instead, the lateral prefrontal cortex appears to be generally involved in strategically recoding incoming information, in order to optimise task performance.