Capacity-speed relationships in prefrontal cortex.

Benjamin P Austin,Lisa E Thomas,Timothy B Meier,Vivek Prabhakaran,Nandakumar S Narayanan,Lin Naing,John D E Gabrieli,Veena A Nair,Bart Rypma

doi:10.1371/journal.pone.0027504

Abstract

Working memory (WM) capacity and WM processing speed are simple cognitive measures that underlie human performance in complex processes such as reasoning and language comprehension. These cognitive measures have shown to be interrelated in behavioral studies, yet the neural mechanism behind this interdependence has not been elucidated. We have carried out two functional MRI studies to separately identify brain regions involved in capacity and speed. Experiment 1, using a block-design WM verbal task, identified increased WM capacity with increased activity in right prefrontal regions, and Experiment 2, using a single-trial WM verbal task, identified increased WM processing speed with increased activity in similar regions. Our results suggest that right prefrontal areas may be a common region interlinking these two cognitive measures. Moreover, an overlap analysis with regions associated with binding or chunking suggest that this strategic memory consolidation process may be the mechanism interlinking WM capacity and WM speed.

Highlights

There was a significant positive correlation between participant’s retrieval time (RT) savings in the high load condition and their Digit Span Forward task (DF) score (r = 0.58, t = 2.12, p = .05) suggesting that higher RT savings are associated with higher capacity
The increased capacity and faster RT in individuals in which this additional activation was observed may be afforded by the activation of these domain independent regions
What might be the role of these domain-independent regions in increasing Working memory (WM) capacity? One possibility is that these activations reflect binding or chunking of to-be-remembered information in the service of increasing WM capacity

Summary

Introduction

Miller was ‘‘persecuted’’ by a magical number which he believed could underlie the basis of human short-term memory capacity His ‘‘Seven, Plus or Minus Two’’ theory was a scientific turning point in the study on the limits of human memory that has lead to a vast amount of research aimed at demystifying this number. In 1966, Saul Sternberg quantified limits on the speed of retrieval of items stored in short-term memory [1] His posited internal serial-comparison process launched a new era in cognitive research aimed at elucidating the mechanism behind processing speed. It wasn’t until 1972, that speed and capacity were inter-linked through an elegant meta-analysis [2], a study which catalyzed the investigation of speed and capacity as interdependent concepts. The current functional MRI study aims to provide insights into the brain bases behind these three concepts and show for the first time brain evidence inter-linking these concepts

Methods

Results

Conclusion