UC Berkeley

Verbal working memory refers to the limited-capacity store responsible for maintaining and manipulating task-relevant information in a verbal form over short time-periods. The cortical regions that underlie this type of memory can be dissociated into control and storage regions. Control processes may involve operations such as the identification and selection of relevant information and the assembly and initial execution of articulatory rehearsal programs. In contrast, storage regions are where the actual task-relevant information is rehearsed. Presumably, control and storage regions transfer information among each other within a functionally connected network.

To study the cortical network formed during the delay period of a verbal working memory task, the first study was specifically designed to investigate functional connectivity. While the functional connectivity results counterintuitively led to a null result, it was demonstrated that middle frontal gyrus (MFG) and superior parietal lobule (SPL) were active during encoding and the start of the delay period, as well as at the end of the delay period, suggesting they were involved in control processes. In contrast, it was demonstrated that inferior frontal gyrus (IFG), premotor (PM) and Sylvian-parietal-temporal junction (area Spt) were active throughout encoding and the entire delay period, suggesting they were involved in storage/rehearsal processes.

In a second follow-up study, results demonstrated that control regions MFG and SPL were not sensitive to manipulations in rehearsal rate, but exhibited non-linear memory load effects. Conversely, storage/rehearsal regions IFG, PM and area Spt demonstrated approximately linear rehearsal rate and memory load effects. Additionally, memory load effects were found to diminish through time in all areas, while rehearsal rate effects persisted through the entire delay period in storage/rehearsal areas. It was also revealed that rehearsal rate and memory load were confounded behaviorally as subjects strategically increased their rehearsal rate to manage an increased memory load, until the load became so great that their rehearsal rate began to decrease. It was also discovered that high capacity individuals have less activity in MFG, IFG and SPL, but more activity in PM.

In a third study, substitute tasks were made for the encoding period (listen word) and delay period (subvocal rehearsal) of standard verbal working memory tasks and the timing and network properties of the active cortical regions were investigated. Results demonstrated that during the listen word condition activity in primary auditory cortex preceded area Spt, and that primary auditory cortex and area Spt formed a functional network. In contrast, during the subvocal rehearsal condition activity occurred first in PM, then in area Spt, followed by IFG. These three regions also formed a functional network in the subvocal rehearsal condition. Together these experiments provide novel insights into the cortical mechanisms underlying verbal working memory.