The neural bases of the short-term storage of verbal information are anatomically variable across individuals

Abstract

What are the precise brain regions supporting the short-term retention of verbal information? A previous functional magnetic resonance imaging (fMRI) study suggested that they may be topographically variable across individuals, occurring, in most, in regions posterior to prefrontal cortex (PFC), and that detection of these regions may be best suited to a single-subject (SS) approach to fMRI analysis (Feredoes and Postle, 2007). In contrast, other studies using spatially normalized group-averaged (SNGA) analyses have localized storage-related activity to PFC. To evaluate the necessity of the regions identified by these two methods, we applied repetitive transcranial magnetic stimulation (rTMS) to SS- and SNGA-identified regions throughout the retention period of a delayed letter-recognition task. Results indicated that rTMS targeting SS analysis-identified regions of left perisylvian and sensorimotor cortex impaired performance, whereas rTMS targeting the SNGA-identified region of left caudal PFC had no effect on performance. Our results support the view that the short-term retention of verbal information can be supported by regions associated with acoustic, lexical, phonological, and speech-based representation of information. They also suggest that the brain bases of some cognitive functions may be better detected by SS than by SNGA approaches to fMRI data analysis.

Figure 1.

Example from subject 7 of SS and SNGA rTMS targets (orange markers; anterior, SNGA; posterior, SS). White blobs on the brain are load-sensitive regions identified by the SS analysis, which have been merged onto this subject's high-resolution T1-weighted anatomical scan, and are visible at this depth of scalp “peeling.” Plots of trial-averaged time series data extracted from each of the target regions and corresponding relevant information from the solution of the general linear model are shown below (SNGA, left column; SS, right column; red/blue lines, 5-letter trials; orange/green lines, 2-letter trials). Plots at the top show trial-averaged time series from each region; plots at the bottom show the delay-period covariates scaled by their parameter estimates, and the individual dots represent residual error at each time point.

Figure 2.

Aggregated results from the task for the rTMS session. A, Results for accuracy data (rTMS-absent, white bars; rTMS-present, gray bars). B, Results for RT data (rTMS-absent, white bars; rTMS-present, gray bars). Error bars depict within-subjects SEM.