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. 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: 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."
Wednesday, October 24, 2007
Brain location of verbal information storage varies between people
A group at Wisconsin has made the interesting observation that group averaged analyses of the sort frequently reported in brain imaging studies may give misleading results if the brain location of the process being studied varies from one individual to the next. Here is their abstract: