Spatio-temporal dynamics of working memory maintenance and scanning of verbal information

Abstract

Objective: During verbal communication, humans briefly maintain mental representations of speech sounds conveying verbal information, and constantly scan these representations for comparison to incoming information. We determined the spatio-temporal dynamics of such short-term maintenance and subsequent scanning of verbal information, by intracranially measuring high-gamma activity at 70-110Hz during a working memory task.

Methods: Patients listened to a stimulus set of two or four spoken letters and were instructed to remember those letters over a two-second interval, following which they were asked to determine if a subsequent target letter had been presented earlier in that trial's stimulus set.

Results: Auditory presentation of letter stimuli sequentially elicited high-gamma augmentation bilaterally in the superior-temporal and pre-central gyri. During the two-second maintenance period, high-gamma activity was augmented in the left pre-central gyrus, and this effect was larger during the maintenance of stimulus sets consisting of four compared to two letters. During the scanning period following target presentation, high-gamma augmentation involved the left inferior-frontal and supra-marginal gyri.

Conclusions: Short-term maintenance of verbal information is, at least in part, supported by the left pre-central gyrus, whereas scanning by the left inferior-frontal and supra-marginal gyri.

Significance: The cortical structures involved in short-term maintenance and scanning of speech stimuli were segregated with an excellent temporal resolution.

Keywords: 4D brain mapping; High-frequency oscillations (HFOs); Intracranial electrocorticography (ECoG) recording; Pediatric epilepsy surgery; Sternberg paradigm; Subdural electroencephalography (EEG).