Event-related potentials during digit recognition tasks.

OBJECTIVE An event-related potential (ERP) correlate of visual short-term memory (VMP) has been identified in our laboratory. This study aims to determine how stimulus load modulates recognition processing of digits. METHODS ERPs were recorded from 117 healthy right-handed subjects during a delayed matching-to-sample paradigm, using number stimuli that were either low load (three digits long) or high load (five digits long). The bootstrap method [R. Srebro, A bootstrap method to compare the shapes of two scalp fields, Electroenceph. Clin. Neurophysiol. 100 (1996) 25-32.] was employed to evaluate the topographic features of the VMP revealed in the current source density (CSD) maps. RESULTS Response times were significantly shorter for matching stimuli than for non-matching stimuli only for low loads; longer response times were related to higher loads compared to low loads only for matching stimuli. The high loads were related to larger ERP responses. The ERP was significantly smaller for matching than for non-matching three-digit numbers, but not for five-digit numbers. The ERP was also reduced to the test stimuli compared to sample stimuli regardless of stimulus load. Both temporal and frontal regions were involved in the recognition of the digit stimuli, and the left hemisphere was more active in the non-matching processing of digits. CONCLUSIONS The VMP spatial pattern in addition to its amplitude is sensitive to stimulus load in the encoding process.