Neural activities of tactile cross-modal working memory in humans: an event-related potential study

In the present study, we examined the neural mechanisms underlying cross-modal working memory by analyzing scalp-recorded event-related potentials (ERPs) from normal human subjects performing tactile-tactile unimodal or tactile-auditory cross-modal delay tasks that consisted of stimulus-1 (S-1, tactile), interval (delay), and stimulus-2 (S-2, tactile or auditory). We hypothesized that there would be sequentially discrete task-correlated changes in ERPs representing neural processes of tactile working memory, and in addition, significant differences would be observed in ERPs between the unimodal task and the cross-modal task. In comparison to the ERP components in the unimodal task, two late positive ERP components (LPC-1 and LPC-2) evoked by the tactile S-1 in the delay of the cross-modal task were enhanced by expectation of the associated auditory S-2 presented at the end of the delay. Such enhancement might represent neural activities involved in cross-modal association between the tactile stimulus and the auditory stimulus. Later in the delay, a late negative component (LNC) was observed. The amplitude of LNC depended on information retained during the delay, and when the same information was retained, this amplitude was not influenced by modality or location of S-2 (auditory S-2 through headphones, or tactile S-2 on the left index finger). LNC might represent the neural activity involved in working memory. The above results suggest that the sequential ERP changes in the present study represent temporally distinguishable neural processes, such as the cross-modal association and cross-modal working memory.