P3, POSITIVE SLOW-WAVE AND WORKING-MEMORY LOAD - A STUDY ON THE FUNCTIONAL CORRELATES OF SLOW-WAVE ACTIVITY

Parietal positivities of the 'slow wave' type are known to emerge afte r the P300 whenever target detection leads to a complex subsidiary tas k. Although the functional correlates of these 'positive slow waves' ( PSW) are not known, it has been suggested that they may index (a) the selection or decision processes, (b) the preparation of the response o r (c) the evaluation of its correctness. We investigated whether PSW c ould be dissociated from each of these putative steps of information p rocessing by means of a paradigm devoid of motor components and needin g very long reaction times. In our protocol, target stimuli acted as t he triggering signal to perform silently one of 4 different tasks, nam ely (a) simple updating of a target count; (b) counting backward in th rees; (c) simultaneous updating of two items (day of the week and ordi nal of the month) and (d) updating of 3 items (the two above plus the month of the year). Reaction times to the same stimuli were obtained i n 5 subjects during separate sessions. The different tasks did not mod ify the latencies of N2 or P3b components, but attenuated the amplitud e of P3 as a mirror image of the subjective difficulty scores. A consp icuous parietal PSW appeared in conditions where two or 3 items had to be updated. This PSW developed 1-2 s earlier than the reaction times to the same experiments and could be therefore dissociated from the se lection and decision processes. PSW latency was correlated with the nu mber of items to be updated, but not with subjective difficulty. In th e present paradigm PSW appeared to index the retrieval of information from working memory; however, in mon general terms our results suggest that PSW is a non-specific activity that signals the completion of an y synchronized operation immediately following target detection. Our d ata suggest a functional link between P3 and PSW, also supported by th e similarity of their respective scalp topographies. The present parad igm proved to be easy to implement and suitable to study the 'executiv e' functions governing attentional and working-memory control during t he performance of multiple tasks. (C) 1998 Elsevier Science Ireland Lt d.