Late components of the visual evoked potential to search in short-term memory

The present paper investigated the changes in the morphology of the VEP as a function of the amount of search in short-term memory. The task required that the subject responds with a "yes" or "no", depending on whether a test digit has been part of a set of digits he had just seen. Six sizes of sets were used, 1, 3, 5, 7, 9 and 11. EEG was recorded from C3, C4, T3, T4, Cz, Pz and Oz, and sampled every 4 msec to obtain VEP to the test digit. Two late components were found to increase in latency with increasing set size: (a) P270, which was masked by the P350 component for small set sizes; it preceded P350 for longer memory sets and its latency increased up to set 7 with an average slope of 20.5 msec per item and (b) P350, which increased in latency up to set size 7 with an average slope of 22 msec per item. It was suggested that P270 reflects the serial search process. When search is terminated another general processor is called into action. This process is reflected in the P300 and is believed to be independent of the memory set size. Amplitude did not correlate with the amount of search. The morphology of the VEP changed for sets above short-terms span, and latency changes in P270 and P350 no longer predicted RT changes. Since subject's strategy was not controlled for supra-span sets, no definite conclusions were offered.