Event-related potentials (ERPs) were recorded in 19 healthy subjects as they completed two Sternberg (1969, American Scientist, 57, 421-457) memory tests. In separate sessions, either single digits (i.e., 0-9) or 10 abstract figures were used as stimuli. In both sessions, memory set sizes were 1 (M1), 2 (M2), or 4 (M4). The amplitude and latency of the parietal P400 and the frontocentral negativity preceding P400 varied significantly with set size, but only between M1 and M2, whereas reaction time increased dramatically from M1 to M2 and from M2 to M4. These findings challenge previous assertions that the ERPs reflect aspects of the exhaustive serial search proposed by Sternberg. A late parietal positivity (P620), which failed to vary with set size, was larger in response to figures than to digits and may represent the search for, or utilization of, semantic traces of the stimuli.
