Difference due to memory

Difference due to memory (Dm) indexes differences in neural activity during the study phase of an experiment for items that subsequently are remembered compared to items that are later forgotten. It is mainly discussed as an event-related potential (ERP) effect that appears in studies employing a subsequent memory paradigm, in which ERPs are recorded when a participant is studying a list of materials and trials are sorted as a function of whether they go on to be remembered or not in the test phase. For meaningful study material, such as words or line drawings, items that are subsequently remembered typically elicit a more positive waveform during the study phase (see Main Paradigms for further information on subsequent memory). This difference typically occurs in the range of 400–800 milliseconds (ms) and is generally greatest over centro-parietal recording sites, although these characteristics are modulated by many factors. The first report of subsequently remembered items eliciting a more positive ERP waveform than subsequently forgotten items during the study phase was by Sanquist et al., in 1980. This paper looked at a subset of the participants' ERPs at the study phase and found those trials subsequently remembered had a more positive waveform in the time range of the late positive complex (LPC), approximately 450–750 ms after stimulus presentation. In the early and mid 1980s, several studies noted modulation of the P300 (P3b) component due to subsequent memory, with items that are remembered having a larger amplitude. In 1987, Paller, Kutas and Mayes, consistent with previous reports, observed that subsequently remembered items elicited more positivity in the later portions of the waveform compared to items later forgotten; they termed these observed differences at the study phase as "the difference due to memory" or Dm effect. Since this seminal paper by Paller, Kutas and Mayes, a wealth of research using ERPs has been conducted using the Dm effect and detailing the multitude of factors that influence the manifestation of the Dm and, by inference, encoding success.