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.
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.
[4][5][6] In 1987, Paller, Kutas and Mayes,[7] 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.
For example, in the study phase of a subsequent memory paradigm, a participant may see the words "frog," "tree," and "car."
The neural activity elicited by the first presentation of "car" and "frog" at the study phase is then compared and the Dm effect is derived from this comparison.
The neural activity for subsequently remembered and forgotten trials is then compared for the first presentation of the items, and a Dm effect is computed.
A large number of Dm ERP studies employ an incidental encoding approach to the subsequent memory paradigm.
In this case the participant pays attention to the items presented during the study phase unaware that a memory test will follow.
Experiments wherein the participant is explicitly told to remember the items presented during the study phase (intentional encoding) because a memory test will follow have yielded slightly differing results.
[10][11] Perhaps the most well known manipulation during the subsequent memory paradigm is how the participant is instructed to encode or process the material during the study phase.
Gonsalves and Paller (2000)[22] interpreted this as indicating that better imagery at encoding led to greater source confusions at retrieval (“did I actually see this or just imagine it?”).
To the extent that greater positivity for subsequently remembered items spans several ERP components (P300, N400, and an LPC), coupled with differing topographical distributions depending on task, it is likely that the neural generators of the Dm effect are widespread in the brain.
Pinning down the location in the brain that gives rise to any ERP component is very difficult if not impossible because of the inverse problem.
[23] Egler et al. (1997)[8] recorded electrical activity directly from the MTL in patients about to undergo surgery for temporal lobe epilepsy.
[2] Taken together, these findings from complementary cognitive neuroscience methods suggest the neural events at encoding that lead to successful later memory are diffuse in the brain and unfold on multiple time scales.
Van Petten and Senkfor (1996)[20] suggest there may be a "family of Dm effects" that occur dependent on a variety of factors, and this seems quite plausible given the wide range of differences observed in the Dm as a function of stimuli used, encoding instructions, orienting tasks and types of retrieval decisions.
Future research using different manipulations of the subsequent memory paradigm, as well as combining methods such as ERPs and fMRI or transcranial magnetic stimulation and fMRI have great potential to lead to further understanding of the Dm effect[1][2] and, more generally, of the neural and cognitive factors that promote later memory under different circumstances.