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.Wagner, AD., Koutstaal, W., & Schacter, D.L. (1999). When encoding yields remembering: insights from event related neuroimaging. Phil. Trans. Soc. Land. B, 354(1387), 1307–24.Paller, KA. & Wagner, AD. (2002). Observing the transformation of experience into memory. Trends in Cognitive Sciences, 6 (2),93–102
History 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.Sanquist, T.F., Rohrbaugh, J.W., Syndulko, K. and Lindsey, D.B. (1980). Electrophysiological signs of levels of processing: perceptual analysis and recognition memory. Psychophysiology, 17, 568–576. 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.Karis, D., Bashore, T., Fabiani, M. and Donchin, E. (1982). P300 and memory. Psychophysiology, 19, 328.Karis, D., Fabiani, M. and Donchin, E. (1984). "P300" and memory: individual differences in the Von Restorff effect. Cognitive Psychology, 16, 177–216.Fabiani, M., Karis, D. and Donchin, E. (1985). Effects of strategy manipulation on P300 amplitude in a Von Restorff paradigm. Psychophysiology, 22, 588–589. In 1987, Paller, Kutas and Mayes,Paller, KA., Kutas, M. & Mayes, AR. (1987). Neural correlates of encoding in an incidental learning paradigm. Electroencephalography and Clinical Neuropsysiology, 67, 360–371. 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. Additionally, the Dm has been studied using intracranial recordingsEgler, C.E., Grunwald, T., Lehnertz, K., Kutas, M., Helmstaedter, C., Brockhause, A., Van Roost, D. and Heinze, H.J. (1997). Human temporal lobe potentials in verbal learning and memory processes. Neuropsychologia, 35, 657–667. and in a variety of functional magnetic resonance imaging (fMRI) studies.
Main paradigms Overwhelmingly, the paradigm used to elicit a Dm effect in ERPs has been the "subsequent memory paradigm." An experiment employing a subsequent memory paradigm generally consists of two phases, a study phase (encoding phase) and a test phase (retrieval phase), with ERPs from scalp electrodes being recorded during each phase, time locked to stimulus onset. In the study phase, a series of items is displayed to the participant, usually one at a time; these items are most often words but pictures and abstract figures have also been used (though with less consistent Dm effects; see "Functional Sensitivity"). The test phase normally mixes together items that were shown during the study phase with others that are being shown for the first time, and the participant must classify each item as being "old" (if it was in the study phase) or "new" (if it is the first time it has been seen).
Critically for the Dm effect, the responses a participant makes to the old items in the test phase are used to backsort trials in the study phase as either "subsequently remembered" or "subsequently forgotten." If during the test phase a participant correctly classifies an old item as old, it falls into the "subsequently remembered" trial type for the study phase. On the other hand, if a person incorrectly calls an old item new at the test phase, or fails to respond "old" to an old item, this item becomes classified as "subsequently forgotten." The ERP waveforms, during the study phase, of all subsequently remembered trials are compared with those of all subsequently forgotten trials and a greater positivity is generally seen for the subsequently remembered trials.
For example, in the study phase of a subsequent memory paradigm, a participant may see the words "frog," "tree," and "car." Following the study phase the test phase occurs and the participant sees the words "shirt," "car," and "frog," and must say if each word is old or new. If the participant correctly classifies "car" as old, it becomes a subsequently remembered trial; however, if the subject incorrectly says "frog" is new, it is a subsequently forgotten trial. 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.
A "continuous recognition paradigm" has also been known to elicit a Dm effect . In the continuous recognition paradigm, study and test phases are not separate entities, but rather, items are continuously presented and the participant is instructed to respond to an item as "old" if it has been seen before (generally presented a second time) in this continual stream of item presentation. Items that were correctly called "old" are the subsequently remembered trials, and items that were "missed" (not called old upon second presentation) make up the subsequently forgotten trials. 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.
Component characteristics Broadly speaking, the Dm ERP effect is any difference in neural activity recorded during the study phase of an experiment that differentiates subsequently remembered items and subsequently forgotten items. Typically, this difference is seen in the form of subsequently remembered items eliciting waveforms that are more positive than subsequently forgotten items during encoding of the item. Most often, the difference between subsequently remembered and subsequently forgotten items emerges at approximately 400 ms post stimulus onset and is sustained until 800 or 900 ms, though this can vary depending on the stimuli used and experimental instructions. The timing of this enhanced positivity suggests that the Dm may be a modulation of several ERP components, including the N400 component, with subsequently remembered items eliciting a less negative amplitude, as well as the P300 or an LPC, where items that are later remembered yield a more positive amplitude in this waveform. In terms of scalp topography, the Dm effect is generally largest over centro-parietal recording sites. However, a Dm effect with a more anterior distribution can be observed by varying the instructions participants receive;Munte, T.F., Heinze, H.J., Scholz, M. & Kunkel, H. (1988). Effects of a cholinergic nootropic (WEB 1881 FU) on event-related potentials recorded in incidental and intentional memory tasks. Neuropsychobiology, 19, 158–168.Friedman, D. (1990a) ERPs during continuous recognition memory for words. Biological Psychology,30, 61–87. this is discussed further below.
Functional sensitivity The canonical characteristics described above of the Dm effect give a general description of the component; however, the strength, timing, topographical distribution and even whether or not the effect is seen is sensitive to a variety of experimental manipulations.
Incidental versus intentional encoding 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. This was the approach used by Paller, Kutas and Mayes in the first Dm study, and this technique reliably elicits a Dm effect. 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. Several studies have indeed recorded a Dm effect using intentional encoding instructions, but this effect sometimes differs from the Dm effect from incidental encoding. In a direct comparison of incidental vs. intentional encoding, Munte et al., (1988) found a stronger Dm effect for the incidental encoding condition. Moreover, the Dm effect for the intentional encoding condition appeared later than the Dm for incidental encoding, and also showed a more frontal topography compared to the centro-parietal distribution observed in incidental encoding. This effect of a delayed and more frontal distribution for intentional encoding paradigms was also seen in two other reports.Paller, K.A. Recall and stem-completion priming have difference electrophysiological correlates and are modified differentially by directed forgetting. Journal of Experimental Psychology: Learning, Memory, and Cognition, 16, 1021–1032.
Levels of processing and rehearsal at encoding 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. Generally speaking, participants may be instructed to observe the items at test and make a judgment regarding each item; crucially, this judgment may be of the "shallow" variety, such as deciding if the word presented contains more than two vowels, or it may be a "deeper" judgment (e.g. is this item edible?). These deeper judgments are more of the semantic variety and typically lead to a better representation of the item.Craik, F.I.M. & Tulving, E. (1975). Depth of processing and the retention of words in episodic memory. Journal of Experimental Psychology, 104, 268–294. This is also reflected in the Dm effect. In the seminal paper by Paller, Kutas and Mayes (1987), participants made shallow judgments based the physical properties of the word or deeper judgments reflective of more semantic information of the word. The Dm effect for words encoded in a semantic fashion was more positive than the Dm effect observed for words non-semantically encoded. A Dm effect can be seen for shallower processing as well, as was the case in one of the shallow processing tasks in the Paller, Kutas and Mayes (1987) paper, as well as in Friedman, Ritter and Snodgrass (1996).Friedman, D., Ritter, W. & Snodgrass, J.G. (1996). ERPs during study as a function of subsequent direct and indirect memory testing in young and old adults. Cognitive Brain Research, 4, 1–13.