Difference due to Memory
Encyclopedia
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.
(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. Additionally, the Dm has been studied using intracranial recordings and in a variety of functional magnetic resonance imaging
(fMRI) studies.
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, 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; this is discussed further below.
. The Dm effect for words encoded in a semantic fashion was more positive than the Dm effect observed for words non-semantically encoded. It is important to note that 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).
In 1997, Weyerts et al. found that both recognition memory as well as the Dm effect was larger for pairs of words that were relationally encoded (e.g. are these two words semantically related) versus non-relationally encoded (e.g. can the color white be associated with one of these words). This further suggests that the Dm effect may be enhanced when items are encoded on a semantic level.
Also, the Dm effect seems sensitive to the type of rehearsal strategies a participant performs. Specifically, Fabiani, Karis and Donchin found that P300 modulation at encoding (particularly for "isolates,"stimuli presented in a deviant font relative to all other stimuli) correlated with later memory for subjects who engaged in rote rehearsal (such as simply repeating the word in one's head) but not for those who undertook elaborative rehearsal, which emphasizes linking the current word to other words presented and pre-existing knowledge. However, in the 1990 report as well as a report by Karis, Fabiani and Donchin (1984), a later positivity emerged in frontal electrodes corresponding to subsequent memory, and this was greater for those in the elaborative rehearsal condition.
In a similar vein, Friedman & Trott (2000) found that young adult participants displayed a robust Dm effect when they not only remembered seeing a word, but could also remember some details of the context of when it was presented. In comparison, a Dm effect for items that were subsequently judged as old, but only from a general sense of familiarity, did not emerge. Interestingly, a Dm effect was found in both conditions for older adults.
.
However, evidence from other cognitive neuroscience techniques can help to shed light on this question. Given that the Dm effect seems to be reflective of mnemonic processes at encoding, one brain area likely to play a role is the medial temporal lobe (MTL), as it is well known this brain area gives rise to the type of memory observed in Dm studies.
Egler et al. (1997) recorded electrical activity directly from the MTL in patients about to undergo surgery for temporal lobe epilepsy. While recording directly from the MTL, participants were shown novel stimuli and then later had a memory test for those stimuli; it was reported that the magnitude of the electrical activity from the MTL during the initial presentation of the stimuli correlated with subsequent memory performance.
Additionally, fMRI studies using subsequent memory paradigms have found evidence suggesting areas of the MTL are involved in the Dm effect, though the precise areas involved and their contributions are unclear. Further, several fMRI studies have reported prefrontal cortex
(PFC) activity during study predictive of subsequent memory, as well as activity in fusiform gyrus
.
Taken together, these findings from complimentary 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. The Dm effect seen in ERPs likely represents a subset of these encoding processes.
and fMRI have great potential to lead to further understanding of the Dm effect and, more generally, of the neural and cognitive factors that promote later memory under different circumstances.
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing 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.
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. In this paper, Sanquist et al. looked at a subset of their 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 complexLate Positive Component
The LPC is a positive-going event-related brain potential component that has been important in studies of explicit recognition memory...
(LPC), approximately 450-750 ms after stimulus presentation. In the early and mid 1980s, several studies noted modulation of the P300
P300
The P300 wave is an event related potential elicited by infrequent, task-relevant stimuli. It is considered to be an endogenous potential as its occurrence links not to the physical attributes of a stimulus but to a person's reaction to the stimulus. More specifically, the P300 is thought to...
(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. Additionally, the Dm has been studied using intracranial recordings and in a variety of functional magnetic resonance imaging
Functional magnetic resonance imaging
Functional magnetic resonance imaging or functional MRI is a type of specialized MRI scan used to measure the hemodynamic response related to neural activity in the brain or spinal cord of humans or other animals. It is one of the most recently developed forms of neuroimaging...
(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 N400N400
The N400 is a component of time-locked EEG signals known as event-related potentials . It is a negative-going deflection that peaks around 400 milliseconds post-stimulus onset, although it can extend from 250-500 ms, and is typically maximal over centro-parietal electrode sites...
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; 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 vs. 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.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. 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 wordWord
In language, a word is the smallest free form that may be uttered in isolation with semantic or pragmatic content . This contrasts with a morpheme, which is the smallest unit of meaning but will not necessarily stand on its own...
. The Dm effect for words encoded in a semantic fashion was more positive than the Dm effect observed for words non-semantically encoded. It is important to note that 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).
In 1997, Weyerts et al. found that both recognition memory as well as the Dm effect was larger for pairs of words that were relationally encoded (e.g. are these two words semantically related) versus non-relationally encoded (e.g. can the color white be associated with one of these words). This further suggests that the Dm effect may be enhanced when items are encoded on a semantic level.
Also, the Dm effect seems sensitive to the type of rehearsal strategies a participant performs. Specifically, Fabiani, Karis and Donchin found that P300 modulation at encoding (particularly for "isolates,"stimuli presented in a deviant font relative to all other stimuli) correlated with later memory for subjects who engaged in rote rehearsal (such as simply repeating the word in one's head) but not for those who undertook elaborative rehearsal, which emphasizes linking the current word to other words presented and pre-existing knowledge. However, in the 1990 report as well as a report by Karis, Fabiani and Donchin (1984), a later positivity emerged in frontal electrodes corresponding to subsequent memory, and this was greater for those in the elaborative rehearsal condition.
Type of Memory at Retrieval
The Dm effect has been shown to be sensitive to how participants are asked to display their memory for previous items. In a 1988 paper by Paller, McCarthy and Wood, a greater Dm effect was observed for items that were freely recalled with no external cues, compared to items that were presented and the subject was asked if he or she recognizes the item as old. This is suggestive of the Dm effect being larger for stronger representations, as recall is generally more difficult than recognition.In a similar vein, Friedman & Trott (2000) found that young adult participants displayed a robust Dm effect when they not only remembered seeing a word, but could also remember some details of the context of when it was presented. In comparison, a Dm effect for items that were subsequently judged as old, but only from a general sense of familiarity, did not emerge. Interestingly, a Dm effect was found in both conditions for older adults.
Stimuli Used
A host of studies have found a Dm effect when presenting words as stimuli. However, experiments using pictures or abstract figures have found less consistent Dm effects. Experiments using a continuous recognition paradigm have found a Dm effect for pictures of everyday objects. Interestingly, Van Petten and Senkfor (1996) did not find a Dm effect when they presented participants with abstract drawings; however, a Dm effect was observed in the same group of participants when words were used as stimuli. A similar pattern of results is described by Fox, Michie and Coltheart, (1990). Coupling the results of Dm effects for words and common pictures and the lack of Dm effects for abstract figures suggests the Dm effect may be contingent on using meaningful stimuli or some pre-existing knowledge of the stimuli.False Memories
In an elegant report by Gonsalves and Paller (2000), the Dm effect was found to be greater for false memories compared to correctly classified memories. In the study phase of this subsequent memory paradigm, participants saw a word which was followed either by a picture of that word or a blank box, in which case participants were asked to imagine a picture of the word they just saw. In the test phase, participants were shown a word and asked if it was presented with a picture during the study phase. 30% of the time participants erroneously said a picture accompanied a word when it had only been imagined by the participant. The waveform at the study phase of trials in which the participant falsely recalled studying the word with a picture elicited a more positive going amplitude compared to the trials where the participant correctly said only the word was presented. Gonsalves and Paller (2000) 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?”). More generally, this study demonstrates that backsorting procedures need not be limited to simply items remembered versus forgotten, but could include a wide range of more complex comparisons as long as test phase behaviors can be linked to specific study phase events.Source
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 problemInverse problem
An inverse problem is a general framework that is used to convert observed measurements into information about a physical object or system that we are interested in...
.
However, evidence from other cognitive neuroscience techniques can help to shed light on this question. Given that the Dm effect seems to be reflective of mnemonic processes at encoding, one brain area likely to play a role is the medial temporal lobe (MTL), as it is well known this brain area gives rise to the type of memory observed in Dm studies.
Egler et al. (1997) recorded electrical activity directly from the MTL in patients about to undergo surgery for temporal lobe epilepsy. While recording directly from the MTL, participants were shown novel stimuli and then later had a memory test for those stimuli; it was reported that the magnitude of the electrical activity from the MTL during the initial presentation of the stimuli correlated with subsequent memory performance.
Additionally, fMRI studies using subsequent memory paradigms have found evidence suggesting areas of the MTL are involved in the Dm effect, though the precise areas involved and their contributions are unclear. Further, several fMRI studies have reported prefrontal cortex
Prefrontal cortex
The prefrontal cortex is the anterior part of the frontal lobes of the brain, lying in front of the motor and premotor areas.This brain region has been implicated in planning complex cognitive behaviors, personality expression, decision making and moderating correct social behavior...
(PFC) activity during study predictive of subsequent memory, as well as activity in fusiform gyrus
Fusiform gyrus
The fusiform gyrus is part of the temporal lobe in Brodmann Area 37. It is also known as the occipitotemporal gyrus. Other sources have the fusiform gyrus above the occipitotemporal gyrus and underneath the parahippocampal gyrus....
.
Taken together, these findings from complimentary cognitive neuroscience methods suggest the neural events at encoding that lead to successful later memory are diffuse in the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
and unfold on multiple time scales. The Dm effect seen in ERPs likely represents a subset of these encoding processes.
Theory
Considering that the Dm is a comparison of neural activity during encoding, and that this activity is predictive of subsequent memory, it is likely the Dm indexes some difference between subsequently remembered vs. forgotten materials at encoding, presumably reflective of learning. The nature of this difference is not entirely clear though. Van Petten and Senkfor (1996) 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 stimulationTranscranial magnetic stimulation
Transcranial magnetic stimulation is a noninvasive method to cause depolarization or hyperpolarization in the neurons of the brain...
and fMRI have great potential to lead to further understanding of the Dm effect and, more generally, of the neural and cognitive factors that promote later memory under different circumstances.
See also
- Somatosensory evoked potentialSomatosensory Evoked PotentialSomatosensory Evoked Potentials are a useful, noninvasive means of assessing somatosensory system functioning. By combining SEP recordings at different levels of the somatosensory pathways, it is possible to assess the transmission of the afferent volley from the periphery up to the cortex...
- C1 and P1C1 & P1 (Neuroscience)The C1 and P1 are two human scalp-recorded event-related brain potential components, collected by means of a technique called electroencephalography . The C1 is named so because it was the first component in a series of components found to respond to visual stimuli when it was first discovered...
- Visual N1Visual N1The Visual N1 is a visual evoked potential, a type of event-related electrical potential , that is produced in the brain and recorded on the scalp. The N1 is so named to reflect the polarity and typical timing of the component. The "N" indicates that the polarity of the component is negative with...
- Mismatch negativityMismatch negativityThe mismatch negativity or mismatch field is a component of the event-related potential to an odd stimulus in a sequence of stimuli. It arises from electrical activity in the brain and is studied within the field of cognitive neuroscience and psychology. It can occur in any sensory system, but...
- N100
- N200N200 (neuroscience)The N200, or N2, is an event-related potential component. An ERP can be monitored using a non-invasive electroencephalography cap that is fitted over the scalp on human subjects...
- N2pcN2pcN2pc refers to an ERP component linked to selective attention. The N2pc appears over visual cortex contralateral to the location in space to which subjects are attending; if subjects pay attention to the left side of the visual field, the N2pc appears in the right hemisphere of the brain, and...
- N170N170The N170 is a component of the event-related potential that reflects the neural processing of faces.When potentials evoked by images of faces are compared to those elicited by other visual stimuli, the former show increased negativity 130-200 ms after stimulus presentation...
- P200P200In neuroscience, the visual P200 or P2 is a waveform component or feature of the event-related potential measured at the human scalp. Like other potential changes measurable from the scalp, this effect is believed to reflect the post-synaptic activity of a specific neural process...
- N400
- P300 (neuroscience)P300 (neuroscience)The P300 wave is an event related potential elicited by infrequent, task-relevant stimuli. It is considered to be an endogenous potential as its occurrence links not to the physical attributes of a stimulus but to a person's reaction to the stimulus. More specifically, the P300 is thought to...
- P3aP3aThe P3a, or novelty P3, is a component of time-locked signals known as event-related potentials . The P3a is a positive-going scalp-recorded brain potential that has a maximum amplitude over frontal/central electrode sites with a peak latency falling in the range of 250-280 ms...
- P3bP3bThe P3b is a subcomponent of the P300, an event-related potential component that can be observed in human scalp recordings of brain electrical activity...
- Late Positive ComponentLate Positive ComponentThe LPC is a positive-going event-related brain potential component that has been important in studies of explicit recognition memory...
- Contingent negative variationContingent negative variationThe contingent negative variation was one of the first event-related potential components to be described. The CNV component was first described by Dr. W. Grey Walter and colleagues in an article published in Nature in 1964...
- Error-related negativityError-related negativityError-related negativity , , is a component of an event-related potential . ERPs are electrical activity in the brain as measured through electroencephalography and time-locked to an external event...
- BereitschaftspotentialBereitschaftspotentialIn neurology, the Bereitschaftspotential or BP , also called the pre-motor potential or readiness potential , is a measure of activity in the motor cortex of the brain leading up to voluntary muscle movement. The BP is a manifestation of cortical contribution to the pre-motor planning of volitional...
- Lateralized readiness potentialLateralized readiness potentialIn neuroscience, the lateralized readiness potential is an event-related brain potential, or increase in electrical activity at the surface of the brain, that is thought to reflect the preparation of motor activity on a certain side of the body; in other words, it is a spike in the electrical...
- Early left anterior negativityEarly left anterior negativityThe early left anterior negativity is an event-related potential in electroencephalography , or component of brain activity that occurs in response to a certain kind of stimulus...
- P600P600The P600 is an event-related potential , or peak in electrical brain activity measured by electroencephalography . It is a language-relevant ERP and is thought to be elicited by hearing or reading grammatical errors and other syntactic anomalies...