Straube Behavioral and Brain Functions 2012, 8:35 http://www.behavioralandbrainfunctions.com/content/8/1/35 REVIEW Open Access An overview of the neuro-cognitive processes involved in the encoding, consolidation, and retrieval of true and false memories Benjamin Straube* Abstract Perception and memory are imperfect reconstructions of reality. These reconstructions are prone to be influenced by several factors, which may result in false memories. A false memory is the recollection of an event, or details of an episode, that did not actually occur. Memory formation comprises at least three different sub-processes: encoding, consolidation and the retrieval of the learned material. All of these sub-processes are vulnerable for specific errors and consequently may result in false memories. Whereas, processes like imagery, self-referential encoding or spreading activation can lead to the formation of false memories at encoding, semantic generalization during sleep and updating processes due to misleading post event information, in particular, are relevant at the consolidation stage. Finally at the retrieval stage, monitoring processes, which are assumed to be essential to reject false memories, are of specific importance. Different neuro-cognitive processes have been linked to the formation of true and false memories. Most consistently the medial temporal lobe and the medial and lateral prefrontal cortex have been reported with regard to the formation of true and false memories. Despite the fact that all phases entailing memory formation, consolidation of stored information and retrieval processes, are relevant for the forming of false memories, most studies focused on either memory encoding or retrieval. Thus, future studies should try to integrate data from all phases to give a more comprehensive view on systematic memory distortions. An initial outline is developed within this review to connect the different memory stages and research strategies. Keywords: False memory, Memory biases, Brain, Functional magnetic resonance imaging, Self-reference, Encoding, Consolidation, Retrieval Introduction Declarative memory is highly flexible, representational Just like perception, memory is a constructive process that and provides a way to model the external world. Since is sometimes prone to error and distortion (e.g., [1-3]). declarative memories are (due to their flexibility) espe- Various memory systems can be distinguished in terms cially prone to errors and distortions, the current re- of the different kinds of information they process and the view focuses specifically on the declarative memory principles by which they operate [4-6]. Consequently system. Declarative memory can be further divided those systems are differentially prone to false memories into semantic memory (facts about the world) and epi- [5,7]. Declarative memory is a long term human memory sodic memory (the capacity to re-experience an event system that refers to memories which can be consciously in the context in which it originally occurred) [7,8]. It recalled such as facts, knowledge or events. An important has been shown that episodic memory requires the principle for the declarative memory is the ability to participation of brain systems in addition to those that detect, encode and retrieve what is unique about a sin- support semantic memory, especially the frontal lobes gle event occurring at a particular time and place. [8-11]. With regard to false memories, it has been assumed that the semantic memory system is less vul- nerable to involuntary transformations and distortions Correspondence: [email protected] Department of Psychiatry and Psychotherapy, Philipps-University Marburg, than the episodic memory system [7]. In line with Marburg, Germany this assumption the evidence about false memories © 2012 Straube; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Straube Behavioral and Brain Functions 2012, 8:35 Page 2 of 10 http://www.behavioralandbrainfunctions.com/content/8/1/35 particularly refers to episodic or “episodic like” aspects of subsequent false memory [28]. These results suggest memories, which often requires that at least two aspects that brain activity of the precuneus - which might indi- of the original encoding situation be remembered (e.g., cate the engagement of visual imagery - during encod- sentence content and body orientation [12]). ing can lead to falsely remembering something that was Since episodic memory is flexible, representational and only imagined. Thus, visual imagery or everything like often contains multiple pieces of information from the ‘fantasy’ can influence memories directly during encod- encoding situation, it comprises an especially construct- ing by providing a biased or mentally changed episode ive process that is prone to error and distortion (e.g., for the episodic memory system, which is likely to be [1-3]). The theory that false memories are a byproduct later remembered in a similarly biased or changed way. of a constructive memory system is supported by a Beside imagery, there are two further theoretical considerable number of behavioral and neuroimaging accounts of false memory encoding (e.g., [35]): The studies (for a review see [13]). It has been demon- ‘spreading activation theory’ (e.g., [31-35]) and the ‘fuzzy strated that human memory is influenced by several trace view’ (e.g., [35,38-41]). These views were discussed factors, such as prior knowledge (e.g., [11,14,15]), particularly with regard to findings of the Deese–Roedi- current mental state or emotions (e.g., [11,13,14,16- ger–McDermott (DRM) paradigm, in which subjects 20]), and therefore does not reflect a perfect documen- were presented with a list of semantically related words tation of the external world. Thus, what is retrieved (e.g., bed, rest, wake, tired, dream, etc.) that converge on from memory can significantly differ from what was a single, non-presented critical item such as sleep initially encoded (e.g., [12,13,21]). The most intriguing [42,43]. In line with the ‘spreading activation theory’ are false memories of specific psychiatric patients (e.g., (e.g., [31-35]), it is assumed, that activation flows from those presenting confabulations [22-24]) and the most list items to critical lures in the semantic network. disastrous are those leading to false decisions in juris- Consequently, the associative activation of related or tic context [25-27]. Specific examples about the forma- similar items is stored and subsequently misattributed tion of false memories will be given in this review to actual encounters. According to the ‘fuzzy trace article to illustrate the variety of memory errors related view’ (e.g., [35,38-41]), studying a list of related words to diverse stages of processing. Different sources of or pictures leads to formation of ‘verbatim traces,’ false memories at encoding, consolidation and retrieval which contain item-specific information, as well as ‘gist will be discussed and compared. This article will finish traces,’ containing the general topic of the list. Both with a research agenda for the comprehensive investi- theories rely on the assumption that the formation of gation of the formation of false memories. false memory traces (spreading activation, gist traces) may be a byproduct of perceptual and semantic elabor- Encoding ation processes [35]. Processes like imagery (e.g., [21,28-30]), self-referential In a recent functional magnetic imaging (fMRI) study, processing (e.g., [12]) or spreading activation (e.g., [31- an adapted version of the DRM-paradigm was used to 35]) can lead to the formation of false memories, already investigate the neural activity associated with the forma- at encoding. tion (encoding) of true versus false memories [35]. The An example for the formation of false memories at materials were 72 categorical 6-word lists, each consist- encoding is seen in errors that occur when a memory ing of the 6 most typical instances (e.g., cow, pig, horse, of an imagined event is later falsely remembered as a chicken, sheep, goat) of a natural/artificial category (e.g., memory of a perceived event (e.g., [21,28-30]). These farm animal). In the test phase, True-word, False-word, errors may arise as a consequence of similarities be- and New-word trials were presented and subjects tween how imagined and perceived events are encoded, responded by pressing one of four keys according to as well as similarities among the event features that are whether the word was judged to be ‘definitely old,’‘un- reactivated during retrieval [36,37]. It has been sug- sure if old,’‘unsure if new,’ or ‘definitely new.’ In their gested, that such confusions may serve as a basic mech- data analyses the authors investigated, which brain anism for the induction of many types of false regions show true memory formation activity (i.e., posi- memories [21,28-30]. It has further been hypothesized tive covariation between encoding trial activations and that brain networks during encoding are related to sub- later hit rate for words from those trials) and which sequent confusion of imagined and perceived events brain regions show false memory formation activity (i.e., [28]. Accordingly, activations in the precuneus and in- positive covariation between encoding trial activations ferior parietal regions in response to words were found and later false alarm rate for semantic associates of during the encoding stage to be greater when partici- words from those trials). pants subsequently claimed to have seen the corre- They found that activity in the left ventrolateral pre- sponding object than when they did not produce a frontal cortex (PFC) and visual areas at encoding Straube Behavioral and Brain Functions 2012, 8:35 Page 3 of 10 http://www.behavioralandbrainfunctions.com/content/8/1/35 contribute to both true (old item) and false memories (e. ventromedial prefrontal cortex (VMPFC), the sub/pre- g., new item of the old category).