False Memories

False memories, also known as confabulations, are a phenomenon in which individuals report having a memory of an event that never happened. This generally occurs due to the constructive nature of memory, where a disruption of that process will result in distortion of the memory. False memories can be significant in a clinical setting, where the production of confabulation may be evidence of underlying neurological problems, but also in a judicial setting, where a witness’ false memory of an event could have drastic consequences on the decision-making in court[1]. These memory corruptions appear to involve different brain correlates at different points in the modal model of encoding, consolidation, and retrieval, but the majority of false memory processes implicate the precuneus and the hippocampus[1]. Although the scientific community is still unsure how and why false memories occur, several theories have been proposed, including the fuzzy trace theory, the spreading activation theory, and the sensory reactivation hypothesis[1]. While these theories propose particular mechanisms for the encoding, consolidation and retrieval of false memories, it is difficult to distinguish these processes due to the nature of behavioural testing of false memories[1]. The video on the right provides an example of a false memory being implanted in an oblivious participant.

1 Encoding

Figure 1
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The precuneus and inferior parietal lobe
Locus of visual imagery in the brain
ImageSource: Wikipedia.org

1.1 Visual Imagery Hypothesis

The visual imagery hypothesis states that the production of false memories occurs due to the encoding of events that are imagined in the mind’s eye as true memories. The theory is based on evidence from fMRI studies showing that regions of the brain usually associated with visual imagery tasks – the precuneus and left inferior parietal cortex, Figure 1 – are activated during the encoding of words that are later falsely remembered, but not during the encoding of words that are later correctly rejected[2], while both true and false memories show activation of the hippocampus at encoding[3]. The theory postulates that the activation of visual processing areas of the brain by imagined events are confused with activation of the visual cortex by the optic nerve, and are subsequently encoded as visual memories.

1.2 Semantic Network Theories

Two theories have been proposed involving the influence of the brain’s semantic networks (Figure 2) on the process of memory encoding, the spreading activation theory, and the fuzzy trace theory. Both theories suggest that that the production of false memories is the unintended consequence of an elaborative semantic encoding process[1]. This is supported by the work of Kim and Cabeza, who found that activation of the left prefrontal cortex, usually associated with semantic elaboration, coincides with the encoding of both true and false memories[4].

Figure 2
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Example of a semantic network, the activation can spread from one node
to the next at encoding, or at retrieval
ImageSource: Institute for Biocomputation and Physics of Complex Systems

1.2.1 Spreading Activation Theory

The spreading activation theory states that in the presentation of a list of words, activation of the semantic network unintentionally spreads from the presented items to semantically related or similar items that have not been presented, which are then mistakenly encoded with the memories for the presented items[5]. This theory explains how the Deese-Roediger-McDermott false recall test is consistently able to produce false memories of critical lures, which have not been presented[5].

1.2.2 Fuzzy Trace Theory

The fuzzy-trace theory states that with the presentation of a list of words, each word will have a verbatim-trace, which encodes specific information about that item, as well as a gist-trace, which encodes the general theme of the presented items[1]. The theory specifies that false memories occur due to an inability to dissociate gist and verbatim-traces at encoding[6], thus explaining the inaccurate retrieval of non-presented gist words in the Deese-Roediger-McDermott false recall test.

1.3 Top-Down Processes

While the process of false memory production may depend on bottom-up factors—such as memory paradigm used, list length, or presentation duration—other factors related to the top-down processing of the presented material also have an effect on the incidence of false memories[7][8][9]. One study has shown that inducing low-arousal moods in participants will prompt greater recollection of false memories, whereas high-arousal moods result in fewer false recollections, possibly due to an increase in the strength of verbatim-traces during encoding[7]. Another study has found that negative affect has an effect similar to high-arousal, decreasing false memory recollection and increasing item-specific memories[8]. These studies provide evidence for an interaction between the amygdala and the hippocampus in the formation of false memories. A further study has found that working memory capacity is inversely related to false memory production in the Deese-Roediger-McDermott false recall test, when participants are warned of the nature of the test before encoding[9]. These studies are just a few examples of the diverse mechanisms of top-down control involved in the process of false memory encoding, other top-down effects on memory processes include lifestyle and stress.

2 Consolidation

2.1 Retroactive Interference

Figure 3
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Effect of sleep on false memory consolidation [12]
Sleep enhances both true and false recall.
Click to Enlarge

As is seen most often in the misinformation paradigm, providing misleading information about an original memory long after it has been encoded can form false memories. The misinformation or new information thus reconsolidates an incorrect version of that memory. For instance, if an individual has had two traumatic experiences in a hospital, he or she might retroactively attribute events that occurred in the second experience to his or her first hospital experience[10]. The theory behind this phenomenon stresses the importance of the distinctiveness of these memories; events that differ not only from their context, but also from other previously encoded events, will be encoded and stored as distinctive[10]. However, it is currently unclear whether the mechanism of distinctive memories relies on differential encoding, limited retrieval, or some other method of consolidation, as it is often difficult to extricate these processes in behavioural memory research[10].

2.2 Sleep Deprivation

While sleep is crucial for the stabilizing and strengthening of newly encoded memories, it is also responsible for the consolidation of memories into long-term storage in the cortex, a process that involves substantial reorganization and restructuring, and can subsequently lead to the formation of false memories[1]. A recent study using an adaptation of the Deese-Roediger-McDermott protocol found that compared to sleep deprivation, sleep appears to enhance both true and false memories, and that only after a night of sleep were both true and false memories formation associated with the hippocampus and retrosplenial cortex[11]. This data suggests a general systems-consolidation function for sleep, rather than the selective enhancement of either true or false memories[11]. Conversely, another recent study using an adapted Deese-Roediger-McDermott protocol found that in individuals with weak general memory performance, both nocturnal sleep and sleep deprivation significantly enhanced false memory recall (Figure 3), demonstrating a dual nature for the effect of sleep on false memory formation[12]. One effect involves the reorganization of memory traces during sleep, and the other involves the restorative role of sleep in the cognitive control processes necessary for true memory consolidation, both of which become evident when general memory performance is weakened[12].

3 Retrieval

3.1 Retrieval Cues

Memories are particularly liable to distortion at retrieval, as is evident in the misinformation paradigm in which the retrieval cue used can have a dramatic impact on the details of the memory that is subsequently retrieved[1]. This effect is related to the retroactive interference effect (see above), in which new information about the event interacts with the memory itself. In a famous experiment from 1974, participants who had watched a video of a car accident were later asked questions about their memory of the accident; participants who were asked the speed of the cars when they hit each other reported lower speeds than participants who were asked the same questions but with the word ‘smashed’ in place of ‘hit’[13].

3.2 Activation Monitoring Theory

Figure 4
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Reactivation of auditory and visual loci [15]
allows researchers to predict the incidence of false memories
Click to Enlarge

The activation monitoring theory relates the spreading activation theory to the context of the retrieval process: the idea is that spreading activation of semantic networks works in tandem with a monitoring process at retrieval, which allows us to determine the source of the activation[1]. In false memories, this monitoring process has been disrupted such that we are no longer able to distinguish the activation source[14]. For instance, in the Deese-Roediger-McDermott protocol, this would cause us to confuse memories for non-presented but semantically related words with our memories of the presented words[14].

3.3 Sensory Reactivation Hypothesis

Some fMRI research has suggested that the only way to distinguish true and false memories may be in the activation of sensory perceptual regions at retrieval, which is supported by the sensory reactivation hypothesis[1]. The hypothesis states that the retrieval of a memory causes a reactivation in the cortex of areas related to the perceptual encoding of the memory[15], thus a memory of Bach’s fifth symphony would activate the early auditory cortex. A recent study by Stark et al. found that by manipulating the modality of the information presented at each stage in the misinformation paradigm, they could accurately predict the retrieval of true or false memories based on fMRI images of the brain at retrieval[15], such as the one in Figure 4. The figure shows activation of the auditory cortex associated with false memories (also known as ‘critical hits’) as a result of the misinformation being presented aurally. This mechanism could also explain the common incidence of false memories in visual syntesthetes.

4 False Memory Research

4.1 Deese-Roediger-McDermott Paradigm

The DRM protocol requires participants to listen to or read a list of semantically related words, for which there is a salient word or ‘critical lure’ that is missing[16]. For instance, the following list that might be used in the DRM protocol is missing the word ‘music’: note, sound, piano, sing, radio, band, melody, horn, concert, instrument, symphony, jazz, orchestra, art, rhythm[16]. In free recall, the participant is simply required to name as many items from the list as they can remember, but in recognition paradigms the participants are presented with some list words, some critical lures and some new words and are asked to make a judgment as to whether the presented word was in the original list[16]. In both free recall and recognition paradigms, participants often report a memory of the critical lures that were not presented[16].

4.2 Misinformation Paradigm

Figure 5
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Dr. Elizabeth Loftus: leading researcher in false memories
ImageSource: brainsandcareers.com

In the misinformation paradigm, participants watch a short vignette or hear a short story that they are asked to memorize[17]. Later, in the misinformation phase, participants are provided with misleading information about the original story, often in the form of questions[17] such as “was the robber holding the gun in his left or right hand?” when the robber had been female, or had been holding a knife. After some time, participants are tested on their knowledge of the original story; the misinformation effect occurs when participants report a false memory that is consistent with that misinformation, such as “I remember seeing the robber put his gun away”[17].

4.3 New Technologies

With the introduction of new technologies such as Transcranial Magnetic Stimulation (TMS) and transcranial Direct Current Stimulation (tDCS) to the study of memory and cognition, researchers have been able to gain new insight into the neurological basis of false memories. A recent study has shown that both unilateral and bilateral stimulation of the anterior temporal lobe results in a significant decrease in susceptibility to false memories in the DRM paradigm, with apparently no effect on true memories[18]. Further research with these new methods could help to distinguish between different mechanisms and effects in the formation and retrieval of false memories.

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