Potential purposes for editing of memories in humans include, 1) reducing emotional consequences stemming from traumatic events, 2) reducing cravings in drug addiction and 3) the enhancement of education. In invertebrates it has been shown to be possible to alter the neural instantiation and behavioral expression of unique memories. This was done through the identification of synaptic changes that underlie the representation of simple associative memories.
A number of challenges must be overcome before memory editing could be safe for humans. Neural representations in vertebrates are much more complex than in invertebrates and techniques used to alter synaptic plasticity in animal models are not safe for use in humans.
Optogenetic approaches have been used in animal models but these techniques are too invasive for clinical use in humans. Another challenge for memory editing in the clinic is that a memory of a single traumatic event such as a car crash is expressed in several ways: episodic details, defensive responses, habitual actions and subjective feelings. Each form of memory may interact and also involve a distinct neural system for storage and expression. For this reason targeting one type of memory representation for editing may or may not alter other forms of memory for the same event.
Most research focuses on two types of representation. Episodic memory mediated by the hippocampus is one and defensive responses mediated by the amygdala is the other. Clinical interventions mainly focus on reducing maladaptive habits and negative feelings associated with psychopathologies.
Memory-editing techniques are thought to work at either of two windows of time when the memory is labile and vulnerable to editing. The techniques are thought to work by modifying the initial storage of memory (consolidation) or by modifying its re-storage after retrieval (reconsolidation). Since consolidation can be prolonged, it can be difficult to decipher whether a technique targets consolidation or reconsolidation in humans especially. Markers of synaptic plasticity which signal memory storage are not able to be assessed.
Neural signatures of even simple memories are complex and distributed throughout the brain, making it impractical to mechanically identify and target. However neural ensembles that represent unique memories are active at certain times and may be vulnerable to editing during those periods. Consolidation is the gradual process of transformation and stabilization and it entails recurrent neuronal reactivations of the memory while a person is awake and asleep. Hippocampal-mediated episodic memories are the type of memories most studied in the context of memory editing during consolidation.
In animal models, when amnestic agents which typically inhibit protein synthesis are administered after learning, synaptic changes that are required for consolidation can be prevented, causing recently encoded memories to be forgotten. In humans, emotional events evoke stress hormones enhance consolidation. Drugs that mimic the effect of or block stress hormones can modulate hippocampal consolidation.
The administration of propranolol, which blocks stress hormones, has been tested as a treatment for humans to after a traumatic event. The treatment is thought to reduce the emotional strengthening of episodic memory. The treatment reduced some indications of physiological arousal when participants recollected the event later but there was not effect on the likelihood of developing symptoms of post-traumatic stress disorder (PTSD).
Adrenaline and glucocorticoids can be given as drugs to mimic the effect of stress hormones. Glucocorticoids have been studied as a method to augment exposure therapy or extinction learning to treat phobia or PTSD. In exposure therapy repeated exposure to a threatening stimulus or memory with no aversive consequences can lead to the subject learning that the stimulus is now safe. The new ‘safe’ memory competes for the expression of the threat memory. Glucocorticoids were given before exposure therapy to strengthen the consolidation of a safe memory. In the study clinical symptoms were reduced but since glucocorticoids also impair retrieval of emotional memories, the mechanism of the action for the observed benefit was not clear.
Targeting memory reactivation (TMR) alters the reactivation of memories in order to strengthen consolidation. TMR involves presenting cues linked to a previously encoded event during periods of sleep or awake rest. In humans this approach has been shown to produce blood oxygenation level-dependent patterns of activity in the hippocampus consistent with the reactivation of neuronal memory and strengthens episodic memory of these events later.
Rather than strengthen episodic memory, TMR is also used to impair episodic memory, by cuing participants during sleep with a tone lined to an intention to forget. Memory control, motivated forgetting or directed forgetting are terms sometimes used to describe these types of memory-editing techniques.
Behavioral tagging is a technique to modulate episodic memory consolidation based on the premise of synaptic tag-and-capture models. Memories that are initially weak are strengthened by the engagement of common neural pathways minutes to hours later. After encoding a stimulus, related stimuli are made relevant by pairing with shock or monetary reward.
Since memory editing techniques that influence consolidation are limited to minutes, hours or days after encoding there is interest in developing techniques for editing memory reconsolidation for clinical interventions. In reconsolidation, reactivation of retrieval of a memory can cause a previously consolidated memory to become labile and prone to modification. The re-storage of the memory requires synaptic plasticity. Reconsolidation has two proposed adaptive functions. The memory is thought to be strengthened and also if the memory is reactivated in a context with new information, the new information is reconsolidated into the old memory. Memory editing that targets human reconsolidation long after the memory is learned is possible and this appears to modify but not erase memories.
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