The pathway of stabilization and destabilization of memory is known as the modal model of memory. Within this pathway, there are many different steps at which memory failure can occur, including at the time of encoding information into short term memory and its subsequent consolidation into long-term memory . This type of memory failure, simply referred to as forgetting, is thought to occur either by a passive decay of the memory trace, or by the active inhibition of consolidation. In the past, it was thought that intentional and incidental forgetting occurred through similar pathways. However, there has been recent evidence for a dissociation of these two types of forgetting. Current research is now focusing on elucidating the cognitive and neural differences between incidental and intentional forgetting. Clarifying these differences is important in order to better understand what leads to memory success, specifically how the brain distinguishes between relevant and irrelevant information.
1 Incidental Forgetting
Forgetting is an important part of cognitive control of memory. The pruning of irrelevant information from memory storage is crucial to the recall of important information. Our brains would be swamped with too much information, most of which would be useless . Therefore, incidental forgetting is a necessary aspect of human memory, and scientists in the field have devoted a significant amount of time to studying this phenomenon.
1.1 Behavioural Theories
There are multiple theories on the processes behind incidental forgetting. The most prominent theory is that unintentional forgetting occurs by various forms of interference. There are two types of interference: proactive and retroactive. Proactive interference occurs when something that is previously learned interferes with recalling information, whereas retroactive interference occurs when something learned later interferes with recalling information. Another theory of incidental forgetting is that of retrieval failure, which could involve a change in context or retrieval cues. Finally, incidental forgetting can just occur by the passive decay of the memory trace .
Others have posited that incidental forgetting, especially in laboratory settings, occurs as a result of lack of attention or a wandering mind, which leads to a failure in encoding . In an article recently published by Rizio et al, the authors wanted to distinguish between incidental and intentional forgetting. They used an item-method directed forgetting paradigm, where the participants are either given instructions to forget or remember an item. Incidental forgetting occurs when the participant is given an instruction to remember, and they cannot recall the item. The authors suggest that, according to their results, incidental forgetting is mediated by a failed attempt of encoding .
1.2 Neuroimaging Results
|Fig. 1: Left Inferior Frontal Gyrus Activation
during Incidental Forgetting
|source: Rizio et al, 2013|
Until recently, intentional and incidental forgetting were proposed to occur through similar pathways and brain regions. Most scientists thought that intentional forgetting, like incidental forgetting, was simply due to the lack of encoding. Rizio et al’s recent study showed that the two different types of forgetting dissociate not only behaviourally, but also in their neural correlates . The authors found that incidental forgetting tends to be left-lateralized, and they specifically saw greater activation in the left inferior frontal gyrus, left superior frontal gyrus, early visual cortex, and left superior parietal lobe. It seems that these areas are related to encoding attempt, leading the authors to their conclusion that incidental forgetting occurs via an attempt, but subsequent failure in encoding . Their neuroimaging results can be seen in Figure 1: the left inferior frontal gyrus is activated during the forgetting of “to be remembered” items in the directed forgetting paradigm.
|Default Mode Networks|
In addition to these regions, it has been proposed that the default-mode network regions may be associated with subsequent forgetting . The default-mode network represents the brain regions that are active in the resting state, or when our minds are not focused on a specific task. These regions include the anterior and posterior midline cortex, temporoparietal junction, and superior frontal cortex . A meta-analysis was conducted regarding the neural correlates involved in subsequent forgetting, and after finding that the default-mode network regions do appear to be involved in subsequent forgetting, the author posited that incidental forgetting may occur due to a wandering mind. Specifically, the author noted that the activation of this network might impair effective recall through the diversion of neural resources . The following video has a good overview on default-mode networks and how they are related to memory.
2 Intentional Forgetting
Intentional forgetting is the motivated inhibition of encoding or recalling memories. The first thing that comes to mind when most people think about intentionally forgetting things is probably related to memory erasure. Popular Hollywood movies have undoubtedly contributed to this view that memories can simply be erased. However, intentional forgetting is something distinct from erasing memories; rather, it seems to be related to inhibitory activity in certain brain regions that prevents the encoding of memories, even before consolidation . Many theories have been proposed on how this type of forgetting occurs, and these are outlined in the next section.
2.1 Behavioural Results
One of the theories proposed for intentional forgetting relates to retrieval suppression. Normally, when one is reminded of a memory, the retrieval of that memory can benefit its retention. However, when one is looking to become unaware of a memory, the benefits normally seen in retrieval are suppressed through cognitive control . In a recent review, researchers implemented a think/no-think paradigm in order to measure positive and negative control effects in retrieval suppression. Positive control effects refer to the increased retention of a memory after a reminder or cue to respond, and negative control effects refer to the diminished retention of a memory after a cue to suppress . Their findings suggest that when an individual does not wish to be reminded of a memory, a cue to respond initiates downstream effects that assist in the retrieval suppression of that memory .
Further, researchers have come up with the idea of thought substitution as a method of improving the ability to intentionally forget. In a study conducted by Hertel et al, participants were subjected to lists of paired words, some of which were to be remembered and some to be forgotten . The first word of the pair was used as a cue. Half of the participants were provided with substitute words for the pairs to be forgotten, and the other half were not provided substitutes. The former was referred to as the aided group, where the latter was referred to as the unaided group. The authors’ results suggest that the aided group were able to more effectively forget the “to be forgotten” words when compared the the unaided group . Interestingly, some of the unaided group were also successful in forgetting the “to be forgotten” words; however, these subjects indicated that they consciously diverted their attention during those trials. The researchers concluded that using thought substitution, either self-initiated or prompted, does appear to increase the ability to forget in this particular paradigm .
Finally, in a recent paper by Rizio et al, the authors propose that intentional forgetting is actually caused by an inhibition of encoding . They also reference previous studies regarding retrieval suppression, and suggest that while it does agree with their findings, this phenomenon may be more important in distractions. The authors used a directed forgetting paradigm and imaged the brains of participants to determine the brain regions involved . These results are discussed in the next section.
2.2 Neuroimaging Results
As mentioned in the previous section, Rizio et al’s paper imaged the brains of participants during a directed forgetting task, and found that the brain regions that are active during intentional forgetting are different from those active during incidental forgetting. In intentional forgetting, the active brain regions tend to be right-lateralized (as opposed to left-lateralized in incidental forgetting), specifically the right superior frontal gyrus and right inferior parietal lobe . Figure 2 shows their neuroimaging results: the right parietal cortex had more activation in intentional compared to incidental forgetting cues. The authors also noted that there is an increase in activity in the right prefrontal cortex that is associated with intentional forgetting, leading to the conclusion that there are right-lateralized inhibitory processes involved in this type of forgetting that hinder the encoding of memories.
|Fig. 2: Right Parietal Cortex Activation
during Intentional Forgetting
|source: Rizio et al, 2013|
Another study conducted by Wylie et al was looking to determine which brain regions were involved in intentional forgetting using an item-method directed forgetting paradigm . Their results agree with the above study: the neural substrates of intentional forgetting, are, in fact, separate from those of unintentional forgetting. Specifically, they found that the parahippocampal gyrus, hippocampus, and superior frontal gyrus were active during intentional forgetting . Taking into account the nature of these brain regions, the authors conclude that these results suggest the role of diversion of attention from the task to the external environment in intentional forgetting .
A more recent study published in 2010 investigated the activity of the mediotemporal lobe during directed forgetting using event-related potentials, and their results contradict those of Wylie and colleagues . They found that the hippocampus is actually inhibited during directed forgetting. Additionally, they noted that this is likely achieved through active inhibitory processes in the frontal cortex, with some involvement from the rhinal cortex .
More studies must be conducted in order to sort out the specific pathways through which intentional forgetting occurs; however, it is clear that the neural correlates of intentional forgetting are separate from those of incidental forgetting, and that the former involves the activation of right-lateralized brain regions .
3 Modulators of Forgetting
Wouldn’t it be great if we could just forget painful memories (without using pills)? Check out the video in this section for an interesting (albeit fictional) commercial that would likely get a lot of attention, were it true or scientifically possible at this time. Even though it would be extremely difficult and highly improbable to be able to selectively erase people or relationships from your memory, your emotions and mood during the encoding of memories does seem to play a role in the forgetting of those memories. This section outlines recent research on some modulators of forgetting.
3.1 Emotional Memories
Few studies have been carried out that examine the relationship between emotion and directed forgetting. In 2007, Depue et al were looking to analyze how emotional memories are suppressed . The authors used a think/no-think paradigm, and they found two different mechanisms through which emotional memories are suppressed. These mechanisms occur at different times: first, the right inferior frontal gyrus is involved in suppressing the visual cortex and the thalamus, and second, the right medial frontal gyrus suppresses the hippocampus and amygdala, or the emotional component of the memory. The authors thus concluded that there is an active process, mediated by areas of the prefrontal cortex, that aid the suppression of unwanted emotional memories .
Further, a recent study published in 2012 used event-related potentials to differentiate the neural substrates involved in the forgetting of neutral versus negative images . The authors found that participants were able to forget both neutral and negative images after a item-method directed forgetting paradigm. Interestingly, they also found that while the inhibition of encoding of all “forget” items occurred, regardless of emotional value, the negative stimuli required a higher cognitive control for the process of successfully forgetting .
Another important modulator of the forgetting process is mood. Researchers in Germany were interested in determining if the mood a person is in during memory encoding can affect the process of forgetting previous memories . They kept the material being studied of neutral emotional value, and induced a positive, negative, or neutral mood prior to the study of this material. Their results show that positive moods negate the directed forgetting effects seen in neutral and negative moods. The authors suggest that this effect is seen because of the associative networks that are activated in positive moods, but not in negative moods .
4 Related Disorders
Considering that higher cognitive processes involving the prefrontal cortex are involved in intentional forgetting, it follows that individuals with disorders that affect the inhibitory processes in the brain would have either impaired intentional forgetting, or alternative pathways to achieve the same end. The following section explores how memory suppression works in individuals affected with attention deficit hyperactivity disorder, and psychogenic amnesia. (link to pages)
Attention deficit hyperactivity disorder (ADHD) is known to confer a deficiency in inhibitory cognitive control, which leads to the assumption that these individuals would have problems when attempting to intentionally forget. A review published in 2012 described a couple of studies conducted regarding ADHD and intentional forgetting, and notes that individuals with ADHD could not activate the crucial brain regions for intentional forgetting, thus leading to the conclusion that these individuals have a hard time with cognitive control and memory suppression .
4.2 Psychogenic Amnesia
|Dissociative amnesia can lead people to forget their names.
In some cases of psychogenic amnesia, individuals “forget” certain events and people from a period in their lives, usually after some sort of stressful trigger. Researchers were interested in determining if this forgetting was an involuntary episode of the mechanisms of intentional forgetting . In a 2010 study conducted by Kikuchi et al, two patients with dissociative amnesia underwent neuroimaging scanning while being shown pictures of people they had forgotten, people who they still remembered, and neutral controls (people they had never met) . The authors found that the same brain regions that are activated in intentional forgetting were also active when the patients were shown the pictures of people they had forgotten. Also, there was a lowered activation of the hippocampus when they were shown the pictures of the people they no longer recognized, leading to the authors to accept the possibility of intentional forgetting mechanisms being involuntarily activated in some cases of psychogenic amnesia .