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Ventromedial prefrontal cortex, adding value to autobiographical memories

The medial prefrontal cortex (mPFC) plays a role in autobiographical memory (AM) recall, recollection of self-relevant information, the imagination of novel scenarios, emotional regulation during autobiographical memory recall and linking self-relevance and value with ventromedial prefrontal cortex (vmPFC) involved.

mPFC also plays a role in representing the values of choice during decision making. Judgements relating to the self are believed to be processed in more ventral mPFC while other-relevant processing is associated with more dorsal mPFC. mPFC might contribute by representing the subjective value of the contents of imagined or recollected scenarios. Increasing the personal relevance might involve more ventral regions of mPFC.

Using fMRI during autobiographical memory task found that:

  1. Activation of vmPFC was modulated by the affective values of items in participants’ memories when they were recalling and evaluating these items.
  2. Unrelated modulation by the participant’s familiarity with the items was also observed.
  3. vmPFC processes self-relevant information, and suggest that it is involved in representing the personal emotional values of the elements comprising autobiographical memories.

In our previous study activity in mPFC was modulated by participants’ subjective evaluation of common items in newly imagined scenarios. In the present study, we replaced the imagined scenarios and imagined items with participants’ real autobiographical memories and the items that were remembered within them.

On day1 we asked participants to recall autobiographical memory including six items within each event that were either liked or disliked at the time of the event. They characterized each memory in terms of its pleasantness, recall vividness, personal significance, recall frequency, recall difficulty, emotional intensity and time since it happened, and also reported the familiarity of the items at the time of the event. On day 2, participants recalled memories and then rated the values of four of the items in each event, and their vividness.

Participants were twenty-five right handed participants from University College London. Cue words used in the interview were 40 nouns chosen from Clark and Pavaio‘s extended norms. All participants took part in the experiment on two consecutive days, the interview on day 1 and recall in the scanner on day 2 to have a better control over factors during memory and item retrieval (like counterbalancing the order in which liked and disliked items were recalled and duration for retrieval of autobiographical memory).

During interview all cue words were presented. Participants were instructed to freely associate one time- and location-specific autobiographical event to each cue word and verbally elaborate the details of the event (age, location, people involved and things happened). Events could range in age from childhood to the day before the interview.

Ratings for each event were also required, including the pleasantness, recall vividness and emotional intensity evoked by the event, as well as its personal significance, recall frequency and difficulty. Some of these ratings are correlated with each other but not identical, like emotional intensity and significance (delicious with no personal meaning). Participants also had to provide three items they liked and three they did not like from each event, as well as rating how familiar they were with each at the moment when the event occured.

Example
Cue word: Journal
Event: Stealing sisters diary
Liked items: cover, carpet, duvet
Disliked: drawer, shoes, light

Scanner day 2
Recall of all 40 memories (encouraged to bring visual, auditory, tactile, olfactory and other details into scenario, sensation of holding, feeling of reading, footsteps).
Followed by focussing attention to specific items.
Four out of the six Items selected by experimenter (time, categorical bias)
Liked item was chosen when same or similar has not been chosen jet, or similar items consisted of disliked items.
Instructed to evaluate how much they liked each item and how vividly they recalled it.

Main Analysis
Preprocessed functional images were analysed with
general linear models (GLM).
Along with
regressor of interest each GLM included 6 movement regressors.

GLM 1 used for hypothesis that mPFC activity is modulated by subjective value of memory. Activity when recalling and evaluating liked items higher compared to not liked items.
Regressors:
– recalling memory
– evaluating liked item from memory
– evaluating a disliked item
– ITI periods
– key presses
One to three averaged and transfered into second level model.
Contrast between recalling a memory and evaluating item used to make sure participants were engaged in autobiographical memory task.

GLM 2 used to further investigate nature of activity in mPFC to see whether activity showed a parametric relationship in the subjective rating of value given for each item.
– recalling memory
– evaluating liked item from memory
– parametric modulator for the item regressor based on the participant‘s value of that item
– ITI periods
– key presses

GLM interrogates BOLD response for liked vs. disliked items.

GLM1 interrogates BOLD response for liked vs. disliked items. GLM 2 interrogates wheather BOLD linearly varies with individual subjective rating of each item.

Further analyses of vmPFC activity

Significantly increased activity in vmPFC in GLM1 and GLM2 for objects with higher subjective value. Weak correlation between ratings of item value, and rating of item recall vividness, and item‘s familiarity at the time of the event. Thus both might contricute to item value effect.

GLM 3 within vmPFC including seven regressors.

– recalling memory
– evaluating item
– vividness parametric modulator of regressor
– familiarity parametric modulator of regressor
– item value parametric modulator of regressor
– ITI
– key presses

Parameter estimates for value were averaged.  First PM allowed to explain unique and shared variance, with subsequent PMs explaining unexplained variance. Subsequent PMs explaining the remaining unexplained variance. Any value effects found in GLM3 is variance uniquely explained by the item value PM.

To assess effects of item familiarity or recall vividness from other factors and from value, we evaluated GLM4 and GLM5.

To examine whether the event-specific item value effects of GLM1 and 2 could reflect the values in everyday life of the types of items, GLM 6 was built.

To investigate relation of event-specific item value during item evaluation to activity during retrieval of corresponding autobiographical memory, we used additional GLMs for each ratings given to characterise the autobiographical memory. Parametric modulator was one of seven memory ratings i.e. memory pleasantness, personal significance, recall frequency, recall difficulty, emotional intensity, recall detail and memory age.

Behavioral results

The memory age ranged from one day to 31 years old. ANOVA on rating of items was conducted to verify the value differences between liked and disliked items, including factors of specifity. There were main effects of both rating specificity, (value of that specific item within the event versus in general life) item type (liked and disliked) and interaction between them. Liked items had higher event-specific value ratings but not higher general value ratings. A paired-samples t-test on recall vividness between liked and disliked items showed that liked items were more vivid. Items with higher values rather had higher familiarity and vividness ratings.

fMRI results

Recall: Search for regions with greater BOLD response when recalling a memory relative to evaluating an item. Large regions of activity throughout network including mPFC, medial temporal lobes, retrosplenial and medial parietal areas.

Subjective Value: vmPFC shows greater activity for liked items and its activity positively correlates with the values of individual items from recalled memories.

Relation of vmPFC value effects to familiarity and vividness: Significantly increased activity in vmPFC for items with higher subjective value (GLM1, 2). Weak correlations between ratings of item value, item recall vividness and familiarity of item at the time of the event (GLM 3). Parametric modulator of item familiarity also revealed a significant unique effect in vmPFC not caused by value or vividness of the items (GLM4). Vividness of item recall does not explain vmPFC activity beyond that explained by familiarity or value (GLM5). No significant effect of general every-day value of the types of items retrieved within vmPFC (GLM6). Item value effect reflected the memory-specific value of the item, rather than gneral preferences for different types of items.

Relation of vmPFC item value effects to the personal emotional significance of the memory: How might the subjective value of items relate to processing during recall of the autobiographic memory? Significant correlations between summed values of the items present in an autobiographic memory and several of the ratings used to characterise that memory overall, including pleasantness, significance, recall frequency, recall difficulty, emotional intensity, detail and age. During recall of a memory, the mean activity in the vmPFC varied with personal significance and emotional intensity of the memory. None of the other memory ratings showed significant modulation of vmPFC during recall.

Discussion

Participants recalled personal autobiographical memories and evaluated their liking for specific items. The liked items were reported as more familiar at the time of the event, and were recalled more vividly during recall. vmPFC activity was modulated by how much  items were liked at the time when the events happened. During recall of the entire memory, activity in the vmPFC location showing item value was modulated by personal significance and the emotional intensity of the memory. vmPFC plays a role in the generation of affective meaning in association of events with emotional responses and in emotional response via self-relevance.

vmPFC involved in self-relevant processing during autobiographical memory recall by assigning personal value to self-related information. One of its roles is to provide subjective values of items present in memory, and these values contribute to overall personal emotional significance of memory itself.

In a previous study the activity of a region in mPFC was modulated by the subjective value of common everyday items, that people were imagining in novel scenarios. This region was more dorsal and anterior than the item value-related region in the present study. A functional gradient along dorsal-ventral axis has been observed in mPFC between making self- or other-related judgements. Self-relevant information is processed more ventral, other relevant information in more dorsal mPFC. vmPFC activity during memory for recent presentation of face stimuli is greater for personally relevant faces. The more ventral location of the item value effect may reflect the greater personal emotional relevance of the items from participants autobiographical memories, compared to photos of common everyday objects.

Speer discovered that mPFC activity was greater during recall of memories that made them feel happy, compared to the recall of neutral memories. In their study, ventral striatum activity was parametrically modulated by affective ratings of memories. Both ventral striatum and mPFC also responded to monetary reward and participants were even willing to lose monetary reward to obtain chances to recall positive memories. Suggesting positive memory recall is valuable, so that reward system was recruited in positive memory recall. This study indicates that vmPFC can represent values of different memory components separately. Also noticed that during recall vmPFC activity was modulated by personal significance and emotional intensity of the memory. So variation in vmPFC activity with subjective value of items relates to the part those items play in the emotional self-reference of the event. If the value-related vmPFC effect reflects items personal relevance, this might explain the reduced involvement of ventral striatum compared to Speer. Assuming that ventral striatum reflects subjective consequences of recalling a positive memory (i.e. feeling of happiness) which has direct value for the current state (equivalent to receiving money), consistent with its association with reward magnitude.

In addition to subjective value, greater levels of activity in vmPFC were reported when participants recalled a familiar memory or imagined personal future events within a familiar contextual setting, compared to imagining personal future events with an unfamiliar contextual setting. vmPFC activity was also modulated by how familiar the items were at the time of the memory in our study. Greater when recalling more familiar items. Unique effects of item value and item familiarity occur indepently in vmPFC, and there were no significant correlation between a memorys personal significance or emotional intensity and summed familiarity of the items within it.

Summary: vmPFC activity modulated by the values of items within autobiographical memories. mPFC represents the values of elements within autobiographical memory and mental imagery, with more ventral mPFC location found reflecting greater emotional self-relevance of objects in autobiographical memories.  Association of vmPFC activity with processing self relevance and contributing of liked objects to personal emotional relevance of autobiographical memories.

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