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Introduction

Neural
structures are believed to hold a vital role in subserving complex
psychological functions such as attention, emotion and memory. A neural network
is a structure of interconnected neurons in the brain and nervous system,
comprised of four lobes – frontal, temporal, parietal, and occipital. Different
areas of the cortex control different cognitive functions, and it is believed
that the amygdala, the hippocampal complex, and the cerebellum are the neural
structures which most significantly influence emotional memory. Neural
structures involved in the encoding, consolidation and retrieval process of
memories are thought to differ in emotional memories than in neutral memories
(Cahill et al., 1995). Memories are sometimes so emotionally stimulating that
they leave a “scar” upon cerebral tissue, which is why the most vivid
autobiographical memories tend to be of emotional events (James, 1890).

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Memory
itself is the psychological function which enables us to record, retain and
retrieve information. There are many different types of memory, including
explicit memory (i.e. declarative memory, which is the conscious retrieval of
information such as facts and events) and implicit memory (i.e. procedural
memory, which is the unintentional retrieval of information from past
experiences). Emotional memory is a type of declarative memory, as it relies on
episodic and autobiographical memories which are the recollection of personal
facts and events, with the influence of semantic memory. It is believed that a high
level of emotional arousal enhances the encoding, consolidation and retrieval
of memories, which is why emotional memories are defined as long-term memories,
not short-term.

 

Whilst
the amygdala is generally thought to employ the greatest role in emotional
memory processing, there are other neural structures of significance such as
the hippocampal complex and the cerebellum, which have also been found to contribute
to processing emotional memory. There are also factors such as the type of
emotion, individual response to arousal, age, gender, mood and/or temperament,
and depression which must be taken into account, as these factors can affect
the role of the amygdala on emotional memory. It is difficult to study the role
of neural structures in subserving memory, as the different research
methodologies employed to examine neurology, such as fMRI studies, have many
weaknesses. Due to a huge influence of individual differences, and of the
unethicality of manipulating neural structures in humans, it is difficult to
study the role of these neural structures.

 

This
essay will examine the different theoretical perspectives on the role of the
amygdala in subserving emotional memory, with consideration of the influence of
other neural structures, individual differences, and issues with the methodologies
employed in this area.

 

Neural Structures

The
amygdala is a small neural structure that is a part of the limbic system,
located medially within the temporal lobe. It modulates sensory cortex
activation, specifically decision making, emotional reactions, and memory
(Amunts et al., 2005). The right hemisphere of the amygdala is the portion associated
with negative emotion, such as processing fear-inducing stimuli. It is also
associated with declarative memory, which is the memory of facts and information
consciously recalled from previous events, and of episodic memory, including
emotional and sensory memory (Markowitsch, 1998). The amygdala engages the
ventral sectors of the prefrontal cortex which subserve emotional attention.
The ventral sectors are specifically involved in semantic and autobiographical
memory retrieval (see Figure 1). This
attention enhances the encoding of emotional material, and the amygdala then
modulates this emotional information through memory consolidation in order to
improve retrieval.

Figure 1: Amygdala mechanisms of mediating
emotional arousal and memory (Nature, 2006).

 

Studies
have found that damage to the amygdala interferes with memory that is
strengthened by emotion, which strongly supports the theory that the amygdala
is an important neural structure in subserving emotional memory. Carlson (2012)
studied a patient with bilateral degeneration of the amygdala, and concluded there
to be less recollection of the stimuli than patients with a functioning
amygdala, despite the stimuli being highly emotional (i.e. violent and
emotionally arousing). This is a strong support of the importance of the
amygdala in subserving emotional memory, as damage to the amygdala here was
found to impair classical fear conditioning, otherwise known as the trained
emotional response of either fight or flight. Emotional memories are stored in
synapses in the brain, and there are thousands of neuronal connections from the
lateral nuclei to the central nucleus of the amygdala. The sensory neurons
project axon terminals to the central nucleus, which is the neural structure
employed in these fear responses, including the fight or flight defence and
also automatic nervous system responses (i.e. raised blood pressure and heart
rate), and neuroendocrine responses (i.e. stress hormone release) (Carlson,
2012). Therefore, a clear link can be concluded between damage to the amygdala
and the emotional memory retention of a fear response.

 

The
amygdala also employs a role in regulating memory consolidation, which is
strengthened by emotional arousal. Research has found a clear correlation
between the amygdala and the consolidation of emotional memory, such as Ferry
et al.’s (1999) study of memory retention in rats. Ferry’s study concluded that
injecting norepinephrine (i.e. stress hormones) into the amygdala of rats
increased their memory retention of inhibitory avoidance training. These stress
hormones activated the noradrenergic mechanisms in the basolateral nucleus of
the amygdala, which stimulated emotional arousal and subsequently enhanced
memory. More recent fMRI research found that an increased activation in the
amygdala was linked with enhanced memory in Buddhist monks who practised
compassion meditation (Lutz et al., 2008). This study of intensive insula
activity concluded that monks who practised a meditation state engaged their
insula (i.e. the area of the cortex in the limbic regions involved in
consciousness) and were able to improve modulation of the amygdala and
subsequently enhanced their emotional memory retention. Although the strength
of activation in the insula was self-reported, a clear association was
concluded between emotional meditation and improved memory due to the increased
amygdala activation and regulation. The amygdala is therefore proposed to
receive information from other neural structures that are important for memory
such as the hippocampus, which is associated with emotional context and
regulation.

 

The
hippocampal structure is part of the larger medial temporal lobe, and it is
theorised to be an important role in encoding the emotional context of
declarative memories from the amygdala. This emotional context is why
revisiting a similar situational context to that of the time of encoding
improves the retrieval process, due to reproduced emotional arousal (Gluck et
al., 2013). The hippocampus is also thought to play an important role in the
consolidation process of emotional memories, as damage to the hippocampus
affects a patient’s ability to form new memories (i.e. anterograde amnesia) and
often the retrieval of past memories (i.e. retrograde amnesia). However, these
patients often retained their more emotionally arousing autobiographical
memories, which suggests that consolidation over time involves a transference
of memories from the hippocampus to other neural structures, such as the
amygdala, which were not damaged in these patients (Eichenbaum, 1993). There is
also a proposed theoretical argument for the role of the hippocampus in the
encoding and retrieval of emotional memories, as more recent research had found
that patients with unilateral hippocampus damage are able to achieve
emotion-based learning, despite their inability to encode novel episodic
memories (Turnbull and Evans, 2006). However, patients with severe bilateral
hippocampus damage are incapable of emotional learning or forming new episodic
memory, which suggests there are separate neural systems which store memories
of emotions and events (Gupta et al., 2009).

 

Whilst
there are other proposed neural structures that influence emotional memory, it
is widely believed that the amygdala employs the most significant role.
Research has found that neural structures such as the cerebellum, for example,
are associated with motor coordination and remembering strong emotions,
particularly the consolidation of the long-term memory of fear (Nijholt et al.,
2003; Sacchetti et al., 2002). Other potential neural structures involved are
the larger prefrontal cortex, which influences mood state and cognitive task
ability (Gray et al., 2002), and the dorsolateral prefrontal cortex, which
activates the surprise-response from exposure to unexpected emotional stimuli
(Fletcher et al., 2001). However, it is the interaction between these neural
structures and the amygdala which is concluded to be the influencing role on
subserving emotional memory.

 

Emotional Memory

There
are different types of emotional memory, such as flashbulb memories and sense
memories, which all fall under the category of autobiographical memory. A
“flashbulb memory” describes a vivid, detailed snapshot of an event, and this
enhanced memory is believed to be due to an increased emotional arousal (Brown
and Kulik, 1977). Emotional stimuli is prioritised in the psychological
functions of perception and attention, and the selectivity of attention, such
as these small flashes of memory, can be explained by the utilisation theory (Easterbrooks,
1959). This theory suggests that a high level of arousal narrows attention,
subsequently enhancing the encoding of central, emotional memories and
diminishing peripheral details, creating a “flashbulb” of emotional memory.

 

Research
has continually supported enhanced memory of emotional events, such as the
challenger explosions (Neisser and Harsch, 1992) and the 9/11 terrorist attacks
(Talarico and Rubin, 2003), due to a high emotional arousal at the time of
encoding. Arousing stimuli is more distinctively encoded in the amygdala, and
situational context is encoded in the hippocampal complex, which results in
enhanced memory recollection (Oschner, 2000). This theory also explains the
phenomenon known as the weapon focus effect, as the increased arousal due to
stress, anxiety or fear – here due to the presence of a weapon in the presented
stimuli – improves the encoding of the event memory (Loftus et al., 1987).
However, due to the self-reported nature of these past emotional memories, they
are often unreliable. Memories of emotion are subject to change over time, as
they are affected by a current appraisal of past events, which can result in a
claim of heightened emotion or impact of an event during later recollection
(Levine and Pizarro, 2004). This means, therefore, that these studies of the
neural processes involved are, to some degree, unreliable due to their
self-reported nature – for example, the Talarico and Rubin (2003) study found
that higher emotional arousal was reported later after 9/11 than was initially
reported at the time of encoding. This self-reported method is therefore not as
reliable as other methodologies employed in examining memory, such as
neuroimaging studies, which can effectively identify neural systems associated
with specific processes, even though changes in activation are sometimes
difficult to interpret (Gabrieli, 1998).

 

Further
support of the role of the amygdala can be found through examination of
emotional suppression, which prohibits memory consolidation and diminishes the
retrieval processes (Richards and Gross, 1999; Tiwari, 2013). This research
concluded that the group who were made to supress their emotions during an
arousing task had a worse performance on a later memory test. This was proposed
to be due to an increase in self-monitoring, as the participant’s attention was
directed to controlling and suppressing their behaviours and emotions, which
reduced their attention to the actual event and prevented exact, enhanced
encoding (Richards, Butler and Gross, 2003). It has been suggested that it is the
release of stress hormones during emotionally stimulating events which activates
the amygdalae receptors. This is known as the Modulation Hypothesis, and
explains how an increase in emotional arousal increases the adrenal gland’s
release of stress hormones (McGaugh, 2004). The subsequently activated
amygdalae receptors modulate hippocampal processing, which improves memory
consolidation. Therefore, emotional suppression prevents increased stress
hormone activation, and the amygdala is not activated which results in
diminished memory consolidation. Research to support this includes stress
hormone manipulations, such as that in the study by LaBar and Cabeza (2006),
which observed an increase in norepinephrine in patients with amygdala damage,
and also the neuroimaging study by Dolcos et al. (2004) which found that
increased prefrontal cortex activation and amygdala interactions results in
enhanced memory after an emotionally arousing event.

                                                            

Following
initial encoding, further consolidation occurs during sleep, and recent
research has concluded that emotional memories are consolidated greater during
sleep than neutral memories (Payne, Chambers and Kensinger, 2012). This has
been examined using the theory of an enhanced consolidation of high valence
(i.e. of positive memories), and also the enhanced consolidation of memories
with ‘future relevance’ (Wilhelm, 2011). An interaction between the amygdala
and the hippocampal complex occurs during memory consolidation, and emotional
memories are more likely to be stored long-term due to elaboration (Kensinger,
2004). This interaction of the amygdala and hippocampus therefore enhances
emotional memory by creating links between novel processed stimuli and previously
existing memories.

 

Emotional
memory is also believed to be enhanced at the time of retrieval, due to
contextual effects – i.e. a similarity between the encoding context and
retrieval context – or mood congruence effects – i.e. an enhanced retrieval
when a person’s current mood is the same as their mood at the time of encoding.
For example, more recent research has found that a depressed mood increases the
retrieval of negative emotional memories (Drace, 2013). This is demonstrated
both in explicit and implicit retrieval. A positive context is associated with
activity in the right fusiform gyrus, and a negative context in right amygdala,
which supports the role of these neural structures in enhanced emotional
memory, specifically at the time of retrieval (Lewis and Critchley, 2003).

 

However,
there are some exceptions where memory is not always enhanced if it is
emotional, and these exceptions complicate the suggested theory of the
significant role of the amygdala in subserving emotional memory. There are
individual differences such as age, gender, mood, and emotion type, which can
affect the encoding, consolidation and retrieval of emotional memories. The amygdala
shows relatively less decline in old age than many other neural structures, meaning
that an enhanced emotional arousal effect on memory recall is maintained among
older adults. However, older age adults also tend to have a positivity bias in
their emotional memory, where they favour positive emotional recall but have a diminished
negative (Mather, 2004). This can be explained by an older adult’s improved ability
to regulate their emotions; hence they have a “selective” emotional memory
(Charles et al., 2003). This also suggests, however, that multiple neural structures
influence emotional memory, as separate processes are applied for the valence
of the memory – i.e. if the memory is positive or negative.

 

Research
has also concluded that gender effects emotional memory, as women are better at
remembering emotional, autobiographical memories than men, and are also more
likely not to consolidate information presented immediately before emotionally
charged encoding – i.e. women are more influenced by emotional content than men.
Men, however, have a significantly reduced emotional memory and tend to favour
neutral declarative memories of facts and events. The neural basis for this
difference is widely unknown. Research from fMRI studies has suggested that sex
differences are associated with different neural systems in men and women, as highly
emotional images were remembered better by women than men as they activated
different neural networks to encode the stimuli (Canli et al., 2002). Men had significantly
more activated structures including their right amygdala, and women had significantly
fewer structures activated including their left amygdala. Women also had significantly
more brain regions activated in general, with great overlap for neural
mechanisms such as the hippocampal complex that are involved in evaluating
emotional arousal and contextual factors. This would seem to suggest that neural
structures other than the amygdala also influence the encoding of emotional
memories, and these processes are employed differently in the male and female
neural systems.

 

Emotional
memory is also affected by valence, which details the positivity or negativity
of the emotion being coded. Pleasant emotions appear to fade more slowly than
unpleasant ones, but this is affected by age and mental health. Those with
depression show an equal fading of pleasant and unpleasant memories, and older
age adults regulate their emotions in the amygdala better than younger people, appearing
to encode less negative information. These exceptions can be explained through
the increased sensory and contextual details encoded with positive memories relative
to negative or neutral ones, which enhance autobiographical memory. A strong sense
of emotion can impair the memory of a less emotional event that is co-occurring,
which suggests that there is a reductionism of neural processes which separate emotional
stimuli into both central and peripheral details (D’Argembeau et al., 2002). Issues
with controlling variables of emotional expression are also subjective and not
easy manipulated, which means these studies must rely on self-reported methods
of emotional memory encoding and retrieval (Ekman, 1992).

 

Emotional
memory is also affected by a person’s emotional state at time of encoding
through mood congruence (i.e. remembering events more clearly if they match
current mood, so a person with depression tends to encode more negative events)
and mood dependence (i.e. memory is enhanced if mood matches at time of retrieval
and encoding, so a person is more likely to remember a depressing event the
next time they feel depressed). Depression, specifically, impairs a person’s ability
to create long-term memories due to a lack of attention and concentration. A
connection was found between depression and memory due to reduced serotonin
levels and neural activity in the frontal lobes (Antikainen et al., 2001), yet
memory problems declined if depression was alleviated. Neuroimaging has been
used to map neural areas associated with depression and has found activation particularly
in prefrontal areas which relate to abnormal cognitive functions including
memory in these neural structures (Grasby, 1999). This all suggests that,
whilst activation of the amygdala is the most significant role in subserving
emotional memory, there are factors of individual differences in other neural
structures due to age, gender and mental health that also affect the consolidation
of emotional memory.

 

Conclusion

Further
research is needed on individual differences to more fully understand the
relative roles of different neural structures. However, this remains difficult,
as methodologies for examining emotional memory have many issues such as the
inability to employ manipulation to brain structures (such as brain lesions) to
examine neural activation, in order to avoid issues of unethicality and
invasiveness. Another issue is that emotion itself is difficult to define and
manipulate, and often relies upon self-reported methodologies, making it
difficult and unreliable to study (Gazzaniga et al., 2009). Generally, however,
although emotional memory is influenced by individual differences such as age, gender,
and mood, research has consistently concluded that the amygdala holds the most
significant role in subserving emotional memory, even though it also relies on
interaction with other neural structures such as the hippocampus and the
cerebellum. The interaction between amygdala-driven and hippocampus-driven
neural structures is therefore currently the most supported explanation for why
emotional arousal can enhance memory encoding, consolidation and retrieval, and
why there is generally a longer persisting emotional memory than event memory
(Koriyama et al., 2010).

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