Thisessay will evaluate the role of neural structures in subserving emotion. The neural structures are part of the nervous system in the humanbrain.
The structure that will be the main focus of this evaluation will be theamygdala. Part of the limbic system, the amygdala is almond-shaped and locatedwithin the temporal lobes of the brain (Amunts et al., 2005). The amygdala has been described by many as beingpart of the neural circuitry whichis critical for emotion (Gallagher & Chiba., 1996) (18) and it is suggested that oneof the principal roles of the amygdala is emotional reactions (is your source for both parts ofthis sentence?). Emotions such as aggression and depression have beenassociated with the amygdala, yet,it is fear and anxiety which are the prime examples of the amygdala beingassociated with emotion, with an overwhelming amount of evidence to supportthis association.
However, it must be noted that it is not the only neural structure atplay, with some researchers finding evidence that undermines the significanceof the amygdala and its link to fear. Additionally, with the staggering amount of research into negativeemotion and the amygdala, it begs the question as to what significance itpossesses concerning positive emotions and their link to this neural structure,with this being explored further in this evaluation. As well as analysing justhow significant the amygdala is at subserving emotion, this evaluation will belooking into research measures and how they may alter the reliability of thevast amount of research into emotion, attempting to create a better evaluation concerning this specificneural structure. Therelationship between the amygdala and aggression emphasises this neuralstructure’s link to the psychological function of emotion. An experiment which explored facial expression recognition inbilateral amygdala-damaged patients supported the idea that the amygdala hassignificant influence on aggression, as these patients were impaired in their processing of facial expressions that wererelated to particular negative emotions, specifically fear and anger (Sato et al.
, 2002). Furthermore, a study whichlooked at the social behaviour of a rat found that lesions to the amygdalalessened the level of shock-induced aggression (Kolb & Nonneman., 1974).
This shows that even early on intothe research of the amygdala there appears to be a link to aggression. This(this twice) researchcomes from animal studies which convey that when the stimulation of the amygdala occurs, itincreases aggressive behaviour. Similarly, studies that have used brain lesionshave conveyed (conveyed twice)that harm to the amygdala may generate the reverse effect. Therefore, it wouldseem that this area of the brain may play a part in the display and modulationof aggression (Brink.
,2008) (15). Stimulation of the amygdala consequences in amplifiedaggressive behaviour, while lesions of this area significantly moderate anindividual’s aggression. A particularlyinteresting study found that the volume of the amygdala may change how aggressive anindividual is. Participants who scored higher in aggression had a 16-18%reduction in amygdala volumes (Matthies et al., 2012). This suggests that the volume of someone’samygdala plays an important role.
There is alsoa study which looked at participants that had a history and record of impulsiveaggressive behaviour, finding evidence showing amygdala-OFC dysfunction inresponse to processing angry faces. This further validates a link concerning a dysfunctional cortico-limbicnetwork and aggression (this means more than just the amygdala, so mayundermine its significance) (Coccaro et al., 2007)(29). However, a study whichcontradicts these findings looked into amygdala volumes in depressed females. Theresearch conveyed that there were no significant findings linked to aggression(Elst et al., 2000)(28).
On the other hand, the study did find that depressed patients exhibitedsignificant enlargement of both left and right amygdala, therefore theamygdala’s involvement in negative emotion is still prevalent. This can be furtherseen in the mental disorder ‘majordepressive disorder’.This disorder has been argued to be linked with volumetric abnormality in theamygdala, and that the size of the amygdala is increased significantly in depressedindividuals compared to healthy people (Hamilton, Siemer & Gotlib.
, 2008) (30). Thereis a vast amount of research which backs the theory that the amygdalastimulates and causes depression-like emotional experiences. However, the specificityas to how the amygdala effects this emotion is still unclear. Yang et al (2010) (31) conducted a study that lookedinto adolescents with depression and the involvement of the amygdala, with their results suggestinga significant level of bilateralamygdala activation in both the healthy and depressed teenagers. In addition,there was greater left amygdala activation in the depressed adolescents incomparison to the controls. More research should be conducted in adolescentsand even children to gain more of an understanding of the amygdala’s role inemotion, seeing as the brain continues to mature and develop well into the 20’s(Johnson, Blum &Giedd., 2009).
It is worthnoting that the amygdala’s involvement in negative emotion has lead researchersto suggest that the amygdala is also involved with memory. When a memory isemotionally significant to an individual it activates brain and hormonalsystems which control the strengthening of obtained new memories. These effectsare incorporated via noradrenergic activation of the basolateral amygdala whichregulates and controls memory consolidation through interactions with variousother brain regions that are linked to consolidating memories of recent experiences(Mcgaugh ., 2008). The fact that the amygdala’s link to negativeemotion is so significant that it is suggested it triggers the obtaining of newmemories emphasises its importance as a neural structure subservingpsychological functions. Inresearch, the emotions fear and anxiety have taken centre stage in beingassociated with the amygdala.
Fear has been defined byEthologists as a motivational state which is stimulated by certain stimuli thatescalate to defensive behaviour or an attempt to escape (McFarland, 1987). Anxietycan be defined as having a generalised reaction to an internal conflict or athreat which is unknown (Craig,Brown and Baum, 1995). The principal function of anxiety and fear is toact as a signal of danger, threat, or motivational conflict, and to triggersuitable adaptive reactions and responses (Steimer, 2002). It has been argued and stated that the amygdala is referred to asthe ‘fear centre’.
This is evident in patients that have a genetic conditionthat is extremely rare; Urbach-Wiethe disease. A particularly well-known casestudy is patient S.M., whosuffered from amygdala damage which appeared to have had a strange affect on their fearreactions, as well as areduced experience of fear. The study concluded that the amygdala plays acentral role in creating a state of fear, and the deficiency of such a neuralstructure prevents the capability of fear itself (Feinstein et al., 2011). As mentioned previously,the amygdala is referred to as the “fear centre”. Somescientists are so sure of this description they suggest that further researchinto the amygdala may be the key to helping those with debilitating fear andstress-related psychopathology (Ressler.
, 2010). This suggests that they have full belief thatthe amygdala is in fact the neural structure that controls fear.There has alsobeen a vast amount of research which looks into the link between the amygdalaand anxiety. The amygdala has been implied in anxiety as well as mentaldisorders such as PTSDand phobias where anxiety is a prominent symptom (Etkin & Wager.
, 2007). (14) Focusing onPosttraumatic stress disorder (PTSD), there is research that conveys it appearsto combine features of both extreme responses to stress, plus either enhanced conditioned fear orincompetence to extinguish, or restrain conditioned fear (Ressler., 2010). Shin andLiberzon (2010)stated moderately amplified amygdala activation in response todisorder-relevant stimuli in social phobia, PTSD, and specific phobia. Inaddition to this, numerous neuroimaging studies have exhibited thatparticipants that suffer with PTSD have superior amygdala activation in comparisonto the control group (Liberzon& Sripada., 2007). When looking at anxiety and the role of neural structures,a study that looked at generalised anxiety disorder (GAD) identified and found abnormalamygdala and prefrontal cortex activation in participants and reducedconnectivity amid these areas.
Additionally, it conveyed amplified ‘grey mattervolume and ‘shrunken ‘structural connectivity concerning these structures.’ (Hilbert et al., 2014). This shows that in Generalised AnxietyDisorder, there are a set of neurobiological alterations that are completelydistinctive, and thatthe amygdala as well as other neural structures play a part in anxiety as awhole.
It is impossible to ignore the overwhelming amount of research that conveysthat the amygdala appears to be the home to fear and also anxiety. However,when analysing anxiety andfear, it must also be noted that the amygdala is not the only neural structure ofsignificance. Rauch and Shin (Davis., 2002) (12) state that numerous brain areas contributeand provide vital feedback to the amygdala, including the medial frontal cortex, thehippocampus and the corticostriatothalamiccircuits.
The hippocampus delivers informationregarding the context of the situation, as well as the medial frontal cortexproviding “top-down” control over the amygdala, therefore enabling attenuationof the response of fear once the danger or threat has passed or when a stimuluswhich could be possibly threatening has changed (Davis., 2002). This conveys that although theamygdala ultimately ends up being central to fear, other neural structures areused in the process. Whilst it mayseem at first glance that there is an overwhelming amount of evidence tosupport the statement that the amygdala is in fact ‘the fear centre’, someresearchers have contradicted this idea.
Firstly, LeDoux (2015), argues that whilst people with damage tothe amygdala are indeed less responsive to threats, they can still feel and experience fear. Heinterprets previous findings by stating that the amygdala is a vital part ofthe circuit that permits the brain to identify and react to the threat but isnot compulsory to feel fear. In addition to this, there are various studiesthat have investigated theneural mechanisms of emotion and have concentrated on individual differences. Aparticular study by Adolphs et al (1999) (11), looked at 9 participants who all suffered frombilateral damage. All participants had issues with identifying the emotion offear, yet, there wasfound to be individual differences in the group of participants.
Someparticipants found difficulty in identifying negative emotions whereas on theother hand two of the patients conveyed little issue with identifying emotionsin facial expression at all despite them both having bilateral amygdala lesions(Adolphs et al., 1999).This supports the ideathat perhaps the amygdala is not the only neural structure that is linked tofear. Neural structures may play various and changeable roles in experiencingemotion subject to the person,and further research needs to be done to explore this idea so that thesesorts of findings become relevant. Furthermore, another study looked intoneural correlates of individual differences in fear and anxiety (Ochsner et al., 2006).
The results conclude that the findings displayedshould help explain firstly the influence of individual differences in emotionon the neural correlates of pain, and the functions of anxiety, pain processingand fear, participated by medial and orbitofrontal systems. With all ofthis in mind, LeDoux (1996)stated that many will be quick to conclude that the amygdala is the centre ofemotion reaction in the brain due to the vast amount of empirical evidence.However, there are clearly other structures and networks that are involved inemotional processing. More research should be done on the role of the neuralsystem in emotion so it can be fully understood. (LeDoux., 1994., LeDoux & Rogan., 1996).
There is also the issue that the amygdalais often only discussed when evaluating negative emotion, suggesting that itsrole in effecting emotion is limited. For example, the amygdala’s role in happiness, whilst slightly contentious,is often found to be insignificant. Some suggest that the neuralprocesses of happiness are the complete opposite to those that processdepression and anxiety (Cunningham& Kirkland., 2013) (25). Thehuman amygdala, when inresponse to emotion, ismost often found to show significantreactions to fearful expression as opposed to happy ones (Morris et al., 1996) (23However,one particular study found that the amygdala and the anterior cingulate weremore active when identifying happy faces that were very quickly ‘masked’ thanthey were identifying faces expressing sadness that was masked in the same way.In fact, when identifying sadness, there was only a reduced level of activationin the left anterior cingulate gyrus, compared to significant bilateralactivation in the amygdala as well as activity in the anterior cingulate whenhappiness was displayed. Theseresults propose that the anterior cingulate and the amygdala are vital factorsof a network involved in identifying affectiveinformation beneath the standard threshold of conscious visual perception (Killgore & Yurgelun-Todd.
,2004) (22). Yet, to some, it remains controversial as to whether theamygdala is involved in emotions like happiness. In a study that looked intoidentifying expressions from unilateral amygdala damage and complete bilateralamygdala damage, the findings conveyed that participants with bilateralamygdala damage performed normally at rating happy faces, whilst the participants with right unilateralamygdala damage performed worse overall than participants with left unilateralamygdala damage (Adolphs& Tranel., 2004).
This leaves an area for slight confusion as towhether individuals with right unilateral amygdala damage could not identifyhappiness. Moreover, an interesting study suggests that happiness is linkedwith a balanced and stable amygdala response to negativity and positivity (Cunningham & Tabitha., 2013)(25). The findings showed thatwhilst all participants, both happy and depressed, responded the same tonegative stimuli, those that were deemed ‘happy’ experienced a higher level ofactivity in the amygdala when exposed to positive stimuli. This would suggestthat the amygdala does play a role in subserving happiness, as well as negativeemotion. However, due to the contentious nature of this role, as well as theconfusion surrounding the amygdala’s influence on happiness, one could stillargue that the amygdala is limited in its significance concerning emotion as awhole, as it is more frequently, and convincingly, associated to negative emotion.
To further understand the role of theamygdala, as well as other neural structures, it may be useful to evaluate themethods of research used in some experiments. When looking into neuralstructures and the brain, the most commonly used apparatus is a PET scan orfMRI scan. In terms of advantages, there is only one clear one that PET hasover fMRI. That is that tiny movements can obscure and ruin fMRI data incomparisons to PET scans,which can afford to have the patient move slightly.
However, the PETscan is lower in resolution. As well as this, PET scanning is a lot moreexpensive since an fMRI can be carried out using an MRI scanner. To continue,fMRI can be used to construct three-dimensional images of activity covering theentire brain, whereas a PET scandoes not actually produce an image of the brain. The scan is a colouredmap showing the amount of radioactivity in the volume pixels that compose thescan, therefore lacking in resolution and detail (Barnes & Pinel., 2014).Although PET scans are better in certain situations in comparison to fMRIs, thedisadvantages of PET scans seem to outweigh the advantages. However, researchconducted into fMRI and looking at the amygdala has shown that there wereconcerns raised about how imaging the amygdala with an fMRI is confounded bymultiple adverse factors, signal drop outs and low signal-to-noise-ratio (Boubela et al., 2015)(19).
As a result, this ultimately makes it challenging to attain consistentand dependable activation patterns in this area. This research brings intoquestion how many conclusions and finding from other studies are perhapsincorrect or reliable if they were relying on fMRI evidence alone. Therefore,when coming to analyse some of the research measures when studying the amygdala, the method that was usedcould come into speculation and effect how reliable the results were. To conclude, it is clear that the amygdalaplays a key role in subserving emotion. Whilst there is a level of confusion surrounding its rolein positive emotions, as well as research that highlights the significance ofother neural structures as well as the amygdala, there is anoverwhelming amount of evidence that conveys that the amygdala is involved inand is shown to be the ‘fear-centre’. This is prominently evident in patientswho suffer from damage to the amygdala and cannot feel fear.
As well as fear,aggression and anxiety have been shown to be involved with the amygdala,suggesting it may be more productive to solely think of the amygdala as aneural structure that effects negative emotions, as it is seen to play a littlepart in positive or neutral ones (Tranel et al., 2006) (26). This is supported by the researchinto happiness displaying little association with the amygdala. Despite thevast amount of literature, there are some researchers who are starting to suggest that factors suchas individual differences maybring into question the role influence the amygdala has on emotion.
However, more research into the idea that it does not control fear needs to becarried out if we are toundermine its significance, as there is currently too little an amount of evidenceto do so. It must alsobe noted that the research measures that examine the brain and neuralstructures must be fully evaluated and questioned on their reliability andeffectiveness. Nevertheless, it is clear that the amygdala is heavily associated with thenegative emotions, specifically fear and anxiety, highlighting its role as a neural structure that subserves psychologicalfunctions.