Emotional lateralization
Emotional lateralization is the asymmetrical representation of emotional control and processing in the brain. There is evidence for the lateralization of other brain functions as well.
Emotions are complex and involve a variety of physical and cognitive responses, many of which are not well understood. The general purpose of emotions is to produce a specific response to a stimulus. Feelings are the conscious perception of emotions, and when an emotion occurs frequently or continuously this is called a mood.[n 1]
A variety of scientific studies have found lateralization of emotions.
Theories of lateralization
Right hemisphere dominance
Some variations of right hemisphere dominance are...
a) The right hemisphere has more control over emotion than left hemisphere.[n 2]
b) The right hemisphere is dominant in emotional expression in a similar way that the left hemisphere is dominant in language.
c) The right hemisphere is dominant in the perception of facial expression, body posture, and prosody.
d) The right hemisphere is important for processing primary emotions such as fear while the left hemisphere is important for preprocessing social emotions. [n 3]
General lesions in the right hemisphere reduce or eliminate electrodermal response (
Subject SB-2046 had part of his right,
Most language production and processing occur in the left hemisphere while the majority of the emotional processing and production of emotion in speech occurs in the right hemisphere. Persons with schizophrenia usually have difficulty processing prosody. These patients also show a remarkable increase in lateralization towards the right hemisphere of both emotionally and non-emotional prosody rich speech.[2] Also, a decrease in right-handedness led to an increase in the right hemisphere lateralization. This right hemisphere lateralization extends beyond prosody to many of aspects of language and speech processing in schizophrenic patients.
Complementarity specialization
The two hemispheres have a complementary specialization for control of different aspects of emotion.[n 2]
a) Left hemisphere primarily process "positive" emotions and right hemisphere primarily process "negative" emotions. A large portion of regions primarily in the right hemisphere are activated during aversive classical conditioning.[n 5]
- While this theory seems to hold true for some emotions, this theory is generally considered outdated; however a few examples exist. For example, a study found that when subjects were primed with positive stimuli before hearing a consonant, the left hemisphere was more active than the right hemisphere.[3] In contrast, when subjects were primed with a negative stimulus, the right hemisphere was more active than the left hemisphere.
b) Other divisions of specialization
- The amygdala plays a role in the conscious awareness of emotion (feelings) resulting in perception of feeling, but experiments suggest the left and right amygdala have distinct roles in conscious and unconscious processing of emotion. The right amygdala plays a role in the nonconscious processing of emotion while the left amygdala was involved in the processing of conscious emotions.[n 6] These results were obtained from studies that masked conditioning stimuli. Stimuli were presented over a very short period of time such that subjects were not consciously aware of the stimuli but were still able to show physiological changes.
- Damage to the left hemisphere in patients results in a marked increase in depression.noradrenergic. Patients with right hemisphere damage had reduced arousal response to painful stimuli.
- Damage to the left hemisphere in patients results in a marked increase in depression.
Homeostatic basis
This model provides a neuroanatomical basis for emotional control and processing. The peripheral
Particularly, cardiovascular function appears to be lateralized and tied to emotional stress. Intense emotional stimuli that cause stress can lead to alterations in cardiovascular function.[6] The right insular cortex probably plays the most significant role in these phenomena. Similar lateralization is probably involved in cardiovascular malfunction in patients with head injury, stroke, multiple sclerosis, brain tumors, meningitis and encephalitis, migraine, cluster headache and neurosurgical procedures.
Lateralization due to lateralization of other functions
"It is unlikely that the brain evolved an asymmetrical control of emotional behavior. Rather, it seems more likely that although there may be some asymmetry in the neural control of emotion, the observed asymmetries are largely a product of the asymmetrical control of other functions such as the control of movement, language, or the processing of complex sensory information,"
Lateralization and sex differences
There may be a difference in cortical activation between men and women. Activity in the right hemisphere was greater in women when exposed to unpleasant images than men, though men showed more activation bilaterally while viewing pleasant pictures.
Evidence of lateralization
The vast majority of the data comes from
Behavioral differences and cortical activation
70% of right handed patients show preference in viewing emotion expressed on the right side of the face (in the left field of view) according to studies using chimeric faces produced using right-right or left-left mirrored faces.[10] The left side of the face seems more fluent in expressing emotions which means the right cortical hemisphere is more fluent in expressing emotions.[n 9] Handedness does not appear to affect the processing associated with viewing facial expressions.[8]
Situations that contradict moral teachings generally produce
Depression or having previously been depressed probably is due to altered brain structure or alters brain structure. Patients who have been depressed or are depressed show more activation to negative stimuli in emotion.[12] When negative stimuli were presented to patients' right hemispheres, the patients were significantly more accurate and quicker to respond to the stimuli. The data in this study shows that psychological disorders are correlated with increased lateralization.
Facial expressions of emotion
Patients with damage to their left amygdala lesions rated fearful faces less fearful than normal subjects.[n 10] Similar findings showed that regional blood flow increased in response to fear faces while decreased to euphoric faces in the left amygdala.
Chimpanzees, other primates, and humans produce asymmetrical facial expressions with greater expression on the left side of the face (right hemisphere of the brain).[13] Researchers also subjectively reported that the left side of the face was expressing more emotion using images of left-left chimeric faces.
Notable lateralized brain structures and regions
Emotion is processed in many different areas of the brain, and a specific emotion may be processed in multiple areas. Regions involved in emotional lateralization appear to follow the general conventions that describe the roles/functions of certain brain regions. Below are a few regions and structure involved in emotional processing that show functional lateralization.
Frontal lobe
Using a
The superior frontal gyrus is the most significantly activated region while processing sadness.[14]
Patients with inferior frontal lobe damage produce less and less intense facial expression when presented with emotional stimuli, and they also have problems reading fear and disgust in other people. People with left inferior frontal lobe damage produced less facial expression and could not analyze emotional situations as well as those with right frontal lobe damage especially with fear and disgust.[n 9] The left inferior frontal gyrus (IFG) plays an important role in anger while the right IFG plays a larger role in disgust.[14]
Patients with dorsolateral frontal cortex lesions have difficulty discerning propositional attitude. Patients with left lesions show further increased impairment.[n 9]
Parietal lobe
Damage to the inferior parietal region including the anterior (
Temporal lobe
The right superior temporal gyrus was the most significantly activated area during the processing happiness.[14] The right superior temporal gyrus increasingly responds to an increasingly happy stimuli, while the left pulvinar increasingly responds to increasingly fearful stimuli.[n 10] The right pulvinar is activated during aversive conditioning.[n 5]
Amygdala
The
Unilateral activation of the amygdala due to fearful stimuli may also produce unilateral activation of other regions.[n 10] The right middle temporal gyrus, right brainstem, left hippocampus, right cerebellum, right fuisform gyrus, and left lingual gyrus were also activated during fearful stimuli. Activation of multiple brain regions both indicates that emotions are processed in many parts of the brain and that emotions are complex.
The amygdala probably plays a role in the conscious processing of emotion. The left amygdala was activated during the processing of conscious stimuli while the right amygdala was active during the processing of nonconscous stimuli.[n 14]
Anterior cingulate cortex (ACC)
The anterior cingulate cortex (ACC) plays a role in a variety of functions including emotional ones. The ACC may be important in conscious awareness of emotion.[n 14] Damage to the ACC is associated with decreased SCR to physical and psychological stimuli. Bilateral and unilateral damage both resulted in decreased SCR indicates that the right and left ACC's may specialized in certain aspects of emotional response.[n 12]
Anterior insula (AI)
The left anterior insula (AI) increasingly responds to increasingly fearful stimuli.[n 10] The AI may also be involved in the conscious experience of emotion.[n 14]
Implications
Phenomena such as emotional lateralization may help describe how emotions arise, persist, and alter our behavior. Understanding emotional lateralization will help scientists understand emotion in general. Emotional lateralization may also play a role in psychological disorders such as depression and schizophrenia. Future treatment of psychological disorders may have more targeted neurological treatments rather than ingested drugs.
Symptoms that arise from confined regions of damage usually have stereotypical emotional and behavioral changes. Diagnosis for locating damaged regions that process emotion may aided by noticeable emotional changes categorized under one of the lateralized emotional control systems. Diagnosis and treatment for cardiovascular irregularities that arises from emotions states could be aided by understanding the physical basis of the psychological influences. Instead of treating the cardiovascular irregularities for psychological issues, treatment could target the lateralized brain regions.
Limitation of Studies
Like all human based scientific experiments there are limitations to what researchers can do. Attempting to study emotions is especially hard since emotions are complex and can lead to subjective response. Since the majority of experiments in emotional lateralization have been on fear this leaves the question of whether other emotions are also lateralized. Below are two major issues associated with many of the experiments studying emotion that require further explanation.
Sample size
A large percent of the human studies are of anomalies due to accidents, tumors, or attempts to cure disease (e.g. seizures) using lesioning. Since very few such cases exist the sample size of human studies of emotional lateralization are generally very limited and may be as small as single person. While these studies may provide a good insight into certain brain regions and their functions their conclusions are not definite. Animal studies may help in understanding this problem but emotions in humans are generally considered more complicated than in most animals.
Functional imaging
There are several limitations when using
Lesioning
Lesions rarely are localized and can affect large areas of the brain. Processing in the brain is generally not localized and requires many areas of the brain to process. Furthermore, lesioning may interfere with pathways that span the lesion site. Thus, lesions are not always a good way to determine what specific brain areas do. Therefore, a degree of skepticism should be kept in mind when viewing lateralization data from lesion studies.
Notes
References
- ^ Lane, R.D., and L. Nadel, ed. Cognitive Neuroscience of Emotion. New York: Oxford University Press, 2000
- ^ Bach, D.R., et al. Altered lateralisation of emotional prosody processing in schizophrenia. Schizophrenia Research. 110: 180–187, 2009.
- ^ Alfano, K.M., and C.R. Cimino. Alteration of expected hemispheric asymmetries: Valence and arousal effects in neuropsychological models of emotion. Brain and Cognition. 66: 213–220, 2008.
- ^ Craig, A.D. Forebrain emotional asymmetry: a neuroanatomical basis? Trends in Cognitive Sciences, 9 (12): 566-571, 2005
- ^ Conesa, J. Electrodermal palmar asymmetry and nostril dominance. Perceptual and Motor Skills, 80: 211-216, 1995
- ^ O, Ozcan, and V. Hachinski. Brain lateralization and sudden death: Its role in the neurogenic heart syndrome. Journal of the Neurological Sciences. 268: 6-11, 2008.
- ^ Vallortigara, G, and LJ Rogers.Survival with an asymmetrical brain: Advantages and disadvantages of cerebral lateralization Behavioral and Brain Science 28: 575–633, 2005.
- ^ a b Rodway, P., L. Wright, and S. Hardie. The valence-specific laterality effect in free viewing conditions: The influence of sex, handedness, and response bias. Brain and Cognition. 53: 452–463, 2003.
- ^ Gasbarri, A., et al. Sex-related hemispheric lateralization of electrical potentials evoked by arousing negative stimuli. Brain Research. 1138: 178-186, 2007.
- ^ "Chimeric debrief". Archived from the original on 2012-03-04. Retrieved 2010-12-04.
- ^ Carla, L.H., and S. Hamann. Neural correlates of regulating negative emotions related to moral violations. NeuroImage. 30: 313 – 324, 2006.
- ^ Atchley, R.A., S.S> Ilardi, and A. Enloe. Hemispheric asymmetry in the processing of emotional content in word meanings: The effect of current and past depression. Brain and Language. 84: 105–119, 2003
- ^ Fernandez-Carriba, S., A. Loeches, A. Morcillo, and W.D. Hopkins. Functional asymmetry of emotions in primates: new findings in chimpanzees. Brain Research Bulletin. 57: 561–564, 2002.
- ^ a b c d Vytal, K., and S. Hamann. Neuroimaging Support for Discrete Neural Correlates of Basic Emotions: A Voxel-based Meta-analysis. Journal of Cognitive Neuroscience 22(12): 2864–2885, 2010.