After these theories were published, they started to receive much attention. This recognition led to more research and the results were not significant enough to hold the limbic system theory as valid. MacLean defined his theory as, “…the region involved in ordering the affective behavior of the animal in such basic drives as obtaining and assimilating food, fleeing from or orally disposing of an enemy, reproducing, and so forth,” (LeDoux, 1998, 93). ). He explained the visceral brain as not having the “intellect” of the “intelligent” brain. It does not have the capabilities of “intelligent” parts of the brain. For example, “intellectual” functions of the brain can recognize the word, “blue”. It is recognized by the shapes of characters (letters) that form what is understood as a word. “Blue” also implies a definition and a specific wavelength of light that is seen and identified as that color, which is a learned understanding. On the contrary, the visceral brain was described as something that perceives stimuli symbolically. It only forms representative meanings. That means, the word “blue” is instead interpreted in a way that may make a person sad or think of water, etc. (LeDoux, 1998). However, research discovered that the hippocampus, an essential function within the limbic system, the mammillary bodies, or anterior thalamus actually has quite a large effect upon long-term memory as opposed to a large effect on human emotion and motivation (LeDoux, 1998, 2000). These results then disprove MacLean’s theory that the limbic system is only involved in simple, representative functions as opposed to “intelligent” processes. In addition, a large part of the reasoning behind the theory was based upon evolution. A “triune brain” was termed and was believed to be the combination of a reptilian, paleomammalian, and neomammalian brain (LeDoux, 1998). These three brains were ranked in a hierarchy according to the theories of evolution. In accordance, only humans, other primates, and advanced mammals had all three brains. It was believed that the “lower” animals did not have a lateral cortex; they only had a medial cortex. The lateral cortex (neocortex) was thought to be in the neomammalian brain, which was essentially the limbic system (LeDoux, 1998). Future research disproved this theory as well. The lateral cortex, thought to be only in “higher” mammals was found in “lower” animals. The cortex was located in a different place than its “higher” counterparts (LeDoux, 1998). This research displayed findings that were incongruent with the theory of past scientists such as MacLean, which presents the question of whether one can actually be sure the limbic system actually exists. If the limbic system was conceptualized as a system created through evolution to accommodate the advances of mankind, then the finding of the limbic system in “lower” species must disprove the existence of the limbic system altogether (LeDoux, 1998).
This paper will continue to use the term “limbic system” for simplicity and constancy reasons. Moreover, not every function in the limbic system is involved with emotion or motivation (LeDoux, 1998). The brainstem, hippocampal system, and amygdala are key factors in emotion (Martin, 1998). The limbic system is also known for being responsible for motivated actions such as fleeing, feeding, fighting, and sexual behavior. Other major parts of the limbic system include the mammillary body, fornix, cingulate cortex, septum, hypothalamus, and olfactory bulb (Pinel, 2003). The amygdala and hypothalamus are two structures that often receive much attention when it comes to emotion and motivation. The amygdala is a vital element in the production of fear. It also assists in maintaining anxiety and many other emotions (Martin, 1998). This structure has been proven to be an essential link in the emotion processes, but not the limbic system (Pinel, 2003). Unilateral or bilateral damage to the amygdala has proven a severe handicap in patients trying to recognize the expression of fear and possibly other emotions (Martin, 1998). Likewise, the hypothalamus is a key concept in the Cannon-Bard theory, which links it as an essential component of the rage response. They discovered a raged response could only be produced with extremely intense stimuli and even then, the results were not typical (LeDoux, 1998). It is also an important factor in controlling effector activities in the endocrine and autonomic nervous systems (Damasio & Hoesen, 1983). LeDoux (1998) concludes that the importance of such structures as the hypothalamus, etc. is a reason for the limbic system theory withstanding the contrasting research that has been brought forth.
Structures involved in emotion and motivation that are included in the limbic system are not functioning independently of other structures and systems within the human brain. This is another reason that the limbic system is not solely responsible for human emotion and motivation. Interpreting stimuli is an indispensable process in emotional experience. One must first experience a stimulus, whether consciously or not, to be able to react appropriately. This reaction is the emotion experienced and the physiological responses that accompany or define it. A structure of the brain involved in an emotion is not always involved in every aspect of that emotion (Pinel, 2003). LeDoux (1998, 77) explains that emotional, “…functions are mediated by interconnected systems of brain regions working together rather than by individual areas working in isolation,”. This is also relative of motivation because several neural systems work collectively to create motivations for people to eat, seek shelter, protect their bodies, and reproduce (LeDoux, 1998). It is suggested that the frontal cortex has an impact upon the perception and expression of certain emotions. The orbitofrontal cortex may also play a role in the emotional response produced after being presented with environmental stimuli (Martin, 1998). Furthermore, the hypothalamus, an important structure in emotion is also associated with cortical and subcortical parts of motor systems. The hypothalamus also has connection(s) to the association areas of the cerebral cortex. Most likely, a result of the hypothalamus’ connections and other major structures within the limbic system having an important role in emotion, it is believed that the limbic system has a connection with practically every structure within the human brain (Damasio & Hoesen, 1983). An example of this is fear conditioning and the auditory system. Lesions made to the auditory thalamus disrupted fear conditioning. This combined with interference in the amygdala make the complications worse (LeDoux, 1998). These two structures have formed a connection that allows fear responses to evade the auditory cortex. The auditory cortex would be thought of as the essential structure because audible sensory information is sent here. However, studies show that it has no effect upon fear conditioning (LeDoux, 1998). The complex connections between the limbic system and other systems and structures of the human brain illustrate the problem with depicting the limbic system as the sole drive behind all human motivation and emotion.
The final argument presented against the limbic system theory is the simple fact that there are endless oppositional studies presented against it. Along with this, is the fact that scientists have not been able to reach a decisive answer on what defines an emotion. There are various lists of different emotions and what constitutes an emotion (LeDoux, 1998). Some research explains the limbic system as something non-complex; a simple system not designed for “intelligent” procedures (Damasio, 1994). It is also said that, “…we are wired to respond with an emotion, in preorganized fashion, when certain features of stimuli in the world or in our bodies are perceived, alone or in combination,” (Damasio, 1994). Another explanation is that a stimulus is encountered and a reaction is sent to the thalamus, which sends a response to the arousal system where it can be dealt with and one response is sent to the cortex also so the event can be recorded. This chain of events produces the emotional experience and a bodily reaction appropriate to the emotion (Parkinson, 1995). This statement leads to another discrepancy. Damasio (1983) explains an emotion as an entire process of first being presented with stimuli, then having cognitive processes, which then creates an emotion “felt” through physiological changes in the body.
In conclusion, the limbic system is not solely responsible for the drive of human emotion and motivation. The limbic system’s existence is created through a faulty evolutionary explanation. Furthermore, the structures in the limbic system are varied and only a portion are involved in emotion and motivation processes. These structures, specifically the amygdala and hypothalamus as well as others, are interconnected with endless structures and systems within the human brain. These connections construct the processes that create emotional experiences, emotional expressions, and motivations. A disruption from any link in a connected chain of events will have an effect upon the expected outcome. Therefore, even an essential structure such as the hypothalamus must rely upon other systems to perform its task otherwise it would be expected that the hypothalamus is capable of performing every function within the human brain system. Current research is making remarkable advancements in this area. Future research should continue to narrow the focus of where human emotion and motivation roots are planted within the human brain as opposed to already assuming it is created from the limbic system. To assume the foundation of such a controversial issue is limiting the possibility of substantial findings. The theory, as it stands, does not allow predictions of emotion and motivation to be made (LeDoux, 1998). A new, revised theory of human emotion and motivation is needed so predictions can be made. After all, if the field of psychology ignores research that helps to predict human behavior, then is it really the science that it was created to be?
References
Damasio, A. R. (1994). Descartes’ error: emotion, reason, and the human brain. New York: G.P. Putnam’s Sons.
Damasio, A.R. & Hoesen, W.V (1983). Emotional disturbances associated with focal lesions of the limbic frontal lobes. In K.M. Heilman & P. Satz (Eds). Neuropsycholgoy of Human Emotion (Vol. 1, pp85-108). New York: The Guilford Press.
LeDoux, J. (1999). The emotional brain. Phoenix: Orion Books Ltd.
Pinel, J.P.J. (2003). Biopsychology. (5th Ed). Boston: Allyn & Bacon
Martin, G. (1998). Human neuropsychology. London: Prentice Hall Europe.
Parkinson, B. & Colman, A.M. (1995). Emotion and motivation. London: Longman
