There are several differences between a healthy brain and the brain of an autistic person. Dr. Joseph Piven from the University of Iowa noticed a size difference . In the autistic brain, the cerebellum is larger and the corpus callosum is smaller. Another study showed that the amygdala and the hippocampus are different in an autistic brain. In an autistic these structures have densely packed neurons and the neurons are smaller than those in a healthy brain. Also, in the cerebellum there is a noticeable reduction in the number of Purkinje cells.
Structure and function can not be separated from one another and changes in one indicate alterations in the other. Because an autistic person has brain defects, a reasonable assumption is made that changes in structure will alter the behavior. An autistic person is characterized by having impaired social interaction, difficulty with communication both verbal and nonverbal, trouble with imagination, and limited activities and interests. By analyzing the abnormal behaviors of the autistic person, the roles that the cerebellum, the corpus callosum, the amygdala, and the hippocampus play in the disease can be inferred.
The cerebellum is usually associated with motor movements. Concerning this topic it is interesting to note the research of Dr. Eric Courchesne. He found that the VI and VII lobes of the cerebellum were smaller in autistics than those of a normal brain. This condition is called hypoplasia. The reverse condition, which is what Piven encountered, is called hyperplasia. Courchesne linked the cerebellum with attention shifting . He proposed that the autistic takes longer time to change the focus of his attention. He believed that this condition was caused by lack of development of the cerebellum in utero caused by perhaps oxygen deprivation, infection, toxic exposure, or genetically. Normal individuals take a second to shift attention but an autistic person may take up to five seconds.
The other difference found in the cerebellum had to do with a reduction in Purkinje cells. These cells are important because they contain seratonin. Seratonin is a neurotransmitter which is responsible for inhibition. It is proposed that the lack of seratonin can be associated with faulty arousal and abnormal mood regulation . However, there is controversy over this issue and its relevance to autistics. According to Celia M. Bibby, autistic children have abnormally high levels of seratonin not low. In fact, when an autistic eats foods with high levels of seratonin an attack is often triggered. Bibby proposes that this is because seratonin plays a role in conditioned reflexes.
The corpus callosum has smaller middle and back lobes in an autistic individual. The function of the corpus callosum is predominately that of intercommunication within the brain. It allows the front of the brain to communicate to the back. It is intuitive that the difference in size also indicates a difference in connectivity. Piven said, "The expected size relationships of various parts of the brain to one another seems to be disproportionate or distorted in autism...This makes you think that those areas might be disconnected functionally." Any difference in connectivity among the neurons is going to result in a defect in communication within the brain and the processing of both outputs and inputs. This indicates general changes in behavior such as responding to inputs in a usual manner. All aspects of autism are most likely caused directly or indirectly by the decreased connectivity within the corpus callosum because the brain's internal communication is diminished.
Two structures of the limbic system are markedly different in the autistic brain. The first is the amygdala which is generally associated with the regulation of emotions and aggression. When the amygdala is removed from an animal, the behavior of the animal is similar to that of an autistic person. Also, the amygdala is linked with response to sensory stimuli. Bibby expands this statement in her essay with the example of face cells. Face cells are found in the amygdala and also in the superior temporal sulcus. Autistic individuals avoid eye contact. This is linked to face recognition because when forced to maintain eye contact, the autistic begins to act aggressively. Face cells enable humans to identify dangerous situations and then the appropriate signals are sent to various brain structures to cause the appropriate response. In the case of autism, the person begins a fight response. This becomes the conditioned response but the autistic recognizes the negative effects such as a feeling of vulnerability and tries to prevent similar reactions by avoiding eye contact which diminished the development of social skills and language.
The second structure of the limbic system that is abnormal in the autistic brain is the hippocampus. The hippocampus is linked to learning and memory. When the amygdala is removed from an animal the behaviors that are exhibited include failure to learn about dangerous situations, and difficulty retrieving information from memory. These behaviors are associated with the hippocampus so it can be inferred that the two structures are connected. Autistics have difficulty learning and storing new information into memory. When the hippocampus is removed from an animal, it will express a series of behaviors classified as self-stimulatory. These behaviors are repetitive body movements or movements of objects . For example, tapping ears, sniffing people, hand flapping, scratching, or rocking back and forth. Two hypotheses of this behavior have been drawn. Either the actions are to stimulate (hyposensitive) or to calm (hypersensitive). In the case of the autistic person, the second hypothesis makes sense. To the autistic the environment is too stimulating and by doing a repetitive motion the environment can be blocked out. The environment is too stimulating because the brain can not process the sensory inputs as fast as they are being received. New information can not be entered into the memory quick enough.
Brain disorders, such as autism, offer scientists a chance to investigate the brain and its functions. When looking at a healthy brain it is difficult to find which structure is responsible for what behavior but by comparing the normal to the abnormal and looking at the difference in behavior and brain structure many conclusions can be drawn. The previously mentioned structures, the cerebellum, the corpus callosum, the amygdala, and the hippocampus clearly play a role in the abnormal behaviors autistics but there are most likely many other parts of the brain that are effected by the disease also. Autism is clearly not a disease that is caused by a defect in only one section of the brain. Many scientists have accepted the idea that autism is caused by a malfunction in the development of the brain which encompasses many regions. Research is still being done to figure out the cause of autism whether it is genetic, or for example caused by biochemical toxicity trauma. Studying diseases is both necessary for trying to find a cure and also useful for gaining insight into neuroanatomy.
Research Finds Size Differences in the Brains of Autistic Individuals.
Edelson, Stephen M. Ph.D. Autism and the Limbic System, The Cerebellum and Autism, Stereotypic (Self-Stimulatory) Behavior.
National Institute of Neurological Disorders and Stroke.
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