Throughout the line of questioning we have been following in our efforts to get "progressively less wrong" in our class wide model of the brain, a constant debate has sparked on the issue of whether brain equals behavior. If the agree that brain truly equals behavior, then we can surmise that the vastly differing human behavior must also translate to differing nuances in the brain. It is a widely conceded point that experience also effects behavior, and therefore experience must also effect the brain. On this point, I have been intrigued: are these differences in the brain mysterious; things as well theorized on by a philosopher as researched by a biologist? Or can an experience actually change the physical structure of the brain? In my web research, I found a partial answer in the concept of plasticity.
According to source (1), "Plasticity refers to how circuits in the brain change--organize and reorganize--in response to experience, or sensory stimulation." There appear be four types of stimuli to which a brain responds with change: developmental, such as in the newly formed and ever evolving brain of a child; activity dependent, such as in cases of lost senses; learning and memory, in which the brain changes in response to a particular experience; and finally injury induced, resulting from damage in the brain, as occurs in a stroke or in the well-know case of Phineas Gage. Although the particular change in the brain is dependent on the type of stimulus, brain plasticity can be widely described as an adjustment in the strength of synaptic connections between brain cells. (1)
The developmental function of brain plasticity is important not only in the world of early childhood, but also has implications for the function of an aging brain. As we age, the synaptic plasticity deceases due to the increased expression of neurotoxins in astrocytes which are responsible for cell-cell communication (2). Similarly, in youth, increased synaptic plasticity accounts for the inordinate amount of growth and learning that must occur in this stage of development.
Much research has been done on injury-induced plasticity, and continues to be done with the hopes of minimizing the effects of an injury on the brain. One case where is in brain injury due to stroke , wherein particular functions of the brain such as motor control, memory, or language may be affected. According to source (3) "a reorganization of brain functions may occur through 'uninjured' brain areas, allowing then-altered functions to be performed differently". If this function of brain plasticity can be exacerbated and emphasized, it is perhaps possible with further research and experimentation to minimize the effects of brain injury such that many or all symptoms are eliminated.
In the area of activity dependent plasticity, a study has been done comparing patients who had gone through a period of deafness and recently received cochlear implants to a control group made up of individuals with normal hearing (4). A positron emission tomography, or PET scan was done on both groups to compare which parts of the cerebral network were engaged in both groups. The results were such that the two groups, when confronted with the same stimuli, would in some cases use entirely different parts of their brains to receive and interpret the same sounds. According to an interpretatio of the study "These data provide evidence for altered functional specificity of the superior temporal cortex [and] flexible recruitment of brain regions located within and outside the classical language areas." (4) This study indicates to me the adaptivity of the brain to outside stimuli in such a light that it become irrefutable.
In reflecting on this newfound knowledge, it becomes clear that experience does indeed account for differences within the brain, modulating synapses such that a certain response is favored as a result of a certain stimuli. Coming from a family of psychologists, I imagine applying this phenomenon to a situation often encountered by my parents: a person with a phobia. Typically, psychologists account phobias to a long ago encounter, sometimes remembered and sometimes long forgotten, which caused the person to always have the protective response of fear when faced with a certain stimulus. Although an explanation such as this is referred to by the general public as psychobabble, the explanation of brain plasticity may lend credence to this psychological theory. Say a child has a fear inspiring encounter with a spider. This single experience has the power to modify the strength of the synapses within his brain such that every time the arachnophobiac is faced with spiders, his fear response is much more potent than the average persons, In this way, I can see the plasticity of learning and memory relating to long held theories about differing human behavior.
The concept of synaptic plasticity is not merely limited to humans, however. In one study done by Sharen McKay, et al., of Yale University, the well known vertebrate enactors associated with synaptic plasticity (neurotrophins) were artificially placed in an invertebrate setting and yet again influenced neuronal growth and plasticity(5). Although invertebrates do not produced neurotrophins, substances with similar effects have been isolated in several invertebrate species. These results suggest that synaptic plasticity is a function that is widely found in many species of life, not merely in a developmental role, but also in the adaptation and modification of adult brains. This article again inspired my theorizing: does the suggestion that "adult plasticity [is] highly conserved across diverse phyla" (5) indicate that the ability of the brain to adapt to learning, memory, and specific experience is an evolutionary advantage?
. In researching and learning about the types of brain plasticity, I found more evidence for the idea that brain equals behavior, and that the experiences which are input into the brain do directly cause changes in synaptic activity and strength. Although I have found the ideas inherent in brain elasticity intriguing, I'm also afraid that they have raised more questions for me than they have answered. I find myself wondering about the evolutionary advantages of plasticity, the amount of time it takes to effect a physical change in the brain, and why, even in the face of plasticity, do certain functions of the brain never seem to adapt to the demands of the modern day world?
References1) Brain Plasticity, on the John F. Kennedy Center for Research on Human Development web site.
2) Glia-neuron intercommunications and synaptic plasticity, on the PubMed web site.
3) A Comprehensive Functional Approach to Brain Injury Rehabilitation, on the Brain Injury Source web site.
4)Functional plasticity of language-related brain areas after cochlear implantation, on the PubMed web site
5) Regulation of Synaptic Function by Neurotrophic Factors in Vertebrates and Invertebrates: Implications for Development and Learning, on the Learning & Memory web site.