Learning from a lifetime of research: Implications to Neurobiology
In his book, In Search of Memory, Eric Kandel explains how neuroscience came into being and how he became fascinated with the study of the brain and higher cognitive processes, most notably with memory. Kandel uses his personal experience and the research compiled over centuries of study to explain his interest in a neurobiological basis of memory. Although we have not discussed memory in detail in this course, there are many overlaps between the topics discussed in class and the topics presented in the book. Kandel provides a detailed account of the history of neurobiology but for the purposes of this paper I will present a few ideas that directly relate to our discussions, both in class and in the course forum. I will expand on the concepts of unconscious processing of sensory input, neural organization, and specialization of brain regions as they relate to class topics and the broader field of neurobiology.
As we have discussed in class, the speed of an action potential (roughly 90 feet per second) is rather slow, relative to the speed of light. The only reason we were able to incorporate this information into our class discussions is because of the work of Hermann Helmholtz. Kandel explains that Helmholtz was the first to quantify the amount of time required for a signal resulting from a tactile stimulus (touching the skin) to reach the brain. From his research, Helmholtz concluded “that a considerable amount of the brain’s processing of perceptual information is carried out unconsciously” (Kandel, 209). This observation has been discussed in detail throughout the course and gives rise to the concept of an “I-function”. The “I-function” is the brains representation of reality, but this representation is rarely an appropriate representation of reality. In light of the vast amounts of sensory information that inundate our brains, it makes sense that a majority of this information becomes “lost”, or edited out of the big picture in our heads. It also makes sense that most of the processes that filter out this seemingly extraneous information occur unconsciously. If our brains had to attend to the vast amounts of sensory input, especially the input that is eventually discarded, we would not be able to devote as much energy to higher-order cognitive processes, such as storing information or being able to recall those memories later. Thus, the “I-function” seems to have an evolutionary advantage in that our brains need only to consciously attend to the information that affects our most immediate situation. However, the question remains as to how this information is processed unconsciously.
Kandel explains that Helmholtz took his observation a step further by inferring that this unconscious processing of information must be due to “signals being routed and processed at different sites during perception and voluntary movement” (Kandel, 209). With this theory, it could be assumed that the information may not be wholly discarded, but rather attended to by brain regions with different functions. From this perspective, it could be implied that content-specific brain regions may actually process the information before conscious attention, or conscious processing, is required. The cocktail party phenomenon can be used as an example. One might unconsciously attend to background noise until they overhear their name in conversation, at which point they would consciously attend to the conversation. Perhaps this could explain the experience of Déjà vu. The unconscious processing of sensory input still informs our mental processes, after all. If someone were unconsciously processing sensory stimuli about a new place or a person they were just meeting, they could consciously feel as though they had been in that place previously or had met that person before.
Unconscious processing may take place at various regions of the brain, casting it outside of the scope of scientific observation and experimentation, but conscious perception of certain sensory input has been quantified. The processing that occurs in the visual system is an example from class discussions. Another example comes from the historical account of neuroscientific research on the somatosensory cortex in the parietal lobe provided by Kandel in his book. Kandel expands on the first experiments that yielded information about the somatosensory cortex and the implications of this research to everyday life. Based on observations made by Wade Marshall while he was conducting research with monkey brains in the late 1930’s, it was concluded that “the entire body surface is represented in the somatosensory cortex in the form of a point-for-point neural map” (Kandel, 111). Similar research conducted by Wilder Penfield in human brains led scientists to conclude that “the parts of the body surface most sensitive to touch are represented by the largest areas of the somatosensory cortex” (Kandel, 111). These findings relate to an interesting phenomenon discussed in class called phantom limb syndrome. Phantom limb is the experience of sensation, usually pain, in a limb that has been amputated. The neurological basis for phantom limb is directly connected to the somatosensory cortex. The area of the somatosensory cortex which used to process sensory input from the amputated limb is no longer in use and is therefore “recycled” by the brain. The neighboring areas of the somatosensory cortex begin to fill in the gap, so to speak. In other words, the processing of sensory input from another body part takes over the abandoned brain region. Sometimes the new sensory input is processed as information coming from the absent limb resulting in perception of sensation in the phantom limb.
Kandel further explains that our modern understanding of how sensory information is organized was greatly influenced by the Marshall’s work. He studied various sensory systems, beginning with the somatosensory system and including the visual and auditory systems. Marshall determined “that the light receptors in the retina of the eye are also represented in an orderly way in the primary visual cortex” and that “the temporal lobe has a sensory map for sound frecuencies, with different pitches represented systematically in the brain” (Kandel, 111). These observations imply that the “boxes”, or neurons, in the sensory systems are organized into a specific order to facilitate processing of the sensory information. As we have learned in class, most sensory systems are distributed systems, meaning that the processing of the sensory input actually occurs across several brain regions that are connected by neural pathways. Does this mean that neurons in other regions of the brain that also participate (to a lesser degree than the primary sensory cortex in question) in the processing of sensory information are also organized in such a way as to facilitate transmission of input and output?
The previous research on these sensory systems has been quite a boon to the field of clinical neurology. As Kandel explains, “[d]isturbances in the sensory and motor systems can be located with remarkable accuracy because of the one-to-one relationship between sites on the body and areas of the brain” (Kandel, 113). This research has helped physicians pinpoint and effectively treat pathology of the sensory systems and motor cortex, as in the case of the “Jacksonian sensory march”. The Jacksonian sensory march refers to a type of epileptic seizure characterized by a spread of numbness throughout the body. This numbness is the result of a surge of abnormal electrical activity which “starts in the lateral area of the somatosensory cortex, where the hand is represented, and propagates across the cortex toward the midline, where the leg is represented” (Kandel, 114). Without the background knowledge of the organization of the somatosensory cortex, physicians would never have been able to explore the neurological basis of this unique form of epileptic seizure.
As can be observed from sensory systems, neural organization gives rise to specialization of certain brain regions. This finding is not specific to sensory systems, but can be applied to various complex cognitive processes, like speech production and comprehension. In the middle of the nineteenth century, two neurologists sought to determine the neurological components to certain disturbances in both speech production and comprehension. Paul Broca found that lesions of a certain region of the frontal lobe resulted in an inability to produce spoken and written speech. This area became known as Broca’s area and deficits resulting from damage to this area are referred to as Broca’s aphasia. Broca’s observation was revolutionary because it “provided the first empirical evidence that a well-defined mental capacity could be assigned to a specific region of the cortex” (Kandel, 122). Interestingly, Carl Wernicke found that lesions in a specific brain region, located more posteriorly in the left hemisphere than Broca’s area, left people incapable of understanding written or spoken language. The inability to comprehend written or spoken language resulting from damage to this area (now referred to as Wernicke’s area) is known as Wernicke’s aphasia. People with Wernicke’s aphasia have no problem producing speech, however their speech is incoherent. As a result of both of these observations, Wernicke concluded that “complex behavior is the product not of a single region but of several specialized, interconnected areas of the brain” (Kandel, 123). Even distributed sensory systems are organized in such a way as to produce specialized areas of the brain.
I do not believe that this evidence supports a localizationist perspective in which each area of the brain is involved in only one mental process. Rather I believe that this evidence should be indicative that the brain is indeed plastic and that specific brain regions are involved in several processes, often simultaneously. As stated so eloquently in Kandel’s summary of Pierre Flourens’ research on the specificity of function of certain regions of the cerebral cortex:
[t]he cortex is equipotential… meaning that every region is able to perform any of the brain’s functions” (Kandel, 120). The fact that certain brain regions have been found to directly correlate with specific functioning and behavior could be a result of the evolutionary process of learning, which results in strengthening of neural pathways. These pathways exist because they have been strengthened, but the desired behaviors could still occur after damage to a specialized brain region because the brain is able to reorganize itself. Once new “replacement” pathways are established, they can be strengthened by repeated use, or learning. However, people with Broca’s or Wernicke’s aphasia rarely ever regain language capabilities. If neural pathways are plastic and speech production and comprehension involve distributed systems throughout the brain, why aren’t language capabilities regained? Perhaps these areas are more critical to the process of speech production and comprehension than other brain regions involved in the distributed system. Research could be conducted on other brain regions connected to both Broca’s and Wernicke’s area to determine if damage to these areas results in the same deficits. This would imply that language production and comprehension are in fact distributed systems which rely on each individual component of the system to function effectively.
In summary, this book is an excellent review of the history of neurobiology and behavior with numerous implications to our class discussions. I was also very intrigued by the connection I found between the three concepts presented in this paper, that of unconscious processing of sensory input, neural organization, and the specialization of these organized neural pathways. The unconscious processing of sensory input may well be a result from a neural organization which filters the input to facilitate processing of the information. This neural organization is subject to change, in keeping with the overall plasticity of the brain, and can result in the specialization of certain brain regions.
Kandel, E. R. (2006). In search of memory: The emergence of a new science of mind.
New York: W. W. Norton and Co.