7 Functional Segregation and Localization of Function
Grace Durling
Learning Objectives
- Define functional segregation.
- Describe Broca’s hemispheric dominance theory.
- Evaluate the evidence presented in this section in support of localization of function. What are the implications of Kanwisher’s research? Does her work undoubtedly prove cortical localization of function?
An Introduction to Functional Segregation: History and Overview
What is Functional Segregation?
The term functional segregation, also referred to as functional neuroanatomy, can be a complex idea to define and comprehend. In the simplest terms, functional segregation is the localization of a particular function to a single region of the brain (Duque-Parra, 2002). The study of functional segregation involves two different, but related, approaches. The first is to describe a neuroanatomical structure at the macro, meso, or micro level, and define the functions thought to be mediated by that structure (Kanwisher, 2010). An example of this on the macro level would be to list behaviors believed to be executed by the right hemisphere compared to the left hemisphere. On the meso level, you might focus on a particular gyrus within a hemisphere and at the micro level you may get even more detailed and examine a select group of neurons within that gyrus.
The second approach to the study of functional segregation is often referred to as the “systems” approach (Kanwisher, 2010). The systems approach involves identifying a particular behaviour and then attempting to define the brain centres and pathways thought to be influential in its expression (Kanwisher, 2010).
Functional segregation focuses on the relationship between structure and function which makes it vital to our understanding of behaviour and emotion. Research in this area has led to a long-standing question – “is the human brain composed of highly specialized components, each carrying out a unique aspect of human cognition, or is it more of a general-purpose device where each component participates in a wide variety of cognitive processes?” (Kanwisher, 2010, pg.1).
The History of Functional Segregation
The notion of localization of function was first entertained in the mid-1700s by the research of the Viennese physician Franz Joseph Gall (Bem Junior et al., 2021). Born in 1758, Gall was convinced that the cerebrum was composed of different functional regions that were all associated with a different faculty of mind (Finger, 2009). Gall proposed that the human mind is made up of highly specialized components that each contain distinct mental faculties, and that he could identify these different regions (Kanwisher, 2010). Subsequently, Jean-Baptiste Bouillaud was the first to showcase cortical localization based on brain lesions. This finding, which was widely accepted, placed the center for fluent speech in the anterior lobes. Following this work, physician Pierre Paul Broca, through the clinical description of two patients with lesions in the same area of the left frontal lobe, concluded that this area was responsible for speech articulation, coming to be known as “Broca’s area” (Bem Junior, 2021). This is discussed more extensively in the following section. In 1874, German physician Carl Wernicke built upon this by describing, for the first time, the existence of the sensory center of speech. This discovery was made by analyzing earlier publications on the motor task of the temporal lobe (Bem Junior, 2021). One of the most important laboratory contributions in the history of cortical localization is the discovery of the cortical motor area in a dog, which was conducted in 1870 by Eduard Hitzig and Gustav Fritsch (Finger, 2009).
Over the next century, a heated debate regarding the localization of function in the brain ensued, with many major figures in the history of neuroscience voicing their opinion on the matter. More specifically, well known neuroscientists Broca, Brodmann, and Ferrier were in favour of this theory, while Flourens, Golgi, and Lashley were opposed (Kanwisher, 2010). The theory of cortical localization of function continued to gain support during the closing decades of the 19th century and by the early 20th century, a consensus emerged that at least basic sensory and motor functions reside in specialized brain regions (Kanwisher, 2010). Cortical localization of function guided changes in neurology, and clinicians encountered new research that supported this theory. Surgical neurology, anatomy, and physiology also supported the new way of thinking, contributing to a better understanding of the functional organization of the cerebral cortex and to clinical neurology, but the debate did not end there (Finger, 2009). Even today, two questions are fiercely debated. First, how functionally specialized are regions of the brain? Functional segregation is not usually noted as an all or none, but a matter of degree. A cortical region might be only slightly more engaged in one mental function than another, or it might be exclusively engaged in a single mental function (Kanwisher, 2010). Secondly, ongoing controversy surrounds the question of whether only basic sensory and motor functions are carried out in functionally specialized regions, or whether the same might be true even for higher-level cognitive functions (Kanwisher, 2010). The current status of this debate is far from clear.
Hemisphere Dominance Theory
As mentioned above, Paul Broca openly advocated for cortical localization of function and coined “Broca’s area” as the brain region responsible for speech articulation (Bem Junior, 2021). This influential discovery came out of Broca’s observations of one very important patient named Louis Victor Leborgne. Louis Victor Leborgne was a patient known around the hospital as “Tan”. Due to his neurologic condition, he had difficulty speaking which resulted in the only word he could speak being “Tan” (Mohammad, 2018). Leborgne was initially admitted to the hospital for psychiatric reasons but was transferred into the surgical service of Broca due to his severe epilepsy. In addition to his speech difficulties, he developed paralysis on the right side of his body and face (Finger, 2009). After his death, a neurosyphilitic lesion was discovered on his left frontal lobe, which ultimately led to the discovery of language localization in the cerebral cortex (Mohammad, 2018). A neurosyphilitic lesion occurs as a result of neurosyphilis, which is an infection that impacts the brain or spinal cord (U.S. Department of Health and Human Services, 2023). Leborgne’s condition was a cornerstone in the evolution of this discovery and in the mid-1800’s, Paul Broca proposed the hemispheric dominance theory. This theory proposed that particular characteristics were associated with each side of the brain (Finger, 2009). For example, the left hemisphere is thought to control functions such as speech, comprehension, arithmetic, and writing. In contrast, the right hemisphere is thought to be responsible for creativity, spatial ability, artistic, and musical skills (Mayfield Brain & Spine Cincinnati, 2018).
Today, hemisphere dominance theory has been studied widely in a variety of populations. The observation of behavioural deficits after localized brain damage is the oldest technique for studying brain laterality (Hellige, 2002). This technique is especially effective because if the brain were completely symmetric in terms of structure and function, an injury to homologous areas of the two hemispheres would have similar effects, which is not the case. Identification of deficits after localized brain damage can also indicate the extent to which one hemisphere is necessary in performing a particular function. For example, language deficits in speech production and perception are more common and more severe after damage to certain temporal and parietal areas of the left hemisphere compared to similar injuries in the right hemisphere (Hellige, 2002). There are also deficits associated with right hemisphere damage such as processing intonation cues.
Studying split-brain patients can demonstrate hemisphere dominance theory. A split-brain patient is an individual whose hemispheres have been surgically disconnected by cutting the corpus callosum, often to control epileptic seizures (Hellige, 2002). It is possible to examine the competence of each hemisphere in split-brain patients allowing researchers to identify the tasks and processes each hemisphere is responsible for. The study of split-brain patients further indicates that the isolated left hemisphere is far more competent in terms of language compared to the isolated right hemisphere. For example, a split-brain patient cannot name a common object such as a pencil if it is placed into the left hand or when its picture is briefly projected to the left visual field (the right hemisphere) but, they can when the object is placed in their right hand or projected to the right visual field (the left hemisphere; Hellige, 2002). Regarding individuals who are neurologically intact, researchers cannot test the competence of each hemisphere in isolation, but they can measure the speed and accuracy of performance as a function of which hemisphere receives information directly (Hellige, 2002).
Current Research on Functional Segregation
Due to the fundamental work of Franz Joseph Gall, alongside other neuroscientists listed above, neurologists and cognitive neuroscientists have investigated cognitive impairments in individuals with focal brain lesions, providing evidence for localization of certain functions in the human brain. Nancy Kanwisher is a cognitive neuroscientist who conducts modern research that investigates functional segregation. She argues that using functional MRI indicates that at least a few specific aspects of cognition are implemented in brain regions that are highly specialized for that process alone (Kanwisher, 2010). This research demonstrates identification of cortical regions that are specialized, not only for basic sensory and motor processes, but also for high-level perceptual analysis of faces, places, bodies, visually presented words, and even for the very abstract cognitive function of thinking about another person’s thoughts (Kanwisher, 2010).
In support of the idea that some areas within the brain are indeed engaged in specific mental functions, Kanwisher and her team have identified a few cortical regions that consistently respond selectively to single categories of visually presented objects. For example, the fusiform face area (FFA), has been shown to response selectively to faces. Kanwisher and her team were able to make this discovery by using fMRI technology to scan Kanwisher’s brain multiple times. In doing this, they found that the signal was higher during the periods when Kanwisher was looking at faces than the periods when she was looking at objects (Kanwisher, 2017). Additionally, the parahippocampal place area (PPA) and the extrastriata body area (EBA) respond selectively to places and body parts respectively (Kanwisher, 2010). This evidence, alongside data collected from other studies, provides strong support for the functional segregation of these regions. Firstly, these regions have been found consistently across multiple studies. Although the theoretical significance of these regions can be debated, the fact that they exist cannot. These regions are part of the basic functional architecture of the brain, demonstrated by the fact that they are found in virtually the same location in every neurologically intact individual (Kanwisher, 2010). Secondly, the ease at which these regions can be located in normal subjects makes it possible to utilize a region of interest (ROI) research strategy. In this strategy, the region is functionally identified in each participant using a short localizer scan and then the response of that region is measured in a number of new test conditions (Kanwisher, 2010). This allows researchers to test specific hypotheses about the exact function of the region of interest. Together, the FFA, PPA and EBA provide some of the strongest evidence that at least some regions process specific classes of stimuli.
In addition to the research of Kanwisher, researcher Rebecca Saxe made a significant discovery in the field of functional segregation. Saxe demonstrated that the region at the junction of the temporal and parietal lobes of the right hemisphere is selectively engaged when one thinks about what another person is thinking (Saxe & Kanwisher, 2003). Saxe and her team identified this region, using the ROI strategy, among hundreds of participants and have labelled it the right temporoparietal junction (rTPJ). The data they collected showed that this region responds strongly when participants were read a scenario describing what an individual knows or thinks, but is so selective that it does not respond when people think about another person’s bodily sensations such as hunger or thirst (Kanwisher, 2010). This is fascinating considering these bodily sensations are mental states, but they do not involve propositional content such as thoughts and beliefs. This discovery serves as further proof of existence that functionally specific cortical regions are not restricted only to sensory and motor areas (Kanwisher, 2010). Furthermore, these findings raise the question of whether other high-level cognitive processes may also be housed in specialized cortical regions.
No complex cognitive process is accomplished in one single location of the brain. These arguments for the specificity of cortical regions do not imply that other areas of the brain have nothing to do with these functions. For example, cortical regions such as the primary visual cortex are also very important in the perception of faces, places, and bodies. In addition, higher areas such as the frontal and parietal regions are likely necessary for information in the FFA, PPA, and EBA to reach awareness and be utilized by other cognitive systems (Kanwisher, 2010).
The evidence presented within this chapter is reflective of only one side of this ongoing debate regarding functional segregation. Despite this, current evidence appears to be strong for cortical regions that are engaged selectively in perception of faces, places, and bodies, and thinking about what others are thinking. Possible specializations of cortical regions for other domains such as numbers, language, or music, are actively being investigated (Kanwisher, 2010). A very exciting component of this research is not where each specific regions is found, but which functions receive their own location of the brain and why some do while others do not? The study of modern functional segregation has invited numerous exciting questions. For example, what are the specific neural circuits that enable regions to conduct their signature function? Are their circumstances where a region arises in a different location due to damage in its original location? How do these regions interact to elicit complex cognition? Could regions be recruited to perform new tasks? (Kanwisher, 2010). At the time of this being written, there are many questions that remain to be answered.
References
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