(Year 1 Coursework - 2019) The Localisation of Different Functions in the Brain
I have written this 1000-word practice essay in my ‘Neural Basis of Perception, Memory and Language’ module in my first-year of undergraduate studies, for which I received a High 1.1 (87).
Introduction
Localization of function refers to the idea that specific brain regions are responsible for specific functions: functions, such as language and memory, can be mapped onto particular brain areas. The first evidence was presented in the 1860s, with the case study of M. Leborgne (“Tan”) by Paul Broca (1861), whom found that damage to the left frontal lobe was linked to speech production impairement. Following him, others found similar causal links between localised damage and specific impairement, such as Fritsch and Hitzig in 1870 (Breathnach, 1992), Wernicke in 1874 (Bogen & Bogen, 1976). Although these works and others enabled the elaboration of detailed maps of the brain, more and more recent studies (Ruff, Driver & Bestmann, 2009) suggest that this modular view (at least alone) cannot be a satisfactory account of the brain, shedding light on alternative views (distributed, equipotential…).
Body I
First of all, different psychological functions seem to be localized to specific areas of the brain, with hemispheric lateralization for some of these functions. For instance, one case study (patient HM), by Scoville and Milner in 1957, suggested the existence of a distinction in brain areas across memory functions (encoding and consolidation on one side, storage and retrieval on the other). To control for his epileptic seizures, patient HM underwent a bilateral removal of the hippocampus, after which he became unable to form new episodic memories; yet, retrieval of old memories was relatively spared (Squire, 2009). This study, along with others (PB and FC: Squire, 2009), showed that while the hippocampus is involved in the encoding and consolidation of new memories, the neocortex seems to be the place where long term memories are stored and from which they are retrieved.
Neuroimaging studies on perception demonstrated that specific brain regions are responsible for perception of specific things. Indeed, Kanwisher et al. (1997), in their fMRI study, showed the existence of a module in the human extrastriate cortex, specialised for face perception: the area, named Fusiform Face Area, was localized in the majority of participants (12 out of 15). Parallelly, a TMS study, conducted by Pitcher et al. (2009), further suggested a “triple dissociation” in perception of faces, bodies and objects, where specific areas contribute only to their “preferred” function (modular view).
Additionally, earlier studies (lesion studies on stroke patients) on language functions had already suggested this modular view of the brain, introduced earlier. In this realm, Broca and Wernicke’s works (Broca, 1861, patient Tan; Wernicke, 1874) in the nineteenth century are regarded as the first example of a double dissociation in the brain: Broca’s area (close to the motor cortex, left inferior frontal gyrus) was linked to speech production; Wernicke’s area (close to auditory areas, left superior temporal gyrus) to speech comprehension. Although Tan seemed to understand language, he could only produce a few words, whereas Wernicke’s patient had severe difficulties in comprehension and a relatively spared speech production (reports of fluent utterances). This distinction in language functions was considered as critical evidence for a functional lateralalization in the brain.
Body II
Although, earlier as well as more recent studies proposed a modular view of the brain, several other studies demonstrated that the systematic mapping of psychological functions onto specific brain areas can be problematic, thus supporting alternative views. The double dissociation, under the Wernicke-Geschwind model, accounts Broca’s area for speech production and Wernicke’s area for comprehension. However, recent research suggests that pathways between brain areas, as well as the areas themselves, are important for language processing (Friederici, 2011). Also, functional neuroimaging research shows that language processing is not extensively left lateralised, as earlier studies suggested: comprehension is more bilateral than production; the deeper the level of processing (amplitude modulation to phonemes, words, phrases, sentences), the more activation occurs in the left hemisphere; right hemisphere is thus also involved (Peele, 2012). Most importantly, the Wernicke-Geschwind model is now outdated, as further studies considerably challenged it: in voxel-based lesion-symptom mapping studies of 101 aphasia patients, Bates et al. (2003) showed that lesions within the insula and deep parietal white matter had the largest impact on speech production and fluency, and not “Broca’s area”; similarly they showed that damage to middle temporal gyrus had the largest effect on auditory comprehension, and not “Wernicke’s area”.
Some recent combined studies, using TMS and fMRI, have indicated that TMS affects both the directly stimulated brain area and remote brain areas connected to it via neural pathways (use of TMS/fMRI in cognitive neuroscience, reviewed by Ruff, Driver and Bestmann, 2009). Thus, the use of TMS in studies on the functional interactions between brain areas shed light on a more distributed approach of the brain: this view states that brain regions are interconnected in networks, “acting” together, rather than each responding to one particular function.
Eventually, several alternative views to the modular approach of the brain have been proposed and developed. The previous studies that we have introduced accounted for a distributed approach, where brain areas can be simultaneously involved in one specific function. A distributed-plus-hub view is similar to the former approach, with, in addition, the existence of a system (”hub”) that organises, manages all distributed areas: these two neuroanatomical distributions of the cortical semantic network were discussed by Patterson et al. (2007) in their study of patient M with semantic dementia. Another influential approach is the equipotential theory, introduced by Lashley (1930), that states that all areas of the brain are equally active in overall mental functioning: this theory suggests that multiple brain areas work together for functions such as memory, undermining the modular view. Alternative recent models to specific functions have also been proposed, such as the MUC model of lamguage (Hagoort, 2013), although not widely established.
Conclusion
Although earlier research showed that critical functions (memory, language functions, etc.) were localized in certain specific areas in the brain, suggesting a modular view of the brain, more recent studies, helped by the use of neuroimaging advances (fMRI, TMS, VBLS mapping), give a limitation to this explanation. Consequently, alternative views to the modular view have been developed.
References
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Bogen, J. E., & Bogen, G. M. (1976). Wernicke’s region–Where is it. Annals of the New York Academy of Sciences, (280), 834-843.
Breathnach, C. S. (1992). Eduard Hitzig, neurophysiologist and psychiatrist. History of Psychiatry, 3(11), 329–338.
Broca, P. P. (1861). Perte de la Parole, Ramollissement Chronique et Destruction Partielle du Lobe Antérieur Gauche du Cerveau. Bulletin de la Société Anthropologique, 2, 235-238. http://psychclassics.yorku.ca/Broca/perte-e.htm
Friederici A. D. (2011). The brain basis of language processing: from structure to function. Physiological Reviews, 91, 1357-1392.
Hagoort, P. (2013). MUC (Memory, Unification, Control) and beyond. Frontiers in Psychology, 4, 1-13.
Kanwisher, N., McDermott, J. and Chun, M. (1997). The Fusiform Face Area: A Module in Human Extrastriate Cortex Specialized for Face Perception. The Journal of Neuroscience, 17(11), 4302-4311.
Lashley, K. (1930). Basic neural mechanisms in behavior. Psychological Review, 37(1), 1-24.
Patterson, K., Nestor, P. and Rogers, T. (2007). Where do you know what you know? The representation of semantic knowledge in the human brain. Nature Reviews Neuroscience, 8(12), 976-987.
Peelle, J.E. (2012). The hemispheric lateralization of speech processing depends on what “speech” is: a hierarchical perspective. Frontiers in Human Neuroscience, 6, 309.
Pitcher, D., Charles, L., Devlin, J., Walsh, V. and Duchaine, B. (2009). Triple Dissociation of Faces, Bodies, and Objects in Extrastriate Cortex. Current Biology, 19(4), 319-324.
Ruff, C., Driver, J. and Bestmann, S. (2009). Combining TMS and fMRI: From ‘virtual lesions’ to functional-network accounts of cognition. Cortex, 45(9), 1043-1049.
Squire, L. (2009). The Legacy of Patient H.M. for Neuroscience. Neuron, 61(1), 6-9.