The Aging Brain: It Starts Earlier Than You Think (3)
The Ghost Of Gall
In the previous post I discussed diverse cognitive functions, some age sensitive, some not. After this post, I will explore how the varying susceptibility of cognitive activities--from fluid intelligence to episodic memory--relate to the aging brain, specifically the cerebral cortex. In this post though, I aim to provide what I hope is some helpful context for the next few posts, on how brain aging relates to cognitive aging.
Historically, there are two fundamentally opposed views as to the mapping of cerebral structures to cerebral functions, including cognitive functions. At one extreme is the view that all cerebral functions can be assigned to very particular cerebral structures. I call this hyper-localization. At the other extreme is the view that all cerebral functions are equally distributed throughout the cerebrum. This view is known as equipotentiality. No contemporary neuroscientists would ascribe to either of these polar positions. But they vary in where along that spectrum their approach to neuroscience lies.
Neuroscience was born with a bias toward localization. In the 1860s the pioneering neuroscientist Pierre Paul Broca extended a theory proposed by the French physician, Franz Joseph Gall, that the cerebral cortex could be divided into non-overlapping areas, each functioning as a separate organ with a single function, a specific psychological category.
Examples of brain organ functional specializations included hopefulness, generosity, mirthfulness, spirituality, combativeness, secretiveness, and self-esteem, to name a few. Gall believed each of these brain organs, like muscles, grew with use, causing bumps on the skull. Therefore, by measuring these skull bumps and their locations a skilled practitioner of what came to be known as phrenology, could divine the personality and other attributes of any individual (Click the link below and scroll up.)
Broca accepted Gall’s general picture of the cerebral cortex as an aggregate of numerous organs each specialized for a single function. But Broca rejected Galls methods for investigating the cerebral cortex, and therefore phrenology. Broca advocated instead that to investigate the brain, you need to more directly access the brain, get your hands wet, as it were. Broca remains famous today for his investigation of one of Gall’s brain organs, the one specialized for language. He studied, post-mortem, the brains of individuals who exhibited a form of aphasia, an inability to produce audible language, what we know as speech. Broca identified lesions in a particular region of the frontal cortex of those who suffered from aphasia. This region of the frontal cortex is now known as Broca’s Area.
In 1874, the German neurologist, Carl Wernicke, identified a different form of aphasia associated with lesions in a different area of cerebral cortex. The aphasia that Wernicke identified is a matter of language comprehension, not language production. Between Broca and Wernicke the localist view of brain functions championed by Gall, received powerful support. This is the story I learned in my first neuroscience class. Versions of this story, modified to varying degrees, continue to be taught in medical schools to this day.
But this attractively simple story is simplistic. Broca’s area is not well defined with respect to its neural substrates, Wernicke’s area even less so. Lesions outside of Broca’s area cause the same aphasic symptoms that Broca described, and lesions within Broca’s area can be asymptomatic (https://dx.doi.org/10.1080/09515089.2017.1322193). The story of Wernicke’s area is even murkier (26567270). It is now known that Wernicke’s area is as responsible for speech production as Broca’s area. Moreover, Wernicke’s area has a host of other functions outside of language processing. In fact, the role of Wernicke’s area in language processing is limited (26567270).
To summarize Broca’s Area and Wernicke’s Area have long been paragons of localized brain functions, the one-to-one mapping of cerebral structures to cerebral functions. Though unbeknownst to many, these two exemplars of cerebral localization have been falsified. Broca’s area is not the cerebral organ for language production, nor is Wernicke’s Area the cerebral organ for language comprehension. Language comprehension, in particular, is a function of neural networks that are distributed among at least three of the four major regions of the cerebrum: frontal cortex, temporal cortex, and parietal cortex. Moreover, these neural networks have multiple computational functions, not just language.
But I hasten to add that this is not support for radical equipotentiality. The language-related neural circuits that traverse large part of the cerebrum are increasingly well-defined. It is the interconnections of these neural networks, not particular brain loci that support our ability to communicate through language.
Which leads to an important distinction to keep in mind in understanding the aging brain going forward: the difference between necessary causes or conditions and sufficient causes or conditions. Broca assumed, because of his localizing predisposition, that Broca’s Area was both necessary and sufficient for speech. Assume that he did in fact identify a specific area necessary for speech. He certainly did not reach the higher bar of demonstrating that Broca’s Area is sufficient for speech. Brain lesion studies and much other evidence of highly localized cerebral structure-function mapping, generally fail to make this distinction.
In the next post I will discuss the aging prefrontal cortex, called by one eminent neuroscientist “the substrate of the highest cognitive functions” (34645980 ). These “highest cognitive functions”, also known as executive functions, are the cognitive processes most likely to be considered jewels in the human cognitive crown.