The prominent debate in the young years of neuroscience was whether or not the brain had specialized functional regions or whether the brain as a whole was responsible for all cognitive functions.
Franz Joseph Gall was the first to assert the radical notion that all behavior emanated from the brain and that specific regions of the cerebral cortex (the large, convoluted structure on top of many pictures of the brain) controlled specific functions. Though, these assertions would be shown to hold water, he falsely viewed the brain as being synonymous to a muscle, with specific areas getting larger with use, which would cause the skull above these areas to bulge, meaning that the surface of the skull could give information about one’s mental state. This became known as phrenology and though the idea of the skull’s shape being indicative of mental capabilities has been conclusively discredited, Gall’s first two assertions have been supported.
In support of Gall’s claims that the brain contained specialized functional areas came the research of Pierre Paul Broca of France and Karl Wernicke of Germany. These two men studied aphasias, which are disorders of language. Broca discovered that patients who displayed difficulty in generating speech, yet had no ailment of the mouth or vocal cords, often had a lesion (damage, such as that caused by stroke) in a very specific site in the left cerebral hemisphere. Wernicke’s patients, who could speak, but not understand language, also had lesions in another specific site of the left hemisphere. Wernicke thus came to the conclusion that the brain was a parallel distributed processor, meaning that complex functions, such as language, are the result of different brain regions working in both serial and parallel manners.
In using using this model of cognitive function, Wernicke successfully predicted another aphasia called conduction aphasia in which these areas were intact, but the connections between them were compromised. Patients with this condition generate nonsensical speech, despite being able to form words and understand language.
If understanding and generating language takes place solely in the left hemisphere, then what occurs at these places on the right hemisphere?
After Broca’s and Wernicke’s areas (as they came to be called) were discovered, scientists probed their right hemisphere complements and found that emotional processing occurred here. Indeed, lesions in the right hemisphere complement of Broca’s area result in the inability to generate emotional content in self expression, while lesions across from Wernicke’s area result in deficits in understanding the emotional content of a message.
We see how complex neural processing can be.
What would happen if, as in the case of conduction aphasia, we cut off communication between the left and right hemispheres, but left the neuronal groups intact to carry out their computations? The brain contains a type of neural bridge between the two hemispheres called the corpus callosum. It was thought that severing this bridge could help patients with epilepsy. However, there are consequences as shown in the video above.
An interesting deficit that occurred in these split-brain patients is that if you present an object to the right hemisphere only, they cannot name the object despite knowing what it is, an occurrence not seen when the object is similarly presented to the language-possessing left hemisphere.
Another, perhaps more disturbing consequence, is what is known as alien hand syndrome. Patients with this condition present with one hand working under their control, and the other working in a completely unpredictable manner and may act against the patients’ intent. Examples of this are trying to button a shirt with one hand, while the other unbuttons it, or trying to read a book while the other hand continuously puts it down. This occurs because we’re considered to have a dominant hemisphere – the left hemisphere in a vast majority of people (no, that doesn’t mean you aren’t creative). The intent and actions of that hemisphere is what we’re conscious of.
Consciousness, as described by Giulio Tononi and the late Gerald Edelman in A Universe of Consciousness must be coherent. Which is why, when our brains become damaged in a way that affect our conscious experience, we may fill in the gap with improper information.
For instance, anosognosia is a condition in which one half of a patient’s body may be paralyzed due to stroke, yet the patient denies the paralysis. These patients will even deny owning the affected body parts if confronted with evidence of the paralysis. Hemineglect occurs when patients with damage to one side of their parietal lobes perceive only one side of the world, such as shaving only one side of their face, yet their consciousness is aware of an entire picture – a fully shaved face.
Here, we’ve seen how complex a biological machine the brain really is, with many different areas carrying out simple functions while working in series and parallel to many other areas to carry out complex functions. We’ve also begin to see how the brain formulates a unified conscious experience.
For more on interesting neuropsychiatric conditions, see pretty much any book by Oliver Sacks, but specifically the Man Who Mistook His Wife For a Hat. For more on the enigma of consciousness, in addition to Edelman and Tononi, see Daniel Dennett’s Consciousness Explained or Christof Koch’s Consciousness: the Confessions of a Romantic Reductionist or The Quest For Consciousness: A Neurobiological Approach.
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