Greg Detre
Monday, November 27, 2000
Describing a cognitive function as �lateralised� is simply making a statement about which hemisphere it is localised in. Gall and Spurzheim made premature steps towards parcelling the brain up geographically into functionally distinct areas in the early 19th century under the spurious guise of �phrenology�, making wild claims about personality traits and their relation to bumps on the skull. On a gross level, the brain looks like a squidgy, homogenous mass, and there is little way of discriminating between the hemispheres. However, as described below, Broca was probably the first to make a serious, reasoned claim about lateralisation of function in the brain.
Paul Broca first argued for there being a language centre in
the left hemisphere (1861). His first patient was a speechless man known as
�Tan� because that was the only syllable he was able to utter. Broca was able
to demonstrate that Tan�s comprehension (of both spoken and written language)
was relatively unimpaired and that Tan�s physiological articulatory mechanisms
were also intact. Tan�s autopsy showed there to be a huge lesion in the
anterior lateral portion of his left hemisphere (now known as �Broca�s area�).
Broca also found lesions in the same location in the left hemisphere in a
number of other patients with similar speech production impairments. He used
his findings of patients with contralateral lesions in the same place (i.e. in
the right hemisphere) who did not suffer language impairments as evidence for
speech production being localised in Broca�s area in the left hemisphere.
�Aphasia� can be defined as a loss or impairment of the faculty of speech or understanding of language (or both) that results from physical damage to the brain, infections or tumors in the brain, and birth defects. Broca�s aphasia has come to be used to describe ability to understand language but impaired ability to speak coherently, as demonstrated in this dialogue (Gardner, 1975):
E: Were you in the Coast Guard?
P:
No, er, yes, yes...ship...Massachu...chusetts..Coast Guard...years. [raises
hands twice with fingers indicating �19�]
E:
Oh, you were in the Coast Guard for 19 years.
P:
Oh...boy...right...right.
E:
Why are you in the hospital?
P:
[Points to paralyzed arm] Arm no good. [Points to mouth] Speech...can�t
say...talk, you see.
E:
What happened to make you lose speech?
P:
Head, fall, Jesus Christ, me no good, str, str...oh Jesus...stroke.
E:
Could you tell me what you�ve been doing in the hospital?
P: Yes, sure. Me go, er, uh, P. T. nine o�cot, speech...two times...read...wr...ripe, er, rike, er, write...practice...get-ting better.
Broca�s aphasia is characterised by disrupted syntax, hesitations and stumbles, and consists mainly of content words, rather like telegraphic speech (the two-word stage of language development that children go through, producing phrases like �sleep blanket�). The disjointed results produced with great effort give rise to the other names for Broca�s aphasia, �non-fluent aphasia� or �expressive aphasia�.
Not long after Broca�s localisation of language production, Karl Wernicke was able to pinpoint a centre for language comprehension (1874). Wernicke�s aphasia is associated with an inability to comprehend language and an inability to produce meaningful discourse. Wernicke�s area is located in the temporal lobe (rather than the frontal lobe), between the primary auditory cortex and the angular gyrus.
In Wernicke�s aphasia, syntax is preserved, but patients have problems in finding the right word. Their speech is peppered with neologisms and seems to be affected by an underlying conceptual deficit, as a sample of a Wernicke aphasic�s speech demonstrates:
Boy, I�m sweating, I�m awful nervous, you know, once in a while I get
caught up. I can�t mention the tarripoi, a month ago, quite a little, I�ve done
a lot well, I impose a lot, while on the other hand, you know what I mean, I
have to run around, look it over, trebin and all that sort of stuff.
The semi-nonsensical babbled stream of Wernicke�s aphasia is also known as �fluent aphasia�.
Wernicke believed that language function was mediated by the two centres, Broca�s area for production and Wernicke�s area for comprehension. The two are connected by a bundle of axons called the arculate fasciculus. Wernicke predicted that if the connection between Broca�s and Wernicke�s areas was ablated, a third sort of aphasia would result, which �
Broca�s experiment on its own is an example of a single dissociation. Here, we know that normal subjects (without lesions or brain damage) are able to perform a given action or possess a cognitive cognitive ability (in this case, produce coherent speech). When the particular area is lesioned, this cognitive ability is impaired. This cognitive impairment cannot be induced by lesioning anywhere else. All this evidence seems to point to the cognitive function being localised. However, this need not be the case at all. The classic analogy is a hi-fi � if one were to remove an important-looking chunk of the insides and observe that when the hi-fi was turned on a high-pitched whistling could be heard, it might seem reasonable to assume that the component was a �whistle suppressor�. In fact, it is much more likely that the component served a very different and specific purpose, and its removal caused other components to receive the wrong inputs or function differently, leading to the whole system behaving strangely, i.e. emitting a whistling sound.
If we have only a single dissociation as a basis for a localisation hypothesis, then it might simply be that one task is harder than the other. In the case of Tan, it might be that speech production is harder than comprehension, so a lesion to the language centre leaves Tan able to comprehend, but not speak. We would probably expect infants to follow a similar progression, being able to comprehend long before they can talk. This does not seem to be the case, and the evidence of Wernicke�s aphasics allows us to make the stronger claim of a functional separation between comprehension and production.
Broca�s and Wernicke�s aphasias provide a good example of a double dissociation, where we can observe subjects in whom one function is impaired while another remains intact, and vice versa. Clearly, if one cognitive ability was harder than the other, then lesioning in a different place would not leave the ostensibly harder ability intact while impairing the easy one. Such double dissociations are important evidence, because they allow us to make an inference of localised functional separation with confidence.
Normally, when working to localise other cognitive functions (e.g. sensory processing or memory), experimenters can also employ animal models. With the greater freedom than they are allowed than in human subjects, they are able to perform careful behavioural measures in highly controlled settings, then ablate specific areas of neural tissue (or use a variety of other techniques to block or enhance activity in these areas) and remeasure performance on the same behavioral tests.
Because there is no animal that has language to work with, neurologists cannot rely on animal studies at all. This is why Broca�s technique of careful observation of symptoms in a variety of patients followed by autopsy to pinpoint the lesioned areas is still vital in aphasiology today. However, we do have the advantage of imaging techniques like PET and fMRI, with a resolution of about 1mm, to resolve some of the methodological problems associated with lesion/deficit studies. Notably, researchers can see which parts of the brain are most active during certain tasks, which are prominently inactive in aphasics, and they do not need to wait until the patient dies, and in the meantime probably acquires additional brain damage, to find the lesion sites.
Modern studies show that the issue of lateralisation is more complicated than first thought, especially in the case of left-handed people. Debate raged throughout the 20th century over whether left-handers� language centres are in their right or left hemispheres, with the resulting aggregate statistics:
About 98% of right handers have left hemisphere lateralisation for language functions.
About 70% of left handers have left hemisphere lateralisation for language functions.
About 15% of left handers are strongly lateralised for right hemisphere language processing.
About 15% of left handers have bilateral representation of language in the right hemisphere.
Oliver Sacks mentions about patients in his casebook on the aphasic ward.
Phrenology is dead � no neuroscientist today seeks to identify high-level social and personality traits with the shape of a person�s skull, because we are so aware that at a macroscopic level, everyone�s brain is more or less alike. The crucial differences lie at the microscopic level, resulting from each individual�s particular neural wiring.
The prevailing paradigm in the 20th century has seesawed between modularity and globalism. Where modularists importantly differ in their information processing models from early diagram-makers is in their understanding that a functionally separate module need not be topologically localised. The degree of connectivity in the brain allows for modules to be spread over a wide area, so that we can talk approximately of auditory centres, say, without necessarily being able to pinpoint a patch of cortex to deal with particular sub-functions.
Globalism takes an opposite stance; in its extreme form, it refuses to admit any localised functions. In such a view, the entire brain forms a fully distributed system.
Given the strong evidence for at least some degree of
hemispheric specialisation, the question remains �Why are we wired up in this
way?�. Hughlings Jackson was the first to point to right hemisphere
specialisation for sensory processing, implying a language/visuospatial divide
that we still retain to some degree. Levy has since argued for an
analytic-holistic distinction between processing paths on the left and right
side, and Rolls sees language as a means of planning for the long-term and
allowing us far more flexible behaviour. It could be that the asymmetry of our
brains reflects a fundamental dual-processing structure as the researchers
above focus on, or simply that bilateral language processing increases
redundancy at the expense of other cognitive functions, and there is no driving
evolutionary force for lateralisation of language in particular.