The first slide allows me to draw attention to the following brain structures:
the four lobes of the cerebral hemispheres: frontal,
temporal, parietal, occipital
the corpus callosum
The next sequence of slides depict basal, midsaggital
and lateral views of the human brain. These are labelled
cartoon versions, but click on each image to get an equivalent view of a real
brain. Some structures you should note are the precentral gyrus - motor cortex,
and the postcentral gyrus - somatosensory cortex. The midsaggital view allows
you to locate Descartes' seat of the soul - the pineal gland.
Studies of the response of the cortex to weak electrical stimulation were conducted by a number of investigators towards the end of the 19th century. It was discovered that stimulation of the area in front of the central sulcus (also known as the Rolandic fissure) caused movements on the opposite side of the body, whereas applying a weak electrical stimulus to the area beind the central sulcus. A Canadian neurologist with the wonderfully evocative name of Wilder Penfield (1891-1976) developed the notion of the homunculus. He worked with patients suffering from epilepsy or brain tumours, mapping the responses of their cortices to electrical stimulation. The motor and sensory homunculi depict the extent and position of the cortex devoted to each part of the body. Glynn makes an important point about the types of brain function that are effectively localized here: only elementary activities, such as opening and closing the hand, swallowing, and leg extension, are generated. No complex learned movements, such as speech, occur as a result of stimulation of the motor cortex. The study of language processing, however, did yield some of the best evidence for functional localization.
In 1861 Paul Broca (1824-1880), a French neuroanatomist, presented a paper to the Societe d'Anthropologie about a 51-year-old man who had died only the day before. The patient, Leborgne, was known as 'Tan' because that was the sound he made when trying to give his name. Tan was epileptic and had lost the power of language production - but not comprehension - when he was 31. Inspection of the surface of Tan's brain revealed a large lesion in the third frontal convolution of the left hemisphere. This area is adjacent to the motor area concerned with the mouth and tongue. The diagnostic category 'Broca's aphasia' survives today, and the group of patients labelled as Broca's aphasics all have difficulty with word finding and production.
Broca's demonstration of such specific brain damage restored some scientific credibility to the functional localization theory, in the wake of Flourens discrediting of the phrenologists. Broca was careful to distinguish his work from that of the phrenologists by pointing out that his speech area was located differently and that it was neurologically based, rather than concerned with bumps on the skull.
Wernicke's Aphasia, named for the German physician Karl Wernicke (1848-1905) differs radically from Broca's. Wernicke's patients produce a fluent stream of language that is often nonsensical, tangential and packed with neologisms and circumlocutions. This simple cartoon of the brain shows the relative positioning of Broca's and Wernicke's areas: Broca's aphasia results from frontal brain damage, whereas Wernicke's aphasia is a consequence of posterior damage in the left temporal cortex, the area adjacent to auditory cortex.
A group of nineteenth century neurologists, including Wernicke, became known collectively as 'the diagram makers' due to their propensity to chart diagrams of language processing. Although the pendulum swung back to the holistic view in the first half of the 20th century, the work of American neurologist Norman Geschwind and his collaborators revived the diagrams of functional lingustic circuitry that involve Broca's and Wernicke's areas. For example, repeating a heard word is thought to stimulate first auditory cortex, then Wernicke's area, travel to Broca's area via the arcuate fasciculus and finally cause activation of the correct motor pattern in the motor cortex. In contrast, speaking a written word involves activation of the primary visual area which is passed on to Wernicke's area via the angular gyrus. If the angular gyrus is damaged the expected disconnection between auditory and written language result: patients with injuries to the angular gyrus may speak and understand speech normally, but have difficulty with written language. Geschwind stated that these diagrams should not mislead us into thinking about these modular areas too rigidly because a considerable degree of recovery is observed, perhaps as a result of other regions taking over these language functions. He hints that this could be through an alternative store of learning on the opposite side of the brain - a sort of understudy. I will discuss lateralization of function and the role of the non-dominant hemisphere at some length below and in the first set of group conferences.
Modern imaging techniques
have freed investigators from the need to study only damaged brains. PET (Positron
Emission Tomography) is a means of visualizing the brain activity of a living
creature. See page of Glynn for more details.
Peterson, Fox, Posner, Mintun & Raichle (1989) performed a PET study of
a particular linguistic task: the generation of verbs. While undergoing a PET
scan their subjects did several things:
|
Experimental
Conditions
|
| 1. The subjects stared at a crosshair on a computer monitor while their brain activity was monitored via PET imaging. |
| 2. Common English nouns appeared on the screen (visual), or were heard over earphones (auditory). The presentation rate was 40 words a minute. |
| 3. The subjects were asked to speak the words that they saw or heard. |
| 4. The subjects were asked to say aloud a use appropriate for the noun they either viewed or heard. e.g., if the word was 'hammer' an appropriate response would be 'hit' |
The PET scans for the different aspects of the tasks were constructed as follows:
| Hearing Words | 2(auditory)-1 |
| Seeing Words | 2(visual)-1 |
| Speaking Words | 3-2 |
| Generating Words | 4-3 |
The resulting scans show considerable functional specilization and specificity. The Hearing words scan shows marked activity in auditory cortex and Wernicke's area. Seeing words generates most activity in primary visual cortex and the occipital-temporal visual association cortex. Speaking words creates a lot of activity in the motor cortex and Broca's area. Generating words generates most activity in the frontal lobes. When you consider all aspects of this relatively simple task together you realize the distributed nature of cortical activation.
As early as the mid-nineteenth century when Broca was making his observations it was realized that the distrubution of functions between the two hemispheres is not even. Broca noted that in the majority of cases of linguistic impairment resulted from damage to the left hemisphere. The British neurologist John Hughlings Jackson (1835-1911) noted a complementary lateralization of some aspects of spatial and visual perception. He realized that his patients with damage to the posterior right cortex had difficulty finding their way around and recognizing things and people on the basis of visual information. He coined the term 'imperception' to refer to these visually related difficulties, but they have come to be referred to as agnosias. The idea grew that the left hemisphere was the intellectual brain and that the right hemisphere subserved functions common to humans and beasts. This was the Zeitgeist in the 1880s when Robert Louis Stevenson wrote The Strange Case of Doctor Jekyll and Mr. Hyde.
It was not until the 1950s that this debate about dual consciousness was subjected to experimental test. Roger Sperry (1913-1994) and his student Ronald Myers performed lesion experiments on cats to examine the effects of severing the corpus callosum. The visual pathways are wired such that information in the visual field to one side of the midline travels first to the cortical hemisphere on the opposite side of the body. This means that stimuli can be presented in one hemifield and only travel to one side of a split brain.
In 1962 a split-brain operation was performed on an ex-paratrooper in his 40s who had been injured over a decade earlier when his parachute did not open during a jump into occupied Holland. This patient suffered from intractable epilepsy and severing of the corpus callosum had been shown to reduce the frequency and severity of grand mal seizures. Sperry and his then student Gazzaniga were able to study this patient post-operatively. They found a striking dissociation: objects presented briefly in the right visual field could be named readily, but objects presented in the left visual field could not be named. The split brain patient was able to correctly identify by touch an object presented in the left visual hemifield. This led to the identification of the right hemisphere as 'silent' or 'non-dominant' for language.
Gazzaniga video clip from Scientific American Frontiers program, Pieces of Mind, Jan. 1997. (If you were not able to attend this lecture, see me about viewing this video clip.)
Gage - The Role of the Frontal Lobes in Emotions. Realaudio of Sylvia Carrdoso's slide show about Gage
Finger, S. (2000) Minds Behind the Brain: A History of the Pioneers and Their Discoveries. New York: Oxford University Press.
Geschwind, N. (1979) Specializations of the Human Brain, Scientific American,
Petersen, S.E.,
Fox, P.T., Posner, M.I., Mintun, M. & Raichle, M.E. (1989)
Positron emission tomographic studies of the processing of single words,
Journal of Cognitive Neuroscience, 1, 153-170.