biol125-lectures/language.md

Paul Broca (1861)

• Believed that functions could be localized in the brain
• Studied patients with aphasia– language disorders found in patients who have had a stroke
• Had a patient that could understand language but could not speak, problems with the organizational aspects of language. Found lesion in posterior frontal lobe (Brodmann areas 44/45)
• This kind of aphasia is called a motor or expressive aphasia
• Eight patients with similar problems all had similar lesions, always on the left side
• "Nous parlons avec l’hemisphere gauche!" "We speak with the left hemisphere!"

Note:

We also define regions of the brain based on studies of brain lesions in patients. Recall the guy with the railroad spike from last lecture. Well the French physician Paul Broca in the 19th c.

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Broca and a patient’s brain

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Carl Wernicke (1874)

• Had a patient that could speak but not understand language. Called receptive aphasia
• Damage was to a different area– left side, posterior part of the left temporal lobe (Brodmann areas 22/39/40)
• Argued that simple perceptual and motor activities were localized to a specific area and that most functions result from interconnections between areas. Idea of "distributive processing"

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Major brain areas involved in the comprehension and production of language

Neuroscience 5e Fig. 27.1

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Arcuate fasciculus: major association fiber tract in CNS that connects Broca’s and Wernicke’s areas

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Characteristics of Broca's and Wernicke's aphasias

Neuroscience 5e Table 27.1

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Broca's aphasia patients have limited writing. Loss of the ability to produce language (spoken or written)

Apraxia

verbal apraxia is difficulty starting and making voluntary movements (motor plans) needed for speech (with no paralysis or weakness of speech muscles)

dysarthria

inability to move the muscles of the tongue and mouth to produce speech

agraphia

inability to write

Agnosia

inability to process sensory information

affects a single modality

syntax :the arrangement of words and phrases to create well-formed sentences in a language

Grammar :in linguistics it is set of structural rules governing the composition of clauses, phrases, and words in any given natural language. The term refers also to the study of such rules, and this field includes morphology, syntax, and phonology, often complemented by phonetics, semantics, and pragmatics

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Other evidence of brain regionalization

• Fritsch and Hitzig (1870)– discrete limb movements in dogs can be produced by electrical stimulation of the contra-lateral motor cortex. Thus the right hand is controlled by the left hemisphere. 'Dominant' hemisphere
• Wilder Penfield (1950)– neurosurgeon, localized motor functions by stimulating specific areas of the brain
• Roger Sperry (1960s)– split brain patient studies

Note:

And there is lots of other evidence for localization of brain function, especially for sensory and motor information for limbs and body. In fact Fritsch…

And the Canadian physician Wilder Penfield performed classical mapping of motor function in the cerebral cortex by localized electrical stimulation.

And then there is the fascinating split brain studies of Sperry and Gazzaniga in the 1960s

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Penfield stimulation studies

Stimululation based brain mapping

Note:

Epileptic patient. mapping the cortical tissue before resecting the site of tissue where the seizures are being generated.

Start about minute 3

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Split brain studies: Nobel prize 1981

• The corpus callosum and anterior commissure are the two axon tracts that connect the two sides of the brain. They are sometimes cut to prevent the spread of severe epilepsies
• Each side of the brain works independently from the other
• Roger Sperry showed that the left hemisphere dominates speech, writing, right hand stereognosis, analysis of right visual field
• Right hemisphere dominates, emotional coloring of language, spatial abilities, left hand stereognosis, analysis of left visual field

R. Sperry

"for his discoveries concerning the functional specialization of the cerebral hemispheres"

Note:

The classic split brain studies which Roger Sperry got the Nobel for in 1981 showed the lateralized localization of language that Broca and Wernicke anticipated as well as several other higher functions. They took advance of the fact that in patients with severe epilepsies, sometimes the commissures connecting the two hemispheres are cut to prevent the spread of seizures.

And since each side of the brain to some degree can work independently of the other

• Humans are 90% right handed as a population and the degree of lateralization among individuals is strong, regardless of left or right-handedness
• 96% of right handers having left hemisphere speech, compared with 70% of left handers
• Twin studies have demonstrated some genetic influence on handedness, but 75% of the variance is nongenetic and individually specific, with only 25% explained by genes [#Bishop:2013]. Even the segregation of of handedness and language laterality suggests a complex polygenic set of factors, with 96% of right handers having left hemisphere speech, compared with 70% of left handers [#Bishop:2013].

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Confirmation of hemispheric specialization for language

Neuroscience 5e Fig. 27.3

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Here is an illustration of the experiment performed by by Sperry and his colleagues for these split brain studies.

After the corpus callosum connecting the two hemisphere was cut to alleviate epileptic seizures, the patients were asked to fixate on a point and name objects presented in each visual field.

Now you haven’t learned about the visual system yet, but just as sensory information from your left hand goes to your right hemisphere, visual information from the lateral part of your left visual field goes to your right visual cortex.

Split brain patients could not correctly name objects presented in their left visual field, presumably because that info could not reach the left hemispheres because the callosal connections were severed. But split brain patients could correctly name an object when presented in their right visual field, because that information was received by the left visual cortex and could be passed onto the language centers.

In all Sperry and his colleagues showed that language, mathematical, and logical reasoning is dominant in the left hemisphere and that shape recognition, spatial attention, emotional processing, and creativity in more dominant in the right hemisphere.

right hemisphere: 'coloring' language with emotive tonal variation, 'prosody'. Adds additional meaning to verbal communication. Mandarin chinese. Monotone professor lecture. Evidence suggest similar areas of the right hemisphere associate with this emotive coloring of language.

Similar areas used in sign language thus this constellation of brain regions specializes in symbolic representation and communication, rather than just spoken language.

PET:

positron emission tomography

detects pairs of gamma rays emitted indirectly by a radioactive tracer injected into bloodstream (positron-emitting radionuclide)

CT:

computerized tomography

a series of X-ray images from different angles

computer processing to create cross-sectional images

Babbling sounds from a baby shows that there is a pattern of sounds produced sequentially that are related to the phones necessary for producing spoken language. Language imitation follows other imitations (mirror neurons?) during developmental learning and behavioral acquistion. Brain is continuously simulating the future based on past experienced training patterns.

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Mapping brain activity with fMRI

Neuroscience 5e Fig. 27.6

Note:

• functional magnetic resonance imaging
• different patterns of brain activity localization depending on what the task is
• Actually sitting inside a small space magnet

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