Language and Cortex Class 11 Friday, October 23, 2009 Marrs

Transcription

1 Language and Cortex Class 11 Friday, October 23, 2009 Marrs Objectives: At the end of this lecture you should be able to: 1) Be able to describe the Wernicke-Geschwind model for language processing. 2) Be able to describe the limitations of the Wernicke-Geschwind model and the more contemporary view of the neural substrates for language in the human brain. 3) Be able to describe different types of aphasia and the cortical regions associated with them. 4) Be able to discuss lateralization of the brain. Which hemisphere is dominant? What tasks are associated with the right hemisphere? What tasks are associated with the left hemisphere? How have decommisurated patients helped us learn about brain lateralization? Introduction Language is a system of communication in which ideas and feelings are encoded into signals of sounds, gestures, signs, or marks that convey meaning within a group or community. Language consists of two components words and grammar. A word is an arbitrary association between a signal and a meaning. Grammar is a system that specifies how words can be combined and how the meaning of a combination of words can be determined. Language Acquisition in Children Humans are born with the ability to perceive the full range of phonemes (the smallest unit of speech sound). Babies make language-like sounds at 5-7 months, babble in well-formed syllables at 7-8 months, and gibber in sentence-like streams by 12 months. They can discriminate speech sounds, even ones not used in their parents language, in their first months. By 10 months, they discriminate only phonemes used by their parents. By age one, babies can begin to comprehend words and have a word vocabulary. By age three, children speak in full sentences and have a rich vocabulary, but still have difficulty with grammar. Older children build up understanding of grammatical structure, rather than simply imitating the speech of others. By age 6, children comprehend about 13,000 words, and high school graduates know 60,000 words. Language development in children led Noam Chomsky to hypothesize in the late 1950 s that the human brain has evolved an innate neural circuitry dedicated to the acquisition of language. Some psychologists and linguists disagree, believing that the capacity for language is one expression of a general cognitive ability to learn patterns, rather than a specific system for language. Whichever view is correct, it is clear that language is an ability that is both innate and learned. There are no homologs to human language in other species of the animal kingdom. The lack of animal models for language has limited our ability to understand the neural basis of language. Consequently, most of our knowledge about which brain regions are involved in -1-

2 language processing and how they function derives from the description of language disorders, or aphasias, caused by focal brain lesions, most frequently stroke or head injury. Lateralization of language processing Early studies of people with stroke or brain trauma found correlations between patients language deficits and the brain areas that had been damaged, determined by an autopsy at the time of death. These studies led to the discovery that most language processing goes on in one hemisphere, called the dominant hemisphere. In over 90% of people (98% in righthanded people and about 65% of left-handed people) the left hemisphere processes grammar, lexicon, phonetics, and speech production. The Wernicke-Geschwind model of language comprehension and speech production The other major discovery revealed by study of patients with aphasias was that two cerebral cortical areas Broca s area in the lateral frontal lobe and Wernicke s area in the posterior superior temporal lobe are major neural substrates of language. A useful, if somewhat simplified, model for understanding how the two language areas interact is the Wernicke- Geschwind model. This model has been quite successful in predicting the effects of damage in several brain regions and how visual information is processed in naming an object. The model posits that Broca s area, which is connected to regions of motor cortex that control the face and tongue, is involved in planning the motor execution of speech, and that Wernicke s area, which is connected to auditory cortex, is involved in recognizing and representing the sound pattern of words. Within this model, the arcuate fasciculus is a unidirectional pathway that brings information from Wernicke s area to Broca s area. According to the Wernicke-Geschwind model, there is a clear, linear flow of information through these two cortical areas for the recognition and production of speech. Wernicke s area contains the auditory codes for words - what they sound like. Broca s area contains the articulatory codes for words - the motor commands that tell the mouth and larynx how to articulate each word. The behavior of repeating a spoken word exemplifies how the Wernicke-Geschwind model understands Arcuate fasciculus language processing. When a spoken word is heard, the sound is processed in auditory cortex and then transmitted to Wernicke's area. -2-

3 There the sound is matched to its auditory code, and it s meaning can be interpreted by other association areas of the cerebral cortex. The auditory code is transmitted to Broca's area through the arcuate fasciculus, where the articulatory code for the word is activated and sent to motor cortex for speech production. Reading comprehension also requires Wernicke s area in the Wernicke- Geschwind model. Visual information is sent to the angular gyrus which translates the visual code to a form accessible by Wernicke s area. Wernicke s area then matches the word to its auditory code, as for spoken speech. Thus, according to this model, reading requires the phonological recoding of words in Wernicke s area. The Wernicke-Geschwind model provided a framework for understanding the neural mechanisms of the aphasias. Patients with damage to Broca s areas cannot produce speech, but they can still comprehend speech because Wernicke's area is intact. Damage to Wernicke's area, on the other hand, produces no problems in speech production because Broca a area is unaffected, but the meaning of the speech of others is improperly understood, and the patient s own speech has virtually no meaning. Other brain areas and pathways involved in language Despite its success predicting clinical outcomes, the Wernicke-Geschwind model has significant limitations. New lesion studies, research in neuropsychology and linguistics, functional imaging (PET, fmri), and direct neural recording techniques (event-related electrical potentials, direct intraoperative recordings from human cerebral cortex) have better defined the brain areas and pathways involved in language processing. -3- It is now clear that there is a direct connection between the parieto-occipitotemporal association cortices and Broca s area. These pathways are involved in the understanding and repetition of written language (i.e., visual recognition). Thus, read words do not need to be transformed into auditory representations. Instead, Broca s area and other higher order association areas can handle this information directly. In addition, while Broca s and Wernicke s areas are important structures for

4 language, they do not operate independently for language production and comprehension to the extent hypothesized in the Wernicke-Geschwind model. The arcuate fasciculus is in fact a bidirectional pathway that connects Broca s and Wernicke s areas to many other regions of cortex in the dominant hemisphere. The prefrontal, premotor, and supplementary motor cortices in the frontal lobes are necessary for higher order aspects of speech planning and production, and for proper syntax of language comprehension and production. Some areas of insular cortex are also related to speech articulation. Wernicke s area has reciprocal connections with the supramarginal and angular gyri of the parietal lobe as well as with areas in the temporal lobe in the dominant hemisphere. These regions are important for language comprehension, and also participate in the mapping of the sounds of words to their meanings. The angular gyrus in the language dominant hemisphere contributes to the understanding of written language. Finally, several sub-cortical structures, such as the thalamus and basal ganglia of the dominant hemisphere, are also critical for language processing. The non-dominant language hemisphere is also involved in many aspects of language. Lesion studies and studies in split-brain patients reveal that the non-dominant hemisphere can understand many words and can take on responsibility for many aspects of language in children with damage to the dominant hemisphere. In normal function, the non-dominant language hemisphere participates in the emotional aspects of comprehension and speech production (e.g., inflections, tone of voice, timing) and in the pragmatics of language (e.g., contextually appropriate speech). It has been suggested that the non-dominant hemisphere codes information in a more general way, representing the overall structure of a stimulus. For instance, the right hemisphere is involved in determining if a statement is funny, which involves analyzing the content of that statement as a whole rather than the individual words. -4-

5 Aphasias and other language disorders Broca s aphasia is usually the product of damage to Broca s area and adjacent structures in the dominant frontal lobe. Damage to this area results in an impairment of language production. Patients can be completely mute or else show slow halting speech that utilizes only key words. The ability to read out loud and write is also impaired. People with Broca s aphasia are generally aware of their condition. Wernicke s aphasia results from lesions to Wernicke s area and surrounding regions of the temporal lobe. Damage to this area results in an impairment of language comprehension. Patients with severe Wernicke s aphasia do not respond appropriately to questions and follow almost no commands. Patients have fluent speech, but they may use the wrong words or make up new ones; language tends to be excessive and devoid of meaning. Patients with Wernicke s aphasia are unaware of their condition, behaving as if carrying on a normal conversation despite their markedly abnormal speech. Conduction aphasia occurs when a language pathway (usually the arcuate fasciculus) is disrupted, although damage is not necessarily limited to white matter. Patients with conduction aphasia show fluent speech, though the number of errors and word substitutions they make are higher than average. They also have difficulty repeating words and reading out loud, but can read silently. Overall, their ability to comprehend language is left intact. Anomic aphasia occurs when the posterior aspect of the left inferior temporal lobe (near the tempero-occipital border) is damaged. This is the rarest of aphasias. Patients with this form of aphasia have difficulty finding the correct words when speaking or writing. Otherwise, their comprehension, production, and repetition abilities are intact. Global aphasia is the most debilitating form of language impairment. Patients show impaired comprehension, production, and repetition. Gobal aphasia is caused by rather widespread damage of the entire perisylvian region, including Broca s area, Wernicke s area, and the arcuate fasciculus. Transcortical motor aphasia is characterized by the inability to produce creative speech. Patients attempt to initiate speech, but produce only a few words. However, they can repeat words and phrases. Comprehension is less disturbed, but writing ability is impaired. This form of aphasia is due to lesions that disconnect Broca s area from the supplementary motor -5-

6 cortex. Transcortical sensory aphasia consists of an inability to read or write, but the patient can repeat spoken language easily and fluently. Comprehension is defective and meanings of words are lost. This results from lesions in the parieto-occipitotemporal junction. Naming deficits occur after damage to the anterior temporal and inferotemporal cortices in the dominant hemisphere. Patients have trouble retrieving words, but do not have grammatical, phonemic, or speech problems. Focal lesions in these regions of temporal cortex can separately affect the recall of names for unique place or person, common names, or the names of specific types of items (e.g., tools) without affecting the recall of words for actions or relationships between things. Insular lesions result in difficulty pronouncing phonemes in their proper order. People with insular damage usually produce combinations of sounds that are not quite correct, even though they have no trouble selecting words and can recognize their mistakes. There is no difficulty comprehending speech sounds. Non-dominant language hemisphere damage disrupts the ability to interpret the emotional content of spoken language and to produce contextually appropriate speech. People with such lesions may produce speech with inappropriate intonation and may have difficulty using socially appropriate language. They may have deficits in interpreting the tone of others speech, understanding jokes, and incorporating sentences into written or spoken narratives. Lateralization of the brain: The functional lateralization of the cerebral cortex was first suggested by Geshwind and Levitsky (1968), who discovered that about 60% of human brains display anatomical differences between the two hemispheres in the posterior temporal lobe (the region which encompasses Wernicke s area). In line with these anatomical observations, language was the first function that was demonstrated to be lateralized in the human cerebral cortex. More recently, various other anatomical and functional differences have been documented between the two cerebral hemispheres. The study of split-brained subjects in the late 20th century provided a great deal of information on the specific roles of the two hemispheres. This condition results from complete lesion of the corpus collosum, known as commissurotomy. Because the callosum is severed, the two hemispheres can no longer communicate with each other and there is no transfer of auditory, visual, or sensory information from one hemisphere to the other. Commissurotomy is sometimes the only treatment available for patients with intractable epilepsy. The procedure can diminish the number and severity of the patients seizures. -6-

7 The corpus collosum is essential for melding the actions of each hemisphere into a unitary whole. Once the callosum is bisected, the hemispheres appear to act independently. The study of this phenomenon has led to some surprising findings in the field of perception and in trying to understand human consciousness. Split-brain experiments often employ a tachistoscope, an instrument that presents visual stimuli only to the patient s left or right visual field (LVF or RVF, respectively). Information about an image presented to the LVF of a split-brain subject only reaches the RIGHT cerebral hemisphere, and vice versa. When the image of an object is projected onto the RVF, the subject can easily name the object because the information is accessible to the left, dominant, language hemisphere. When the image is projected onto the LVF, the subject cannot verbally identify it and may even deny ever seeing it - the language areas of the left hemisphere are unaware of the image. However, the patient can readily identify the object nonverbally, such as pointing to it with the left hand, or using tactile cues to distinguish it from several other unseen objects. This suggests that while the right hemisphere cannot talk, it is able to perceive, learn, remember, and issue commands for motor tasks, even when the subject is not consciously aware of these processes. In another set of experiments, the split-brained patients ability to identify written words was examined. Words presented to the RVF were easily read out loud. On the other hand, the patients were unable to pronounce the word if it was presented to the LVF. Interestingly, some patients could write words projected onto the LVF, but not the RVF. This finding demonstrates that some people possess language skills in the right hemisphere, and that writing may require non-dominant hemisphere skills such as creativity or artistic ability. -7-