Chapter 16.pdf

Chapter 16: Lateralization, Language, and the Split Brain EVOLUTIONARY PERSPECTIVE OF CEREBRAL LATERALIZATION AND LANGUAGE • Theories of the Evolution of Cerebral Lateralization • Most are based on the same general premise: that it is advantageous for areas of the brain that perform similar functions to be located in the same hemisphere • Analytic-Synthetic Theory • The analytic-synthetic theory of cerebral asymmetry holds that there are two basic modes of thinking, an analytic mode and a synthetic mode, which have become segregated during the course of evolution in the left and right hemispheres respectively • Its vagueness is a problem • It is not possible to specify the degree to which any task requires either analytic or synthetic processing, it has been difficult to subject the theory to empirical tests • Motor Theory • The motor theory of cerebral asymmetry, the left hemisphere is specialized not for the control of speech specifically but for the control of fine movements, of which speech is only one category • One shortcoming is that it does not suggest why motor function became lateralized in the first place • Linguistic Theory • The linguistic theory of cerebral asymmetry, posits that the primary role of the left hemisphere is language; this is in contrast to the analytic-synthetic and motor theories, which view language as a secondary specialization residing in the left hemisphere because of that hemisphere’s primary specialization for analytic thought and skilled motor activity, respectively • When Did Cerebral Lateralization Evolve? • There is evidence of lateralization of function in many vertebrates that evolved long before we humans did • Right-handedness may have evolved from a preference for the use of the right side of the body for feeding • A left hemisphere specialization for communication is also present in species that existed prior to human evolution • What Are the Survival Advantages of Cerebral Lateralization? • Cerebral lateralization must have survival advantages • Two fundamental advantages: • First, it may be more efficient for the neurons performing a particular function to be concentrated in one hemisphere • Second, two different kind of cognitive processes may be more readily performed simultaneously if they are lateralized to different hemispheres • Once the control of some abilities becomes lateralized, this may make the lateralization of more abilities advantageous • Evolution of Human Language • Human language is a system allowing a virtually limitless number of ideas to be expressed by combining a finite set of elements Chapter 16: Lateralization, Language, and the Split Brain • Once the ability to discriminate particular speech sounds is lost, it is difficult to regain • Vocal Communication in Nonhuman Primates • Each nonhuman primate species has a variety of calls, each with a specific meaning that is understood by conspecifics • Moreover, the calls are not simply reflexive reactions to particular situations: They are dependent on the social context • There is typically a substantial difference between vocal production and auditory comprehension • Even the most vocal nonhumans can produce a relatively few calls, yet they are capable of interpreting a wide range of other sounds in the environments • This suggests that the ability of nonhumans to produce vocal language may be limited, not by their inability to interpret sounds, but by their inability to interpret sounds, but by their inability to exert fine motor control over their voices • Motor Theory of Speech Perception • The motor theory of speech perception proposes that the perception of speech depends on the words activating the same neural circuits in the motor system that would be activated if the listener said the words • The main thesis of the motor theory of speech perception is that motor cortex plays a role in language comprehension • Gestural Language • Language in nonhuman primates might be mainly gestural, rather than vocal • Chimpanzees’ gestures were much more like human language than were their vocalizations CORTICAL LOCALIZATION OF LANGUAGE: WERNICKE-GESCHWIND MODEL • Language lateralization refers to the location within the hemisphere of the circuits tat participate in language-related activites • Historical Antecedents of the Wernicke-Geschwind Model • Broca hypothesized that programs of articulation are stored within this area and that speech is produced when these programs activate the adjacent area of the precentral gyrus • According to Broca, damage restricted to Broca’s area should disrupt speech production without producing deficits in language comprehension • There is language area in the left temporal lobe just posterior to the primary auditory cortex • This second language area, which Wernicke argues was the cortical area of language comprehension, subsequently became known as Wernicke’s area • Wernicke suggested that selective lesions of Broca’s area produce a sundrome of aphasia whose symptoms are primarily expressive - characterized by normal comprehension of both written and spoken language and by speech that retains its meaningfulness despite being slow, labored, disjointed, and poorly articulated • This hypothetical form of aphasia became known as Borca’s aphasia • Wernicke suggested that selective lesions of Wernicke’s area produce a syndrome of aphasia whose deficits are primarily receptive - characterized by poor comprehension of both written and spoken language and speech that is Chapter 16: Lateralization, Language, and the Split Brain meaningless but still retains the superficial structure, rhythm, and intonations of normal speech • This hypothetical form of aphasia became known as Wernicke’s aphasia • Wernicke reasoned that damage to the pathway connecting Borca’s and Wernicke’s areas - the arcuate fasciculus would produce a third type of aphasia, one he called conduction aphasia • He contended that cmprehension and spontaneous speech would be largely intact in patients with damage t the arcuate fasciculus but that they would have difficulty repeating words the have just heard • The left angular gyrus is another cortical area that has been implicated in language • Alexia - the inability to read • Agraphia - the inability to write • The left angular gyrus is responsible for comprehending language-related visual input, which is received directly form the adjacent left visual cortex and indirectly from the right visual cortex via the corpus callosum • The Wernicke-Geschwind Model • Wernick-Geschwind model: primary visual cortex, anguar gyrus, primary auditory cortex, Wernickes area, arcuate fasciculus, Borca’s area and primary motor cortex - all of which are in the left hemisphere • When you are having a conversation • primary motor cortex --> wernickes area --> to reply --> left arcuate fasiculus --> broca’s area --> primary motor cortex • When reading aloud • primary visual cortex --> left angular gyrus --> wernickes area --> arcuate fasiculus --> brocas area --> motor cortex WERNICKE-GESCHWIND MODEL: THE EVIDENCE • Effects of Cortical Damage on Language Abilities • Study of patients whom discrete areas of cortex have been surgically removed has been particularly informative about the cortical localization of language, becuase the location and extent o these patients lesions can be derived wit reasonable acuracy from the surgeons reports • The study of neurosurgical patients has not confirmed the predictions of the Wernicke-Geschwind model • Patients with excisions of Broca’s area often regained conscousness with their language abilities fully intact only to have serious language-related problems develop over the next few hours which subsided over the following weeks • Permanent speech difficulties were not produced by discrete surgical lesions to the arcuate fasciculus, and permanent alexia and agraphia were not produced by surgical lesions restricted to the cortex of the angular gyrus • In some cases a good portion of Wernickes area has been removed without lasting language-related deficits • Larger lesions did produce more lasting language deficits but in contrast to the predictions of the Wernicke-Geschwind model, problems of articulation were just as likely to occur following parietal or temporal lesions as they were following comparable lesions in the vicinity of Broca’s area Chapter 16: Lateralization, Language, and the Split Brain • Major findings: • No aphasic patients have damage restricted to Broca’s area or Wernickes area • Aphasic patients almost always have significant damage to subcortical white matter • Large anterior lesions are more likely to produce expressive symptoms whereas large posterior lesions are more likely to produce receptive symptoms • Global aphasia is usually related to massive lesions of anterior cortex, posterior cortex, and underlying whit