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Cerebral Physiology. Jonathan W. Bekenstein, M.D., Ph.D. May 26, 2008 M1 Neurosciences. Objectives:. Know basic concepts about localization of function and the limits of the concept for many higher cortical functions.
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Cerebral Physiology Jonathan W. Bekenstein, M.D., Ph.D. May 26, 2008 M1 Neurosciences
Objectives: • Know basic concepts about localization of function and the limits of the concept for many higher cortical functions. • Understand localization of language and the dominance of the left hemisphere in most humans, based on neuroanatomical and developmental observations. • Know that lesions in particular parts of cortex produce clinical syndromes that can help localize the lesion. • Recognize that neural networks are important for higher cortical function and that many brain regions may be involved for complex tasks.
How we got where we are: • Study autopsy brains from those with brain injury. • Study of cytoarchitecture (Brodmann areas). Broca’s area (44) and Wernicke’s (22) • Electrical stimulation during resective brain surgery. • Functional imaging (PET, fMRI, amobarbital testing, diffusion tensor imaging (fiber tract analysis), magnetic source imaging)
Early Work in Localization • Many believed that the brain itself had localized functions. In the 1860s, French anthropologist Paul Broca associated expressive speech impairments with left frontal lobe injury. He and others were working with autopsy brains and made clinical correlations. Wernicke distinguished receptive (related to understanding language/words) from expressive disorders (where language/words are produced or spoken/written). At about the same time, the English neurologist John Hughlings Jackson described fluent and non-fluent aphasias.
Early Doubters of Localization • It took a great deal in the 1900’s to convince many that localization of function in the brain exists. Some biological functions are localized more discretely than others. • While neuroscientists agree that localization exists, most functions require integration of neuronal activity from different locations in the cortex.
How Does Cerebral Dominance Develop? • Dominance develops gradually. In young children with brain injury in one hemisphere, or who undergo hemispherectomy for treatment of intractable epilepsy, the remaining or healthy hemisphere can develop the language, motor, and sensory functions of the lost or damaged hemisphere. • An inherent anatomic asymmetry is present in the brain. The planum temporale is larger in 65% of brains on the left. This may initially favor language localization in the left hemisphere.
Lateralization • Lateralization, in theory, may allow for more efficient “wiring” and for faster processing speed; however, there is no concrete evidence for this. • Einstein Syndrome: he didn’t speak until age 5. There is a popular theory that late language development allows areas normally used for language to develop into areas used for mathematical calculations.
Anatomical Considerations of Language: Basic features and concepts • Language is everywhere in the brain, but specialized areas are particularly prominent in the left hemisphere. More than 90% of right handers are left hemisphere language dominant and more than 70% of left handers are still left hemisphere dominant.
Language and Localization • Cortical and subcortical representation of language is separate from the circuits involved in motor control of pharynx, larynx, tongue and mouth. • Letter sequences that do not make words primarily activate visual cortex. • Tones that are not words activate auditory cortex in the superior temporal gyrus, usually on the left side of the brain. • The human brain can determine words from sounds from either visual or auditory representations, as written or spoken words always activate Wernicke’s area (Brodmann area 22).
Language is More Diffusely Located • Thus, there is prominent lateralization. • Neural networks are important. Studies indicate that multiple brain regions are involved in language tasks at the same time.
Early Definitions of Aphasias • Broca aphasia describes effortful production, loss of grammatical modifications, and relatively preserved comprehension. • Wernicke aphasia denotes fluent speech typified by circumlocution and neologisms, and is classically associated with more severe comprehension impairment. • Conduction aphasia indicates minimally disturbed comprehension and expression, but with relatively impaired repetition. Global aphasia (or severe aphasia, total aphasia) indicates profoundly impaired general language functions with minimally deficient other cognitive functions.
Conduction Aphasia • Conduction aphasia (termed because it was thought to result from impaired signal conduction between Wernicke's and Broca's areas in the left hemisphere) is a fluent aphasia typified by phonemic paraphasia and impaired repetition, but with relatively preserved speech comprehension. Conduction aphasics are aware of their deficits, and they try repeatedly to correct their speech errors through minor modifications of single words.
Hyperlalia • Hyperlalia or excessive fluency may appear following right hemisphere injury with the patient often appearing apathetic and indifferent. Alternatively, hyperlalia may result from posterior left hemisphere injury, in which case its coexistence with impaired comprehension suggests Wernicke aphasia.
Fluency: • Absent (no word production on naming tasks) is termed anomia. • Reduced • Excessive (Hyperlalia)
2 Types of Paraphasic Errors: • Incorrect responses when asked to name are Paraphasic errors. • Phonemic: a letter or two is wrong • Semantic: the wrong word is produced/spoken. • With naming objects, retrieval of word memory is involved. With repetition, acoustical analysis of words must be performed to produce identical words.
Left vs. Right Anatomical Differences • Interhemispheric anatomical asymmetries may reflect left hemisphere specialization for language. The Sylvian fissure is longer in the left hemisphere than in the right in most individuals.
Asymmetry: • Quantitative assessment of fiber tract morphology indicates that the left arcuate fasciculus is larger and has a greater degree of anisotropic molecular movement along its bundle than the right hemisphere in humans (Powell et al 2006). • This suggests that the left arcuate fasciculus is more active than the right and is consistent with left hemispheric specialization for language.
More Asymmetries • Asymmetries are less marked or reversed among individuals with right hemisphere language specialization. These findings suggest that anatomical specialization for basic language is associated with relative enlargement of contributing anatomical structures.
History of Brain Evolution:The Radiator HypothesisDean Falk, Florida State University • Humans have a vast emissary vein network that penetrates the skull and allows the brain to “vent” temperature, much like a radiator. • “The radiator hypothesis is mechanistic, i.e., it suggests that the dramatic increase in brain size that occurred in Homo was facilitated (rather than directly caused) by the release of thermal constraints that previously kept brain size in check. The radiator network of veins is thus seen as a prime releaser, not a prime mover of human brain evolution (Falk, 1992). One must therefore look elsewhere (Falk et al., 2000) for the behaviors that were selected for once the brain had acquired an adjustable radiator and could get bigger.”
Classic Model of Language • Broca's and Wernicke's 19th century clinical observations inspired a serial processing model of left hemisphere language function for most right-handers that was influential in inspiring current concepts of structural-functional relations in general neuroscience, but is beginning to be regarded largely incomplete (Damasio et al 2004).
Early Model of Language • The model posited that 2 areas are vital for language: (1) Broca’s area in the left frontal opercular region, which encodes phonology for expression, and (2) Wernicke’s area in the left posterior superior temporal gyrus, which associates heard speech with meanings. Finally, a white matter bundle that connects these areas, the arcuate fasciculus, is incorporated indicate how Wernicke’s area may control speech expression. This fiber bundle is thought to be a pathway to connect Wernicke’s and Broca’s areas.
New: Parallel Distributed • This network, or parallel distributed processing model (Absher and Benson 1993), posits that the brain has regionally specialized functions, but that particular cognitive operations emerge from reciprocal neuronal communication among diverse areas. Thus, a restricted lesion does not abolish a regionally specific function, but instead impairs it.
Cerebellar Involvement in Language • The right cerebellum is metabolically linked with the left frontal cortex on language-related tasks, as demonstrated on positron emission tomography studies of word generation and speech discrimination. • May also see cerebellar activation during SPECT scan with WADA testing.
Cerebellum • It is, therefore, not surprising that left frontal injury resulting in nonfluent aphasia is associated with contralateral cerebellar hypometabolism, and that this is less common in fluent aphasia.
Cerebellum • Lesions of the right cerebellar hemisphere lead to verbal deficits, while those of the left lead to nonverbal deficits. The generally greater impairment of those patients with a right-sided lesion was interpreted as resulting from the connection of the right cerebellum to the left cerebral hemisphere, which is dominant for language functions and crucial for right hand movements. Jansen and colleagues, using fMRI with healthy subjects, corroborated that crossed cerebral and cerebellar language dominance is a typical characteristic of brain organization (Jansen et al 2005).
Cerebellum in Multi-Lingual Humans • Both cerebellar hemispheres participate in language processing and the extent and laterality of activation differ between native and non-native languages. • A greater involvement of left cerebellar hemisphere to overall cerebellar activation in the non-native language (English) than with the native language (Spanish).
Thalamus: • Language disorder syndromes noted following thalamic damage can be categorized into 3 subtypes: (1) medial (left paramedial thalamic area, involving dorsomedial and centromedian nuclei), (2) anterior (left anterolateral nucleus), and (3) lateral (left lateral thalamus). • Thalamic nuclei and systems are involved in multiple processes that directly or indirectly support cortical language functions: lexical-semantic functions, working memory, visual processing in reading, and category-specific naming (Crosson 1999).
More Thalamus • Infarcts in the left posterolateral territory have also been associated with aphasia (Carrera et al 2004). • Patients may present with classical symptoms suggesting aphasia following thalamotomy (repetition, comprehension, fluency, and naming abnormalities.)
Another Organizational Scheme: Extrasylvian Aphasias • Preserved repetition represents the hallmark of extrasylvian aphasias. Language repetition defects in perisylvian aphasia are due to a diversity of factors (phonemic discrimination errors, verbal memory defects, and verbal apraxia). Patients with extrasylvian aphasia do not present with these defects responsible for the language repetition impairments observed in perisylvian aphasias. • Hemorrhage is the most common subcortical pathology associated with aphasia.
Transcortical Aphasias (extrasylvian) Transcortical motor aphasia: • 2 different language disorders: lack of verbal initiative associated with left prefrontal pathology (Luria dynamic aphasia) and defects in language initiation observed in cases of damage in the left supplementary motor area. • Subtype I (dynamic aphasia associated with left dorsolateral prefrontal pathology) and subtype II (supplementary motor area aphasia)
Extrasylvian Sensory Aphasia • Repetition and echolalia are the cardinal features of this aphasia . • Conversational language in extrasylvian sensory aphasia is fluent, often featuring paraphasias (especially neologistic and semantic substitutions) and emptiness in content.
Extrasylvian Sensory Aphasia (cont.) • Repetition sometimes is a true echolalia. Frequently, these patients will incorporate words and phrases uttered by the examiner into the ongoing output when apparently failing to understand the meaning of the words. Reading comprehension, on the other hand, is severely defective. • Writing is usually defective and, in general, resembles the disturbance in written language observed in Wernicke aphasia patients.
Motor and Sensory System Localization • Primary somatosensory cortex (areas 3, 2, and 1) correspond roughly to the postcentral gyrus of the parietal lobe and a small portion of the precentral gyrus. • There is point to point (somatotopic) representation of the body’s periphery in the motor and sensory cortex. This is termed the “homunculus” (a “little man”).
