Neuroscience/Objectives/Lecture 51

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Language, aphasia, and neglect

Describe the contributions of the left hemisphere and of the right hemisphere to language processing.

Both hemispheres contribute to language, but one is dominant (usually the left). The dominant hemisphere is responsible for the following:

The non-dominant hemisphere (usually right) is responsible for:

• some single-word comprehension
• prosody (ie, intonation, rhythm, etc.)
• pragmatics of speech acts
• abstract language use (eg, metaphors, humor)

Characterize the main distinction between aphasias caused by anterior lesions and aphasias caused by posterior lesions.

The main distinction between anterior (frontal) and posterior (parietal, temporal) lesions is in degree of fluency. Other distinctions include grammatism, articulation, and paraphasias (speech errors):

Explain why the terms "expressive" and "receptive" aphasia are not useful classifications of the aphasias.

Both anterior and posterior regions of the perisylvian area contribute to both language comprehension and speech generation. So both anterior and posterior lesions will result in some loss in the ability to generate speech or comprehend language. Calling one "expressive" and the other "receptive" is therefore inaccurate, because essentially all types of aphasia present with some degree of expressive or receptive deficits.

Identify the main features of Broca's aphasia and of Wernicke's aphasia.

Broca's aphasia

• Non-fluent
• Agrammatic speech
• Short, single-word phrases
• Poor articulation
• Fairly good comprehension (not perfect)
• Lesion must extend deep to include periventricular white matter and possibly inferior motor strip

Wernicke's aphasia

• Fluent (sometimes at an excessive rate)
• Normal articulation
• Literal and semantic paraphasias common (but neologisms, jargon, and semantic paraphasias possible as well)
• Paragrammatism
• Poor comprehension
• Lesion always includes the entire posterior superior temporal gyrus (BA 22), often extending into the middle temporal gyrus and nearby parietal regions

List the three primary dimensions of language that are used to distinguish the eight major categories of aphasia.

Fluency

Discussed below.

Repetition

Poor repitition in patients who speak and comprehend well suggests phonological processing deficit. Phrases with little semantic content (eg, no ifs, ands, or buts) are often used to test abilitity to repeat. Patients may have difficulty with phonologically difficult phrases, long sentences, and numbers. Relative repetition deficit helps distinguish between aphasic patients.

Comprehension

Present to some extent in all aphasic patients, suggesting that both anterior and posterior perisylvian regions are involved with comprehension. Tests include sentences that are difficult to comprehend (eg, The lion killed the tiger. Which one died?). Relative comprehension deficit helps distinguish between aphasic patients.

Define fluent and non-fluent speech.

Fluent speech

Associated with posterior lesions, fluent speech is characterized by empty speech (ie, using few nouns and verbs), normal or greater than normal output, normal phrase length, and normal articulation.

Non-fluent speech

Non-fluent speech is associated with anterior lesions and is characterized by the use of few syntactic words (ie, using mostly nouns and verbs), limited output, and short phrases.

Locate the following and identify their role in the organization of language in the brain.

Wernicke's area

Located on the posterior aspect of the left superior temporal gyrus (BA 22), Wernicke's area processes auditory patterns of words. It is activated when a spoken word is recognized and when an intended word is retrieved for speech.

Broca's area

Located on the posterior aspect of the left inferior frontal gyrus (BA 44,45), Broca's area converts auditory patterns into motor commands. It is also important for processing syntax.

Arcuate fasciculus

The arcuate fasciculus is a fiber bundle connecting Wernicke's and Broca's areas. It is important for relaying information from Wernicke's to Broca's area. Damage to the arcuate fasciculus is associated with conduction aphasia, which is characterized by a difficulty in speaking.

Supplementary motor area

The rostral portion of BA 6 contains the supplementary motor area, which is important for the initiation of spoken language.

Explain the phenomenon of unilateral tactile anomia that may be seen in split-brain patients.

Unilateral tactile anomia is the inability to identify an object held in the left hand. It results from severing of the corpus callosum, preventing the transfer of somesthetic information between the cerebral hemispheres. The neuroanatomical explanation for the condition depends on the fact that the left hand is represented in the right hemisphere, while language is generated in the left hemisphere. In split-brain patients, the tactile information about an object held in the left hand reaches the right hemisphere, and the patient even has some knowledge of the name of the object, but because this information cannot cross to the left hemisphere, the patient is unable to generate the word for that object. Once the object is placed in the right hand, the information reaches the left hemisphere and is readily accessible to areas of the left hemisphere needed for speech generation.

Describe some methods of studying language organization in the brain.

Normal studies

This is an older mechanism of study using tachistoscopes that present a brief pulse of light in a specific region of the visual field. The patient is then asked to identify the location of the light pulse on a piece of paper that contains a map of the tachistoscopic presentation. When a split-brain patient is presented a pulse of light in the left visual field, they can only identify it on the map using their left hand. The underlying reason is that the left visual field is received by the right hemisphere (and vice versa) and that the hemispheres control the contralateral limbs.

Wada studies

In a Wada test, ambobarbital is injected into the carotid artery and essentially knocks out one hemisphere. This is useful for establishing cerebral language dominance.

Electrical stimulation

Particular areas of the brain may be electrically stimulated to elicit particular speech behaviors. For example, stimulation of Broca's area may elicit particular patterns of movement that resemble speech.

Aphasics

Aphasics present with particular lesions that allow localization of language areas. For example, anterior and posterior lesions show characteristic aphasic symptoms. Aphasic studies are used to further localize language within a hemisphere.

PET and fMRI

Can be used to localize which regions of the brain are most active during language comprehension, generation, etc.

Split-brain studies

Discussed earlier.

Define the neurobehavioral phenomenon of neglect.

Unilateral damage near the temporo-parietal junction is associated with contralateral hemineglect which is characterized by inability to consciously perceive the contralateral visual field (also auditory, spatial, etc.). Hemineglect patients shown a clock will consistently draw only one side of the clock because their consciousness of the other side is essentially absent.

List and briefly describe three types of neglect.

• Unilateral visual heglect
• Unilateral neglect of representational space
• Extinction to double simultaneous stimulation