1 - Aphasia, agnosia and apraxias PDF

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This is a summary of a topic that I had to revise for my neuroscience exam....

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Aphasia, agnosia and apraxias 2) Agnosia Agnosia = higher level sensory deficits Two main types of visual agnosia: a) Associative agnosia b) Apperceptive agnosia In associative agnosia a patient cannot recognize objects, name them or use objects appropriately but can identify objects by selecting the correct drawing and can draw the object accurately. In contrast, in apperceptive agnosia a patient can name an object if the appropriate perceptual cues are present but cannot draw the object. They are also impaired at ‘naming’ the object if presented in an unusual view. Object recognition deficits can be category specific. McCarthy and Warrington described patients that could recognize inanimate objects but not living objects. It is now clear that impairments in recognition memory are associated with damage to rhinal cortex (perirhinal and entorhinal). (Fig 12) within the medial temporal lobe. Therefore, the projections from inferotemporal cortex (TEO/TE) into the rhinal cortex (Fig 13) are crucial for visual recognition memory Fig 12: Mean score on the DNMS performance test (ie the three longer delays and three list lengths). Chance is 50 percent correct. Abbreviations: N, unoperated control monkeys; Rh, monkeys with rhinal cortex lesions; A + H (+ Rh), monkeys with aspiration lesions of the amygdala, hippocampus and parahippocampal gyrus that included much of the ventrally subjacent rhinal cortex; A + H (IBO), monkeys with excitotoxic lesions of the amygdala and hippocampus intended to spare the ventrally subjacent rhinal cortex. Insert diagram of fig 12 graph Insert diagram adjacent to fig 13 from handout - The rhinal cortex may be part of a larger system within the temporal lobes important for storing knowledge about objects that may be analogous to a semantic knowledge or specialized knowledge system in humans. Note possible relevance to associative agnosias

3) Aphasias Aphasia = Disorders of language, very often in the absence of additional cognitive impairment or the inability to move the muscles used in speech Broca, in 1861, that established a region within the frontal lobes as a key region essential for language production, based upon the study of a patient who was almost entirely unable to speak. Broca also (hemispheric specialisation) Broca’s aphasia: patients speech laboured and slow, articulation impaired. However, often such patients can communicate as their selection of words, especially nouns, is often correct even though they are difficult to understand. Verbs, as well as grammatical conjunctions are less well selected and may be missing altogether. They are also impaired at repeating complex sentences spoken to them, BUT, these patients appear to comprehend the words and sentences they hear. In severest form patient can utter almost no words Damage includes: Broca’s area (Brodmann’s areas 44/45), see Fig 3, surrounding frontal fields, underlying white matter, insula and basal ganglia. Damage restricted to Broca’s area produces a far less severe and transient aphasia. Wernicke’s aphasia: Unlike Broca’s aphasia, the speech of patients with Wernicke aphasia is effortless, melodic and produced at a normal rate. The content however is unintelligible because of frequent ‘wrong’ words and phonemes (individual or combined letter sounds, ‘a’, ‘ch’ etc). These patients also have difficulty comprehending sentences and this aphasia was originally thought to be a deficit in auditory comprehension. Damage includes: posterior sector of the left auditory association cortex (brodmann’s area 22), see Fig 3. in severe, persistent cases there is also damage to middle temporal gyrus and underlying white matter. Deficit in understanding sentences with complex grammar is not simply a deficit in ‘grammar’ as some grammatical error types can be detected by Broca’s aphasic’s. Particular problem in using grammar when elements to be linked are in different parts of the sentence thereby requiring short term memory

Wernicke’s original model (Fig 3): proposed that the two different aspects of language processing were associated by fibre tracts – arcuate fasciculus. CAN REPEAT SENTENCES AND MAKE GRAMMATICAL CORRECTIONS TO SENTENCES THEY DONT UNDERSTAND Broca’s area needed for articulatory speech ‘memories’ and aphasias arise from damage to this area resulting in problems with production of speech. Wernicke’s area needed for auditory speech ‘memories’ and damage to this area results in comprehension problems causing empty error filled production. Fibre tracts (arcuate fasciculus) link these areas together - damage to this results in conduction aphasias. Wernicke’s model proposed to explain this (page 3) Shown by split brain patients Lichtheim’s model Extended by Wernicke-Geschwind model Problems with Wernicke-Geschwind model (very briefly outline this)

Apraxias (lecture 10) Apraxias: These are motor disorders in which there is a difficulty in performing purposeful or voluntary movements. 4 major types of apraxia: limb, oral, agraphic and constructional. Limb: problems with arm, hand and finger movements; oral: problems with programming movements of the tongue, lips and throat to produce sequences of speech, agraphic: particular type of writing deficit (not discussed in detail in these lectures), constructional; inability to copy mental or visual pictures Also impairments in learning new sequences of actions (Kimura box test) associated with damage to posterior parietal cortex and frontal premotor areas as well as the connections between them Constructional apraxia: right parietal damage, limb apraxia: bilateral damage to parietal or premotor cortex Deficits in execution AND recognition of movements associated with parietal damage whilst deficits in execution but NOT recognition of movements, associated with premotor damage Posterior parietal cortex: areas 5 and 7. Rostral part mainly concerned with integration of somatosensory and proprioceptive information relating the relative position of body segments to their movement, whilst the posterior part is dominated by the integration of visual information about events located in the external environment. One of the main functions of the posterior parietal cortex with respect to the control of action (as concluded by Mountcastle) is in the control of reaching into extra-personal space, ie. it mediates between spatial perception and the direction of action.

Also involved are the LPM and SMA as they also connect with posterior parietal cortex : Lateral premotor cortex (PM) and supplementary motor area (SMA): Both regions have connections with motor cortex and posterior parietal cortex. In addition, lateral premotor cortex is connected with the cerebellum (more so than with basal ganglia) whilst SMA is connected with the basal ganglia (more so than with cerebellum). They both have some outputs to the subcortical motor systems as well as to the cortico-spinal tract via primary motor cortex, area 4. Both regions are thought to participate in the programming of actions ie. sequencing the temporal order of movements, selecting the appropriate action when there are alternatives, and optimising the conditions for their performance, eg. postural adjustments and inhibiting irrelevant movements. The two areas differ however in their specialization. SMA is particularly important for bimanual coordination and contributes to movement when there are no external cues (internal generation of action), whilst lateral premotor cortex makes the greater contribution when a subject has to use external cues to direct action. Lateral premotor cortex, ventral regions, contain mirror neurons that are active not only when monkey makes a movement but also when same movement produced by others. First reported by Rizzolatti. Mirror neurons have since been found in inferior parietal cortex too. Proposed to contribute to an animal’s ability to determine the intention of others. Importance shown through ablation of SMA — deficit in bimanual co-ordination, (ii). failure to orient hands and fingers accurately as they approach food, (iii). failure to raise hand (in the absence of external cues) in order to get peanut reward; also shows alien hand syndrome, SMA is far more activated when subjects are performing a learned sequence of finger movements rather than when relying on external cues to signal a novel sequence of finger movements. See Old>New condition in Figure 3. Ablation of PM — deficit in performing hand actions based on external cues, PET scans show greater activation in PM when subjects are relying on external cues to determine a sequence of finger movements compared to when they are performing a sequence of finger movements from memory. See New>Old condition in Figure 3 Also involved is prefrontal cortex for planning and executive control, which interacts with rest of brain: (i). the specialized processing modules in the posterior cortex including parietal (spatial attention) and inferotemporal (feature attention) areas, (ii). the declarative memory systems in the temporal lobes including the rhinal cortex (recognition memory) and hippocampus (scene/episodic memory) (iii). the limbic structures involved in emotional processing including the amygdala and hypothalamus (iv). the basal ganglia that are involved in the higher-order control of action. Prefrontal cortex regultes working memory — Lesions of various regions of pfc impair delayed response tasks that require monkeys to remember, over a brief delay, spatial, object or proprioceptive information. Sample stage during which monkey is shown peanut being hidden in one of 2 locations, then shown one of two objects, then required to press lever 1/5 times

Delay of few seconds —> choice stage: monkeys choose spatial location of food they have just seen hiden, choose object they've just seen or make same response they've just made (1/5 presses of lever) . PFC lesions dont impair monkeys ability to choose the coret response if theres no delay - Similar findings in spatial delayed response in humans with damage to dorsal pfc — These findings have led Goldman-Rakic to suggest that the pfc is important for holding information, (that is relevant to action), ‘on-line’ or ‘in mind’. Conclusion :)...