Behavioral Neurology
|
The left hemisphere’s predominance in language processing is complemented by the right hemisphere’s specialization for spatial attention. Studies with split-brain patients have shown right hemisphere superiority for various tasks involving such components as part-whole relations, spatial relationships, apparent motion detection, and mirror image discrimination, among others. Both hemispheres are equally able to perform many visual tasks provided that they lack a spatial component.
Each disconnected hemisphere may have a separate sense of self. The most intriguing example of self-referential hemispheres is the left hemisphere promptness in detecting a partial self-image, even one that is only slightly reminiscent of the self, whereas the right brain virtually needs a complete picture of the self before it recognizes the image as such. That is to say, the left hemisphere has a linear relationship between the amount of self in a portrait and the probability of detecting self. Conversely, the right hemisphere does not recognize any images as self until they contain more than 80% self. The next pages in this chapter will suit the bulk of our left-brain readership, whose left-brain interpreting powers will make the best of the partial information available and make a judgment call on the referencing value of the whole. If the left hemisphere requires less self in an image for self-recognition, it might require fewer pages to grasp and retrieve the body of knowledge that is to follow.
Behavioral Neuroanatomy
| |||||||||||||||||||||
Broca aphasia, from a lesion in the left Brodmann area (BA) 44 and 45, leads to nonfluent speech, agrammatism, paraphasias, anomia, and poor repetition. Lesions anterior, superior, and deep to, but sparing, Broca area (anterior cerebral artery [ACA]-middle cerebral artery [MCA] watershed) produce abnormal syntax and grammar but preserved repetition and automatic language, as well as uninhibited echolalia, a disorder known as transcortical motor aphasia. Pseudopsychopathic disorder is the impairment of social conduct from defects in social inhibition, judgment, and emotional reactions caused by orbitofrontal lesions. Memory disturbances only develop with lesion extension into the septal nucleus of the basal forebrain. Appreciation of verbal humor is most impaired in right frontal polar pathology. Lesions in the superior medial prefrontal region, particularly the cingulate cortex, are associated with lack of initiative, which ranges from apathy and abulia to akinetic mutism.
| ||||||||||||||||
The hippocampal complex is responsible for the acquisition of new verbal (left) and nonverbal (right) knowledge. The amygdala registers emotional information (in facial expressions, for instance) to be used in appropriate social settings. KBS is recognized by hyperorality, hypersexuality, visual agnosia, hypermetamorphosis, and memory deficits in patients with herpes encephalitis, frontotemporal lobar degeneration (FTLD), Alzheimer disease (AD), temporal lobe epilepsy, and head trauma. Retrograde memory localizes to nonmesial, nonhippocampal, and anterior and lateral temporal lobe regions. Prosopagnosia stems from bilateral occipitotemporal (fusiform gyri) damage but can reflect isolated right inferomedial (lingual gyrus) temporal lobe injury.
| ||||||||||||||||||||||
Bálint syndrome, from biparieto-occipital lesions corresponding to the PCA MCA-PCA territories, is recognized by the striking combination of simultanagnosia, ocular apraxia (psychic paralysis of gaze), and optic ataxia (disturbance of visually guided reaching behavior). Simultanagnosia alone occurs when occipital cortices are affected without parietal extension. Astereopsis (lack of visual depth perception) occurs only in the setting of full Balint syndrome. Achromatopsia (black-and-white world) results from bilateral damage to the hue discrimination area, located in the junction of the lingual with the fusiform gyrus. Unilaterally damaged patients (hemiachromatopsia) are usually unaware of the deficit. In optic aphasia, qualitatively similar to associative agnosia, the lack of visual naming is associated with preserved recognition in which the language centers are disconnected from the visual recognition centers due to a lesion involving both the left optic pathways and the splenium.
| ||||||||||||||||||
Wernicke aphasia results from a lesion in BA 22 (posterior part of left superior temporal gyrus), 40, and 39 (supramarginal and angular gyrus, respectively), known as the greater Wernicke area, and is characterized by fluent, paraphasic speech, impaired repetition, and defective aural comprehension. Lesions anterior, superior, and deep to, but sparing Wernicke area (PCA-MCA watershed), produce a transcortical sensory aphasia, a Wernicke-like aphasia with preserved repetition. Bilateral area 22 lesions lead to the syndrome of auditory agnosia or inability to identify the meaning of verbal and nonverbal auditory signals, including spoken words and familiar environmental sounds such as telephone ringing or door knocking. The left inferior parietal lobe stores the movement representations memories and its damage will lead to ideomotor apraxia. These patients are likely to have conduction aphasia, characterized by a disproportionate repetition defect in otherwise mildly paraphasic speech. Neglect may present as anosognosia, which in turn can lead to excessive lack of concern and minimization of personal deficits, known as anosodiaphoria.
Vascular Syndromes Leading to Classical Behavioral Complications
|
Wernicke aphasia. This 67-year-old right-handed man with history of atrial fibrillation and remote right parietal ischemic stroke developed acute onset of “nonsensical speech.” Of note, treatment with rivaroxaban had been held 5 days prior to elective surgery for left lower limb ischemia.
|
Gerstmann syndrome. This 54-year-old right-handed man with history of obstructive sleep apnea and patent foramen ovale developed sudden inability to recall his password to log into his computer and operate a TV remote control. Neurological examination showed deficits on tests of working memory, processing speed, and executive functioning, as well as finger agnosia, right-left disorientation, dyscalculia even for simple mathematical equations. MoCA score was 25/30.
Amnesia
Amnestic syndrome applies to a state in which memory and learning are affected out of proportion to other cognitive functions in an otherwise-alert and responsive patient.
Korsakoff Syndrome
Korsakoff syndrome (KS) is a persistent amnestic syndrome of nutritional origin whereby profound anterograde memory loss affects the formation of new memories. In addition, impairment of retrograde memory, particularly source memory (where and when was the memory formed), leads to confabulation. Thiamine depletion underlies the onset of KS and the often-preceding acute neuropsychiatric disorder, Wernicke encephalopathy (confusion, ataxia, nystagmus, and ophthalmoplegia). The characteristic neuropathology consists of neuronal loss, microhemorrhages, and gliosis in the paraventricular and periaqueductal gray matter as well as the mammillary bodies. Persistent memory impairment is associated with involvement of the anterior (rather than the dorsomedial) nucleus of the thalamus.
Herpes Simplex Encephalitis
The encephalitic damage to the medial temporal lobe, including the hippocampi, amygdalae, and the entorhinal and parahippocampal cortices, produces a retrograde memory loss with a “flatter” temporal gradient (i.e., less sparing of early memories). Impairments in semantic memory, naming, and reading (surface dyslexia) result from the involvement of the left inferolateral temporal lobe.
Hypoxia
The classic anterograde amnesia of anoxic encephalopathy appears to result from severe loss of pyramidal cells in the CA1 region of the hippocampi with additional damage to the cerebellum and basal ganglia.
Vascular Disorders
Anterograde amnesia with minimal retrograde amnesia results from anterior thalamic infarction, variably supplied by the polar or paramedian arteries, which are, ultimately, branches of the PCA. The anterior and posterior choroidal arteries, branches of the internal carotid artery (ICA) and PCA, respectively, supply the hippocampi. Unilateral damage leads to material-specific memory loss.
Transient Global Amnesia
Most commonly occurring in middle-aged or elderly individuals, transient global amnesia (TGA) consists of a sudden temporary period of acute amnesia and repetitive questioning—without personal identity loss—lasting up to 24 hours, preceded by headache, stressful events, medical procedures, or vigorous exercise. Anterograde amnesia is profound and retrograde amnesia is relatively brief and restricted to recall of episodes instead of facts. TGA is presumed to result from transient dysfunction in limbic-hippocampal circuits presumably due to ischemia, migraine, or venous congestion. When attacks are brief (<1 hour) and multiple, epilepsy may be the underlying cause (transient epileptic amnesia). Interictal memory deficits may be present.
Memory Primer
Anterograde memory depends on the abilities to encode, consolidate, and retrieve new information. Encoding difficulties are thought to be responsible in part for the memory deficits associated with attention or perceptual dysfunction. Encoding is assessed by tasks of immediate recall. The hippocampus is considered the key structure for memory consolidation. Deficits of memory consolidation are characterized by impaired recall with no benefit from cueing or recognition. Disproportionate deficits in recall compared with recognition indicate retrieval deficits and are observed in patients with frontal lobe and subcortical lesions:
Memory recall (based on contextual features of time and spatial location) localizes to the Papez circuit: hippocampi, fornices, mammillary bodies, mammillothalamic tract, and anterior thalamic nuclei.
Recognition memory (based on familiarity judgments) depends on the integrity of the temporal (entorhinal and perirhinal cortices) and frontal lobes.
Retrograde memory can be dissociated (differential episodic and semantic impairment) and exhibits a temporal gradient (remote memories preserved better than recent ones), as demonstrated in patients with KS, TGA, and anoxia.
Working memory is used in the temporary and flexible manipulation of information for such cognitive tasks as reasoning and arithmetic, updated and retrieved within a few seconds but of limited storage capacity and rapidly decayed if not rehearsed. It is also considered an executive function. (Testing: serial sevens, digit span backward, Corsi Blocks backward).
Declarative memory: information is stored on a permanent or semipermanent basis in medial temporal lobe structures (severely affected in KS and herpes simplex encephalitis [HSE]) and is relatively impervious to the effects of decay or interference.
Episodic or explicit (long-term) memory refers to autobiographical that become part of a conscious awareness, processed in the hippocampus. (Testing: logical paragraph recall, California verbal learning test, Benton delayed recall)
Semantic memory refers to the storage of facts, concepts, and language, acquired over time across without a temporal or spatial context in the acquisition of the memory (e.g., the capital of Argentina) and supported by the entorhinal and perirhinal cortices in the anterior temporal lobe (testing: category fluency task, Pyramids and Palm Trees test, famous faces test).
Implicit memory refers to learning of skills that are expressed in the course of task performance without requiring awareness of the basis of this learning. It is relatively preserved in amnesic patients but impaired in certain subcortical dementias such as Parkinson disease (PD) and Huntington disease. It includes
Classical conditioning or repetition priming refers to the facilitation in performance which results from previous exposure to task stimuli.
Repeated practice or procedural memory, motor and perceptual skill learning acquired with practice. It has been localized to the motor and prefrontal cortices, basal ganglia, and cerebellum (testing: pursuit rotor learning, reverse-mirror tracking).
Agnosias
Agnosia is a recognition failure specific to one sensory channel that cannot be attributed to a primary sensory defect, a generalized cognitive deficit, a language disorder, or a lack of prior knowledge of the stimulus.
Visual agnosias are a group of visual recognition disorders without primary visual defects where tactile and auditory stimuli are correctly identified.
Apperceptive visual agnosia refers to the failure to achieve a structured description of the shape of the object, face, or color. The lesion is located in the right ventral occipital region. Denial of blindness and visual confabulation (Anton syndrome) are common after bilateral PCA infarcts.
Associative visual agnosia is the inability to attribute meaning to a correctly perceived object (object-associative amnesia; good copies of it can be made), face (prosopagnosia), or color (color agnosia). Object and color agnosias arise from damage to the left occipitotemporal junction, whereas prosopagnosia to the bilateral occipitotemporal junction or right medial occipitotemporal regions.
The callosal syndrome of hemisphere disconnection consists of left-hand tactile agnosia, ideomotor transitive apraxia, and agraphia with constructional apraxia of right hand and intermanual/intentional conflict (aka, diagnostic dyspraxia: conflict between the desired action and the actually performed act).
Tactile agnosia is a disorder of tactile object recognition by their texture, size, and shape, also called pure astereognosis, that occurs in the absence of basic sensorimotor impairment (it could be named stereoanesthesia) or other cognitive dysfunction. Bilateral involvement results from unilateral damage to either parietal operculum (corresponding to the ventrolateral somatosensory association cortex [S2] since lesions in the primary somatosensory cortex [S1] produce impairment in basic and intermediate sensory modalities.
Lesions to the sensory cortex or its thalamocortical projections lead to cortical sensory loss, which is translated into lack of sensory discrimination: loss of position sense (discrepant with the relative preservation of touch, pain, temperature, and vibration sensation), astereognosis, agraphesthesia, and poor two-point discrimination.
Auditory agnosia is the failure to recognize verbal (pure word deafness) or nonverbal (environmental sound agnosia: dog barking or keys jingling) sounds when audiometry is normal. They occur when patients with cortical deafness from bilateral Heschl gyri lesions (which is, by definition, not agnosia since hearing itself is ultimately affected) begin to recover. Patients recovering from Wernicke aphasia may also go through a state of pure word deafness.
Apraxias
Limb apraxia is the inability to perform previously learned or skilled movements in the absence of weakness, deafferentation, or elemental movement disorders. This term may be a misnomer in “apraxia of eyelid opening” (probably focal eyelid dystonia) and “gait apraxia” (locomotion is a repetitive motor pattern that is not consciously learned). Apraxia of speech, due to insular lesions, is distinguished from conduction aphasia, due to mid-posterior sylvian lesions (angular and supramarginal gyri), by more self-monitored articulatory errors, making speech more halting, effortful, and less prosodic.
Limb-Kinetic Apraxia
These patients are unable to make fine, precise finger movements of the hand contralateral to a frontoparietal cortical lesion. The thesis that limb-kinetic apraxia is a disorder between paresis and apraxia is supported by the observation that monkeys with corticospinal tract lesions may have clumsiness without weakness.
Ideomotor Apraxia (Patient Knows What to Do But Not How to Do It)
Patients perform movements with spatial (positioning, orientation of the limb) or temporal (wrist rotation prior to arm extension when “using” a key) errors. The body-part-as-tool error is the most common. It improves on imitation and with use of the actual tool. Subjects may not be able to recognize the purpose of certain gestures. Transitive gestures (demonstrating tool/object use) are typically more impaired than intransitive gestures (not involving a tool/object), especially in AD. Unilateral ideomotor apraxia is an early sign in corticobasal syndrome. Ideomotor apraxia is primarily caused by left inferior parietal lobe lesions, which often produce bilateral but asymmetric deficits. Lesions to the corpus callosum can lead to an ideomotor apraxia known as disconnection subtype.
|
Ideational or Conceptual Apraxia (Patient Does Not Know What to Do)
Patients fail to use tools and objects correctly due to content errors or impairment in the ideation of tool use (e.g., using a razor as a comb) despite knowing what tools are being used (absent agnosia). Ideational apraxia represents a disruption of the praxis conceptual system (poor conceptual knowledge of tool use) rather than the praxis production system, as in ideomotor apraxia. Bilateral parieto-occipital damage is the underlying pathology, often associated with AD.
Constructional apraxia (drawing disturbance) is tested by drawing intersecting pentagons and is significantly worse in patients with dementia with Lewy bodies (DLBs).
Aphasias
Perisylvian language zones are at the mercy of MCA perfusion. Lesions within the MCA territory produce nonrepetitive aphasias (i.e., repetition is impaired):
Broca aphasia (inferior-posterior left frontal cortex, BA 44 and 45) consists of impaired articulation (“apraxia of speech”) and simplification of sentence structure (agrammatism).
Wernicke aphasia (superior-posterior left temporal cortex, BA 22) produces a fluent and well structured but meaningless or paraphasic speech and writing. Patients are typically unaware of the errors.
Conduction aphasia (arcuate fasciculus, supramarginal gyrus with surrounding cortex, primary auditory cortex) consists of isolated repetition defect with phonemic paraphasias (e.g., “splant,” “plant,” “plants,” for pants).
Lesions outside the perisylvian region are called transcortical aphasias, occur in the watershed of vascular territories, and produce repetitive aphasias:
Transcortical sensory aphasia (damage to parietal convexity)
Transcortical motor aphasia (damage to cingulate gyrus and supplementary motor area)
Mixed transcortical aphasia is comparable to global aphasia but with preserved repetition. Patients are echolalic
| ||||||||||||||||||||||||||||||||||||||||
Related posts:
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
