Neuropsychiatry and Behavioral Neurology

What are the different stages of fact-based memory storage, such as recalling a list of words?

Clinically, memory can be conceptualized in a three-stage model. Registration (sensory) memory holds large amounts of incoming information briefly in sensory stores, such as the visual or auditory cortex. This information is then transferred to “working memory,” also called short-term memory, in the dorsolateral prefrontal cortex, which holds memory from seconds until we change our “train of thought.” Working memory functions by holding information, internalizing it, and using it to guide behavior. It can be tested with instruments such as the digit span. It remains intact in pure amnesia, such as early Alzheimer’s disease (AD). Short-term memory is then converted to long-term memory, in the process of “consolidation,” which sends memory to the cortex for long-term storage.

What is declarative memory and how does it differ from nondeclarative memory?

Declarative or explicit memories are facts and events that are available to consciousness. They require awareness, allow conscious recollection, utilize the hippocampal system, and are the type of memory damaged in amnesia. Declarative memory can be either semantic or episodic.

Semantic memory refers to fund of knowledge information, language usage, and practical knowledge, whereas episodic memory refers to memories that are localizable in time and space. Autobiographical memories stored within the past few hours to few months are clinically termed as recent memory, whereas older memories dating from early childhood are termed as remote memory. Interestingly, source (contextual) memory is the knowledge of where and when something was learned and is a subtype of declarative memory. Prospective (capacity for remembering to remember) memory and future episodic memory (creation of scenarios requiring drawing upon past experiences to guide what might happen in the future) are other subtypes of declarative memory.

Nondeclarative or implicit memories are passively acquired. They do not require the hippocampal circuitry and are not consciously accessible. One is unaware of them, they are inflexible, and they remain intact in amnesia. Examples are motor skill learning, such as learning to play golf through repetition, conditioning, priming, cognitive skill learning, and habit learning. This system requires the cerebellum, basal ganglia, and association cortices.

In healthy individuals, both systems work together. In amnesia, implicit or nondeclarative learning remains intact; this fact can be utilized for rehabilitation. Even patients with AD, for example, can learn through this system—the repetition of facts, rather than single presentations, may allow their storage.

Which areas are responsible for encoding, storage, and retrieval of declarative information?

The hippocampal system is concerned with encoding and consolidating information.

Long-term storage generally occurs in the temporoparietal cortices. The left hemisphere stores primarily verbal or general knowledge (i.e., semantic/lexical information), and the right stores nonverbal and autobiographical information.

Retrieval uses prefrontal and distributed temporoparietal networks; the role of the hippocampal system in retrieval is time limited.

What is amnesia and injury to which areas can cause an amnestic syndrome?

Amnesia is a severe, isolated disturbance of declarative memory in the absence of other forms of cognitive dysfunction. Patients are unable to acquire new memories (anterograde amnesia) or recall recent memories (retrograde amnesia). Other memories remain intact, including remote memory, working memory, and semantic memory.

Bilateral lesions of the Papez circuit and related areas can cause an amnestic syndrome, while unilateral lesions can produce a milder, but often clinically relevant, memory deficit. The Papez circuit includes the medial temporal lobes (hippocampus and entorhinal cortex), the diencephalon (fornix and mammillary bodies; dorsomedial and anterior nuclei of the thalamus), and the basal forebrain cholinergic nuclei (medial septal nuclei and the diagonal band of Broca).

The isolated reduced ability to learn new information is referred to as amnestic mild cognitive impairment (aMCI). Many patients with aMCI develop Alzheimer’s dementia over the next 5 years.

What are the most common etiologies for amnestic syndromes?

The etiologies for amnestic syndromes can be divided according to their anatomic location:

• Medial temporal lobes—hypoxia, herpes simplex encephalitis, early AD, posterior cortical atrophy (PCA), strokes (thalamic and temporal lobe), surgery

• Diencephalon—Korsakoff’s syndrome (thiamine deficiency), thalamic strokes, surgery

• Basal forebrain—anterior communicating artery aneurysm bleed or clipping (with damage to the small perforating arteries)

• Substances—alcohol, benzodiazepines (transient, not permanent)

What are the clinical characteristics of transient global amnesia ?

Patients develop a sudden, isolated amnestic syndrome without structural brain abnormalities (anterograde and retrograde amnesia), which usually has a duration of 12 to 24 hours. Afterward, patients will not remember the episode because they are unable to encode new memories during it. Working memory is normal during the episode. On positron emission tomography (PET) or single-photon emission computed tomography (SPECT), bilateral temporal hypoperfusion can be demonstrated. The cause of this benign syndrome remains unknown. However, because it occurs more often in migraineurs, it could be a migraine equivalent. It is also more likely in those with epilepsy, patients at risk for transient ischemic attack, and during exercise. Risk of recurrence is low but higher than in the general population.

What are the features of psychogenic amnesia ?

In most cases of psychogenic amnesia, patients exhibit biologically unlikely patterns of impairment. Commonly, autobiographical memory is disproportionately affected, sparing memories of political and entertainment events. Patients often exhibit deficits in remote memories, which are normally very resistant to damage. New learning (anterograde memory) is often spared. Nevertheless, reversible abnormalities on PET with temporal hypometabolism have been found in some of these patients.

What types of memory difficulties occur with frontal lobe lesions ?

Frontal lobe damage can lead to important cognitive, emotional, and social dysfunction. The frontal lobes are organized into three basic subdivisions: precentral, premotor, and prefrontal.

The prefrontal division is crucial for higher order functions like planning, judgment, reasoning, decision making, emotional regulation, and social conduct. It can be further subdivided into ventromedial prefrontal cortex, dorsolateral prefrontal cortex, and superior medial prefrontal cortex.

The dorsolateral prefrontal cortex is important for working memory and “metamemory,” which has executive control over the memory apparatus. For example, it decides whether a retrieved memory is plausible for a given context, does strategic searching of the memory store, and temporally orders memories. Working memory holds about 7 to 10 bits of information as long as they are constantly repeated. Prefrontal damage and difficulty with working memory may manifest as interference with learning and impairment in performing tasks requiring delayed responses.

An impairment of declarative memory in conjunction with frontal lobe dysfunction may cause confabulation , which is the inability to distinguish a true memory from a false memory or from a memory inappropriate for the context. Confabulations are a common occurrence in alcoholic Korsakoff’s syndrome.

Frontal cortical lesions can also cause disorders of one or more functions that facilitate memory, such as learning strategies , retrieval strategies , and learning efficiency .

What is the definition of aphasia and how does it differ from dysarthria ?

Aphasia is an acquired disturbance of language whereas dysarthria is an acquired disorder of speech production. An example of dysarthria is what happens after having anesthesia for dental work and then being unable to enunciate words.

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Which are the nonfluent aphasias and which are the fluent aphasias?

The anterior aphasias are nonfluent and include Broca’s, global, and mixed transcortical and transcortical motor. The posterior aphasias are fluent, including Wernicke’s, transcortical sensory, and usually thalamic ( Table 17-1 ).

Which aphasias spare repetition and which have impaired repetition?

Repetition is spared in those outside the perisylvian fissure area , including transcortical motor, transcortical sensory, and thalamic aphasias. Repetition is impaired in the perisylvian aphasias , including Broca’s, Wernicke’s, conduction aphasia, and pure word deafness.

What are the clinical characteristics of nonfluent aphasia ?

Impaired articulation, impaired melodic production, reduced phrase length (five or fewer words per phrase), and decreased grammatical complexity. The production of fewer than 15 words per minute is used to define nonfluent aphasias.

What are the clinical features of Broca’s aphasia and where is the lesion responsible for it?

Speech is nonfluent, effortful, agrammatic, and telegraphic, with poor ability to name, semantic and phonemic paraphasic errors, impaired repetition, and relatively spared comprehension. The lesion is in Broca’s area (the frontal operculum, Brodmann areas 44 and 45), inferior left frontal gyrus, the surrounding frontal areas, and the underlying white matter and subjacent basal ganglia.

What are the clinical features of aphemia and where is the lesion that underlies it?

Aphemia refers to poor speech output with sparing of comprehension and writing. Speech output can be slow and halting. Aphemia has been reported with lesions of the lower motor strip (cortical dysarthria), supplementary motor cortex, and several other areas.

What are the clinical features of anomic aphasia , and where is the lesion that underlies it?

Persons with anomic aphasia have isolated deficits in word finding. Otherwise, their speech is fluent with good comprehension and good repetition. Lesions producing this aphasia localize less specifically than other aphasias. They may be in the temporo-parieto-occipital association area. This is also a common chronic residual syndrome for other acute aphasias after rehabilitation.

What are the clinical features of conduction aphasia and where is the lesion that produces it?

Conduction aphasia is a fluent aphasia, with good comprehension, poor repetition, paragrammatic errors, anomia, paraphasic errors, good recitation, and good reading aloud. While any type of paraphasia may be seen, the vast majority of substitutions involves phonemes resulting in literal (phonemic) paraphasic errors. The lesion usually involves the left inferior parietal lobule, especially the anterior supramarginal gyrus. Often the lesion is in the subcortical white matter, deep to the inferior parietal cortex, involving the arcuate fasciculus or the extreme capsule immediately below it. Both structures are connected to the temporal and frontal cortex.

What are the clinical features of Wernicke’s aphasia and where is the lesion responsible for it?

Wernicke’s aphasia is characterized by fluent speech, good articulation, good or sometimes exaggerated prosody (the expressivity of language), impaired naming, phonemic and semantic paraphasias, poor auditory and reading comprehension, impaired repetition, and fluent but empty writing. It is usually caused by damage to the posterior sector of the left auditory association cortex, Brodmann area 22. Often there is involvement of Brodmann areas 37, 39, and 40, or all three.

What are the clinical features of transcortical motor aphasia and where are the lesions responsible for it?

Spontaneous speech is nonfluent, with good repetition and good comprehension, delayed initiation of output, brief utterances, semantic paraphasic errors, and echolalia. Lesions around Broca’s area in the supplementary motor area or its connections to Broca’s area are responsible.

What are the clinical features of transcortical sensory aphasia and where is the lesion responsible for it?

Spontaneous speech is fluent, with good repetition, echolalia, impaired auditory and reading comprehension, right visual field deficits, and rare motor and sensory deficits. Lesions outside of Wernicke’s area in the surrounding temporoparietal area are responsible.

What are the clinical features of mixed transcortical aphasia , where is the lesion responsible for it, and what is its vascular territory?

There is absent spontaneous speech, impaired comprehension, and intact repetition. Stock phrases, such as “you know” and “the thing is,” and echolalia are produced. The lesion includes the areas that cause transcortical motor and sensory aphasias: the dorsolateral frontal region anterior to the motor cortex and the temporal–parietal–occipital junction. This lesion may be caused by hypoperfusion in the distribution of the left internal carotid artery, which produces a watershed stroke.

What are the clinical features of global aphasia and where are the lesions that produce it?

Spontaneous speech is nonfluent, with poor repetition and poor comprehension. The output is restricted to meaningless speech sounds or stereotypes. These lesions involve Broca’s and Wernicke’s area. They may be combined cortical–subcortical or purely subcortical.

What is alexia and how does it differ from dyslexia ?

Alexia is an acquired disorder of written language comprehension, i.e., difficulty in reading. Dyslexia refers to developmental difficulties with reading.

What is alexia without agraphia and where is the lesion responsible for it?

Alexia without agraphia (pure word blindness or acquired pure alexia) is the inability to read despite preserved ability to write. It is associated with a lesion in the dominant occipital lobe (frequently producing a homonymous hemianopia), and a disconnection of the nondominant occipital lobe from the dominant parietal lobe via a lesion of the inferior splenium of the corpus callosum. Alternately, it can occur with lesions of the dominant lateral geniculate body and splenium of the corpus callosum or with a single lesion of the dominant occipitotemporal periventricular white matter behind, beneath, and beside the occipital horn of the lateral ventricle. It is most often associated with infarction in the distribution of the dominant hemispheric posterior cerebral artery.

Where is the lesion responsible for alexia with agraphia ?

Alexia with agraphia is usually associated with a lesion of the angular gyrus.

What percentage of people are left handed ?

Fewer than 5% of people use their left hands for all skilled tasks. Sixty percent are strongly right handed and 35% have a mixed hand preference.