Semantic memory refers to knowledge about people, objects, actions, relations, self, and culture acquired through experience. The neural systems that store and retrieve this information have been studied for many years, but a consensus regarding their identity has not been reached. Using strict inclusion criteria, we analyzed 120 functional neuroimaging studies focusing on semantic processing. Reliable areas of activation in these studies were identified using the activation likelihood estimate (ALE) technique. These activations formed a distinct, left-lateralized network comprised of 7 regions: posterior inferior parietal lobe, middle temporal gyrus, fusiform and parahippocampal gyri, dorsomedial prefrontal cortex, inferior frontal gyrus, ventromedial prefrontal cortex, and posterior cingulate gyrus. Secondary analyses showed specific subregions of this network associated with knowledge of actions, manipulable artifacts, abstract concepts, and concrete concepts. The cortical regions involved in semantic processing can be grouped into 3 broad categories: posterior multimodal and heteromodal association cortex, heteromodal prefrontal cortex, and medial limbic regions. The expansion of these regions in the human relative to the nonhuman primate brain may explain uniquely human capacities to use language productively, plan, solve problems, and create cultural and technological artifacts, all of which depend on the fluid and efficient retrieval and manipulation of semantic knowledge.

/pdf/where-is-the-semantic-system-a-critical-review-and-meta-4aaglbm4tj.pdf

Where Is the Semantic System? A Critical Review and Meta-Analysis of 120 Functional Neuroimaging Studies

Cognition and comportment are subserved by interconnected neural networks that allow high-level computational architectures including parallel distributed processing. Cognitive problems are not resolved by a sequential and hierarchical progression toward predetermined goals but instead by a simultaneous and interactive consideration of multiple possibilities and constraints until a satisfactory fit is achieved. The resultant texture of mental activity is characterized by almost infinite richness and flexibility. According to this model, complex behavior is mapped at the level of multifocal neural systems rather than specific anatomical sites, giving rise to brain-behavior relationships that are both localized and distributed. Each network contains anatomically addressed channels for transferring information content and chemically addressed pathways for modulating behavioral tone. This approach provides a blueprint for reexploring the neurological foundations of attention, language, memory, and frontal lobe function.

Large-scale neurocognitive networks and distributed processing for attention, language, and memory

The use of positron emission tomography to measure regional changes in average blood flow during processing of individual auditory and visual words provides support for multiple, parallel routes between localized sensory-specific, phonological, articulatory and semantic-coding areas.

https://bi.snu.ac.kr/Courses/4ai05s/Essay/1-25.pdf

Positron emission tomographic studies of the cortical anatomy of single-word processing

By utilizing new information from both clinical and experimental (lesion, electrophysiological, and gene-activation) studies with animals, the anatomy underlying anterograde amnesia has been reformulated. The distinction between temporal lobe and diencephalic amnesia is of limited value in that a common feature of anterograde amnesia is damage to part of an “extended hippocampal system” comprising the hippocampus, the fornix, the mamillary bodies, and the anterior thalamic nuclei. This view, which can be traced back to Delay and Brion (1969), differs from other recent models in placing critical importance on the efferents from the hippocampus via the fornix to the diencephalon. These are necessary for the encoding and, hence, the effective subsequent recall of episodic memory. An additional feature of this hippocampal–anterior thalamic axis is the presence of projections back from the diencephalon to the temporal cortex and hippocampus that also support episodic memory. In contrast, this hippocampal system is not required for tests of item recognition that primarily tax familiarity judgements. Familiarity judgements reflect an independent process that depends on a distinct system involving the perirhinal cortex of the temporal lobe and the medial dorsal nucleus of the thalamus. In the large majority of amnesic cases both the hippocampal–anterior thalamic and the perirhinal–medial dorsal thalamic systems are compromised, leading to severe deficits in both recall and recognition.

/pdf/episodic-memory-amnesia-and-the-hippocampal-anterior-50qwk8ap9b.pdf

Episodic memory, amnesia, and the hippocampal–anterior thalamic axis.

on to develop full blown Parkinson's disease with rigidity and bradykinesia in the next few years. For those interested in the mechanisms of tremor, there are the customary authoritative reviews by Llinas, De Long, Lamarre, Rothwell and Deuschl, but uncertainty remains with respect to the relative importance of central autonomous generators and instability of peripheral reflex loops. Well written chapters are also included on primary orthostatic tremor and its relationship to essential tremor, writing tremor, neuropathic tremor, midbrain tremor and the increasingly acknowledged psychogenic tremors. Complex interrelationship between dystonia and postural tremor is also covered in depth. This cornucopia will be coveted and dipped into by those neurologists with a special interest in abnormal movement disorders, but who would not consider themselves to have a research interest in tremor. However, for the majority of clinicians involved in the hurly burly of clinical practice, I suspect that regrettably time and cost factors will conspire together to keep this excellent book out of reach. ANDREW LEES

https://jnnp.bmj.com/content/jnnp/59/3/346.1.full.pdf

Contributions to Neuropsychological Assessment

Eight patients with acquired agraphia were studied using the same writing tests and were compared with normal and brain-damaged controls. Four patients fulfilled the criteria for lexical agraphia and on CT scan had lesions of the posterior angular gyrus that spared the supramarginal gyrus. The other 4 fulfilled the criteria for phonological agraphia. They had lesions on CT scan that were similar to those found in previously described patients with phonological agraphia. Their lesions involved the supramarginal gyrus or insula deep to it and spared the angular gyrus. These studies support the hypothesis that there are two dissociable spelling systems and that these spelling systems are disrupted by focal lesions in separate but distinct brain regions. Further studies investigated the relationships between phonological agraphia and phonological dyslexia (alexia), and lexical agraphia and surface dyslexia (lexical alexia). The data support the hypothesis that individual systems subserve the four processes of phonological spelling, phonological reading, lexical spelling and lexical reading.

Lexical agraphia. Further support for the two-system hypothesis of linguistic agraphia.

Two writing routes (phonological and lexical) have been postulated. We studied four patients who had disruption of the phonological route (ie, inability to write pronounceable nonwords) but with a preserved lexical route. Results showed that the phonological route has two components: segmentation and phoneme-grapheme conversion. Disruption of either function may induce phonological agraphia. The preserved lexical route uses a whole-word mechanism and is strongly affected by semantic factors, such as imageability, emotionality, and grammatic class. Results of CT suggest that a portion of the supramarginal gyrus is the most likely anatomic substrate of phonological agraphia.

Phonological agraphia: writing by the lexical-semantic route.

We studied the delayed neurologic complications of left atrial myxoma in 2 patients and analyzed 47 previously published cases. Embolic cerebral infarctions may occur before surgical removal of the primary cardiac tumor, but aneurysm formation and hemorrhage may be delayed, whether or not there is evidence of neurologic disorder at the time of initial surgery. The true incidence of these delayed complications is unknown. Resultant aneurysms may show progressive enlargement, and treatment with doxorubicin failed to prevent aneurysm enlargement in one patient. Better guidelines are needed for clinical management in these patients.

Delayed neurologic complications of left atrial myxoma

It is controversial whether a focal lesion can specifically induce Gerstmann's syndrome (dyscalculia, left-right disorientation, finger agnosia, and agraphia). Also, Gerstmann's tetrad has been attributed to other cerebral symptoms, particularly aphasia. We examined a patient who had all four symptoms of Gerstmann's syndrome, without other symptoms or signs, and who had a discrete left parietal lesion.

David P. Roeltgen

Papers

Lexical agraphia. Further support for the two-system hypothesis of linguistic agraphia.

Phonological agraphia: writing by the lexical-semantic route.

Delayed neurologic complications of left atrial myxoma

Pure Gerstmann's syndrome from a focal lesion.

Apractic agraphia in a patient with normal praxis.