[4] However, it is fairly rare for patients with semantic dementia to develop category specific impairments, though there have been documented cases of it occurring.
[7] SD is one of the three variants of primary progressive aphasia (PPA), which results from neurodegenerative disorders such as FTLD or Alzheimer's disease.
SD was first described by Arnold Pick in 1904 and in modern times was characterized by Professor Elizabeth Warrington in 1975,[8] but it was not given the name semantic dementia until 1989.
[2] Syntax is spared, and SD patients have the ability to discern syntactic violations and comprehend sentences with minimal lexical demands.
[16] With Alzheimer's disease in particular, interactions with semantic memory produce different patterns in deficits between patients and categories over time which is caused by distorted representations in the brain.
[18] Clinical signs include fluent aphasia, anomia, impaired comprehension of word meaning, and associative visual agnosia (inability to match semantically related pictures or objects).
[20] SD patients sometimes show symptoms of surface dyslexia, a relatively selective impairment in reading low-frequency words with exceptional or atypical spelling-to-sound correspondences.
This theory is supported by the atrophy of the anterior temporal lobe, which is believed to contain a component of the semantic system that integrates conceptual information.
Others hypothesize that the damage is predominantly to the ventral temporal cortex, since SD patients remember numbers and music, but have trouble associating visual cues to concrete words.
[2] Due to the variety of symptoms dementia patients present, it becomes more difficult to assess semantic memory capability especially with regard to musical elements.
[22] Meta-analyses on MRI and FDG-PET studies confirmed these findings by identifying alterations in the inferior temporal poles and amygdalae as the hotspots of disease - brain regions that have been discussed in the context of conceptual knowledge, semantic information processing, and social cognition.
[24] Damage to white matter tracts connecting the anterior temporal cortex to the inferior longitudinal, arcuate, and uncinate fasciculi, which are regions of the language network, is also seen using diffusion tensor imaging.
[2] Imaging also shows the integrity of the axonal connections from the anterior temporal cortex to frontal and posterior associative areas to be altered.
Overall, the results of these studies suggest that the neurobiological basis of musical semantic memory is bilaterally located in the cerebral hemispheres, likely around the fronto-temporal areas of the brain.