Neural basis of category-specific semantic deficits for living things: evidence from semantic dementia, HSVE and a neural network model

Abstract

Studies of patients with semantic impairments following brain damage offer key insights into the cognitive and neural organization of semantic memory. Especially important in this regard are studies of category-specific semantic impairment. We report a direct comparison of semantic deficits in two groups suffering from different diseases: semantic dementia (SD) and herpes simplex virus encephalitis (HSVE). Although pathology in both disorders is centred on the anterior temporal lobes bilaterally, category-specific semantic impairment is rarely observed in SD yet commonly found in HSVE. Using a combination of neuropsychology and computational neuroscience, we tested the possibility that category-specific deficits for living things depend not solely upon the location of damage within the cortical semantic network but also critically upon the type of impairment. When the semantic representations within the model are degraded or 'dimmed' then a generalized, global semantic impairment results (as found in SD) but when the representations are distorted then a category-specific pattern emerges (as per HSVE). Three novel predictions from this model were tested and confirmed, thereby adding weight to the hypothesis that both type and distribution of pathology can be critical in producing neuropsychological phenomena.

Tracking neural coding of perceptual and semantic features of concrete nouns

“…The other factor singled out by Just et al (2010), word length, was also consistently predicted in this paper. Moreover, several other studies have shown the importance of animacy in the neural representation of concrete nouns (Lambon Ralph et al, 2007; Mahon and Caramazza, 2009; Moss et al, 2005). In the future, we plan to narrow down the number and type of semantic features being predicted in order to fully characterize a minimal semantic space that is sufficient to obtain comparable results.…”

Progression of logopenic variant primary progressive aphasia to apraxia and semantic memory deficits

“…The different neural substrates for semantic memory may reflect category-specific semantic systems. Although previous cases with deficits for living things often resulted from anterior temporal lobe damage [37], deficits for non-living things or man-made objects have been reported after left temporo-parietal damage [38]. From a lesion study involving lexical retrieval, words for living things are associated with the left anterior temporal lobe, whereas those for tools are associated with the left posterior temporal lobe [39].…”

Linguistic Aspects of Primary Progressive Aphasia

“…Some forms of stroke and closed-head trauma may resemble svPPA, but these are easily distinguished because of their sudden onset and nonprogressive nature. Other causes of a pattern of semantic memory difficulty resembling svPPA include herpes encephalitis (Lambon Ralph et al 2007, Noppeney et al 2007, Patterson et al 2015), but this is often subacute in progression and associated with the stigma of an infection.…”

“…Evidence consistent with this claim has been reported in svPPA patients with selective deficits for all aspects of a category of knowledge within semantic memory, known as a category-specific deficit. Single cases and small series of cases have been reported with a deficit for concepts involving living things (e.g., animals) versus nonliving things (e.g., tools, furniture) (Carroll & Garrard 2005, Lambon Ralph et al 2007, Libon et al 2013, Rogers et al 2004). The reverse pattern of impairment—greater difficulty with manufactured than natural objects—has also been described in svPPA (Cappa et al 1998, Moss & Tyler 2000, Sacchett & Humphreys 1992), suggesting that natural objects are not simply more difficult to name and understand than manufactured objects.…”