Objectives/Aims Semantic dementia (SD) is characterised by impairment in conceptual (semantic) knowledge due to anterior temporal lobe (ATL) neurodegeneration.1 Here we show, by analysis of the drawings-from-name of 19 patients with SD, that knowledge of individual object characteristics is lost in a transmodal, graded fashion, correlating with volume loss in the anterior-inferior temporal lobe.
Previous studies1 in SD have used the technique of delayed copy drawing: showing patients a line drawing of a camel; removing it from view; then 10 seconds later, asking: ‘Please draw what you were looking at.’ This yielded informative results, but drawings in this procedure can rely on visuospatial working memory and episodic declarative memory, as well as conceptual knowledge. Here we gave requests like: ‘Please draw a picture of a camel,’ where there is no perceptual connection between the word and the object other than through semantic knowledge.
Methods Line drawings of 19 patients with diagnoses of SD at Addenbrooke’s Hospital were analysed and compared with drawings of the same items from 3 age-matched healthy controls. For a subset of 10 patients, MRI-brain results were used to correlate semantic performance with cortical atrophy.
Qualitative analyses Patient drawings demonstrated a pattern of degradation similar to that observed in previous delayed-copy studies. Rare and distinctive features (such as the hump on a camel) were lost earliest in disease course, and there was an increase in the intrusion of prototypical features (such as normal-sized ears on an elephant) with more advanced disease, either cross-sectionally or longitudinally.
Crucially, patient drawings showed a continuum of degradation rather than a binary ‘present’ or ‘absent’ state (figure 1).
Quantitative analyses Core features of each object were identified by studying commonality across control drawings, and patient drawings were scored to quantify the proportion of core features correctly drawn by patients, normalised to object familiarity and difficulty. This composite score was then regressed against MRI-extracted grey-matter volume within ventral stream regions-of-interest. Left inferior temporal gyrus atrophy was significantly (p = 0.01) correlated with drawing score.
Conclusions Progressive atrophy of the inferior temporal lobe correlated quantitively and qualitatively with a transmodal continuum of degradation in semantic knowledge. These results therefore support the idea of a population code for semantic memory, whereby a graded ATL ‘hub’ binds modality-specific features of concepts stored across various cortical areas (the ‘spokes’).2
Bozeat S, et al. A duck with four legs: Investigating the structure of conceptual knowledge using picture drawing in semantic dementia. Cogn. Neuropsychol.2003; 20: 27–47.
Ralph MAL, Jefferies E, Patterson K, & Rogers TT. The neural and computational bases of semantic cognition. Nat. Rev. Neurosci. 2017;18:42–55.
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