What is semantic in semantic dementia? The decay of knowledge of physical entities but not of verbs, numbers and body parts

Background: Conceptual knowledge does not decay randomly in\r\npatients with cerebral damage, suggesting that dedicated neural\r\nsubstrates may support different categories of knowledge.\r\nSemantic dementia is an optimal natural model for studying the\r\norganization of semantic memory. Nevertheless, in a pathology\r\nprimarily characterized by a semantic memory disorder categorical-\r\nand modality- specific effects are not obvious findings. In fact,\r\nthere is no clear evidence of categorical effects, at least concerning\r\ntwo broad categories of knowledge, that is, natural items and\r\nartifacts. Furthermore, transmodal deficits do not seem to be the\r\nrule in SD. Also quite robust is the observation that some conceptual\r\ndomains are relatively spared in this pathology, that is,\r\nnumerical knowledge, abstract words, and action verbs.\r\nAims: To explore category specific and modality specific deficit in\r\nSD and to support the evidence that semantic degradation in SD\r\nprimarily involves knowledge of the objects in the real world,\r\nwhereas categories of knowledge whose items can be less easily\r\nidentified by surface attributes, such as verbs, numbers and body\r\nparts, are more preserved.\r\nMethods and Procedures: We investigated the semantic impairment\r\nin 8 patients with Semantic Dementia (SD). Voxel-based morphometry\r\n(VBM) in each patient was also obtained\r\nOutcomes and Results: In some patients manmade objects were\r\nsignificantly more preserved than natural items, verbs more preserved\r\nthan nouns and the number system entirely preserved; the\r\nbody parts category was the least impaired in all subjects; finally,\r\nin three patients visual semantic knowledge was significantly more\r\npreserved than verbal semantic knowledge. VBM showed that\r\natrophy of the anterior inferior temporal regions was insufficient\r\nto impair knowledge about verbs, numbers and body parts, whose\r\nimpairment was associated with more widespread atrophy. In\r\nsubjects whose verbal semantic knowledge was significantly\r\nmore impaired than visual semantic knowledge, atrophy was principally\r\ndistributed in the left hemisphere. In patients with significant impairment for natural items compared to manmade\r\nobjects, atrophy was not confine in the temporal lobes.\r\nConclusion: We conclude that in SD semantic decay primarily\r\ninvolves the real-world items whose knowledge is processed by\r\nsurface sensorifunctional features and that this is the type of\r\nknowledge stored in the temporal lobes. Our data support a\r\nmodel that associates a semantic hub with modality/category\r\nspecific neural substrates.