What the left and right anterior fusiform gyri tell us about semantic memory

Abstract

The study of patients with semantic dementia, a variant of frontotemporal lobar degeneration, has emerged over the last two decades as an important lesion model for studying human semantic memory. Although it is well-known that semantic dementia is associated with temporal lobe degeneration, controversy remains over whether the semantic deficit is due to diffuse temporal lobe damage, damage to only a sub-region of the temporal lobe or even less severe damage elsewhere in the brain. The manner in which the right and left temporal lobes contribute to semantic knowledge is also not fully elucidated. In this study we used unbiased imaging analyses to correlate resting cerebral glucose metabolism and behavioural scores in tests of verbal and non-verbal semantic memory. In addition, a region of interest analysis was performed to evaluate the role of severely hypometabolic areas. The best, indeed the only, strong predictor of semantic scores across a set of 21 patients with frontotemporal lobar degeneration with semantic impairment was degree of hypometabolism in the anterior fusiform region subjacent to the head and body of the hippocampus. As hypometabolism in the patients' rostral fusiform was even more extreme than the abnormality in other regions with putative semantic relevance, such as the temporal poles, the significant fusiform correlations cannot be attributed to floor-level function in these other regions. More detailed analysis demonstrated more selective correlations: left anterior fusiform function predicted performance on two expressive verbal tasks, whereas right anterior fusiform metabolism predicted performance on a non-verbal test of associative semantic knowledge. This pattern was further supported by an additional behavioural study performed on a wider cohort of patients with semantic dementia, in which the patients with more extensive right-temporal atrophy (when matched on degree of naming deficit to a set of cases with more extensive left temporal atrophy) were significantly more impaired on the test of non-verbal semantics. Our preferred interpretation of this laterality effect involves differential strength of connectivity between different regions of a widespread semantic network in the human brain.

Triangulation of the neurocomputational architecture underpinning reading aloud

“…However, although these studies provide persuasive evidence for the co-occurrence of semantic impairment and exception word reading deficits, they provide limited data regarding the exact neuroanatomical locus of such effects. A number of recent neuroimaging studies have highlighted the involvement of the ventral anterior temporal lobe (ATL), i.e., the inferior temporal and fusiform gyri, in semantic processing of verbal and nonverbal stimuli (29)(30)(31) in an area that coincides with the peak neural correlate of semantic impairment in patients with SD (32). The ATL is rarely activated in studies of word reading.…”

Word meaning in the ventral visual path: a perceptual to conceptual gradient of semantic coding

“…3 associative cortices (sometimes referred to as "semantic hubs" ) such as the inferior frontal cortex (Devlin et al, 2003), the anterior temporal cortex (Mion et al, 2010), or the inferior parietal cortex (Bonner et al, 2013), but also in primary and secondary sensory-motor cortices, in a way that appears proportional to the relevance of perceptuo-motor attributes (Pulvermuller, 2013). Researchers capitalizing from both machine learning techniques and Representational Similarity Analysis (RSA) frameworks have shown that it is possible to discriminate between words belonging to different semantic categories (e.g., animals vs tools) as well as sub-categorical clusters (e.g., mammals vs insects) using distributed patterns of brain activation (Shinkareva et al, 2011;Bruffaerts et al, 2013;Devereux et al, 2013;Fairhall and Caramazza, 2013;Simanova et al, 2014), but they did not determine if such discriminations were driven by conceptual or/and by correlated perceptual information (Naselaris and Kay, 2015).…”

The neural substrates of inferential and referential semantic processing

“…Patients with greater left than right temporal atrophy were more impaired than patients with greater right than left atrophy on both referential tasks (Thompson et al, 2003) or on picture naming only (Mendez et al, 2011). In addition, Mion et al (2010) showed that the left anterior fusiform metabolism most contributed to predicting scores on picture naming and category fluency, whereas the right anterior fusiform metabolism was most responsible for scores on a nonverbal test (pictureepicture matching). Some additional evidence points to a role of the right temporal lobe for referential tasks.…”

“…Recently some research showed that nonverbal tasks are more affected by right temporal damage (Butler et al, 2009;Ikeda et al, 2006;Mion et al, 2010; but see Thompson et al, 2003), verbal tasks by left temporal damage (Hosogi Senaha et al, 2007;Mion et al, 2010;Thompson et al, 2003).…”