At the Edge of Semantic Space: The Breakdown of Coherent Concepts in Semantic Dementia Is Constrained by Typicality and Severity but Not Modality

Abstract

Hub-and-spoke models of semantic representation suggest that coherent concepts are formed from the integration of multiple, modality-specific information sources with additional modality-invariant representations-most likely stored in the ventrolateral anterior temporal lobe (vATL). As well as providing the necessary computational mechanisms for the complexities of feature integration, these modality-invariant representations also license a key aspect of semantic memory-semantic-based generalization. Semantic dementia allows us to investigate this aspect of conceptual knowledge because (a) the patients have a selective and progressive semantic degradation and (b) this is associated with profound ventrolateral ATL atrophy. Specifically, the boundaries between concepts become degraded in semantic dementia and, when tested using the appropriate materials, the patients make simultaneous under- and overgeneralization errors. We found that the rate of these errors were a function of typicality and pseudotypicality of the items as well as the severity of the patients' semantic impairment. Following the modality-invariant nature of the vATL hub representation, we also confirmed that the patients were impaired on both verbal- and picture-based versions of the same task.

Language, executive function and social cognition in the diagnosis of frontotemporal dementia syndromes

“…Overall, this variant of PPA is characterized by a progressive loss of semantic knowledge (Hodges et al, 1992; Julien et al, 2008; Kashibayashi et al, 2010; Mayberry et al, 2011; Snowden et al, 1989; see also Gorno-Tempini et al, 2011). Thus, patients with svPPA lose the meaning of words, seen particularly in the context of naming and single-word comprehension (Adlam et al, 2006; Hodges et al, 1992; Kertesz et al, 1998, 2010).…”

Going beyond Inferior Prefrontal Involvement in Semantic Control: Evidence for the Additional Contribution of Dorsal Angular Gyrus and Posterior Middle Temporal Cortex

“…The "hub" in bilateral ventrolateral ATL extracts modality-invariant conceptual representations from information about the visual, auditory, motor, and verbal attributes of concepts (stored within distributed motor and sensory areas-i.e., the "spokes"; Mayberry, Sage, & Lambon Ralph, 2011;Lambon Ralph et al, 2010;Pobric et al, 2010;Patterson et al, 2007;Rogers et al, 2004;McClelland & Rogers, 2003). Recent evidence suggests that the key site for the semantic hub may not be the temporal pole but, instead, is centered on the anterior fusiform ( Visser & Lambon Ralph, 2011;Binney et al, 2010;Mion et al, 2010).…”

Reconceiving conceptual vehicles: Lessons from semantic dementia

“…I am sympathetic to this concern, but think that even if such a distinction exists, the ATLs are likely to play a representational role in at least certain kinds of conceptual processing. Specifically, the ATLs seem to play an important role in comprehending abstract concepts and judging category membership (Mayberry, Sage, & Lambon Ralph, 2011;Pobric et al, 2009). Mayberry et al (2011) hypothesized that two kinds of category judgments, those involving pseudotypical exemplars and atypical exemplars, might require access to representations that go against the grain of perceptual similarity.…”

“…Specifically, the ATLs seem to play an important role in comprehending abstract concepts and judging category membership (Mayberry, Sage, & Lambon Ralph, 2011;Pobric et al, 2009). Mayberry et al (2011) hypothesized that two kinds of category judgments, those involving pseudotypical exemplars and atypical exemplars, might require access to representations that go against the grain of perceptual similarity. An example of an atypical exemplar is a sea sponge-it does not look like an animal, but it is.…”

“…An example of a pseudotypical exemplar is a dolphin-it looks like a fish, but it is not. Mayberry et al (2011), who hypothesized that the ATLs make an amodal contribution to category formation, found that patients with more severe SD were prone to both overgeneralization errors (e.g., a dolphin is a fish) and undergeneralization errors (e.g., a sea sponge is not an animal). The effect was robust across different modalities (e.g., picture sorting and naming tasks) and different conceptual categories (e.g., animals, fruits, tools, etc.…”