Graded modality-specific specialisation in semantics: A computational account of optic aphasia

Abstract

A long-standing debate regarding the representation of semantic knowledge is whether such knowledge is represented in a single, amodal system or whether it is organised into multiple subsystems based on modality of input or type of information. The current paper presents a distributed connectionist model of semantics that constitutes a middle ground between these unitary-versus multiple-semantics accounts. In the model, semantic representations develop under the pressure of learning to mediate between multiple input and output modalities in performing various tasks. The system has a topographic bias on learning that favours short connections, leading to a graded degree of modality-specific functional specialisation within semantics. The model is applied to the specific empirical phenomena of optic aphasia-a neuropsychological disorder in which patients exhibit a selective deficit in naming visually presented objects that is not attributable to more generalised impairments in object recognition (visual agnosia) or naming (anomia). As a result of the topographic bias in the model, as well as the relative degrees of systematicity among tasks, damage to connections from vision to regions of semantics near phonology impairs visual object naming far more than visual gesturing or tactile naming, as observed in optic aphasia. Moreover, as in optic aphasia, the system is better at generating the name of an action associated with an object than at generating the name of the object itself, because action naming receives interactive support from the activation of action representations. The ability of the model to account for the pattern of performance observed in optic aphasia across the full range of severity of impairment provides support for the claim that semantic representations exhibit graded functional specialisation rather than being entirely amodal or modality-specific.

Triangulation of the neurocomputational architecture underpinning reading aloud

“…Neuropsychological studies and neuroanatomically constrained computational models also implicate the lateral ATL in semantically mediated speech production (50,53). This role is consistent with the view that (i) as a result of the convergence of multiple white-matter tracts and connections, the ATL region as a whole acts as a representational system and (ii) there are graded specializations in different parts of this system, as a consequence of systematic variations in structural connectivity across the region (54,55).…”

An Exception to Mental Simulation: No Evidence for Embodied Odor Language

“…In this account, a semantic level separate from modality‐specific information is postulated, and this is where featural information is bound together (Vigliocco et al., 2004; see also Plaut, 2002, for a graded account of modality‐specificity in semantics). If comprehension of odor‐words does not involve odor simulation, other methods may be used to ground meaning when describing odors, such as gesture or metaphorical language (Majid, 2013).…”

Semantic memory in Alzheimer's disease and the frontotemporal dementias: A longitudinal study of 236 patients.

“…Lambon argued that semantic representations are bilaterally distributed, but phonological representations for speech production are left lateralized. On the general principle that close connections in the brain are more influential than distant ones (Plaut, 2002), this hypothesis predicts that-even if subgroups of patients with SD with greater left-versus right-sided abnormality have comparable comprehension deficits-the subgroup with predominantly left-temporal atrophy will have a disproportionate deficit in naming and in expressive (i.e., propositional) speech more generally, even if postsemantic mechanisms of verbal production are comparatively spared.…”