A structural connectivity convergence zone in the ventral and anterior temporal lobes: Data-driven evidence from structural imaging

Abstract

The hub-and-spoke model of semantic cognition seeks to reconcile embodied views of a fully distributed semantic network with patient evidence, primarily from semantic dementia, who demonstrate modality-independent conceptual deficits associated with atrophy centred on the ventrolateral anterior temporal lobe. The proponents of this model have recently suggested that the temporal cortex is a graded representational space where concepts become less linked to a specific modality as they are processed farther away from primary and secondary sensory cortices and towards the ventral anterior temporal lobe. To explore whether there is evidence that the connectivity patterns of the temporal lobe converge in its ventral anterior end the current study uses three dimensional Laplacian eigenmapping, a technique that allows visualisation of similarity in a low dimensional space. In this space similarity is encoded in terms of distances between data points. We found that the ventral and anterior temporal lobe is in a unique position of being at the centre of mass of the data points within the connective similarity space. This can be interpreted as the area where the connectivity profiles of all other temporal cortex voxels converge. This study is the first to explicitly investigate the pattern of connectivity and thus provides the missing link in the evidence that the ventral anterior temporal lobe can be considered a multi-modal graded hub.

Keywords: Anterior temporal lobe; Connectivity; Laplacian eigenmapping; Semantics; Spectral reordering.

Fig. 1

The bottom row shows similarity space plots of the temporal lobe seed voxels according to the similarity of their connectivity profiles. The colouring is a ‘ranked’ RGB representation of a voxel's location in the connective similarity space. The top row displays the results projected onto the temporal lobes, with a given voxel's RGB colour retained as it moves between similarity and anatomic spaces Note that the colouring is ranked in order to improve visualisation; see supplementary animation for the raw colours and a depiction of the mapping between spaces.

Fig. 2

Left and right hemispheric connectivity space highlighting regions around the centre of mass in red. This region in the centre of the connectivity space is the area of least difference, on average, to all other areas.

Fig. 3

The graded convergence of information A) as proposed by the (Plaut, 2002) model; B) proposed convergence based on white matter connections by (Binney et al., 2012). C) the results of the current Laplacian eigenmap showing the convergence of connectivity in the ventral ATL. Panels A and B were originally published in the journal of cognitive neuroscience (Binney et al., 2012); reproduced with permission.

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