Abstract
The human cortical regions for processing high-level visual (HLV) functions of different categories remain ambiguous, especially in terms of their conjunctions and specifications. Moreover, the neurobiology of declined HLV functions in patients with Alzheimer's disease (AD) has not been fully investigated. This study provides a functionally sorted overview of HLV cortices for processing “what” and “where” visual perceptions and it investigates their atrophy in AD and MCI patients. Based upon activation likelihood estimation (ALE), brain regions responsible for processing five categories of visual perceptions included in “what” and “where” visions (i.e., object, face, word, motion, and spatial visions) were analyzed, and subsequent contrast analyses were performed to show regions with conjunctive and specific activations for processing these visual functions. Next, based on the resulting ALE maps, the atrophy of HLV cortices in AD and MCI patients was evaluated using voxel-based morphometry. Our ALE results showed brain regions for processing visual perception across the five categories, as well as areas of conjunction and specification. Our comparisons of gray matter (GM) volume demonstrated atrophy of three “where” visual cortices in late MCI group and extensive atrophy of HLV cortices (25 regions in both “what” and “where” visual cortices) in AD group. In addition, the GM volume of atrophied visual cortices in AD and MCI subjects was found to be correlated to the deterioration of overall cognitive status and to the cognitive performances related to memory, execution, and object recognition functions. In summary, these findings may add to our understanding of HLV network organization and of the evolution of visual perceptual dysfunction in AD as the disease progresses.
Highlights
The human visual cortex is primarily located in, but not confined to, the occipital lobe
Convergent brain regions were identified from five activation likelihood estimation (ALE) analyses of face, word, object, motion and spatial visual functions (Table 1, Figure 1) and were mainly distributed in the bilateral inferior and middle temporal gyrus (Brodmann area 19, BA 19) and the middle and inferior occipital gyrus (BA 18, 19)
The performances of category fluency-animal and trail making test A & B were found to be positively correlated with the gray matter (GM) volume in most region of interest (ROI) of both “what” and “where” visual cortices
Summary
The human visual cortex is primarily located in, but not confined to, the occipital lobe. It extends into the temporal and parietal lobes and shows complicated cortical distribution. The dorsal pathway, known as the “where” stream, is an occipito-parietal network which lies between the early visual cortex and those specialized cortical structures involved in the processing of spatial and motion information (Kravitz et al, 2013). The ventral pathway, known as the “what” stream, is an occipito-temporal network that bridges the early visual cortex and is involved in processing visual identity and feature information (e.g., faces, object identities, colors, and words; Kravitz et al, 2013). The motion-selective area of V5 (hMT+; Morrone et al, 2000; Huk et al, 2002), the spatial specific brain region of V3a (Tootell et al, 1997; Backus et al, 2001), as well as areas for words (Liu et al, 2008) and faces (Grill-Spector et al, 2004), are anatomically distinct in fMRI
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