White matter connectivity changes between visual and higher-level cortical regions in association with perceptual learning revealed by diffusion tensor tractography

Perceptual learning (PL) is defined as a long-term increase of performance in a visual task as a result of training and is regarded as a manifestation of plasticity in the visual system. It has been shown that some types of PL are associated with an increase of BOLD signal in the region of V1, which corresponds to the trained location (e.g., Yotsumoto et al, 2008). However, it is not clear whether structural plasticity occurs in association of PL. Neither is it whether plasticity occurs only locally within the visual cortex or globally in connectivity between the visual cortex and higher stages. To address these questions, we used diffusion tractography to identify several white matter pathways running between the visual cortex and the anterior part of the brain. Seven young adults underwent 14 daily behavioral training sessions of a texture discrimination task (TDT: Karni and Sagi, 1991). Subjects also underwent four diffusion tensor imaging scans: pre-training, after 1, 6 and 14 training sessions. Superior longitudinal fasciculus (SLF), inferior longitudinal fasciculus (ILF) and inferior occipito-frontal fasciculus (infOFF) were selected for investigation because these tracts are known to be involved in visuospatial processing, and can be reliably identified by deterministic diffusion tractography in vivo. Fractional anisotropy (FA), which represents the integrity of white matter on the identified pathways was obtained across training periods and was normalized relative to the whole brain. Behavioral results indicated that performance in TDT significantly improved by training. Results by the tractography indicated that FA values of the right infOFF and the left ILF were significantly increased after 14 training sessions. These results suggest that axonal connectivity changes occur between the visual cortex and the frontal or temporal areas, while functional activity changes occur mostly in the visual cortex, in association with PL of TDT.