Training-Induced Functional Homogenization in the Occipitotemporal Cortex: Differential Cross-Modal Mechanisms in Blindness vs. Severe Low Vision

No visual motion is available to totally blind (TB) individuals, raising questions about the utilization of the territory of the visual motion complex (hMT+) in the absence of vision, and its potential reorganization through training. This study explores whether such reorganization differs between TB individuals and those with some residual vision, such as severe low vision (SLV). Methods: TB and SLV subjects underwent five sessions of the Cognitive-Kinesthetic Memory-Drawing Training for spatial navigation. Pre- and post-training, whole-brain scans (Siemens 3T Prisma) were conducted while subjects (i) haptically explored and memorized raised-line tactile maps (30 s); after a 20 s rest, they (ii) drew-from-haptic-memory (30 s) the maps using the opposite hand with a stylus. Results: Despite the absence of visual input, hMT+ was robustly activated bilaterally in TB individuals in the right-hand blind Memory-Drawing task. However, the left-hand Haptic Exploration task activated only right hMT+, an unexpected interhemispheric functional asymmetry. Furthermore, following the training, significant brain reorganization occurred in the lateral occipitotemporal cortex, forming clusters of cortical areas TPOJ 1-3, FST, MTG and LOd expressing the same task-response asymmetry as hMT+. In SLV individuals, on the other hand, although a similar large-scale functional clustering occurred, hMT+ was surprisingly excluded and even unilaterally suppressed. Granger Causal connectivity analysis revealed a complex interplay between hMT+, its surrounding cluster, and motor, somatosensory, and memory areas. Conclusions: The multidimensional findings shed light on non-visual hMT+ functionality, its novel interhemispheric asymmetries, and their implications for functional brain architecture and its reorganization through learning. Furthermore, the results reveal for the first time the emergence of training-induced functional-homogenization of extended clusters of occipitotemporal areas around hMT+ in the visually deprived, which in the sighted are functionally distinct, demonstrating a mechanism for a novel type of cross-modal functional reorganization, offering crucial insights into neuroplasticity and sensory compensation.