Causal evidence for dorsal stream involvement in object shape recognition: A tractography-guided ccPAS Registered Report study.

Visual experience requires orchestration of intricate brain connections. One of the most influential models of human visual information processing posits two segregated streams and postulates that conscious object recognition relies exclusively on the ventral visual stream. However, recent evidence suggests that the dorsal stream is involved in encoding crucial attributes of visual object recognition, such as global shape. The specific role of the dorsal stream is controversial in this context. Through an individualized multimodal approach, we investigated the contribution of the dorsal pathway to object recognition. Combining MRI-based diffusion-weighted imaging tractography with cortico-cortical paired associative stimulation protocol (ccPAS), we causally manipulated V1-to-IPS dorsal stream connectivity by strengthening synaptic efficacy between targeted regions through Hebbian-like plasticity mechanisms. Thirty-two healthy participants underwent three tractography-guided ccPAS sessions targeting right V1-to-IPS connectivity with different interstimulus intervals (9 ms, 0 ms, 30 ms). Before and after each session, participants performed a two-alternative forced-choice task requiring discrimination of either global shape or local features. Results showed that the critical 9 ms V1-to-IPS ccPAS protocol improved global shape discrimination, as indexed by accuracy and d', relative to the 0 ms control condition, with no effect on reaction times or response bias. Although task specificity was not fully supported, exploratory analyses revealed no changes in local feature discrimination. These findings provide causal evidence that strengthening dorsal V1-to-IPS connectivity enhances global shape processing, supporting a direct contribution of the dorsal stream to object recognition and potentially guiding the development of targeted rehabilitative interventions after visual system damage.