Despite the importance of short association fibres (SAF) for human brain function, their structures remain understudied. It is not known how SAF are organised across the brain, and how consistent their geometries and locations are across individuals. To address this gap, we mapped the precise structures of SAF in the primary (V1) and secondary (V2) visual cortex in a group of participants in vivo and a post mortem specimen. We assessed the consistency of SAF geometries and their expected structural and functional topography using probabilistic tractography on sub-millimetre-resolution diffusion-weighted MRI combined with functional MRI retinotopic maps in vivo . We found that dense SAF connected V1 and V2, forming sheet structures with retinotopic topography and bearing consistent geometries that resembled the local V1–V2 cortical folding. In vivo findings were corroborated by the robust and fine-grained post mortem reference. Our in vivo approach provides important insights into SAF organisation and could be applied to studies across species on cortical and SAF reorganisation and support neuronavigation.

The thalamus is primarily known as a relay for sensory information; however, it also critically contributes to higher-order cortical processing and coordination. Thalamocortical hyperconnectivity is associated with hallucinatory phenomena that occur in various psychopathologies (e.g., psychosis, migraine aura) and altered states of consciousness (ASC; e.g., induced by psychedelic drugs). However, the exact functional contribution of thalamocortical hyperconnectivity in forming hallucinatory experiences is unclear. Flicker light stimulation (FLS) can be used as an experimental tool to induce transient visual hallucinatory phenomena in healthy participants. Here, we use FLS in combination with fMRI to test how FLS modulates thalamocortical connectivity between specific thalamic nuclei and visual areas. We show that FLS induces thalamocortical hyperconnectivity between lateral geniculate nucleus (LGN), early visual areas, and proximal upstream areas of the ventral visual stream (e.g., hV4, VO1). Further, an exploratory analysis indicates specific higher-order thalamic nuclei, such as anterior and mediodorsal nuclei, to be strongly affected by FLS. Here, the connectivity changes to upstream cortical visual areas directly reflect a frequency-dependent increase in experienced visual phenomena. Together, these findings contribute to the identification of specific thalamocortical interactions in the emergence of visual hallucinations.