Occipito-temporal connections in the human brain

Diffusion tensor MRI (DT-MRI) provides information about the structural organization and orientation of white matter fibres and, through the technique of 'tractography', reveals the trajectories of cerebral white matter tracts. We used tractography in the living human brain to address the disputed issue of the nature of occipital and temporal connections. Classical anatomical studies described direct fibre connections between occipital and anterior temporal cortex in a bundle labelled the inferior longitudinal fasciculus (ILF). However, their presence has been challenged by more recent evidence suggesting that connections between the two regions are entirely indirect, conveyed by the occipito-temporal projection system--a chain of U-shaped association fibres. DT-MRI data were collected from 11 right-handed healthy subjects (mean age 33.3 +/- 4.7 years). Each data set was co-registered with a standard MRI brain template, and a group-averaged DT-MRI data set was created. 'Virtual' in vivo dissection of occipito-temporal connections was performed in the group-averaged data. Further detailed virtual dissection was performed on the single brain data sets. Our results suggest that in addition to the indirect connections of the occipito-temporal projection system: (i) a major associative connection between the occipital and anterior temporal lobe is provided by a fibre bundle whose origin, course and termination are consistent with classical descriptions of the ILF in man and with monkey visual anatomy; (ii) the tractography-defined ILF is structurally distinct from fibres of the optic radiation and from U-shaped fibres connecting adjacent gyri; (iii) it arises in extrastriate visual 'association' areas; and (iv) it projects to lateral and medial anterior temporal regions. While the function of the direct ILF pathway is unclear, it appears to mediate the fast transfer of visual signals to anterior temporal regions and neuromodulatory back-projections from the amygdala to early visual areas. Future tractography studies of patients with occipito-temporal disconnection syndromes may help define the functional roles of the direct and indirect occipito-temporal pathways.