Short frontal lobe connections of the human brain

Advances in our understanding of sensory-motor integration suggest a unique role of the frontal lobe circuits in cognition and behaviour. Long-range afferent connections convey higher order sensory information to the frontal cortex, which in turn responds to internal and external stimuli with flexible and adaptive behaviour. Long-range connections from and to frontal lobes have been described in detail in monkeys but little is known about short intralobar frontal connections mediating local connectivity in humans. Here we used spherical deconvolution diffusion tractography and post-mortem dissections to visualize the short frontal lobe connections of the human brain. We identified three intralobar tracts connecting: i) posterior Broca's region with supplementary motor area (SMA) and pre-supplementary motor area (pre-SMA) (i.e., the frontal 'aslant' tract - FAT); ii) posterior orbitofrontal cortex with anterior polar region (i.e., fronto-orbitopolar tract - FOP); iii) posterior pre-central cortex with anterior prefrontal cortex (i.e., the frontal superior longitudinal - FSL faciculus system). In addition more complex systems of short U-shaped fibres were identified in the regions of the central, pre-central, perinsular and fronto-marginal sulcus (FMS). The connections between Broca and medial frontal areas (i.e. FAT) and those between the hand-knob motor region and post-central gyrus (PoCG) were found left lateralized in a group of twelve healthy right-handed subjects. The existence of these short frontal connections was confirmed using post-mortem blunt dissections. The functional role of these tracts in motor learning, verbal fluency, prospective behaviour, episodic and working memory is discussed. Our study provides a general model for the local connectivity of the frontal lobes that could be used as an anatomical framework for studies on lateralization and future clinical research in neurological and psychiatric disorders.