Abstract

The anatomical connectivity pattern of a brain region determines its function 1 . Although invasive tracer studies have produced a large body of evidence concerning connectivity patterns in non-human animals 2-4 , direct information concerning brain connections in humans is very limited. Injection of fluorescent dyes post mortem allows tracing of tracts, but only for distances of tens of millimeters 5 . Longer-distance connections can be investigated by dissection of major tracts or histological studies of remote degeneration following a focal lesion 6 , but such work is based on a relatively small number of informative patients. A specific, important focus for investigation is the thalamus because nearly all incoming information to the cortex is routed through this deep gray-matter structure. The thalamus is subdivided into cytoarchitectonically distinct nuclei which have different patterns of anatomical connectivity that are well characterized for non-human animals Diffusion imaging characterizes the apparent diffusion properties of water Here, using a probabilistic tractography algorithm, we were able to infer anatomical connectivity that progresses fully into gray matter. We thus provide a comprehensive description of the connections between thalamus and cortex in the human brain in vivo. An additional result of this approach is the discrimination of human thalamic subregions on the basis of their connections with the cortex. RESULTS We used a fully automated probabilistic tractography algorithm (see Methods) to form connectivity distributions from individual voxels within the thalamus of a single subject. From these distributions, we traced pathways all the way to the cortex Commonly connected thalamic subregions We segmented the cortex into large, anatomically defined regions (see Methods) corresponding to known connection areas of the major thalamic nuclear groups in non-human primates