Ventral-dorsal interactions in the context of action recognition in humans

Background: A common idea of brain organization states that visual input is processed separately within a ventral stream for object perception and recognition, and a dorsal stream for guiding actions towards visible objects (Milner and Goodale 1995). Within such a network structure, the role of the mirror neuron system (MNS), which is accepted to be relevant for context-dependent action recognition, is yet not clear. The pars opercularis (PO, BA44) of the inferior frontal gyrus (IFG) and the inferior parietal lobule (IPL) are considered as anatomical substrate of the human MNS. In this context, one of the implications of different visual processing streams might be that PO functionally and anatomically show interactions with the “ventral stream“. To address this issue, we analyzed the white matter pathway architecture structurally connecting occipital, parietal, and temporal areas with inferior frontal gyrus, representing areas relevant for action observation.

Methods: 60 normal subjects were scanned in a block-design while observing a video of a grasping movement. DTI scans were obtained from each subject. Individual region-to-region specific white matter tracts between peak voxel of significant clusters were extracted from combinations of single probabilistic connectivity maps as described previously (Kreher et al. 2008). Tracking results were then normalized before binary group tracking maps were generated.

Results: We obtained significant BOLD activity for visual (V1, V4, V5) cortex, inferior temporal cortex, IPL, AiP, superior parietal cortex (SPC), and PO as well as pars triangularis (PT, BA45) of IFG. Region-to-region specific fiber tract extraction revealed two main streams of long association fiber tracts: towards PO, temporo-occipital and parietal seed points were connected via the SLF/arcuate fascicule, whereas PT additionally received projections via a ventral pathway along the extreme/external capsule.

Conclusion: It was suggested that, within the visual dorsal stream, both the context of an action and action recognition are processed within parallel pathways between IPL and premotor cortex (Rizzolatti and Matelli 2003). Here, we refine this view: our results suggest that, in the context of goal-directed action observation, the “dorsal stream“ gains access to temporal areas of the “ventral stream“ via fibers of the SLF and arcuate fascicle, representing an anatomical bridge for “ventral-dorsal“ interactions for action recognition.