New evidence points to an independent, specifically social visual pathway
If you studied some psychological in college, you probably learned about the two primary visual pathways in humans: the “what” ventral pathway, used to identify objects (e.g., persons or buildings) and the “where” dorsal pathway, used for locating and manipulating objects (e.g., reaching, throwing, pushing).
Traditionally, it has been thought that social information was distilled from these two pathways. However, a growing body of empirical evidence obliges us to revise this model. A recent paper published in Trends in Cognitive Science attempts to organize these studies to provide a coherent report of our current understanding of this third, social pathway, and what important questions still remain to be answered.
Most of the evidence for a third pathway comes from non-human primate studies, especially regarding macaques. However, some human-based evidence has also been found.
Tractography studies, which use MRI data to create 3D models of the ‘nerve tracts’ or pathways in the brain, have identified a pathway leading from the visual cortex to the superior temporal sulcus (STS), a portion of the brain involved in social processing, theory of mind, biological motion and face perception.
Additional evidence comes from differences in the way facial information is relayed between the ventral (“what”) pathway and this third, social pathway, including the fact that the latter demonstrates greater connectivity between left and right visual fields. This leads the authors to conclude that “the third visual pathway has evolved to compute face information across the entire visual field to support social interaction, which, by its very definition, is a dynamic and continually changing process.”
Indeed, the evidence supports a preference for and unique capacity to understand moving biological objects, including moving faces. The “face-selective” region of the third pathway responds much more strongly to moving faces than static ones. Disrupting the ventral pathway has no effect on the STS in this regard, meaning they operate largely independently.
In addition to processing moving bodies and faces, the third pathway seems to be implicated in a number of higher socio-cognitive processes. The STS has been implicated in perceiving, evaluating and responding to facial expressions and eye gaze, bodies and point-light walkers (a collection of moving dots or lightpoints that reproduces the experience of a moving body), language, and the audiovisual integration of speech.
The social aspects of vision are undeniable. It may seem in hindsight that models forgoing a specifically social pathway in favor of the purely physical, locational and mechanical have been short-sighted, but this only goes to show how profoundly social experiences have impacted the evolutionary trajectory of the human race.
Understanding the implications and inner workings of this pathway may help us better understand certain developmental disorders, like autism spectrum disorder, and get a clearer picture of how our perceived environment influences our perceptions of those around us.
The study, “Evidence for a Third Visual Pathway Specialized for Social Perception“, was authored by David Pitcher and Leslie G. Ungerleider.