Holes, objects, and the left hemisphere

The human visual system is remarkably efficient at extracting useful information, especially in detecting objects in our environment. Many models have been developed over the years to account for this amazing ability, most following the sensible hierarchy of detecting features first and then integrating them to build objects (1, 2). A notable exception to this general scheme is the theory of topological perception proposed by Lin Chen (3). During the last 25 years, Lin Chen and colleagues have argued for the importance of extracting global topological properties as primitives in object perception (4). The emphasis on global properties in perception is not new [e.g., Gestalt theory of perception (5)], but the topological perception theory specifically defines the global properties as topological invariants. In addition, this theory states that the primitives of visual form perception are geometric invariants at different levels of structural stability under transformations. Thus, a more stable property would be more primitive and more important to extract early in the process. Topological properties are the most stable in relation to other geometrical properties such as projective, affine, and Euclidean properties. In a recent issue of PNAS, Lin Chen and colleagues (6) reported the intriguing discovery that the human visual system's sensitivity to topological properties is superior in the left hemisphere, at least for right-handers (Fig. 1).