from http://www.scholarpedia.org/article/Pinna_illusion :
Pinna illusion is the first visual illusion showing a rotating motion effect. In Figure 1 the squares, delineated by two white and two black edges each, are grouped by proximity in two concentric rings. All the squares have the same width, length, and orientation in relation to the center of their circular arrangements. The two rings differ only in the relative position of their narrow black and white edges forming the vertexes. More precisely, the two rings show reversal of the vertex orientation and, consequently, opposite inclination of the virtual or implicit diagonal orientation polarity obtained by joining the two vertexes where black and white lines meet (Pinna, 1990; Pinna & Brelstaff, 2000).
related to the aperture problem
The Pinna illusion and the related effects represent an opportunity within the context of vision science and cognitive neuroscience (Gazzaniga, 2004; Purves & Lotto, 2003). If the task of a sensory system is to provide a faithful representation of biologically relevant events in the external world, the previous phenomena show that visual perception contrives, through complex neural computations, to create informative and efficient representations of the external environment. These representations are at the same time simpler and richer than the raw signals transduced by receptors. They are simpler because they simplify the enormous quantity of raw measurement information submitted to the central nervous system (see Section 2). They are richer because they contain properties of events and objects abstracted from the primitive sensory signals (see Sections 3 and 4). Therefore, the first opportunity suggested by the previous effects concerns the basic encoding of the features of the stimuli, i.e. the nature and meanings of the signals carried by single neurons, the maps and areas where they operate (see Section 2) and the pattern of motion of objects, surfaces, and edges in a visual scene due to the relative motion between an observer and the scene (optical flow, Gibson, 1979). Furthermore, they are good tests to understand the perceptual context within which a specific element is perceived, namely “what is ‘figure and what is ‘background”, “how separated elements of a visual event are combined and organized in a sensory representation” (see Section 4).