Research Focus Symmetry perception in humans and macaques Diane M. Beck, Mark A. Pinsk and Sabine Kastner Department of Psychology, Center for the Study of Brain, Mind, and Behavior, Princeton University, Green Hall, Princeton, NJ 08544, USA The human ability to detect symmetry has been a topic of interest to psychologists and philosophers since the 19th century, yet surprisingly little is known about the neural basis of symmetry perception. In a recent fMRI study, Sasaki and colleagues begin to remedy this situation. By identifying the neural structures that respond to symmetry in both humans and macaques, the authors lay the groundwork for understanding the neural mechanisms underlying symmetry perception. Bilateral symmetry pervades both the animal and plant kingdom as well as the man-made world, so perhaps it is not surprising that humans show a remarkable sensitivity to visual symmetry [1]. Infants as young as 4 months are able to discriminate mirror symmetry from other forms of symmetry, and in adults symmetry has been shown to influence various perceptual processes from figure– ground segregation [2] (see Box 1) to judgments of facial attractiveness [5]. Moreover, sensitivity to symmetry does not appear to be a uniquely human trait and, in fact, it not only crosses class but also phyla boundaries, with preferences for bilateral symmetry being shown in various species of birds as well as bees [6]. In a recent study, Sasaki, Vanduffel, Knutsen, Tyler and Tootell [7] not only extend this list to include rhesus macaques (Macaca mulatta), but also take an important step in under- standing its neural underpinnings by identifying the cortical regions that respond to mirror symmetry in both humans and macaques. Symmetry-related activity in humans Using fMRI, Sasaki and colleagues found that symmetric dot patterns evoked more activity than random dot patterns in human visual cortex, more specifically in areas V3A, V4, V7 (an area just anterior to V3A), and the lateral occipital complex (LOC). It is interesting to note that symmetry-related activity was absent in early visual cortex, and instead was confined to more advanced extrastriate areas with large receptive fields that may mediate the integration of information from large portions of the visual field. Importantly, the activity within this extrastriate network of areas correlated strongly with the subjects’ perception of symmetry. Stimuli that were perceived as being more symmetric evoked greater activity than stimuli that were perceived as being less symmetric, suggesting that the response differences did indeed reflect selectivity for symmetry. Interestingly, the Box 1. Symmetry and perceptual organization Mathematically, a figure is symmetrical if it is unchanged across a class of Euclidian transformations, which include translations, rotations and reflections. However, often what is meant by the term symmetry is more specifically reflectional or ‘mirror’ sym- metry, and in fact Ernst Mach [3] suggested that observers are more sensitive to mirror symmetry than to repetition (i.e. translational symmetry). Since then, symmetry in general, and mirror symmetry in particular, has been the cornerstone of several theories of how we organize our perceptual world. Symmetry has been shown to help observers segregate figures from their background [2]. All else being equal, observers will often perceive a mirror symmetrical region as figure over a non-symmetrical region. It has been suggested that symmetry can influence perceived shape. The shape in Figure Ia can be seen as either an upright diamond or a tilted square [3], depending on your perceptual frame of reference. It has been suggested that there is a bias to choose an axis of mirror symmetry as the frame of reference [4]. The square/diamond is ambiguous because it has multiple axes of symmetry. However, if the square/diamond is placed in a context like the one in Figure Ic it looks much more like a square – it seems that a tilted frame of reference is adopted in this context. Again, this can be explained by appealing to symmetry, but this time in terms of the global axis of symmetry: as a group, the set of three shapes in Figure Ic have a diagonal axis of symmetry and thus a diagonal frame of reference. Similarly, the same square as in Figure Ib looks more like a diamond in Figure Id because the symmetry axis of the group (i.e. the diagonal) defines the orientation of the individual shapes [4]. TRENDS in Cognitive Sciences (b) (a) (d) (c) Figure I. Examples of symmetry influencing shape perception. (a) This shape can be seen as either an upright diamond or a tilted square, depending on the perceptual reference frame adopted. Embedded in the context shown in (c) it appears more like a square. Similarly, the square in (b) appears more like a diamond in (d). These effects have been explained as resulting from a strong bias to choose the global axes of symmetry in (c) and (d) as the perceptual frame of reference. Corresponding author: Kastner, S. (skastner@princeton.edu). Available online 2 August 2005 Update TRENDS in Cognitive Sciences Vol.9 No.9 September 2005 www.sciencedirect.com