A salient image region is a part of the scene that stands out relative to neighboring regions. By that we mean that a human observer would experience a salient region as being more prominent. It is, however, important to quantify saliency in terms of a mathematical quantity that lends itself to measurements. Different metrics have been shown to correlate with human fixation data. These include contrast, brightness, and orienting gradients calculated at different image scales. In this paper, we show that saliency can be measured by the transformations pertaining to the dihedral group D₄, which is the symmetry group of the square image grid. Our results show that salient features can be defined as the image features that are most asymmetric in their surroundings.