of limbic system activation and reinforcement. Grouping Perceptual grouping to delineate a figure from the background may be enjoyable. The source of the pleasure may have come about because of the evolutionary necessity to give organisms an incentive to uncover objects, such as predators, from noisy environments. For example, when viewing ink blots, the visual system segments the scene to defeat camouflage and link a subset of splotches together. This may be accomplished most effectively if limbic reinforcement is fed back to early vision at every stage of visual processing leading up to the discovery of the object. The key idea is that due to the limited attentional resources, constant feedback facilitates processing of features at earlier stages due to the discovery of a clue which produces limbic activation to draw one's attention to important features. Though not spontaneous, this reinforcement is the source of the pleasant sensation. The discovery of the object itself results in a pleasant 'aha' revelation causing the organism to hold onto the image. An artist can make use of this phenomenon by teasing the system. This allows for temporary binding to be communicated by a signal to the limbic system for reinforcement which is a source of the aesthetic experience. Contrast Extracting contrast involves eliminating redundant information and focusing attention. Cells in the retina, the lateral geniculate body or relay station in the brain, and in the visual cortex respond predominantly to step changes in luminance rather than homogeneous surface colors. Smooth gradients are much harder for the visual system to detect rather than segmented divisions of shades resulting in easily detectable edges. Contrasts due to the formation of edges may be pleasing to the eye. The importance of the visual neuron's varying responses to the orientation and presence of edges has previously been proven by David H. Hubel and Torsten Wiesel. This may hold evolutionary significance since regions of contrast are information rich requiring reinforcement and the allocation of attention. In contrast to the principle of grouping, contrasting features are typically in close proximity eliminating the need to link distant, but similar features. Perceptual problem solving Tied to the detection of contrast and grouping is the concept that discovery of an object after a struggle is more pleasing than one which is instantaneously obvious. The mechanism ensures that the struggle is reinforcing so that the viewer continues to look until the discovery. From a