![]() The second term that must be defined is 'illusion.' The usual concept of an illusion is a percept that fails to agree with the real world measurements made with devices such as photometers, spectrophotometers, rulers, protractors, and so on. Perception is thus used here in the conventional sense of our visual experience or phenomenology. Of course many visual stimuli result in appropriate behavioral responses even when we are not aware of having seen them indeed the majority of visual processing and visually guided behavior falls in this category (think of all the visual information processed and responded unconsciously to keep your car on the road when driving to work while preoccupied with other thoughts). The first term that needs to be defined in discussing visual illusions is 'percept.' The simplest definition of visual percepts is "visual experience" which works well in ordinary discourse, vision science, and thinking about visual illusions. 3 Explaining illusions in empirical terms.2 Approaches to understanding visual illusions.An implication of this approach is that the distinction between appearance and reality as discussed by philosophers like Plato (360 BC) and Kant (1787) finds new support. Nonetheless, to be successful, visually guided behavior must deal appropriately with the physical sources of light stimuli, a quandary referred to as the “inverse optics problem.” As briefly explained here, visual illusions appear to arise primarily from the way the visual system contends with this problem. In consequence, the patterns of light in retinal stimuli cannot be related to their generative sources in the world by any logical operation on images as such. 1 the same conflation obtains for illumination, reflectance and transmittance see Purves & Lotto, 2003). In contemporary terms, information about the size, distance and orientation of objects in space are inevitably conflated in the retinal image (Fig. ![]() The overarching obstacle in the evolution of vision, however, was documented several centuries ago when George Berkeley pointed out that information in retinal images cannot be mapped unambiguously back onto their real-world sources (Berkeley, 1709/1975). Among these are: (1) the limited resolution of photoreceptor mosaics (thus the input signal is inherently noisy) (2) the limited number of neurons available at higher processing levels (thus the information in retinal images must be abstracted in some way) and (3) the demands of metabolic efficiency (thus both wiring and signaling strategies are sharply constrained). The evolution of biological systems that generate behaviorally useful visual percepts has inevitably been guided by many demands. Catherine Howe, University of Washington, Dept of Psychiatry, Seattle, WA, USA Wojtach, Center for Cognitive Neuroscience, Duke University, Durham, NCĭr. Dale Purves, Center for Cognitive Neuroscience, Duke University, Durham, NCĭr.
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