Photoreceptors and Image Processing: Part 1B Lecture Notes
Key Words and Terms
eye, retina, visual cortex, fovea, eye movements, photoreceptor topography, ganglion cell receptive field, center-surround receptive field, Hering illusion, motion detector, Ouchi illusion, stereoscopic depth perception, depth from shading, figure-ground organization, visual art
The human retina has a specialized central region, the fovea, which has a high cone density and is used for high acuity vision.
Fixational eye movements serve to bring the image of the object of interest onto the fovea. Monitoring these eye movements reveals the visual search strategy associated with a given visual task. Eye movements also largely compensate for head and body motion.
The antagonistic center-surround spatial organization of many retinal ganglion cell receptive fields enhances the detection of spatial contrast and accounts for the Hering illusion. This organization, and its elaboration at higher levels within the visual system, probably accounts for the effectiveness of simple line drawings and portraits, which, in essence, are creating an image that mimics the brain’s processed representation of the scene.
Other retinal ganglion cells respond selectively to motion in a particular direction, and some of these cells respond only to local motion. Retinal ganglion cells that are direction-selective motion detectors mediate the Ouchi illusion.
Binocular vision (i.e. vision with two eyes) serves as the anatomic basis of stereoscopic depth perception. Charles Wheatstone, a British engineer, first recognized this phenomenon in the early 19th century. Because of the different locations of the two eyes within the head, the images projected onto the left and right retinas differ, and those differences allow the brain to calculate the relative depths of objects within the scene. How and where in the brain this calculation takes place is still not entirely clear.
A variety of simple perceptual effects that are both subconscious and effortless appear to involve learned responses. For example, judgments about three-dimensionality based on shading are made with the hidden assumption that the source of illumination is from above (a generally correct assumption for the real world); and judgments about absolute size are strongly biased by our assessment of how far away the objects are.
Perception in the context of ambiguous or competing ways of organizing the scene suggest that the visual system attempts to synthesize a single, unifying, and consistent perceptual explanation for any given scene.
We can extend the theme summarized above regarding simple line drawings. Thus, abstraction in visual art probably works because it mimics some of the perceptual strategies used in the brain’s higher visual centers.