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            In class we learned that the image produced by the absorption of light on the retina is not the same picture we ‘see’. That image is modified in the brain through the process of imaging and is the result of photoreceptors as well as other inputs and expectations in the nervous system. One difference between these two pictures is the dimensions; the image on that appears on the retina is two dimensional, but we experience a three dimensional world.

            I did some searching to account for this difference and found that Stereopsis is an important cue to depth perception in. Stereopsis refers to the ability to distinguish the relative distance of objects with a physical displacement between the objects. The lateral displacement of the eyes provides two slightly different views of the same object. Because each eye views the visual world from slightly different positions, each eye's image differs from the other. Objects at different distances from the eyes project images in the two eyes that differ in their positions. This is retinal disparity and is important in the perception of depth by stereopsis. Retinal disparity within the zone of focus in each eye can be fused resulting in the convergence of the optic axes of the two eyes. Due to the lateral displacement of our eyes, slightly dissimilar retinal images result from the different perception of the same object from each eye.  Stereoscopic depth perception results from fusion of the images in each of the eyes.

            One draw back to this process is that when the two images don’t match up (that is there is an object in one and not the other) suppression, superimposition or binocular rivalry may occurs. Suppression occurs to eliminate one image in order to prevent confusion. Superimposition results in one image presented on top of the other image. Binocular rivalry describes alternating suppression of the two eyes resulting in alternating perception of the two images. This usually occurs when lines are presented to the two eyes differ in orientation, length or thickness. These modifications in the fusion process account for some of the optical illusions we experienced in class.