Fields of Vision

Seeing an object and recognizing it for what it is involves image processing by cells in the retina and brain. Three type of cells are involved in this processing: simple, complex and hypercomplex. Simple cells respond best to a clearly defined slit of light (or dark) or to edges between dark and light areas, but the images have to be exact locations in the receptive field of the cell and in the proper orientation. Complex cells respond to slits and edges as well, but they generally react to any location or orientation of the image. The hypercomplex cells respond best to even more specific features such as corners, angles and lines of exact length, orientation and location. The area which encompasses the sight of both eyes is called the "field of vision". Capturing an object, whether standing still or moving, and converting it to nerve signals is only part of the process of seeing. In order for us to see, the brain must analyze the signals and draw meaningful information from them. Some nerve signals are passed from the retina to the temporal lobes of the brain, near the sides of the head, and these contribute to the processes of visual recognition and memory. Another section of the brain acts to control visual attention. Objects on the periphery of vision are detected and the head and eyes are instructed to turn and bring the object into focus inside the visual field. Another set of nerves carries signals to the cerebellum, the part of the brain responsible for muscular coordination. For full depth perception, the stereoscopic vision by two eyes is a necessity. Each eye views a scene from a slightly different angle, and the fusion of these two images gives a three-dimensional effect. With objects less than 200 feet away, vergence comes into play, so that each eye is angled in a little to focus directly on the object; the larger the object, the closer it is. When objects are moving, the brain knows that close objects pass faster than those farther away, and it can, therefore, compare the apparent speeds. Perfect 20/20 vision will not be enough to pass an eye test given to military pilots. It also involves "contrast sensitivity." One must be able, for example, to see a white cat walking in the snow.