Neuroscientific Studies of Vision and Consciousness
Your visual field has a hole where the optic nerve leaves your eye. Have you seen it—the blind spot? Our brains mask it with something contextually appropriate. The masking may be imperfect, but we seldom notice. To notice the blind spot, stare at the right dot with your left eye while covering your right eye and move your head closer to the page. At appropriate distance the left dot vanishes.
Some take this masking as illustrating how our world of experience is a systematically constructed illusion.
More evidence of the illusory character of the visual world comes from empirical studies of change blindness, a phenomenon involving not noticing visual changes. It works in the following way. Suppose I display a picture to you, then a brief flicker (some visual noise), then an altered version of the picture. With appropriate timing, you might not detect any changes, although they could be substantial. Our visual system comes with change detection mechanisms that work well for detecting movement. However, they work with continuously evolving scenes and can be knocked out by sudden visual noise, as many drivers have experienced. With a splash of water on the windshield, an animal crossing the road up ahead enters our consciousness too late. With knocked-out visual change detection, the driver relies on visual attention, but it is too limited, and suddenly the animal hits the windshield. Where did it come from? Drivers in these situations learn how visual attention is more limited than it seems.
When we take a walk in a park and admire the scenery, feel the fresh air around us, and hear rustling leaves, we have a sense of encountering it all, not just the things we happen to focus on. Change blindness studies challenge this. They show we are aware of less than we think. Studies of a similar phenomenon—inattentional blind- ness—provide further evidence for how visual experience is illusory. In one study, subjects were asked to pay attention to one of two teams playing basketball in a video clip. In it, a person in a gorilla suit walks into the game, waves to the camera at center stage, and walks off. After watching the clip, most participants didn’t report anything unusual (Simons and Chabris 1999).
Our eyes don’t move smoothly over the visual field. They jump around—sac- cade—from place to place. Vision is dependent on an incomplete series of snapshots the brain pieces together. Under these circumstances, full visual awareness is an illusion. Visual awareness is not all that is perplexing about human perception.
-  Our eyes move smoothly, continuously over the visual field, only when we are tracking a movingobject, but then all we see in detail is the object we are tracking.