How do we see the world? Our eyes convert light waves into electrical signal and send it to the brain. From this signal, different patterns are recognized such as lines, edges, borders, shades, colors, objects, things, faces, persons, motion, etc. How are these patterns recognized by the brain? As we grow up, these patterns are slowly learned step by step and stored as NC-s (Neuronal Circuitry). These NC patterns are models of the world. Ultimately, we perceive these models. The following simple illusions vividly show the difference between the reality and our brain constructed models.
(Click on the pictures to maximize it)
(Color of A and B are exactly same. Take a printout of it, then cut the two areas and place them side by side)
(Hollow Face. This is not a trick. As face is usually projected outwards, our brain sees even the hollow side as the projected outward face )
If seeing is recognizing different patterns from the light signal, then how do we see immediately? We do not see immediately. It takes approximately 50 milliseconds to start recognize something. Different elements of visual recognition such as color, motion, etc. are recognized at different locations in the brain and it takes different time duration to recognize them (includes both serial and parallel computations). So, we actually never see live (always delayed) and never see continuously. A snapshot of visual image is taken approximately every 50 milliseconds and processed. Our brain creates an illusion of continuity. That is why we can enjoy movies – continuous snapshots of picture frames as motion picture!
If recognizing different things takes time, how do we recognize so many things so fast, for example when we enter a room? This is also an illusion. We do not recognize all things immediately. Based on the context and history, our brain uses already stored templates and fills different things one by one depending on the needs and importance. That is why it takes a while to find the differences between two similar images.
(If you still can't find the face, a hint: at the bottom. Please try the following test sincerely)
(Major percentage of people miss it. Older people miss it easily. Though we seem to see the entire picture, our brain picks certain things and misses others depending on the situations)
Brain neuronal pulse cycle speed is in the order of tens to few hundreds of hertz (compare to Giga-hertz of current computer speed). Then, how does the brain recognize things so fast? As mentioned earlier, the brain uses many techniques such as parallel computation, template filling, etc. Why is neuronal pulse cycle speed limited? Neurons use chemical ions to charge and discharge to create electric pulses. Building up this charge potential (action potential) takes certain time. Moreover, the brain consumes huge amount of energy to function (as high as our legs). So, the brain normally works at certain minimum required speed that is needed for our normal day to day operations. But during certain high-alert critical situations (such as escaping from a danger or an accident), the brain works faster than normal speed, and hence it can process/recognize lot more information. During this period, things seem to go in slow motion. Action in the external world goes at the same speed. Since the brain processes it faster than usual speed, things seem to go in slow motion. Perhaps, most of the good sports players able to do this during simulated situations such as sports games. As we get older, this brain speed slowly goes down and hence older people perceive outside world as going faster.
For more information: