2007-10-09 Perception
Annenberg CD collection
How is our brain tricked by visual illusions? What makes us misjudge depths? Sensation and perception. (We just need Alex Ketteman doing tricks for the whole period and we'd be good instead of a video). Lights, camera, perception. Perception is our way of discovering what is happening outside of our body. All species have developed sensory organs. Bald eagles have good vision, dogs can smell things that are hundreds of times lower in concentration than we can, and bats can use SONAR to catch tiny, fast moving insects. Psychologists study all sensory processes: hearing, smell (olfaction), and taste. And then there's visual perception.
Absolute threshold- the minimum signal that can be detected at least half the time. That includes a candle flame seen at 30 miles on a dark, clear night. The tick of a watch, on quite conditions, at 20 feet. One teaspoon of sugar in two gallons of water. One drop of perfume in a three-room apartment. Wing of a bee falling from a distance of 1 centimeter. There are many millions of specialized receptors spread throughout our body: eyes, ears, nose, tongue, skin, muscle. Our joints and tendons, our inner ear, and even certain parts of our digestive tract. Each receptor is designed to detect physical energy like light waves or sound waves. This is then converted into electrical code (neural impulses) which are transported to the brain's cortex. The cortex has the job of putting all of the information together. Different regions of the cortex translate the information differently.
Visual information is processed in the occipital lobe. Hearing and smell in the temporal lobe. Speech reception in the frontal lobe, and body senses in the parietal lobe, which then project their results to the thalamus (relay station) which then transmits them back to the cortex in various areas. That's where abstract information processing takes place, where we can connect the 'new' with the 'old'. Visual perception takes place in the retina (back surface), pathways in the brain, and the part of the cortex in the back of the brain- the visual cortex.
Images are formed on the retina full of holes, upside down, and kind of blurry. The distal stimulis is the "thing out there". Information derived from the object's stimulation of a receptor, this stimulation is called the proximal stimulus. The true nature of the actual object, the distal stimulis, so here, the brain must eliminate confusing signals and put it all into perspective. These transformations occur instantly and continually.
Mapping the action of receptor cells from the retina to the cortex. Turn the energy into electrical signals. 125 million rods and cones in each retina. In the visual pathway, you start with the retina, and the output is the optic nerve, which end up in a few regions, which are each then going to send a cable of a few million axons that connect to other regions, and even in the visual cortex, there are various ways of connecting them, and this whole thing is a pathway, and the primary cortex, beyond 7 stages from the retina, both cells react to visual stimuli, only if a line falls on a retina, and it has to be a particular orientation, like an edge between bright and light, and the position of the line and so on are terribly important, and so some cells respond and some don't.
You can show that there's a different amount of stimulation in certain parts of the brain due to whether a line is horizontal or vertical. This shows that visual patterns are mapped to hundreds of millions of various neurons. (They showed a cat watching a scanning, white line). Stanford University-> study of stages of light. Using computer graphics, he has demonstrated how the brain can break down the visual information, into millions of bits, into a coherent vision that we can recognize. There is complexity and difficulty. The visual system uses edges, lines, and other characteristics. The brain looks for vertices, rigidity, etc.
The brain is computing size, distance relationships, boundaries, edges, identifying figures within backgrounds, removing us from objects we need to avoid, and many of these are made without conscious experience of what's going on. The brain is doing all of this automatically. When a quarterback sees a receiver, he automatically, unconsciously, calculates angle, velocity, etc., and his muscles adjust accordingly. Getting accurate information from the world around us, is a major task. To sense, perceive, we utilize sensory receptors that detect external stimulation, and we call this bottom-up processing and then there's top-down processing where we use semantics, classification, meaning, etc., and use memory to try to figure out what's going on.
Perceptual constancy- size, shape, orientation, brightness, are seen relatively constant, even if the projected image is weird (like a newspaper with a shadow on it). We have an active construction of reality-- we do not see the world as it is, we see it as we are.
We actively perceive via context. This can even determine the nature of the object itself. Visual perception would be too slow if we had to wait for our brain to take in the "entire" scene (an impossibility). So we speed up our brain at the risk of missing some stuff. Subjective contours- phantom patterns.
Too much unfamiliar information to absorb- surprise, like what happens to eye witnesses. Analytic vs. holistic, which together make a paradox. Next: learning, profiting from experience, mice, men, and women.