Lecture 7 Waber Fractions the Retina and Color Vision Theories Real Change 1 1 correspondence Perceived change Waber Fraction It was discovered that you could do psychophysical scaling mapping the perception of the individual the psychological component to the physical component We are not precisely calibrated to our environment we don t register changes in the environment in our perception in exactly the way they occur We perceive the world we don t directly experience it Vision the Retina The retina is a layered set of neural tissue in the back of the eye we pick up information through the retinal the retinal absorbs light Information processing begins at the retina and the retina sends information on to the brain The optic neurons that connect the retina to the primary visual cortex an area in the rear of the brain where visual information is processed in the very early stages these optic nerves pass through a hole in the center of the retina called the optic disk Optic disk is responsible for the phenomenon called the blind spot there are light receptors on the retina but the optic disk has no light receptors so we are blind to everything on the optic disk This can be a problem when driving This is a negatively accelerated function it takes more and more and more real change to see a perceived change the slope of the function is different for each stimulus This was the first time that we could objectively measure a psychological mental event Real Change Perceived change There are two types of receptors on the retina What happens in the real world 1 Rods are specialized for low light do not process color very well have low visual acuity fuzzy picture there are about 120 million of these 2 Cones are specialized for color and bright light they have high visual acuity give sharp clear pictures there are about 7 million of these The receptors are not spread evenly across the retina the cones are in the center of the retina the phobia and the rods are at the edges Thus you see things better by looking right at them in light Your peripheral vision outside of retina is where the rods are so this is good in low light basically you should look to the side of what you want to see in the dark so you can see it with your rods Information processing begins at the retina since there are 127 million receptors but only 1 million optic nerves thus the information from 127 million inputs must be condensed into 1 million fire no fire nerve actions This is a selection of what information the brain gets 127 mil 1 mil Each optic nerve has a receptive field a group of receptors that it is sensitive to Center Surround System center surround light light Light hitting the surround has a different affect than light hitting the center thus if most of the light hits the center the nerve will do one on thing say increase rate of fire while if most of the light hits the surround the nerve will do another thing decrease rate of fire light falling equally they cancel each other out cell does nothing A system like this can enhance boundries a difference between light falling on different parts of the object Another control mechanism is when neurons are laterally antagonistic which means that the neurons are connected in such a way that as the firing in one becomes stronger it inhibits the firing of the other as one gets stronger it shuts the other one off This enhances the difference between light intensity Edge detection is really important for visual object identification Perception starts virtually immediately Color is a phenomenon of perception We take input from the world and we translate that input into color How do we perceive color Color Vision Tri chromatic theory Early Mid 1800 s you can create any color by mixing the wavelengths of light from green blue and red Not the colors seen in art class because there are two ways to produce color additive color mixing where you add wavelengths into the light stream and subtractive color mixing where you take wavelengths out of the light stream We could perceive color by having specialized receptors in the eye for red green and blue then depending the number of receptors of each color that are activated you can see all of the colors Theory perceive all the colors because we have specialized receptors for processing the wavelengths of red green and blue and by mixing and matching how those receptors are firing we can produce all of the colors that humans can see Prediction we don t have rods and cones we have rods and three different kinds of cones which are specialized to respond to specific light frequencies that correspond to red green and blue Results we have three kinds of cones which are maximally sensitive to red green and blue Opponent Process Theory Tri chromatic theory doesn t explain opposite color after image thus opponent process theory was developed Opposite color after image is when you stare at a picture and then look at a blank screen and you will see the same image with the colors reversed Instead we have three channels rather than a red line a green line and a blue line Each channel can register one of a pair of colors there is a yellow blue channel a black white channel and a red green channel The channel is arranged in an antagonistic relationship such that blue inhibits yellow and vise versa black inhibits white and vise versa and red inhibits green and vise versa This explains opposite color after image because when we get an extremely bright light light a flash bulb or something that you stare at without moving your eyes for a long time the resources in the channel that is processing the information becomes exhausted and when you remove the bright light the exhausted color in the channel is much weaker than the other so for a short period of time the stronger channel takes over and sends a signal even when there is no input to the retina Which theory is correct Both theories explain part of the process of color vision but neither is sufficient on its own We need a theory that combines both of them
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