Visual Coding and Receptors Perception in general Most of us perceive springtime as green There is no up So much of what we believe is what we perceive and how we perceive things People get really angry about perception If we don t perceive things as they do they get mad Do we all perceive the same thing The Matrix is one of the most interesting movies that deals with perception With any stimulus out there it reaches a receptor It goes through 3 steps It goes through reception of that stimulus to perception of that stimulus First step is reception whatever it is you see an absorption of that physical energy by a receptor for it Following reception one sees transduction Transduction is a conversion of that physical energy to an electrical chemical pattern in the neurons Finally one sees coding which is a one to one correspondence between some aspect of that physical stimulus and some aspect of nervous system activity Certain neurons in the brain respond when you see a blue light of a certain intensity other neurons on seeing a red light at the same intensity Each sensor receptor is designed to receive one type of energy Like visual receptors receive light auditory receptors receive sound And if that receptor is stimulated it will produce what is called generated potential kind of like EPSP If you see enough stimulus you will see an action potential generator potential This specificity of sensory neurons has been known about for a very long time like 1800s There s a law about it law of specific nerve energies Simply put said that the brain will see activity of the optic nerve will smell activity of the olfactory nerve it hears activity of the auditory nerve Closing one s eyes and rubbing them will cause one to see flashes of light which is brain seeing activity of visual receptors The law is still true for the most part today Know more about vision than any other sense Light enters the eye through the pupil focused by the cornea and lens onto the retina rear surface of the eye The retina is lined with visual receptors Light from the right side of one s world strikes the left side of the retina and vice versa Light from the bottom part of one s world strikes the top of the retina and vice versa So one gets an upside down and backwards vision of the world but the brain doesn t process the world that way so we don t see it that way The path of light for reception is slightly inside out When light enters the eye it passes through a lot of cells Goes through all these cells before it even reaches the receptors These cells are very translucent and have no effect on the light Light reaches receptors along the cones Also reaches cells called bipolar and ganglion cells The axon of the ganglion cells move around exit the eye at the rear at a point called the blind spot Blind spot is so named because there are no receptors there Blind spots at each eye brain fills each spot in for you and you don t notice it You would think it would screw up your vision It doesn t happen because information from the axons reaches the brain at the same time because it travels faster In humans in the center of the retina one will find the fovea The fovea is specialized for very acute detailed vision One to one correspondence of cells in the fovea bipolar to ganglion cell Fovea is responsible for about 70 of visual input into the brain That s in humans Other animals are very different The big cat cheetahs they have a visual streak not fovea That s because those animals spend a lot of time scanning horizons Hawks have really good visual systems The eyes take up a lot of space in the head they have two foveas Hawks also have more visual receptors at the top of their eyes hawks spend a lot of time in flight looking down for prey Many types of prey like field mouse have more visual receptors on the bottom of their eye always looking up Differences depending on species and evolutionary needs Ending of the rod in the cone Rods are abundant in the periphery in humans peripheral vision They are sensitive to dim light Cones are in the fovea respond to bright light and responsible for color vision These receptors contain what are known as photopigments chemicals that release energy when struck by light These photopigments contain 11 cis retinal These 11 cis retinal are bound to opsins chemical bound protein When light strikes sensory neurons causes that 11 cis retinal to change to all trans retinal When those sensory receptors respond what one actually sees is the closing of a lot of ion channels basically inhibition Inhibition is just as important as excitation Color vision We have a lot more rods than cones 20 1 ratio Cones are responsible for most of what we see Three theories of how color vision works First theory is more general of how color vision works trichromatic theory of color vision Attends to color vision at the level of the cones 3 different types of cones short wavelength cones medium wavelength cones long wavelength cones Relative rate of responding of these cones that actually determines the color we see Most humans do have 3 types of cones Have more medium and long wavelength than short wavelength cones Short wavelength cones are distributed fairly evenly across the retina the others are more haphazardly scattered Mice have one kind of cone they re colorblind Opponent process theory They were devised to explain a particular phenomenon of color vision Developed to explain negative afterimages Idea is that we perceive colors as paired opposites level of bipolar cells The thought is that you fatigue those cells by staring at a certain color for a long time you will start to see the opposite color that s paired with the original The paired opposites are red to green yellow to blue white to black This theory may be partially correct but certainly is not totally correct This theory is trying to explain color vision at the level of the bipolar cell The reason it has to be more than this to be a correct encompassing theory is that you can actually see negative afterimages to illusions And there is nothing hitting your eye from looking at illusions So something is going on cortically to cause seeing optical illusions things that aren t even there Retinex theory Devised solely to explain color constancy Color constancy is our ability to recognize colors despite changes in lighting Brain subtracts out that constant so to see the colors as they actually are Some people suffer from color vision deficiency When