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PSYCH 1X03 (1,053)
Joe Kim (987)

5. Vision 1 and 2

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McMaster University
Joe Kim

PSYCH 1XX3 Mar. 3 /11 3. Vision 1 and 2  Light moves at about 300 000 km/sec  The height of each wave is the amplitude  The distance between waves of the successive waves is the wave length  Variations in amplitude affect the perception of brightness, the higher the brighter  Variations in wavelength affect the perception of colour  Wavelength is measured in nanometers  Smaller wavelengths refer to a light wave with a higher frequency because there is less distance between successive peaks  Larger wavelengths refer to light waves with a lower frequency Wavelength  Humans are only sensitive to a tiny portion of the total range of wavelengths of electromagnetic radiation, this tiny portion is called the visible spectrum  The shortest wavelength that we can see is around 360 nanometers (violet) and the longest is 750 nanometers (red)  Bees can see shorter than 360 in the ultraviolet spectrum and may see flowers that look all the same to us to be perceived the same to them  Snakes can see waves longer than 750 which allows them to find prey in the dark by being able to see the body heat that is emitted by the prey Purity  A light said to be made up of a single wavelength is said to be pure and the perceived light would be completely saturated ex. (f f f f f)  We could have a light that is a combination of all the wavelengths, this light would be perceived as white and would be completely desaturated  Most of the colours we see in our everyday life are not pure but a mixture of wavelengths and are less intense than pure colours The Eye  Light first passes through the curved cornea o Focusing process o Transparent window at the front of the eye  The sclera covers the rest of the eye, it is the white part and is the tougher membrane  After the cornea light passes through the pupil of the eye  The iris or the coloured part of your eye controls the size of the pupil o The iris is a band of muscles that is controlled by the brain so that if not enough light is reaching the retina, this band causes the pupils to dilate, if too much light is entering then it causes the pupil to contract  After the pupil light passes through the lens a transparent structure that does the final focusing of light on the retina The Lens PSYCH 1XX3 Mar. 3 /11 3. Vision 1 and 2  The curvature of the lens causes images to land upside down and reversed from left to right on the retina  There is a correction that allows us to see a properly oriented image  Accommodation o If the object is close the lens of your eye gets fatter or rounder to produce a clear image o If the object is far away the lens of your eye gets elongated to focus the image on the back of your eye The Retina  Light passes through the vitreous humor, which is the clear jelly like substance that comprises the main chamber inside the eyeball until it finally lands on the retina (neural tissue that lines the back of the eye)  Light becomes neural impulses  Paper thin sheet that covers the back of the eye and is made up of a complex network of neural cells arranged in three different layers  The layer at the very back of the eye, farthest away from the light, is where the photoreceptors are located o Cells in the retina that are responsible for translating the physical stimulus of light into a neural signal that the brain can understand  To reach the photoreceptors, light must pass through the other 2 layers of the retinal tissue which are transparent  Inside out? o Has to do with where the photoreceptors get their nutrients from which is a layer of cells at the very back of the eye called the retinal pigment epithelium  The photoreceptors would die without access to the RPE  If photoreceptors were located at the front of the retina, facing the light than they would not have access to the RPE that they need to live Photoreceptors  Rods are plentiful and cones are less plentiful  Cones o High light intensities o Day vision o 6 million o Colour o Good visual acuity (sharpness of detail) o Concentrated in fovea (provides more detail)  Rods o 125 million o Night vision PSYCH 1XX3 Mar. 3 /11 3. Vision 1 and 2 o No colour o Poor visual acuity o Concentrated in periphery Physical to Neural  Photo pigment- complex molecule which is sensitive to light o 4 kinds of photo pigments, 1 for rods and 3 for cones o When a photon of light is absorbed it changes the chemical state of the photo pigment and splits it into 2 component molecules which sets off a bio chemical chain reaction leading to an electrical current flowing across the membrane o The 2 molecules that had originally split can reconnect so that the photo pigment is ready to light again o When exposed to very bright light, the rate of spitting of photo pigments exceeds the rate at which they are being combined, this is why it takes a few seconds to get your eyes adjusted to the dark (dark phenomenon) o After this process, the photoreceptors then send their information to the next layer of cells in the retina called bipolar cells, by means of a transmitter substances o The bipolar cells send their information onto the next layer of cells in the retina called the ganglion cells Ganglion Cells  Collect information from a larger segment in the retina and the axons of these cells all converge on one point of the eye, called the optic disc and then leave the eye to join the optic nerve which travels all the way to the brain The Retina (con’t)  Some amount of visual processing is done in the retina before it enters the brain  Receptive field o The collection of rods and cones in the retina that when stimulated affect the firing of a particular ganglion cell o Variety of shapes and sizes, but most are donut shaped o Light falling in the centre of the donut will either excite (increased compared to baseline) or inhibit (decrease compared to baseline) the cell and the opposite of the charge will be on the outside of the donut o Strikes inhibitory- decrease in the firing rate of the ganglion cell o Strikes excitory- increase in the firing rate of the ganglion cell o Strikes both- cancel each other out same rate as baseline  Horizontal cells activated by photo receptors  Amacrine cells activated by the bipolar cells  Lateral inhibition has the effect of sharpening edges and is supported by both the horizontal and Amacrine PSYCH 1XX3 Mar. 3 /11 3. Vision 1 and 2 Visual Fields and Hemispheres  Before reaching their respective hemisphere, the axons from the inner region of the retina have to cross over to the opposite hemisphere  The point at which the optic nerves from the inside half of each eye cross over to the opposite is called the “optic chiasm”  When it arrives in the opposite hemispheres the optic fibers split and travel along 2 pathways  Most of the retinal or ganglion cell axons travel along the main pathway and synapse in the lateral geniculate nucleus (LGN) which is part of the thalamus that receives visual information  After processing the visual signals are sent to the areas in the occipital lobes that make up the primary visual cortex  A smaller portion of the axons takes a detour to an area in the midbrain called the superior colliculus  Information is sent upwards to the thalamus and onto the occipital lobe or downward structures in the brainstem  This pathway deals with coordinating visual input with information coming in with other senses, as well as localizing other objects in space through head and eye movements and helping to guide those movements Main Pathway  Two subdivisions o Magnocellular Pathway  Processes movement  Runs to 2 layers of the LGN o Parvocellular Pathway  Processes colour and form
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