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Lecture 3

PSC 131 Lecture 3: First Stages of Seeing

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PSC 131
Geng Joy

I. Lecture 3 – First Stages of Seeing (April 10, 2017) A. Introduction 1. Two Properties of Seeing: a. Sensitivity i. Have you ever seen a star in the “corner of your eye” that disappeared when you tried to look at it? ii. Sensitivity reflects the dimmest light you can detect  In your peripheral vision, you can detect dimmer light b. Acuity i. Acuity: The smallest spatial detail that can be resolved ii. Better in central vision than peripheral vision iii. Ex: Snellen E chart test (eye-doctor test) 2. Light 3. Retina a. Optics of the eye and visual angle b. Optic nerve and the blind spot c. Rods and cones d. Sensitivity vs Acuity e. Dark adaptation 4. Diseases of the Retina B. What is Light? 1. Light – a form of electromagnetic radiation a. Can be described as waves and protons 2. Wave – when light moves around the world (colors) 3. Stream of photons (particle that = 1 unit of light energy) – when light is absorbed 4. Electromagnetic spectrum a. Visible light wavelengths between 400-700 nanometers i. Our biology only allows us to see waves in this range b. Infrared spectrum in rage of 1000-100,000 nm c. Ultraviolet below 200 nm II. Retina A. What is the Retina? 1. The retina contains light sensitive cells, which convert light to electrical activity that is sent to the brain (transduction) a. Neurons collect visual information and transmits it down to the optic nerve of the brain (network of cells) 2. Objects have spatial organization that is preserved as it is projected into the eye; each part is relative during the stages of processing 3. Optic Nerve: blind spot and filling in a. Macula – center of the fovea, contains cones b. Optic Nerve – (blind spot) where (really long) axons of ganglion cells exit the eye, bringing sensory input from other cells to the brain i. There are no photoreceptors to detect anything in the optic nerve—fiber nerve bundles take up the space ii. It allows all of the input collected to be sent to the brain down one pathway c. Blindspot – you don’t see a whole even when you close one eye d. Ex: Finding your blind spots – Close one eye. Fixate the 3, move back and forth until the yellow dot disappears. Fixate other numbers and repeat i. When the yellow dot disappears, you only see the background ii. Your cells are filling in information to make up for what your blind spot can’t see B. Rods and Cones 1. Transduction – occurs when 1 photon of light is absorbed by a light sensitive molecule in a photoreceptor and translate it into chemical signals (neurotransmitters) a. This produces an enzyme cascade that reduces release of glutamate and signals that light has been captured to the bipolar cells, which activate ganglion cells whose axons form the optic nerve 2. Two types of photoreceptors: Rods and Cones a. They have different visual pigments that are sensitive to different wavelengths of light b. They capture light and release neurotransmitters for the bipolar cells and ganglion cells to pick up those signals and send all the information through the optic nerve to the brain for processing 3. Rods – more about detection of light, but not wavelength; very sensitive a. Rhodopsin (protein) c. Scotopic (dim) illumination b. Can respond to 1 photon d. More dense in peripheral retina 4. Cones – need more signal in order to decide it’s enough to release neurotransmitters a. 3 pigments sensitive to long, medium, and short wavelengths (will be important for color vision) b. Requires greater number of photons to respond (need more light to respond) c. Photopic (bright) illumination d. More dense in the fovea 5. Density a. As light enters the eye, all the info will be picked up by rods and cones in the retina b. Density is not evenly distributed c. Lots of cones in central vision (fovea) d. More rods as you move farther from the center (more rods in peripheral vision) C. Sensitivity vs Acuity 1. Sensitivity – the ability to detect the presence of light a. Allows us to see faint stars in the periphery that disappear if we look directly at it b. Provide vision under scotopic (dim, dusk) conditions c. Depends mostly on rods 2. Acuity – the ability to detect detail a.
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