Perception, Week 7 Notes
Perception, Week 7 Notes 222
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This 3 page Class Notes was uploaded by Sarah Kincaid on Tuesday October 18, 2016. The Class Notes belongs to 222 at Boston University taught by Rucci in Winter 2016. Since its upload, it has received 7 views. For similar materials see Psychology of Perception in Psychology at Boston University.
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Date Created: 10/18/16
Review: Light goes into eye, converges to single point if you are emmetropic (too short - myopic, too long - hyperopic) Lens modified through accommodation to tune its refractive properties (issues as we age - loses elasticity) Cataracts - opacities/crystallin irregularities that absorb light so less light gets processed = blurry images Retina: Light travels through all neurons to photoreceptors where light --> electrical signals Photoreceptors: rods & cones (structure, function, photo-activation, distribution around fovea [~300 mm] - periphery better in dark bc rods) Release most of photons in dark (less when its light) measure size of visual stimuli by how big image appears on retina instead of how large object physically is Retinal size measured by "degrees of visual angle" Care about visual angle (distance from object not important): fingernail at arms' length = 1 degree (thumb is 2 degrees) Visual angle is combo of actual size and distance from observer Blue (s-cones), green (M-cones), and red (L-cones) of human retina Humans have 3 types of cones S-cones are 5-10% of total number of cones; not in fovea (s for short wavelength, ~425 nm) 2x more L-cones as M-cones (m for medium, ~525 nm; l for long, ~575 nm) Actual number & distribution of cones vary among people Cones are fragile: loose functionality o Age-related macular degeneration (AMD) Wet AMD: abnormal blood vessels grow under macula and leak blood and fluid, raising macula and impairing central vision; leak; macula displaced = straight lines look wavy Dry AMD: more common; macula cones degenerate; dry AMD can --> wet AMD; advanced dry AMD = no treatment Damages fovea (issue reading) Scotoma = no vision in area o Retinitis pigmentosa Don't notice because influence periphery (bump into things) death & degeneration of rods & cones in periphery; forms rings Scotoma = no vision in this ringed area Can read but difficulty moving around world bc poor vision outside fovea Retina: Horizontal pathway o Horizontal cells: specialized retinal cells running perpendicular to photoreceptors; make contact with photoreceptors & bipolar cells Responsible for lateral inhibition, which creates center-surround receptive field structure of retinal ganglion cells o Amacrine cells: synapse horizontally between bipolar & retinal ganglion cells Transfer info from one bipolar to another (30-40 types, don't know what exactly they do) Vertical Pathway o Bipolar cells: synapses with one or more rods or cones (not both!) and with horizontal cells; passes signals onto ganglion cells On/Off cells: ON cell respond to increase in light; OFF cells respond to decrease in light (50/50); two separate channels that go to cortex Diffuse bipolar cell: receive input from multiple photoreceptors (up to 50); high sensitivity, low acuity; periphery Midget bipolar cell: small cell receives input from one cone; high acuity, low sensitivity; fovea o Ganglion cells: retina output stage; humans only have ~1,250,000 ganglion cells (1:100 of ganglion cells: photoreceptors) P ganglion cells: ~70%, connect to parvocellular ("small cell") pathway (fine visual acuity, color, shape processing; poor temporal resolution, good spatial resolution); input from midget bipolar cells M ganglion cells: ~10%, connect magnocellular ("large cell") pathway (motion processing, excellent temporal resolution, poor spatial resolution); input from diffuse bipolar cells Koniocellular ganglion cells: primordial pathway? Remaining cells o Photoreceptors: rods (periphery) & cones (fovea) Convergence: how many neurons projecting to each other; low in fovea but high in periphery (causing sensitivity & acuity) Because of high light sensitivity in periphery, we have low acuity in periphery. Similarly, in fovea, we have high acuity but low light sensitivity. -give and take With electrode on/near neuron, we can measure changes in membrane potential (action potentials) Receptive field: region in which stimuli activate neuron; spikes = activity; fovea (small) & periphery (large) ON-center ganglion cells respond a lot with light in center; low response with light in periphery o As size of spot of light increases beyond receptive field of center, becomes less effective in stimulating cell = contrast (not luminance) Care about properties of object in contrast to that of surroundings (measuring brightness of center vs periphery) Latera inhibition: what gives the on-center/off-surround or vice versa (off-center/on-surround structure) Action potential works when light/dark edge is on edge of center/surroundings Input pattern of luminance superimposed with input signal/filter to get predicted level of neural response (engineer filters) OFF-center ganglion cells respond a lot to light in periphery; low response with light in center Retinal ganglion cells (RGCs) Each ganglion responds best to spots of specific size - RGCs "filter" info coming from brain RGCs most sensitive to differences in light intensity between center & surround (mostly unaffected by average intensity) o Luminance variations smooth w/in objects and sharp between objects o Center-surround receptive fields emphasize object boundaries In response rate vs position graph, A and C have no response bc A has no light and C has equal light/dark ratio so they balance out (no overall difference) and E is all light (so almost no response, due to less contrast, Rucci would have made this no response as well) Change in luminance: intensity Changing contrast: change of luminance in at least two surfaces Mach bands: believed to be product of visual system's center-surround receptive fields; caused by latera inhibition (theory) Light Adaptation: Photopic: day vision (much light); concentrated around fovea; high acuity, low sensitivity; only 4-5 million cones Scotopic: night vision (little light); outside fovea; low acuity, high sensitivity; 90 million rods We are privileged to have both types of vision Mechanism of dark/light adaptation (luminance range) 1. Pupil dilation (2 mm (bright illumination) - 8 mm (dark), factor 4, area factor 16) 2. Photoreceptors replacement (# of photoreceptors/pigments available: rods and cones; once molecule absorbs pigment, it changes shape & takes a while before can be used again) a. Time vs Threshold: photoreceptors are very efficient at time 0, rods are more sensitive to light and thus take longer to adapt to a change in light for them to function in the light change, cones adapt faster but do not detect light at extremely low illumination levels like rods so rods take over after the cone- mediated adaption period
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