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Week 5 Notes

by: Shane Ng

Week 5 Notes PSY 201

Shane Ng
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About this Document

Chapter 5: Sensation and Color Vision notes.
Class Notes
Psychology, color vision, sensation




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This 5 page Class Notes was uploaded by Shane Ng on Monday February 8, 2016. The Class Notes belongs to PSY 201 at University of Oregon taught by Sereno in Winter2015. Since its upload, it has received 14 views.


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Date Created: 02/08/16
Sensation The Eye - Constructed like a camera - Cornea and Lens focus images onto the back of the eye, where the sensitive cells of the retina (Photoreceptors) respond to light. - The retinal output cells (the retinal ganglion cells) send messages out the optic nerve to the thalamus. - - Fovea: Area of retina where the receptor cells are especially densely packed. o What you are looking directly at appears on the fovea o The best, most detailed, vision is at the fovea - Where the optical nerve leaves the eye, there is no room for receptors. This is the blind spot. How we see - Rods and Cones are receptor cells. Sensitive to light. - Light waves have 2 aspects: o Wavelength (determines color) o Amplitude (determines brightness) - Rods o Outside of the fovea o Not sensitive to color o Very sensitive to light o Operate at night - Cones o Concentrated in the fovea o Less sensitive to light o Operate during the day o Responsible for color vision: Blue, Green, Red. Each sensitive to a different wavelength (color) of light Visual Pathways - Signals leave eye via the optic nerve. - At the optic chiasm, the nasal ganglion cell axons cross over to the other side of the brain; the temporal ganglion cell axons stay on the same side of the brain. - These fibers project to the lateral geniculate nucleus (LGN) of the thalamus then to the primary visual cortex (V1). - Therefore, neurons in left cortex get signals form the right side of the visual field and vice versa. Coding of Contrast and Contour - Purpose of vision is not simply to detect the presence and absence of light, but to detect and identify objects. - Objects are defined mainly by their contours/boundaries, which in turn are defined by the contrast between one side of the boundary and the other. The Visual system is designed to exaggerate contrast. Stage 1 - Retinal ganglion and LGN cells have a concentric “on/off” receptive field organization. o In the “on” (+) portion, light increases activity from spontaneous rate o In the “off” (-) portion, light decreases activity from spontaneous rate - Mechanism: Lateral Inhibition - These center-surround mechanisms are thought to be responsible for a number of contrast effects. o Induced Grating o Simultaneous or Brightness Contrast o Hermann Grid o Mach bands Stage 2 - Primary visual cortical neurons (area V1) respond well to light or dark bars or edges of particular orientations: o Simple cells Generalization about Visual Processing - Feature Detection: o As proceed to higher stages in the visual system cells:  Respond to more complex visual features - Functional Specialization: 2 “streams” of processing in primate visual cortex. o Ventral/ “what” pathway  Processes info concerning pattern identification and recognition.  Ends up in Temporal lobe o Doral/”where” pathway  Processes info about motion, space, and depth.  Ends up in Parietal lobe - Functional Specialization o Evidence in Monkeys  Temporal cortex lesion (not parietal) impairs object discrimination task.  Parietal cortex lesion (not temporal) impairs object location discrimination task. o Evidence in humans  Visual Agnosia: can see but can’t organize or recognize what is seen. “Psychic blindness”. Damage to inferior temporal cortex.  Unilateral Neglect: perceive world but ignore or fail to attend to parts of the world. Damage to (right) parietal cortex. Color Vision - Color vision is the ability to discriminate between different wavelengths of light when no brightness cues are present - “Color” is a psychological construct. Physical stimuli may differ in wavelengths they reflect, but those wavelengths have no intrinsic hue (color). Physical vs. Psychological (perceptual) - Wavelength  Hue (color) - Amplitude  Brightness Color and Wavelength - Experience of color depends on wavelengths of light that hit the eye. - See light/color: o Directly from light sources (i.e., the sun, light bulbs, TV/computer screen) o From light bounced off of (i.e., reflected from) surfaces. Color Mixing - Mixing of pigments are called subtractive color mixing o When mix 2 pigments, only those wavelengths absorbed by neither pigment will be seen. o Additive color mixing occurs when colored lights vs. pigments are mixed.  This is additive because all wavelengths are added to each other. (i.e., lights projected onto a projection screen.) Two Theories of Color Vision 1. Trichromatic Theory a. Three primary laws of color b. Color matching experiments: can select (no less than) 3 different wavelengths, and match the color of any visible wavelength by mixing them in various proportions (i.e., intensities) c. Hypothesis: Color vision depends on 3 receptor mechanisms, each with different spectral sensitivities. i. Light of a particular wavelength stimulates the 3 receptors to different degrees. ii. Each wavelength, therefore is represented by a unique pattern of activity in the 3 receptors. iii. Hypothesis is true  3 cone-types in retina. 2. Opponent-Process Theory a. Proposed to explain perceptual phenomena in which the colors red/green and blue/yellow seemed to be paired with each other as opposites: i. Opposing afterimages ii. Red  green afterimage iii. Yellow  blue afterimage b. Visualizing colors: it is easy to visualize certain colors such as bluish green (turquoise) or reddish yellow (orange) but impossible to visualize other colors such as reddish green or bluish yellow. c. Hypothesis: Color vision is based on 3 mechanisms that respond in opposite ways to light intensities or wave lengths: i. Black-White; Red-Green; Blue-Yellow. d. Hypothesis is true  i.e., +R center and –G surround retinal ganglion cells. e. The function of opponent cells is to enhance the ability to distinguish between different wavelengths. Sum - Perception is shaped/determined by neural processing. - Color perception is both trichromatic (as measured by color- matching experiments) and opponent (opponent effects). - Both theories are right, the represent mechanisms at different neural stages of color processing.


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