Vision - Henderson State Universityfac.hsu.edu/ahmada/3 Courses/3 Physiological/1 PhysioNotes/Vision...
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1 Vision Seeing is in the mind ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ 2 Stimulus: Light ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ 3 Light Characteristics 1. Wavelength (hue) 2. Intensity (brightness) 3. Saturation (purity) ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________
Transcript of Vision - Henderson State Universityfac.hsu.edu/ahmada/3 Courses/3 Physiological/1 PhysioNotes/Vision...
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VisionSeeing is in the mind
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Stimulus: Light
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Light Characteristics
1. Wavelength (hue)2. Intensity (brightness)3. Saturation (purity)
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Wavelength (Hue)Hue (color):
dimension of color determined by wavelength of
light.
Wavelength the distance from the peak of one wave to the peak of the
next.
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Wavelength (Hue)
Different wavelengths of light resultin different colors.
400 nm 700 nmLong wavelengthsShort wavelengths
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Intensity (Brightness)Intensity:
Amount of energy in a
wave determined by
amplitude relates to perceived
brightness.
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Intensity (Brightness)
Blue color with varying levels of intensity.As intensity increases the blue color looks
more “washed out”.
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Purity (Saturation)
Monochromatic light added to green and redmake them less saturated.
Saturated
Saturated
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Saturation
Saturation:Purity of hues. Red would be a pure color, but
bluish red would be saturated
color.
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Characteristics
Saturation (mix of wavelengths)
Different wavelengths mix to change thepurity of the final wavelength.
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Color Solid
Represents all three
characteristics of the light
stimulus on model.
http://www.visionconnection.org
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The Eye
Sclera:Outer layer of the eyeball
Iris: constricts or dilates by varying amounts of light.
Pupil: regulates amount of light entering the eye.
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Anatomy of the Eye1. Cornea: Transparent tissue
where light enters the eye.2. Iris: Muscle that regulates the
amount of light3. Pupil: The aperture4. Lens: Focuses the light rays
on the retina.5. Retina: Contains sensory
receptors that process visual information and send it to the brain.
6. Fovea: Maximum concentration of cones for resolution
7. Optic disk: Where the optic nerve leaves the eye. Blind spot.
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Fundus Picture
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Blind Spot
Check your blind spot
http://www.valeoraytheon.com/lanechange1.htm
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Retina1. Photoreceptors: Light sensitive
rods and cones.2. Bipolar cells: Connect
photoreceptors with ganglion cells
3. Ganglion Cells: Process visual images into neuronal impulses and carry them to the thalamus
4. Horizontal Cells: Horizontal cells connect photoreceptors laterally.
5. Amacrine Cells: Amacrine cells connect ganglion laterally.
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Photoreceptors
Rods:Low light visionSensitivityGreater in number(120 million)
Cones:Bright light visionResolutionFewer in number(8 million)
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Cone Surface
http://webvision.med.utah.edu/sretina.html
Cones in fovea
Ves
sels
aro
und
fove
a
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Photoreceptor Distribution
1. Large number of cones in the center of the retina. Their density decreases in the peripheral retina
2. Fewer numbers of rods in the central retina. Their density increases in the retinal periphery
3. No photoreceptors at the optic disk (blind spot) Fovea Middle
retinaPeripheral
retinaMiddleretina
Peripheralretina
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Gray Shades
The photoreceptors are bombarded with shades of light and darkness. The intensities of these shades can be depicted as number say 9 (light) to 2 (dark).
Can you tell what do these numbers represent?
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Photoreceptors
Outer segments of photoreceptors consist of disks,
embedded with rhodopsin molecules that go through
changes when light falls on them.
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Experimental Setup
Experimental set-up to record from individual photoreceptor
cells.
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Phototransduction
1) Light transforms rhodopsin. 2) Replaces GTP instead of GDP. 3) Activated G-protein binds to PDE. 4) PDE hydrolyzes cGMP. 5) Na+ channels close. Photoreceptor hyperpolarizes.
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GlutamateGlutamate release is decreased when the photoreceptor
hyperpolarizes in light. Glutamate increases when photoreceptor is depolarized in darkness.
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Graded Response
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Visual Pathways
Central Visual Pathway(Visual Information)
Optic nerveLateral Geniculate Nucleus
(Thalamus)Visual Cortex
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Visual Pathways1. Projections to the Pretectum (Pupillary Reflex)2. Projections to the superior Colliculus (saccadic eye movements)
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Visual Fields
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Lateral Geniculate Nucleus
1. Parvocellular Pathway (cone mediated, color, high resolution)
2. Magnocellular Pathway (rod mediated, b/w, sensitivity)
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How does the brain sees?
Hubel and Wiesel (1960s)
Used microelectrodes to record from neurons in
the primary visual cortex cats and monkeys.
Discovered feature detector cells: neurons that
respond selectively to lines, edges.
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Photoreceptor RF
Off-cell
Photoreceptorreceptive field
On-cell
Photoreceptorreceptive field
(Dowling, 1987)
Receptive field is an area of avisual cell in which it is responsive
to visual stimuli
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Ganglion Cell RF
Off-center
Receptive field ofbipolar, ganglionand LGN neurons
Surround
Center
(Dowling, 1987)
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Visual Cortex Cell’s RF
LGN neuronCenter-surround
RF
Visual Cortex NeuronRF of a simple cell
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Complex Cell
Receptive fields of other neurons in visual
cortex are more complex. Bars of light
moving in one direction lead to strong
response, but a weak response in the other
direction.
Strong
Weak
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Streams of Visual information
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Color Vision
Different wavelengths of light resultin different colors.
400 nm 700 nmLong wavelengthsShort wavelengths
Wavelength determines color.
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Color Vision
Most important aspect of primate vision. Also present in many birds. Why is color
vision important?
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Food
Identification of food.
http://personales.ya.com/botanical/Malus_domestica1.jpg
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MatesAttracts mates
http://www.cranefest.com/assets/thumbs/pheasant.jpg
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Why Color Important?Camouflage to avoid predation
Quail
http://www.longhorn-cattle.com/camo.html
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Theories of Color Vision
1. Trichromatic theory (Helmholtz)2. Opponent-Process theory (Herring)
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Blue Green Red
Trivariancy Property
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Three ReceptorsHelmholtz suggested
(based on his psychophysical
experiments) that there has to be three
receptors in the retina that are sensitive to red, blue and green
colors.Helmholtz (1821-1894)
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PhotoreceptorsRedGreenBlue
Longwave
Mediumwave
Shortwave
Edwards MacNicholand his colleagues and George Wald and Paul Brown (1967) measured directly the absorption spectra of visual pigments of single cones obtained from the retinas of humans.
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Photoreceptor Response
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Addition of ColorsIf three primary colors (lights) are mixed the
wavelengths are added and the result in white color.
http://www.arborsci.com/images/RGBlight.jpg
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Subtraction of Colors
If three primary colors (pigments) are
mixed it results in subtraction of all
wavelengths and the result is black color.
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After-images
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Another Example
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Opponent Process theoryHering, proposed that we process four primary colors opposed in pairs of red-green, blue-yellow, and black-
white. That is why we see red, blue and white afterimage of Britainʹs flag because green’s afterimage
is red, yellow’s blue and blackʹs white.
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Color Blindness
Ishihara Test
Genetic disorder in which people are blind to green or red colors.
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Color Blindness
http://members.shaw.ca/hidden-talents/vision/color/colorblind1.htmlNo red No green
Normal
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Perceiving familiar objects as having consistent color, even if changing illumination alters the wavelengths
reflected by the object
Color Constancy
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Theories of Color Vision
1. Trichromatic theory (Young and Helmholtz)
Electromagnetic spectrum contains continuousspectral colors that are infinite
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Blue Green Red
Trivariancy Property
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Three Receptors
There has to be three receptors in the retina that are sensitive to red, blue and
green colors and that combination of these three colors will lead to generating
any color in the spectrum.
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PhotoreceptorsRedGreenBlue
Longwave
Mediumwave
Shortwave
Edwards MacNicholand his colleagues and George Wald and Paul Brown (1967) measured directly the absorption spectra of visual pigments of single cones obtained from the retinas of humans.
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Photoreceptor Response
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After-images
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Opponent Process theory
Four color primaries in the retinal ganglion
cells formed by three photoreceptors.
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Color-opponency in Ganglion Cells
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Color Blindness
Ishihara Test
Genetic disorder in which people are blind to green or red colors.
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Normal Color Vision
http://members.shaw.ca/hidden-talents/vision/color/colorblind1.html
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Protanomaly
http://members.shaw.ca/hidden-talents/vision/color/colorblind1.html
Weak in red
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Deuteranomaly
http://members.shaw.ca/hidden-talents/vision/color/colorblind1.html
Weak in green
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Protanopia
http://members.shaw.ca/hidden-talents/vision/color/colorblind1.html
No red
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Deuternopia
http://members.shaw.ca/hidden-talents/vision/color/colorblind1.html
No green
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Perception
Perception: selection, organization, and interpretation of neural information
Gestalt psychologists suggested that perception was organized.
The same visual stimulus can result in very different perceptions
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Two Faces or a Vase?
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Old lady or young?
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Can you see it now?
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Proximity
Elements that are close together are grouped together.
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Continuity
Elements in smooth continuation are grouped together.
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Simplicity
Elements are organized in simplest possible ways.
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Closure
Missing elements are supplied to close or complete a familiar figure.
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How do we perceive?
Feature detection theorySuggests that perception is based on a
process of breaking stimuli into features which are put back together in the brain.
Bottom-up processing
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Features
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Bottom-up processing
Detect specific featuresof the stimulus
Combine specific features into more
complex forms
Recognizestimulus
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How do we perceive?
Gestalt theorySuggests that perception is based on what
we already know, or what we hypothesize about the stimulus
Top-down processing
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Top-down processing
Make hypothesis about the stimulus
Select featuresto check hypothesis
Recognizestimulus
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Read below!
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Depth Perception
Binocular cuesWhen we use both eyes to perceive
depth
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Convergence
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Retinal Disparity
http://www.vision3d.com/stereo.html
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Depth Perception
Monocular cues Perception of depth using one eye
1. Motion parallax2. Accommodation
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Pictorial Depth Cues
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Perceptual Constancy
Stable perceptions amid changing stimuli
Size constancy
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Shape Constancy
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IllusionsFalse perception:
discrepancy between visual appearance and
physical reality.Ponzo Poggendorf
Muller-Lyer
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Impossible Figure
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Are illusions innate?
Cultural differencesPerceptual hypotheses at work
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