Title: The Visual System
1The Visual System
2What stimuli are required for vision?
- Light- which can be thought of as discrete
particles (photons) or traveling waves.
3Human Visible Spectrum
- Humans can detect waves of energy traveling
through space between 380 760 nanometers). - Wavelengths outside this spectrum are
undetectable to the human eye. - Some organisms do detect wavelengths outside our
visible spectrum. - E.g., Rattlesnakes detect in infrared!
- bees detect ultraviolet light.
4Electromagnetic Radiation
5What are the 2 properties of light that influence
visual perception?
- 1. Wavelength is associated with our perception
of color. - 2. Intensity is associated with our perception of
brightness.
6Reflectance of light
- Light is reflected off of the surface of objects
to the eye. - Light energy is converted into neural energy
then processed by the brain.
7(No Transcript)
8(No Transcript)
9Parts of the eye
- Outer Layer
- 1. Sclera White fibrous layer
- 2. Cornea the clear protruding structure in the
front of the eye that is curved. - Bends light rays is responsible for 70-80 of
eyes focusing ability.
10Cornea
11Middle Layer
- 1. Choroid vascularized layer that provides
nutrition for retinal cells. Is located between
the retina sclera. - 2. Pigment epithileum a black pigment found
between the choroid retina. - Traps photons from light to prevent scattering
of photons along the retina, which reduces
distortion. -
12Middle layer (contd.)
- 3. Iris smooth ring of muscle with a central
opening (pupil). Gives us our eye color! - 4. Pupil Pupil changes in size depending on
intensity of light. - -Intense light, small constricted
pupils - -Dim light, dilated pupils
- 5. Lens focuses light on retina (convex).
- -lens is round (nearby objects)
- -lens is flatter for distant objects
13Inner layer
- 1. Retina contains receptor cells needed for
neural processing of light. - A. Fovea indentation on retina.
- -fine discrimination colors detail.
- B. periphery area on either side of
fovea of - retina.
- -detection of light
- 2. Optic disk place where axons exit eye
forming optic nerve.
14(No Transcript)
15The Photoreceptors Rods Cones
- Cones-specialized for color vision detail
(fovea). - Rods-sensitive to light (periphery)
- 126 million receptors total!
- 120 million are rods 6 million are cones
- sensitivity to light enhanced by convergence
ratio. Many rods converge on a single retinal
ganglion cell than do cones. - Rods-big receptive fields
- cones-small receptive fields
16How does visual information get from the eye to
the brain to be processed?
- Two vision pathways
- Geniculostriate Tectopulvinar
17Visual Pathways
- 1. Geniculostriate pathway-
- optic chiasm----LGN---Primary Visual cortex
- involved in patter perception, color
vision - 2. Tectopulvinar pathway-
- optic chiasm---superior colliculus---Lateral
- posterior pulvinar---PVC
- detection of light spatial orientation
18Primary Visual Cortex
19How do our eyes adapt to the dark?
- The photopic scotopic systems adjust their
sensitivity as a function of time in the dark. - The cones become more sensitive during the first
5-10 minutes after being in the dark. - Rods continue becoming more sensitive over period
of 20-30 minutes.
20How is a dark adaptation experiment conducted?
- Step 1 The subject (S) is exposed to bright
light of a known intensity. - Step 2 S is then placed in total darkness
asked to detect a spot of light (controlled
luminance). - Step 3 Ss detection threshold is plotted as a
function of time spent in dark.
21We have three cone wavelengths
- 1. Short wavelength peaks at 419 nm (blues).
- 2. Medium wavelength peaks at 531 nm (greens).
- 3. Long wavelength peaks at 558 nm (reds).
- The primary colors are blue, green, red
22What are the cells in the retina?
23What is relationship between activity in the
retina the brain?
- An electrode is inserted into various parts of
the visual system (retina, cortex) of an animal.
- The cells activity in response to the
presentation of visual stimuli (lights, bars,
complex images) to the animals retina is
recorded.
24(No Transcript)
25What is a receptive field of retinal ganglion
cells?
- The receptive field for these cells is the region
of the retina that, when stimulated excites or
inhibits the cells firing pattern.
26Retinal ganglion cells
- Kuffler (1953) presented spots of light to retina
cells in the cat recorded their responses. - The cells have a Concentric circle configuration!
- usually called center-surround cells
- On-center, off-surround cell has an excitatory
center, inhibitory surround - Off-center, on-surround cell has an inhibitory
center excitatory surround
27(No Transcript)
28Are receptive fields of cortical cells like those
of retinal ganglion cells?
- No!!
- Our visual cortex cells respond to more complex
stimuli (e.g., bars of light).
29Hubel Wiesel (1950s 60s)
- Recorded cortical cells in the visual cortex of
cats in response to visual images they presented
to the cats retina. - They found three types of cells with different
receptive fields (bar detectors).
30Receptive Fields of cortical neuronsPrimary
Visual cortex
- 1. Simple Cells
- --respond to points of light or bars of
light in a particular orientation -
- 2. Complex cells
- --respond to bars of light in a particular
orientation moving in a specific direction. - 3. Hypercomplex Cells
- respond to bars of light in a particular
orientation, moving in a specific direction, of
a specific line length.
31Simple Cells
32Complex Cells
33What is the organization of the visual cortex?
- Hubel Wiesel found that the visual cortex is
organized into columns. - Location specific For each place on the retina
there is a column of cells in cortex. - Two columns next to one another in the cortex
respond to stimulation of two adjacent points on
the retina.
34Orientation Ocular Dominance columns in Primary
Visual Cortex
35The Visual cortex has a retinotopic map
- Visual cortex has a map of the retinas surface.
- More cortical neurons are devoted to fovea of
retina. - As fovea only has cones, they are widely mapped
on cortexs surface. - The reason cones allow us to see detail color.
36How do we see in color?
- Two Theories
- 1. Young-Helmholtz Trichromatic theory of color
vision. - The pattern of activity among the three
cone-types determines the color we perceive. - The cones all respond equally to white light
(which contains all wavelengths).
37Evidence for Trichromatic theory
- 1. We have three types of cones sensitive to
different wavelengths of light (short, medium,
long wavelengths). - 2. There are 3 types of color-blindness.
- 3. Approximately any color can be matched by
mixing varying amounts of red, green, blue
light.
38Additive Color mixing with lights
39Results of Additive Color Mixing
40Subtractive Additive Mixing
41Why mixing blue yellow pigments, yields the
color green.
42Color-blindness
- Results whenever we are either missing one of our
cones or one of our cones doesnt work properly.
432. Opponent-Process theory
- We have opposing mechanisms that allow us to
perceive colors. - Evidence for this theory
- 1. Incompatible colors cannot be seen Explains
why we cant see certain colors (reddish-green,
bluish-yellow) color afterimages. - We have 3 opposing mechanisms red-green,
yellow-blue, black-white. These are called
complimentary colors put together they produce
yellow or white.