Transcript How Do We See: Part 2 - Department of Cognitive Science

COGNITIVE
SCIENCE
17
The Visual
System:
Color Vision
Part 2
Jaime A. Pineda, Ph.D.
Visible Spectrum
• Color we perceive an
object to be is determined
by which wavelengths of
light are reflected or
absorbed by object
• Only reflected
wavelengths reach our eye
and are seen as color
• Referred to as spectral
reflectance
Theories of Color Vision
• Young-Helmholtz
Trichromatic theory
(1802)
Based on the existence of
three types of receptors
that are maximally
sensitive to different, but
overlapping, ranges of
wavelengths
Light mixing vs pigment mixing
• Yellow + blue paint produces green paint
(mixing pigments)
• Yellow + blue light produces white light
(mixing light)
Cones of visual system
Cones of Visual System
•
•
•
•
•
•
Cones are photopic (high light)
3 different cone types allow for color vision
Each sensitive to different wavelengths of light
L (long wavelength cones) - red
M (medium wavelength) cones - green
S (short wavelength) cones - blue
Three cones
• S correspond to
“blue”
• M corresponds to
“green”
• L corresponds to “red”
• Throughout whole
retina, ratio of L & M
cones to S cones is
100:1
• Eye less sensitive to
blue end of spectrum
Theories of Color Vision
• Opponent-process theory
– Cells in the visual system respond to redgreen and blue-yellow colors
– A given cell might be excited by red and
inhibited by green, while another cell might be
excited by yellow and inhibited by blue
Opponent processing of color
• Proposed by 19th century physiologist Ewald
Hering (1905)
• Certain colors not perceived together (don’t mix)
– Reddish green or bluish yellow??
• Antagonism between colors occurs in retina
• 4 unique hues fundamental (primary colors):
– red/green and yellow/blue are opposed
Opponent processing of color
• Four unique hues of red,
green, yellow and blue
arise from the 3 types of
cones
• Input of L and M cones
combined contribute to
lightness or darkness
• Mixtures account for all
shades and tints we
perceive
Genetic Defects in color vision
• Result from anomalies in one or more of the
three types of cones.
• Because some defects are mainly in the X
chromosome and males only have one they are
more susceptible to defects.
• Protanopia  confuse red/green (see the world
tinged with yellow/blue; red cones filled with
‘green’ opsin.
• Deuteranopia  confuse red/green; green
cones filled with ‘red’ opsin
Protanopia/Deuteranopia test
Cannot read right digit  deuteranopia
Cannot read left digit  protanopia
Ganglion cells of retina
• On-center cells:
excited (depolarized) when light is directed to
cones in center of receptive field; inhibited when
light hits the surround
• Off-center cells:
inhibited when light is directed to the center of
receptive field; excited when light is directed to
center
Ganglion cells of retina
• Two major classes of ganglion cells within retina:
• M & P cells – named for separate projections to
magnocellular ( large cell) and parvocellular
(small cell) layers of lateral geniculate nucleus
• Account for 90% of all ganglion cells
• More P than M cells
• M cells
large; simple antagonistic
receptive fields, some offcenter, some on-center but
in both types the center
and surround have similar,
broad spectral sensitivities.
Concerned with gross
features of a stimulus
and its movement
• P cells
color information
carried almost
exclusively by these
cells. These are
smaller, have smaller
receptive fields;
respond selectively to
specific wavelengths
Primarily involved
in analysis of fine
detail of visual
image
Dorsal (“Where”) and Ventral (“What”)
Visual Streams in Monkey
Parietal (Dorsal) and Temporal
(Ventral) Processing Streams
Areas MT and V4 in the
Macaque Brain
Dorsal (“Where”) and Ventral (“What”)
Visual Streams in Human (PET)
Dorsal (where) pathway
shown in green and blue
and Ventral (what)
pathway shown in yellow
and red serve different
functions. (Courtesy of
Leslie Ungerleider).
Retinal and Thalamic Precursors of the
Dorsal and Ventral Visual Pathways
Magnocellular (dorsal)
and parvocellular
(ventral) pathways from
the retina to the higher
levels of the visual
cortex are separate at
the lower levels of the
visual system. At higher
levels they show
increasing overlap.
Primary visual cortex
• Visual area 1 (V1); Brodman’s area 17
• Located at posterior pole of cerebral hemisphere
around calcarine sulcus
• Striated; consists of 6 layers of cells
• Organizes retinal inputs into building blocks of
visual images (columns)
• About ½ of V1 is devoted to fovea and retina
region just around the fovea
• Allows for great acuity of spatial discrimination in
central part of visual field
Some Human Cortical Visual Regions:
V1, V2, V3, V4, V5 (MT)
Receptive Fields of Lateral Geniculate
and Primary Visual Cortex
Beyond V1
Extrastriate Cortex
Unfolded Map of Monkey Cortex Highlighting
Extrastriate Visual Cortex
Multiple Cortical Areas Devoted to Visual Functions
David Van Essen
developed the
technique of unfolding
the cortex to better
appreciate the many
areas that contribute to
vision.
Colored areas are
devoted to visual
function and brown
areas are devoted to
other functions.
Extensive Interconnections Between
Areas in Primate Brain
Separation and Integration of Function
Areas of the
monkey visual
system (shown
previously on
unfolded cortex)
are heavily
interconnected.
The Visual System,
Light/Dark Cycles, and
Circadian Rhythms
Retinohypothalamic Pathway: Visual
Input Maintains Circadian Rhythms
Pathway from retina to the suprachiasmic nucleus (SCN) carries information
about the light-dark cycle in the environment to the SCN. The size of the
SCN is enlarged for viewing. Axons from the left eye are labeled in red and
from the right eye in green. Both eyes project so diffusely to the two
overlying SCN that they are outlined in yellow. (SCN photograph courtesy
of Cynthia L. Jordan).