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 red-green 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 testCannot read right digit deuteranopiaCannot 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

• 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

Ganglion cells of retina

• M cells large; simple antagonistic

receptive fields, some off-center, 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).