Visual Cognition
If we had the
sensory
apparatus of
some other of the
earth's
organisms,
'reality' would
seem quite
different - Irvin
Rock
Visual Cognition: In Humans
Camera eye
Compound eye
The Problem
• How to turn an upside down, 2D, warped
mirror-reflection and turn it into a right-side
up, straightened, aligned 3D world?
Sensitivity to Light Across Biological Organisms
• Birds have the highest resolution of visual
acuity, with cones eight times smaller than
ours.
• Basic feature design for predators (cats):
Both eyes in front
• Prey (rabbits, horses): eyes on the sides of
the head
Sensitivity to Light Across Biological Organisms
• Rattlesnakes detect in infrared;
•
bees detect ultraviolet light.
“Function Follows Form”
• Two visual systems with(mostly) distinct:
–
–
–
–
–
–
photoreceptors
regions of the eye
Perceptual characteristics (acuity v. sensitivity)
Convergence ratio
neural pathways
target regions
Two Photoreceptors:
Rods & Cones
• Rods
– Less dense
– Sensitive: brightness, movement
• Cones
– Dense
– Sensitive: acuity / edges, color
• Trade-off of Sensitivity for Acuity
• The Purkinge
Shift
Regions of the Eye
Retina: Whole inner eye
containing all photoreceptor
cells.
• Fovea indentation on retina,
mostly Cones.
– fine discrimination; colors &
detail.
• Periphery area outside fovea,
mostly rods
– Sensitivity to brightness,
movement
–
Optic disk: “Design Flaw” place
where axons exit eye forming
optic nerve.
Rods & Cones: Regions of the Eye & Convergence Ratio
Cones-specialized for color vision
& detail fovea.
Rods-sensitive to light periphery
• 126 million receptors total with 6
million cones.
• Cones = 5% of photoreceptors, but
25% of brain dedicated to them.
Specialization of light processing
determined by convergence ratio.
Rods-big receptive fields
Cones-small receptive fields
How Does Visual Information Flow?
• Lateral Inhibition
Information Flow: Lateral Inhibition
• Mach Bands
Luminance Comparison
Both squares are identical in luminance gradient.
The four squares are identical in luminance gradient.
Portions indicated by arrows have the same mean luminance.
Rods & Cones: Convergence Ratio
Specialization of light processing determined by “convergence ratio.”
Many rods converge on a single retinal ganglion = Sensitivity
Few Cones converge on a single retinal ganglion = acuity (detail)
Rods-big receptive fields
cones-small receptive fields
What is a receptive field of retinal
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”
Information Flow: Receptive Fields
Edges and Receptive fields
Receptive fields at work.
Receptive fields at work.
As your fovea lands on a white
intersection, the black corners of the
neighboring squares fall on the retina,
with its larger receptive fields. With
each of the four corners falling into a
receptive field, the receptive field
sums to ‘dark’.
How the Hermann Grid Illusion Works
• (from a student response) The retina contains collections of
photoreceptors, some of which are activated by light and
others which are activated in the absence of light. The two
types are usually arranged to encircle each other, dark ones
around light ones and vice versa, and are spread throughout
the retina. The gray spots appear in the intersection of the grid
due to the competing effects of the dark and light
photoreceptors.
• When looking at the grid as a whole, the majority of which is
dark, it causes the more numerous dark photo receptors to
activate, overiding the light activated ones, and causing a
subtle darkening effect. When you focus directly on the white
space at the intersections between the squares, it narrows the
field of vision, with the smaller resulting receptive fields able
to “fit” within allowing the light detecting photoreceptors to
function without interference from the dark activated ones.
Receptive fields in Art
Mona Lisa’s beguiling
smile results from the large
receptive fields signaling
darkness at the corner of
her mouth, drawing your
attention, directing your
eye to foveate on the
corner. The fovea has
smaller receptive fields,
which do not sum with
darkness, making the smile
look like it is disappearing.
Apparent movement of the streams is
created by afterimages as our eyes shift to
examine the picture.
Two Visual Systems
• Geniculostriate & Tectopulvinar
Visual Pathways
• 1. Geniculostriate pathway– optic chiasm LGN  Primary Visual Cortex  Ventral
“What”
– Signals from fovea & cones (mostly); Parvo
– involved in patter perception, color vision
• 2. Tectopulvinar pathway– optic chiasm---superior colliculus---Lateral Posterior Pulvinar-- Visual Cortex  Dorsal “Where/How”; Magno
– Signals from periphery & rods (mostly)
– detection of light; spatial orientation
Information Flow
Optic Chiasm
Retinal- Lateral
Calcarine Geniculate
pathway Nucleus
Dorsal Pathway
Parvocellular
Retina (rods &
cones)
Magnocellular
Visual
Cortex 1
(V1)
Tectal
Pathway
Superior
Colliculus
Ventral
Pathway
Parvo and Magno Cellular Pathways:
Example of Double Dissociation
• Lesions to the parvo-cellular pathway affect
perception for color, and fine detail (small spatial
frequencies); Lesions to magno-cellular pathway do
not.
• Lesions to magno-cellular pathway affect perception
of movement, brightness contrast (flicker); Lesions to
parvo-cellular pathway do not
• An engineer’s
box diagram of
the neural
circuitry in visual
perception, from
the retina to the
hippocampus.
Information Flow
• Topographic
Information Flow
• Visual Hemifields
measurement
• cones: vision under bright illumination
– Cone time constant=100ms, 1,000 photons can be
presented in a brief period (say, 1000 photons within a 1
ms period) or over a long period (such as 200 photons in
each of five 20 ms period) for the same visual effect. The
cones cannot tell the difference.
– Rod time constant=400 ms, meaning the photon catch
extends over a longer time interval working like a slow
shutter speed on a camera (well, cameras once had shutters
that had a “click” that digital cameras artificially reproduce
even though there is nothing to “click” any more.)
Sensation vs Perception
• The difference between sensation and
perception is the difference between light and
color
• It requires senses to detect light
• It requires perception and cognition to see
color
• Color happens both in the eye and in the brain
Measurement in Vision Science
• light measured in nanometers (nm)
nan·o·me·ter (năn'ə-mē'tər)
– 1nm=1billionth meter
– eyes sense from 360nm to 780nm on the infinite
scale of wavelength – nothing.
– “light” is whatever energy falls in that range “light” by definition, is anthromorphized.
• 1st feature of light
– wavelength:480=blue, 540=green,
565=yellow,590=red
Electromagnetic Radiation
What 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.”
Visual Cognition:
Describing Light
• Hue or Value (Color)
• Luminance
(Brightness)
• Saturation (Purity)
We 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
Additive Color mixing with lights
Results of Additive Color Mixing
Two Theories of Color Cognition
• Young-Helmholtz Trichromacy
Theory
• Proposed in 1802 and confirmed in
1983
– You should be able to create any color
by combining three basic colors, red,
green and blue.
• Evidence: three different colored
pigments in the fovea (electron
micropscope)
Two Theories of Color Cognition
Trichromacy does not explain:
• Incompatible colors cannot be seen. Why can’t we
see certain colors (reddish-green, bluish-yellow)
• Color afterimages.
Opponent Process
We have 3 opposing mechanisms: red-green, yellowblue, & black-white. These are called
complimentary colors & put together they produce
yellow or white.
+
Opponent Process in a Movement
Illusion: Waterfall Effect
• http://video.google.com/videoplay?docid=6294268981850523944&ei=r5P
RSNGPD6fcqAPS48y6Ag&q=spiral+visual+illusion&vt=lf&hl=en
• http://video.google.com/videoplay?docid=2927422796086500362&vt=lf&hl=en
Color Perception
• Each set of colored rectangles, against the solid background, is
the same
•
http://www.youtube.com/watch?v=mf5otGNbkuc&feature=em-subs_digest-newavtr-vrecs
Assume Surrounding Colors are
Constant
In this illusion, the second
card from the left seems to be
a stronger shade of pink in the
top picture. In fact they are the
same color, but the brain
changes its assumption about
color due to the color cast of
the surrounding photo.
Color-blindness
• Results whenever we are either missing one
of our cones or one of our cones doesn’t
work properly.
Color Helps To Recognize Objects
Color Helps To Recognize Objects
Color Helps To Recognize Objects
Color Helps To Recognize Objects
Color Helps To Recognize Objects