1. COGNITIVE
SCIENCE 17
The Visual
System
Part 1
Jaime A. Pineda, Ph.D.

2. Our vision is optimized for: 1
Our vision is optimized for: Receiving the most abundant spectral radiance the sun emits ( nm) and Responding to the most prevalent speed at which events occur. Evolution? 3. color is a function of a. Hue, saturation, brightness

3. DEFINITIONS Hue: wavelength; each associated with a different hue
Brightness: intensity of light
Saturation: purity of the light; only one wavelength  pure or fully saturated
if light contains all wavelengths  white

4. The Visual Brain

5. The Eye

6. What's the Big Deal? Vision seems easy. It is effortless for us.
Understanding how the brain processes visual information is hard.
We still understand only the most basic computations.
To understand why it is so difficult, we must examine the problem.

7. The Properties of Light
Light is emitted from one or more sources
Point sources or more distributed sources of light.
Light hits surfaces and some of it is reflected and some absorbed.
The reflected light may bounce off multiple surfaces before reaching the eye.
Only some of the light rays will eventually be focused on the retina.

8. The Eye as a Pinhole Camera
We can approximate the image formation performed by the lens of the eye as a pinhole camera.
Light rays from an object project through a single point (the center of projection) onto the retina (or image plane).
This creates a 2-D image of a 3-D object.

9. Ill-posed Problem
It is not so hard to compute a 2D image from a 3D scene.
It is very difficult to compute the original 3D scene from the 2D image.
Many aspects of a 2D scene are consistent with multiple (or even infinite) possible arrangements of the 3D scene.
Because there is no single solution, the problem is ill-posed.

10. Visual Difficulties
An image of a given size can represent a small, nearby object or a large, distant object.

11. Visual Difficulties (cont)
How do we know which edges belong to which objects?
When you move your eyes, the image moves.
How do you know the world is not moving?

12. How Do We Solve It?
To solve ill-posed problems, we must make assumptions about the scene and the objects in it.
These assumptions are also known as heuristics or constraints.
We can determine which assumptions are made by the human visual system by performing psychophysical experiments.

13. Height in the Visual Field
In normal images, things that are farther away often have images that are higher in the visual field.
The visual system uses this height in the visual field as a cue to distance.
We then adjust our interpretation of the size of the object based on this distance.

14. A small man
When we remove the height cue, the second man looks small indeed!

15. Assumption of Right Angles
The visual system has a tendency to interpret angles as right angles.
We see this as a right angle
This allows us to construct interesting visual illusions such as...

16. The Ames Room
Ever wonder how the hobbits are made to look so short in "The Lord of the Rings" movies?

17. Sometimes, we assume something exists when in reality it doesn’t…

19. Human Eye and Retina

20. The Human Eye

21. The Octopus has a comparable eye to ours--an example of parallel evolution

22. Insect visual simulation at 2-3 m
Image projected onto compound eye
Albert Einstein
Insect views AE

25. Cones--color sensitive, form sharp images, require photons, 5 million
Rods are intensity but not color sensitive, form blurred images, 120 million
Birds--big eyes, more cones, “faster” eye muscles, more support, and best vision (8x better than ours)
Nocturnal animals have big eyes & more rods/fewer cones
Retina Rods & Cones

26. Photopigments – night vision
Opsin (protein) + retinal (a lipid)
Rhodopsin  from vitamin A; found in rods
Light splits rhodopsin causing a change in membrane potential; closes Na+ and Ca++ channels; and reduces the rate of release of glutamate

27. Human Vision -- could be better
to processor
Not optimal design--lots of nerve cells in pathway

28. Overlapping Sensitivities of Cones
S-cones M-cones L-cones

29. Ganglion cells
A) ON cells
B) OFF cells
C) ON/OFF cells

30. Center-Surround A consequence of lateral inhibitory connections
Receptive fields of a cell

31. Visual Field and the Human Visual System
The visual field represented on the retina is flipped on the vertical and horizontal axis. That is, up is inverted down and right is represented on the left.
Signals from the right half of the retinas (left visual field) of both eyes travel to the primary visual cortex in the right hemisphere.
Signals from the left retinas (right visual field) travel to the left hemisphere.
Primary visual cortex lies along the calcarine sulcus.

32. Lateral geniculate (part of thalamus)

33. Visual streams Magnocellular  dorsal (where) stream
Parvocellular  ventral (what) stream
Koniocellular  biorhythms

34. Cortical Functions and Their Measurement: Vision as a Prototype
Autoradiography 2-deoxyglucose (2-DG)
fMRI