1. Visual Computation I. Physiological Foundations
II. Elements of Visual Computation
III. Synchronized Oscillation in Visual Cortex

2. I. Physiological Foundations
Visual perception:
“What you see is not what is out there in the outside world, but instead what has been created all inside of your head as the nervous system’s response to external stimuli.”

3. “Visible” Spectrum
Not all ranges of the light spectrum is perceived. Why?

4. The Eye

5. The Retina

8. Steps of Visual Computation
S1: Rod & Con cells (light detection)
S2: Horizontal & Bipolar cells (preprocessing)
S3: Ganglion cells (preprocessing)
S4: Lateral geniculate nucleus (LGN) cells (relay station)
S5: Visual cortex (conscious perception)

9. Rods and Cons Rods: Cons:
- Discriminate B/W brightness in low illumination
- Contribute to peripheral vision
Cons:
- Discriminate Colors
- Contribute to central vision

10. Three Types of Con Cells

11. Each wavelength gives rise to a different combined activity pattern of the three cone types, thus giving experience of different colors.
Why three cone types?
Why not one or two or even several?
What would be like one-cone perception?
Color-misnomer phenomenon.

12. One Type of Con Cells

13. Two Types of Con Cells

14. II. Elements of Visual Computation
1. Receptive field (RF)
2. ON-center/OFF-surround RF
3. Edge detector
4. Orientation detector
5. Location-invariant detector
6. Ice-cube model of visual cortex
7. Color computation

15. 1. Receptive Field
The receptive field (RF) of a neuron is the area of retina cells that trigger activity of that neuron.

16. 2. On-center/OFF-surround RF

17. 3. Edge Detector

18. 4. Orientation Detector
(Huble & Wiesel, 1959)

19. Hypothetical Wiring Diagram

20. 5. Location-invariant Detector

21. 6. Ice-cube Model of Visual Cortex

22. Orientation Selectivity of Cortical Neuron

23. Face-detecting “Grand Mother” Neuron

24. 7. Color Computation
Transformation: (B,G,R) -> (W, B-Y, R-G) -- no information loss, then why?

25. Take-home Message:
“What you see is not what is out there in the outside world (ie., not like “taking a picture”), but instead a result of visual computation -- only those computations that are critical for survival, shaped by the evolution.”

26. What about Illusions? -> “Unintended” consequences of the computation

27. III. Synchronized Oscillation in Visual Cortex
1. Sensory Binding Problem
2. Neural Assemblies
3. Synchronized Firing

28. 1. Sensory Binding Problem
In the brain, spatially different areas are functionally specialized for processing visual attributes such as shape, color, orientation, and direction of motion.
Binding Problem:
How does the brain then “bind” together the sensory attributes of an object to construct a unified perception of the object?

29. Binding Problem

30. 2. Neural Assemblies
Neural assembly: a group of neurons whose activities are highly correlated/inter-dependent to one another.

32. 3. Synchronized Firing
Neural synchrony: in which neurons in a neural assembly tend to fire in phase-lock fashion or in unison.

34. Global Synchronization via long-range excitatory connections

36. … Meaning that the following was the input stimulus,

37. Synchronized Neural Assembly Coding
->Solution to the Binding Problem
Two or more assemblies, each of which is tuned to a particular feature/attribute present in a stimulus object, contribute to the representation of that object by jointly firing synchroniously and simultaneoulsy.
Biological plausibility:
well-documented (e.g., Gray et al, 1989).

42. Vision is a computation.