1. Understanding Psychophysics: Spatial Frequency & Contrast
How does differential neural firing to differing orientations of “bars” cause us to “perceive” our environment?
Understanding the relation between physical stimuli (i.e., what is “Spatial Frequency” and “contrast”?) and our perceptions of the world.

2. Low Spatial Frequency:
Shapes of buildings
Closer buildings
high Spatial Frequency:
Size of the windows
Far buildings
Large Contrast:
Dark building among light-colored buildings
Small Contrast:
Shading differences among small background buildings

3. Frequency - cycles per second (time domain)
Spatial Frequency – cycles per degree of visual
angle (space domain)

4. square-wave grating
sine-wave gratings
the unit of measure is cycles per degree (of visual angle)

5. Visual Angle and Spatial Frequency: The angle of an object relative to the observer’s eye
The closer an object is, the larger it’s visual angle, or
The larger an object is (relative to smaller object) the larger the visual angle

6. Size (visual angle) is related to distance from the viewer (e. g
Size (visual angle) is related to distance from the viewer (e.g. how “big” are the windows?)

7. I x cycles/deg 2x cycles/deg Low-contrast High-contrast M A
contrast = A/M

8. Our sensitivity to contrast varies as a function of spatial frequency
Contrast Sensitivity
0.1
0.2 1 10 50
Spatial Frequency

11. Panel (a): strips are wider than photoreceptors, hence brain can “reconstruct” vertical lines.
Panel (b): strips are narrower than photoreceptors, resulting in perception of gray.
Retinal ganglion cells and striate cortex have “frequency sensitive” cells

13. Contrast Sensitivity Function
Lowest contrast with lowest spatial frequency
Contrast Sensitivity Function
Lowest contrast with highest spatial frequency
highest contrast with lowest spatial frequency
Highest contrast with highest spatial frequency

14. How does the brain analyze visual information?

15. How does the brain analyze visual information
How does the brain analyze visual information? Breakdown the scene by its spatial frequency (and contrast) “components”

16. Breakdown of spatial frequencies & contrast components scene
How does the brain analyze visual information?
Mathematical technique call Fourier Analysis
Breakdown of spatial
frequencies
& contrast components
scene

17. How the brain analyzes visual information
-any scene can be broken down into spatial frequency components - a series of sine waves
= Fourier analysis
square-wave
Freq= f; Amp = a =
sine-wave f, a +
3rd harmonic 3f, a/3
5th harmonic 5f, a/5
+ all odd harmonics

18. Our brain does Fourier Transfer Functions via “firing preferences” of different (1) retinal, (2) LGN receptive field sizes and (3) cortical “frequency analyzers”

19. Adaptation & physiological measurement experiments reveal -- Spatial Frequency Channels (Spatial Freq. “Analyzers”)
Contrast Sensitivity
0.1
0.2 1 10 50
Spatial Frequency
-single-cell recording experiments show that simple
cells in visual cortex respond to a narrow range of spatial frequencies

20. Retinal ganglion cells are also sensitive to specific spatial frequencies as a function of the size of the center-surround field.

21. Breakdown of spatial frequencies & contrast components scene
How does the brain analyze visual information?
Mathematical technique call Fourier Analysis
Breakdown of spatial
frequencies
& contrast components
scene

22. Organization of Visual Cortex
1. Retinotopic Maps

24. Figure 4. 13 Retinotopic mapping of neurons in the cortex
Figure 4.13 Retinotopic mapping of neurons in the cortex. When the electrode penetrates the cortex obliquely, the receptive fields of the neurons recorded from the numbered positions along the track are displaced, as indicated by the numbered receptive fields; neurons near each other in the cortex have receptive fields near each other on the retina.

25. Retinotopic Organization
vertical penetration
Visual Field A B C F E D
oblique penetration A A B C D E F

26. Organization of Visual Cortex
1. Retinotopic Maps
2. Magnification Factor

27. Magnification of the fovea in the visual cortex
50,000 Ganglion cells are found in 1 mm from the fovea
1,000 Ganglion cells are found in 1 mm from the periphery of the retina
Need for a lot more cortex to process foveal information

28. Magnification Factor
The apportioning of proportionally more space on the cortex to central vision (cones), compared to peripheral vision (rods).
Figure 4.14 The magnification factor in the visual system: The small area of the fovea is represented by a large area on the visual cortex.

29. Figure 3.26, page 95 Magnification factor, packing density
Copyright © 2002 Wadsworth Group. Wadsworth is an imprint of the Wadsworth Group, a division of Thomson Learning

30. Magnification of the Fovea
vertical penetration
Visual Field A B C D E F
oblique penetration A B C A D A E A F A

31. Organization of Visual Cortex
1. Retinotopic Maps
2. Magnification Factor
3. Orientation Columns

32. “Location column” (A): all receptive fields are from the same point on the retina
“Orientation column”: includes simple, complex and end-stopped cells (informing about spatial frequency, contrast, movement & length) that are sensitive to only one orientation A

33. Organization of Visual Cortex
1. Retinotopic Maps
2. Magnification Factor
3. Orientation Columns
4. Ocular Dominance Columns
5. Hypercolumns (location Columns)

34. Ocular Dominance & Hypercolumns
Ocular (left or right eye) Dominance Columns
80 percent of cells fire to both eyes, but…
Cells have preference for left or right eyes
Left v. right preference set up in columns
Hypercolumns – one spot on the retina
Full set of everything
Orientation
Ocular Dominance (preference for left v. right)

35. The Hypercolumn 1 mm color blobs (pegs) R L Ocular dominance columns
(R or L)

36. Retinotopic Organization indicates the existence of “location columns”
vertical penetration
Visual Field A B C F E D
oblique penetration A A B C D E F

37. “Location column” (A): all receptive fields are from the same point on the retina
“Orientation column”: includes simple, complex and end-stopped cells (informing about spatial frequency, contrast, movement & length) that are sensitive to only one orientation A

38. “L” left eye dominant “R” right eye dominant Loc 3 Loc 2 Loc 1
Across the retinal field (each L/R pair is one retinal location)
Loc 3
Loc 2
Loc 1
“L” left eye dominant
“R” right eye dominant

39. Figure 4.22 Schematic diagram of a hypercolumn as pictured in Hubel and Wiesel’s ice-cube model. The light area on the left is one hypercolumn, and the darkened area on the right is another hypercolumn. The darkened area is labeled to show that it consists of one location column, right and left ocular dominance columns, and a complete set of orientation columns.

40. Short-, Medium- & Long-Cone densities

41. Only pieces of an object overlap with any one hypercolumn

42. Figure 4.24 How a tree creates an image on the retina and a pattern of activation on the cortex. See text for details.

43. Figure 4. 25 How the trunk of the tree pictured in Figure 4
Figure 4.25 How the trunk of the tree pictured in Figure 4.24 would activate a number of different orientation columns in the cortex.