1. Spatial Vision (continued)3
Spatial Vision (continued)

2. Visual Acuity: Oh Say, Can You See?
Spatial frequency: The number of cycles of a grating per unit of visual angle (usually specified in degrees)
Another way to think of spatial frequency is as the number of times a pattern repeats per unit area

3. Visual Acuity: Oh Say, Can You See?
Why sine gratings?
Patterns of stripes with fuzzy boundaries are quite common
The edge of any object produces a single stripe, often blurred by a shadow, in the retinal image
The visual system breaks down images into a vast number of components; each is a sine wave grating with a particular spatial frequency

4. Figure 3.7 The contrast sensitivity function (red line): our window of visibility; and Figure 3.8 A modulated grating X X
e.g. contrast sensitivity = 1/contrast threshold (CT)=100 for
CT=1% difference between dark&light stripes
Visual acuity threshold = 0.017degrees
Spatial frequency = 1/0.017 = 60 (limit determined by cone spacing) X X

5. Retinal Ganglion Cells and Stripes
The response (right) of a ganglion cell to gratings of different frequencies (left): (a) low, (b) medium, and (c) high
How do the center– surround receptive fields respond to sine wave patterns with different spatial frequencies?

6. Retinal Ganglion Cells and Stripes
Not only is the spatial frequency important, but so is the phase
Phase: The phase of a grating refers to its position within a receptive field

7. Figure 3.11 The primate lateral geniculate nucleus
LGN: relay station between retina and cortex
Six layers: looks like a stack of pancakes bent in the middle
wolfe2e-fig jpg

8. Left side of both retinas Right visual field
Figure Input from the right visual field is mapped in an orderly fashion onto the different layers of the left LGN, and vice-versa
wolfe2e-fig jpg
Left side of both retinas
Right visual field
Right side of both retinas
Left visual field

9. Striate Cortex
Striate cortex: Also known as primary visual cortex, area 17, or V1
A major transformation of visual information takes place in striate cortex
Circular receptive fields found in retina and LGN are replaced with elongated “stripe” receptive fields in cortex
It has about 200 million cells!

10. Figure Striate cortex }
LGN
wolfe2e-fig jpg

11. Striate Cortex
Two important features of striate cortex:
Topographical mapping
Cortical magnification:
Dramatic scaling of information from different parts of visual field
The amount of cortex devoted to processing the fovea is proportionally much more than the amount of cortex devoted to processing the periphery

12. Figure 3.14 The mapping of objects in space onto the visual cortex
wolfe2e-fig jpg

13. Receptive Fields in Striate Cortex
Hubel and Wiesel: tried to map the receptive fields of neurons in striate cortex
Used spots of light that worked as stimuli for ganglion cells
No neuronal activity was observed!
Where did the signals from ganglions disappear???
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