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Chapter 3

Chapter 3 Notes

Course Code
Matthias Niemeier

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Chapter 3: Spatial Vision: From Stars to Stripes
Visual system in cerebral cortex translate images into stripes
From Primary visual cortex we separate into Where and What
Flow of info for form and colour
Retinal ganglion cells Thalamus LGN Primary visual cortex visual association
Higher-order visual assoc. cortex
Visual Acuity: Oh Say Can You see?
Contrast: The difference in luminance btw an object and background or light/dark
Acuity: smallest detail we can see
oFor scientists, smallest visual angle of cycle we can see
Cycle: one rep. of black and white stripe
Visual angle: Angle subtended by an object at the retina
oangle from top and bottom of object onto center of lens and onto retina Fig 3.3
p. 53
we can see one cycle of 2mm at about 1 minute of arc (0.017 )
othis is limit is b/c of spacing of photoreceptors
Sine wave gratings: grating with a sinusoidal luminance profile
oe.g. fig. 3.4 p. 54
oIf cycle falls on separate cones, we can make out grating
oWhen cycle falls onto one cone we see gray field or aliasing
Aliasing: see cycles as longer than they really are
Cones are separated by 0.5 min of arc (0.008 ) why we need 2 cones to see cycle
Acuity in peripheral worse b/c rods and cones are packed less tightly there
A Visit to the Eye Doctor
Snellen invented 20/20 system of measuring acuity
Letters are 5 times larger than strokes
20/40 means distance person can see letters / distance normal vision can see letters
oE.g. 20/40 means you have to be at 20 ft. to see letters ppl can see at 40 ft.
If you can read 20/20 latter you can see detail that subeneds 1 min of arc
Most healthy adults have acuity of 20/15
Acuity for Low-Contrast Stripes
Spatial frequency: # of cycles of a grating per unit of visual angle
oE.g. Visual angle btw each pair of white stripes is 0.25 . Therefore spatial
frequency is 1/0.25 = 4 cycles per degree

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Cycles per degree: # of dark and bright bars per degree of visual angle
oTighter bars is higher cycle per degree, higher spatial freq.
Found that people dont always find it easier to distinguish low contrast stripes of
lower spatial freq.
oContrast Sensitivity Function: Describes how sensitivity to contrast
depends on spatial frequency size
Contrast threshold: Smallest amount of contrast needed to detect pattern
oe.g. if dark stripes have to be 1% darker than white stripes A light stripes
reflecting 1000 photons would have dark stripes reflecting 990 photons to be
At 60 cycles/degree we need 100% contrast, corresponds to cycle width of 1 min., arc
Why Sine Wave Gratings?
Patterns of stripes with fuzzy boundaries are common, e.g. trees in forest
We break down images into singe wave gratings
Retinal Ganglion Cells and Stripes
Ganglion cells respond well to certain types of stripes or gratings
ON ganglion cell - When spatial frequency is right, bright bar fills center and dark
bars fill surround, positive response
Ganglion cells are tuned to a spatial freq.
Response depends on phase of grating
oPhase: Gratings position within RF
When grating has light bar filing RF center and dark bars in surround ON center
cell responds positively
oIf bars are half in center, no response
The Lateral Geniculate Nucleus
Axons of ganglion cells synapose in two LGNs
Lateral geniculate nucleus (LGN): In thalamus, receives input from ganglion
cells and has I/O connections w/ visual cortex
oLike relay from retina to cortex
oMagnocellular layers: Bottom 2 layers of LGN
oParvocellular layers: Top 4 layers
Get input from M ganglion cellsGet input from P ganglion cells
Responds to large, fast moving objectsProcesses static things
Left LGN gets projections from left sides of retina in both eyes
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