Lecture 5.1: Spatial Vision
-
Light
-
Visible Light
-
Spectral Curves
-
dominant wavelength/hue
-
brightness/lightness
-
purity/saturation
-
Human Visual Physiology
-
Human eye
-
overall structure
-
cornea: most of focussing (> 80%)
-
iris: focusses incoming light further
muscles contract and relax to change shape (optical power ==> accommodation
nonuniform transmission of wavelengths (absorbs more blue)
chromatic aberration (blue seems farther)
-
optic nerve
-
retina w/fovea: layer of light sensitive cells
-
retina
-
mechanism: pigment which reacts to light
-
rods: perceive intensity at low light lievels (scotopic vision)
-
cones: perceive light at normal light levels (photopic vision)
3 types: sensitive to different wavelengths (420,530,560 nm)
luminous efficiency curves: S much less sensitive than M or L
smaller and pack more densely than rods
-
distribution of receptors
just cones in foveola (1/3 degree visual angle)
more rods in periphery
-
linking and processing cells
-
ganglia: gather inputs, process and transmit
-
transform incoming RGB into opponent color responses
R-G
B-Y (Y = R + G)
W (W ~= R + G) ==> basis for pattern vision
-
characteristics (continue down visual pathways)
concentric receptive fields
logarithmic response of receptors
adaption
within receptor (Weber's law)
with neighbors (lateral inhibition)
-
LGN (lateral geniculate nuclei)
-
2 LGN (one for each side of visual field)
-
located deep within brain
-
acts as relay station to visual cortex, some upstream data flow
-
arranged in 6 layers
-
2 monochromatic (one from each eye) ==> beginning of magnocellular
-
2 chromatic (two from each eye) ==> beginning of parvocellular
-
magnocellular pathway
-
characteristics
achromatic
large RF centers
fast and transient
-
role in vision
identify objects and boundaries
depth perception
motion perception
-
Visual Cortex
-
located at back of head (but 90% of brain responds to visual stimuli)
-
cells sensitive to
-
orientation
-
end-stopped
-
occular dominance
-
spatial frequency (scale)
-
feedback from cognitive levels to earlier stages of pathway
Human Visual Characteristics
-
adaption
-
to deal with large dynamic range of light
-
we become acclimated to intensity, hue, orientation, spatial frequency
-
contrast sensitivity influenced by spatial frequency
-
contrast sensitivity functions
how big is jnd?
dI/I nearly constant (Weber's law)
-
grating
-
max ~3-5 cycles/degree
-
influenced by adaption
-
communication between neighboring receptors
-
edge completion: subjective contours
-
relative judgements
-
constancy
-
lightness
-
simultaneous contrast
-
tolerant of noise
-
illusions
-
feedback from higher visual processes
-
3D interp of 2D drawings
-
expectations from experience