rhess home - McGill Vision Researchmvr.mcgill.ca/Robert/site/files/lightDarkAdaptation.pdf ·...
Transcript of rhess home - McGill Vision Researchmvr.mcgill.ca/Robert/site/files/lightDarkAdaptation.pdf ·...
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http://mvr.mcgill.ca/Robert/rhess_home.html
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Luminance and retinal illumination
The range of luminances (left) and retinal illumination (right) found in the natural world
Sun
Fluorescent light/bulbs
White paper, full sunlight Candle flame
Comfortable reading
Print read with difficulty White surface, moonlight Threshold for cone vision
White surface, moonless night
Visual threshold
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How can we see over 15 log units of illumination?
• 1. Duplex function • 2. Cellular adaptation
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Rod and cone operating ranges
cone vision
day
LUMINANCE RANGE
rod vision
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Duplex function
• 1. Rods are more sensitive than cones (x50)
• 2. There are more rods than cones (x10)
• 3. Ganglion cells have larger RFs for rods than cones (i.e. more post-receptoral summation)
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2. Distribution of rods and cones visual eccentricity (deg)
spatial density (cells/square mm)
macula lutea
cones
rods
retinal eccentricity (mm)
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Convergence
receptors130 million
bipolars20 million
ganglion cells1 million
optic nerve fibres
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3. Spectral sensitivity curves for rod and cone vision
Rela
tive
sen
siti
vity
Wavelength (nm)
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Purkinje effect
• A shift in the colour appearance at dusk. • Reds look darker, blues look brighter
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1. Contrast sensitivity functions at three different light levels
Spatial Frequency (cycles/mm on retina)
Spatial frequency (cycles/degree) Se
nsit
ivit
y (1
/thr
esho
ld c
ontr
ast)
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Contrast sensitivity and operating range
Large operating range but poor contrast sensitivity
Relative light intensity Intensity
Brig
htne
ss
Brig
htne
ss
Good contrast sensitivity but small operating range
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Receptoral adaptation
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Ganglion cell adaptation
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I1 I2 I3 Re
spon
se (i
ps)
1 1.5 10 15 100 150 Flash intensity log
Mean light levels
I I
I = 0.5 I = 5 I = 50
A single neuron can shift its operating range according to the mean light level. The light increment (delta I) required to obtain a criterion response is scaled up or down, according to the mean light level. This is known as GAIN CONTROL.
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Increment threshold curve
WEBER’S LAW
Log background intensity (I)
Log
incr
emen
t th
resh
old
(del
ta I
)
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Weber’s Law
∆ I/I = constant
• Our sensation is determined by the percentage difference in the luminance of a surface relative to its background
• This holds over a wide range of background (ambient) luminances
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4. Sensitivity to light of rods & cones: Dark Adaptation
Low Lo
g. l
ight
sen
siti
vity
High Time in dark (min)
7 minutes
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Dark adaptation curves Low Lo
g. l
ight
sen
siti
vity
High Time in dark (min)
Cones Rods
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Rods & cones: 4 key differences between scotopic and photopic vision • Contrast sensitivity
• Distribution of rods and cones
• Spectral sensitivity of rods and cones
• Sensitivity to light of rods and cones.