color perception in the intermediate periphery of the visual field thorsten hansen, lars pracejus...
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Color perception in the intermediate periphery of
the visual field
Thorsten Hansen, Lars Pracejus & Karl R. Gegenfurtner
Abteilung Allgemeine Psychologie
Introduction Perception in the periphery
Contrast sensitivity
Fine details (high spatial frequencies) are harder to detect in the periphery.
This can be compensated by magnifying the image.
Introduction
Cone density in the human retina Curcio et al. (1990)
Steep decline towards periphery.
Nasal retina has a higher density than temporal retina.
Nasal Fovea Temporal
1000 /mm²
Introduction Color opponency in the fovea
Derrington (2001).
The RF center of a parvo ganglion cell in the fovea is driven by a single cone.
Introduction Color opponency at larger eccentricities
…is expected to be weaker and will eventually be zero when the center and surround is driven by an equal ratio of L:M cells
Fovea Periphery
Introduction Psychophysics vs. physiology
Psy
chop
hysi
cs (
Hum
an)
Cel
l rec
ordi
ng (
Mac
aque
)
Macaque ganglion cells remain color sensitive, while human performance drops.
Martin et al. (2001). Nature.
Introduction Psychophysics
Mullen, Sakurai & Chu (2005). Perception.
»Thus we conclude that there is little or no L/M cone opponent response measurable psychophysically beyond 20–30 deg of
eccentricity in the nasal visual field.«
LumS−(L+M)
L−M
Methods
DKL color spaceAn achromatic axis: Lum
Two chromatic axes: L−M and S−(L+M))
L−M
S−(L+M)Lum
L−MS−(L+M)
Derrington Krauskopf Lennie
Short presentation stimuli (500 ms)
forced choice (4AFC)
threshold measured by standard stair-case procedure
Exp 1 & 2: Detection & Identification Exp. 3: Discrimination
Methods Procedure
Comparison color varied along 8 chromatic directions
Methods Chromatic discrimination
Ellipse was fit to the data to characterize discrimination performance
Results Control: Foveal discrimination
…as expected:
Best at the adaptation point
Elongated along the saturation axesKrauskopf & Gegenfurtner (1992). Vision Res.; Hansen, Giesel & Gegenfurtner (in press), J. Vision.
Results Discrimination at 50°
Larger size of ellipses: Discrimination is worse, but not absent!
Greatest increase along L−M axis
Leads to rounder shapes of ellipses off the L−M axis
Summary Chromatic processing in the periphery
• Detection
• Identification
• Discrimination
As long as the stimuli are large enough, peripheral color vision is just like foveal vision.
Introduction Cortical representation
Cortical magnification factor
The central part of the visual field (10 deg) is represented by about half of all neurons in primary visual cortex V1
Introduction Physiology
• Size of the receptive fields
• Cortical representation
• Center – surround ratio
• Random wiring
• Results physiology – psychophysics
Discussion Biased sample? (Martin et al. 2001)
34/53 overt red-green response
28/35 cone-opponent
11 not significantly different from foveal cells
Introduction Color opponency at larger eccentricities
Several factors can contribute to preserve color opponency
• selective wiring (vs. random wiring)
• elongated RFs
• unequal/random distribution of cone types at each retinal location
Introduction Selectivity by elongated RFs
Martin et al., 2001
Midget RF centers are elongated and may rotate to sample one cone type more than the other to increase cone-purity