Contrast Matching With Complex and Natural Stimuli
of Varying Angular Size
Bridget C. Hendricks, James Comerford, Frank Thorn, and Eli Peli
The New England College of Optometry, Boston, MA
Introduction
Discussion
Results
Observers demonstrate contrast constancy for suprathreshold natural stimuli
despite defocus and aberrations induced by viewing images which subtend small
angular retinal sizes. The amount of constancy is consistent across images of
varying sizes, for all images that are able to be resolved. However, contrast
constancy seems to fail for stimuli near threshold. The seeming failure of contrast
constancy for stimuli near threshold was also found in our previous study in which
sine wave grating stimuli were used.
Defocus and aberrations degrade the retinal image of high spatial frequencies
compared to low spatial frequencies, resulting in decreased contrast sensitivity at high
spatial frequencies.
Georgeson and Sullivan (1975) found that observers matched the contrast of high and
low spatial frequencies so that high contrast images of all frequencies appeared
subjectively to have equally high contrast despite effects of retinal blur. They call this
effect contrast constancy.
From previous studies we know that induced refractive defocus and scatter blur
have little effect on contrast constancy, when tested using high contrast gratings of
varying spatial frequency.
Contrast constancy has obvious implications for vision with uncorrected refractive
error and may also be important for low vision patients (perhaps patients can have the
perception of sharp contours despite low contrast optical and neural images).
These results suggest that there is an adaptive mechanism responsible for
maintaining constancy of image perception across varying spatial frequencies as
we scan our natural environment.
Webster et al (2002) studied the effects of contrast adaptation as it applies to stimuli
such as simple edges and grayscale images of natural scenes. Their results show
large and rapid changes in the perception of image focus when people view images
with altered spatial statistics.
Image 1
Both Images have a characteristic distribution of spatial contrasts as captured by the 1/f amplitude spectra typical of natural scenes.
Reduced images were generated using a Bicubic interpolation algorithm for image resampling.
Contrast constancy has important implications for the study of perceptual
adaptation in low vision due to either optical or neural blur.
The standard image and the experimental image were presented side by side on a 19
inch Samsung monitor, at a distance of 3 meters. Subjects adjusted the experimental
image to match the standard image in contrast by using the up/down arrows on a
keyboard. An ascending and descending method of limits experimental paradigm
was used.
Results
The averaged results of our 14 observers are presented on the right. While contrast
sensitivity functions typically show a decrease in sensitivity at high spatial frequencies
relative to lower spatial frequencies, contrast matching functions were unaffected by
the high spatial frequencies represented in the ½ standard and ¼ standard images,
relative to the lower spatial frequencies presented in the 1X and 2X standard images,
thus demonstrating contrast constancy.
Results for image 1 (face) and image 2 (trees) were similar. Results were similar for
all 14 observers. Subjects reported that the 1/4x size target, during the lowest
contrast condition (standard = 0.075 Michaelson contrast), was difficult to resolve.
Stimuli that were reportedly difficult to resolve may represent sub-threshold stimuli.
Contrast constancy appears to fail as stimuli approach threshold.
Stimulus Contrast (RMS)
The perceived contrast of these images was matched with the same image
subtending different angular retinal sizes (2x, 1x, 0.5x, and 0.25x standard size
image). The “2x” image subtended a visual angle of 2.86 degrees; the “1x” image
subtended a visual angle of 1.43 degrees; the 1/2x image subtended a visual angle of
0.76 degrees; and the “1/4x” image subtended a visual angle of 0.38 degrees.
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2X
1X
10
1/2X
1/4X
1
1
10
Standard Contrast (RMS)
References
IMAGE 2
IMAGE 1
Method
Fourteen observers were presented with a grayscale image of a face (image 1) and a
forest (image 2) at Michaelson contrasts of 0.075, 0.15, 0.30, and 0.60.
Image 2
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Stimulus Contrast (RMS)
Our previous experiments have explored the effects of refractive defocus and scatter
blur on contrast matching, demonstrating contrast constancy with suprathreshold
simple grating stimuli. We further explore this effect as it relates to more natural
images by evaluating effects of varying retinal image size (i.e. shifting the spatial
frequency spectra of the image) on contrast matching of complex, natural stimuli.
Significant amounts of compensation for blur have previously been shown to
occur rapidly, and exhibit almost complete interocular transfer (Mon-Williams et al,
1998; Webster et al, 2002). It has been suggested that the mechanism for blur
compensation is a function of gain control for spatial frequency-selective channels
within the visual cortex (Georgeson & Sullivan, 1975; Mon-Williams et al, 1998).
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Burton, G.J., Moorehead. I.R. Color and spatial structure of natural scenes.
Applied Optics. 26. 157-170, 1987.
Georgeson, M.A. and Sullivan, G.D. Contrast Constancy: deblurring in human
vision by spatial frequency channels. J. Physiol, 252, 627-656, 1976.
10
Mon-Williams, et al. Improving vision: Neural compensation for optical defocus.
Proc. R. Soc. Lond. B 265, 71-77, 1998.
Webster, et al. Neural adjustments to image blur. Nature Neuroscience.
Sept. 5(9): 839-840, 2002.
1
1
10
Standard Contrast (RMS)
Contrast matching performance for images subtending visual angles of 2.6 degrees (2x image), 1.43 degrees (1x image),
0.76 degrees (1/2x image), and 0.38 degrees (1/4x image), relative to a 1.43 degree (1x image). Contrast matching
functions were unaffected by the high spatial frequencies represented in the 0.38 degree and 0.76 degree images relative
to the lower spatial frequencies represented in the 2.6 degree and 1.43 degree images. Contrast matching functions for
image 1 (face) and image 2 (forest) are similar.
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Acknowledgments
Funded by the New England College of Optometry, and the following Grants:
NECO Vision Core Grant: 5 R24 EY014817
NIH/NEI Grant: 5 T35 EY007149
Special thanks to Image Specialist, Hyunjeong (Jamie) Han, for generation of
natural image stimuli, and image analysis.