Bridget C. Hendricks, James Comerford, Frank Thorn, and Eli Peli The New England College of Optometry, Boston, MA

1. Contrast Matching With Complex and Natural Stimuli of Varying Angular Size Bridget C. Hendricks, James Comerford, Frank Thorn, and Eli PeliThe New England College of Optometry, Boston, MA Introduction Results Discussion 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. 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. 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. 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). Contrast constancy has important implications for the study of perceptual adaptation in low vision due to either optical or neural blur. 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. 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). 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. 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. Image 1 Image 2 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. References Method 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. 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. 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. 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. 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. Acknowledgments Results 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. 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. 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.