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When a dark chromatic contour delineating a figure is flanked on the inside by a brighter chromatic contour, the brighter color will spread into the entire enclosed area. This is known as the watercolor effect (WCE). Here we quantified the effect of color spreading using both color-matching and hue-cancellation tasks. Over a wide range of stimulus chromaticities, there was a reliable shift in color appearance that closely followed the direction of the inducing contour. When the contours were equated in luminance, the WCE was still present, but weak. The magnitude of the color spreading increased with increases in luminance contrast between the two contours. Additionally, as the luminance contrast between the contours increased, the chromaticity of the induced color more closely resembled that of the inside contour. The results support the hypothesis that the WCE is mediated by luminance-dependent mechanisms of long-range color assimilation.

Chromatic contrast thresholds for spatially varying patterns of various spatial frequencies (0.5, 1, 2, and 4 cycles per degree) were measured for ten older (65-77 yr of age) and ten younger (18-30 yr of age) observers. The stimuli were Gabor patches modulated along S-varying or (L - M)-varying chromatic axes. Thresholds were determined for two sets of stimuli. For one set of stimuli, the mean chromaticity and luminance were equated at the cornea for all observers. The second set of stimuli was corrected for ocular media density differences to equate stimulation of each of the three cone types at the retina for each individual. Chromatic contrast thresholds were higher for older observers for all stimuli tested. The magnitude of this difference showed little dependence on spatial frequency. When stimuli were equated at the cornea, this difference was greater for S-varying stimuli. When stimuli were equated at the retina, the age-related difference in thresholds for S-varying stimuli was reduced. Both optical and neural factors contribute to these age-related losses in spatial chromatic contrast sensitivity.

Age-related changes in the spatial chromatic contrast sensitivity function of detection, measured along S and L - M cone axes, were demonstrated in a companion paper [Hardy et al., J. Opt. Soc. Am. A 22, 49 (2005)]. Here senescent changes in chromatic contrast appearance were assessed by contrast-matching functions (CMFs). Luminance and chromatic CMFs (S and L - M axes) were compared for younger (age 18-31 yr) and older (age 65-75 yr) trichromatic subjects by using stimuli that were perceptually anchored to the same physical standard contrasts. Subjects matched the contrast of test gratings of various spatial frequencies (0.5-8 cycles per degree) to the standard stimuli under natural viewing conditions. Because of changes in the visual system with age, the standard stimuli were closer to threshold for older subjects; however, in general, the shapes of the CMFs were similar for both groups. The results suggest that the perception of relative contrasts across spatial frequencies is stable with age.

Previous studies have demonstrated an inverse relation between the size of the complete spatial summation area and ganglion cell density. We hypothesized that if this relation is dynamic, the spatial summation area at 6° nasal would expand to compensate for age-related losses of retinal ganglion cells but not in the fovea where age-related loss in ganglion cell density is not significant. This hypothesis was tested by measuring contrast thresholds with a series of Gabor patches varying in size. The spatial summation area was defined by the intersection of the segments of a two-branched, piece-wise linear function fitted to the data with slopes of -0.5 and 0 on a plot of log threshold vs. log area. Results demonstrate a 31% increase in the parafoveal spatial summation area in older observers with no significant age-related change in the fovea. The average foveal data show a significant increase in thresholds with age. Contrary to the foveal data, age comparisons of the parafoveal peak contrast thresholds display no significant difference above [corrected] the summation area. Nevertheless, as expected from the increase in summation area, expressing the parafoveal thresholds as contrast energy reveals a significant difference for stimuli that are smaller than the maximal summation area.

BackgroundOur lab has previously demonstrated losses in contrast sensitivity to low spatial frequencies under scotopic conditions with older adults. It is not clear, however, whether the temporal frequency of a stimulus alters the relation between age and the spatial contrast sensitivity function (sCSF) under scotopic conditions.MethodsA maximum-likelihood, two-alternative, temporal forced-choice QUEST procedure was used to measure threshold to spatially and temporally modulated stimuli in both young (mean = 26 years) and old (mean = 75 years) adults.ResultsIn general, the shapes of the spatial and temporal CSFs were low-pass for both young and old observers; contrast sensitivity decreased at approximately the same rate with increasing spatial frequency and temporal frequency for both age groups, although the overall sensitivity of the old group was lower than that of the young group. The high-frequency resolution limit was lower for the old group compared to the young group.ConclusionsThe differences in contrast sensitivity between the young and old groups suggest a uniform loss in sensitivity of the channels mediating spatial and temporal vision. Because of this loss, the spatial and temporal window of visibility for the older adults is compromised relative to the younger adults.

Accurately processing orientation information is a fundamental component of visual performance. Single-unit recordings have shown that the orientation tuning of individual neurons in macaque cortical areas V1 and V2 is reduced dramatically with age (M. T. Schmolesky, Y. Wang, M. Pu, & A. G. Leventhal, 2000; S. Yu, Y. Wang, X. Li, Y. Zhou, & A. G. Leventhal, 2006). These researchers suggest that losses in single-unit orientation selectivity result in declines in orientation discrimination and object recognition in older humans. Three experiments were conducted to determine whether human performance is affected by putative age-related changes in tuning of cortical neurons. Ten younger and ten older observers participated in this study. Experiment 1 demonstrated significant differences in the contrast sensitivity of the two age groups. Experiment 2 showed significant differences in orientation discrimination thresholds. However, when thresholds were plotted in terms of multiples of detection threshold, age-related differences were not observed. In Experiment 3, perceptual orientation tuning curves did not significantly differ in shape for younger and older subjects. As in Experiment 2, at any given contrast, there is a large difference in sensitivity between younger and older adults. This implies a model of orientation processing that allows the adult visual system to maintain consistent and reliable orientation information at the network and ultimately the perceptual level.

Color assimilation with bichromatic contours was quantified for spatial extents ranging from von Bezold-type color assimilation to the watercolor effect. The magnitude and direction of assimilative hue change was measured as a function of the width of a rectangular stimulus. Assimilation was quantified by hue cancellation. Large hue shifts were required to null the color of stimuli < or = 9.3 min of arc in width, with an exponential decrease for stimuli increasing up to 7.4 deg. When stimuli were viewed through an achromatizing lens, the magnitude of the assimilation effect was reduced for narrow stimuli, but not for wide ones. These results demonstrate that chromatic aberration may account, in part, for color assimilation over small, but not large, surface areas.