Pupil responses to colorfulness are selectively reduced in healthy older adults

Abstract

The alignment between visual pathway signaling and pupil dynamics offers a promising non-invasive method to further illuminate the mechanisms of human color perception. However, only limited research has been done in this area and the effects of healthy aging on pupil responses to the different color components have not been studied yet. Here we aim to address this by modelling the effects of color lightness and chroma (colorfulness) on pupil responses in young and older adults, in a closely controlled passive viewing experiment with 26 broad-spectrum digital color fields. We show that pupil responses to color lightness and chroma are independent from each other in both young and older adults. Pupil responses to color lightness levels are unaffected by healthy aging, when correcting for smaller baseline pupil sizes in older adults. Older adults exhibit weaker pupil responses to chroma increases, predominantly along the Green-Magenta axis, while relatively sparing the Blue-Yellow axis. Our findings complement behavioral studies in providing physiological evidence that colors fade with age, with implications for color-based applications and interventions both in healthy aging and later-life neurodegenerative disorders.

Figure 1

Procedure of the color pupillometry experiment. The experimental stimuli consisted of 26 broad-spectrum color fields which were shown in a black-out room on an Eizo ColorEdge CG2420 24-inch LCD monitor in pseudo-randomized to prevent that variations of the same color category were shown in direct succession (e.g., light blue and dark blue). There were 26 trials in total. Each trial consisted of a 5-s baseline condition (a) and an experimental condition (b) during which a color field was shown for 5 s. Pupil diameters were measured continuously at a frequency of 1000 Hz with an SR Research Eyelink 1000 Plus table-mounted eye-tracking camera. Further information on the experimental design and color selection can be found under “Materials and methods” and in the Supplementary Materials.

Figure 2

Pupil response profiles relative to baseline of young and older adults plotted against the duration of the stimulus presentation (5000 ms) at a sampling rate of 100 Hz, and parsed in 3 time windows aligning with distinct processing phases of aesthetic stimuli: 0–250 ms; 250–750 ms; 750–5000 ms. Only the recorded pupil data during the 750–5000 ms time window, indicated by the shaded areas in the graphs, were used in the linear mixed models (see further detailed under the subsection Data Analyses under “Materials and methods”). Pupil responses to the color stimuli have been grouped by modifications in color lightness (dark vs light) and colorfulness (saturated vs muted)—See also Fig. 5.

Figure 3

Pupil responses to color lightness (L*) and chroma (C*) in young and older adults, defined in the CIELCh color space. Sustained pupil response to the color stimulus was calculated as the log of the ratio of average pupil size in the right eye under experimental condition, to pre-trial baseline right-eye pupil size. The vertical markers on each eye symbol indicate the confidence intervals of the percent change compared to baseline pupil size. (a) Shows the pupil responses to increasing levels of lightness (L*) and chroma (C*) in young adults. (b) Shows the pupil responses to increasing levels of lightness (L*) and chroma (C*) in older adults. Older adults’ pupil responses to increases in chroma (C*) were significantly weaker compared to young adults (statistical significance indicated with ).

Figure 4

Pupil responses to increases in saturation of the four polar CIELAB hues (Green, Magenta, Yellow, and Blue) in young and older adults, plotted to their maximum saturation level in the color selection. Sustained pupil response to the color stimulus was calculated as the log of the ratio of average pupil size in the right eye under experimental condition, to pre-trial baseline right-eye pupil size. The vertical markers on each eye symbol indicate the confidence intervals of the percent change compared to baseline pupil size. (a) Shows the pupil responses to increases in relative saturation levels of Green, Magenta, Yellow, and Blue in young adults, with relative Green or Magenta saturation levels having stronger effects than relative saturation levels of Blue or Yellow. (b) Shows the pupil responses to increases in relative saturation levels of Green, Magenta, Yellow, and Blue in older adults, with significantly weaker constriction responses—and a range of responses that also include pupil dilations—to relative Magenta or Green saturation levels compared to young adults (statistical significance indicated with ). In older adults, relative Magenta saturation level had the strongest effect on pupil responses, while relative Green saturation appeared to have a similar effect as relative Blue saturation on pupil responses in older adults.

Figure 5

Overview of the 26 broad-spectrum color fields used as experimental stimuli in the pupillometry experiment.