Short-wavelength light sensitivity of circadian, pupillary, and visual awareness in humans lacking an outer retina

Abstract

As the ear has dual functions for audition and balance, the eye has a dual role in detecting light for a wide range of behavioral and physiological functions separate from sight. These responses are driven primarily by stimulation of photosensitive retinal ganglion cells (pRGCs) that are most sensitive to short-wavelength ( approximately 480 nm) blue light and remain functional in the absence of rods and cones. We examined the spectral sensitivity of non-image-forming responses in two profoundly blind subjects lacking functional rods and cones (one male, 56 yr old; one female, 87 yr old). In the male subject, we found that short-wavelength light preferentially suppressed melatonin, reset the circadian pacemaker, and directly enhanced alertness compared to 555 nm exposure, which is the peak sensitivity of the photopic visual system. In an action spectrum for pupillary constriction, the female subject exhibited a peak spectral sensitivity (lambda(max)) of 480 nm, matching that of the pRGCs but not that of the rods and cones. This subject was also able to correctly report a threshold short-wavelength stimulus ( approximately 480 nm) but not other wavelengths. Collectively these data show that pRGCs contribute to both circadian physiology and rudimentary visual awareness in humans and challenge the assumption that rod- and cone-based photoreception mediate all "visual" responses to light.

Figure 1

Neuroophthalmology and Ocular Anatomy of the Blind Female Subject and a Normal Control The left panel shows fundoscopy findings of the 87-year-old blind female subject (A) and a representative ocular-coherence tomogram for the peripheral retina (C) and central macula region (D) of the left eye, compared with a normal age-matched sighted control (B, E, and F). Her retina is abnormally thin (less than 160 microns) and there is no identifiable outer nuclear layer or photoreceptor layer, suggesting that photoreceptors are absent, and the choroid has abnormally high reflectivity (Ch) in contrast to the normal age-matched subject (E and F), where stratification within the neurosensory retina, particularly the outer nuclear layer (ONL), can be seen. By contrast, the ganglion cell and nerve fiber layers of the inner retina of the blind woman are of normal thickness, and there is no cellular disruption, allowing clear recognition and delineation of normal histo-architecture in both retinal periphery and macula. In (G), comparison of the normal macula profile in an age-matched individual (within green limits, as shown in OCT image in [F]) illustrates loss of normal macular contour in the blind subject (black line, as derived from [D]). The normal distribution percentile correlates the color-coded areas of the figure to percentages of age-matched people who might possess retinae within that region. V = vitreous, NR = neurosensory retina. The right panel shows electroretinographic responses from the female subject (A, C, and E) and an age-matched, normal eye (B, D, and F) for dark-adapted (rod-photoreceptor predominant) responses (A and B); dark-adapted, light-adapted (mixed photoreceptor) responses (C and D); and light-adapted (cone predominant) responses (E and F) to 30 Hz flicker stimuli. White-light stimuli at 3.0 cd s/m² intensity were used for all tests and began at the start of recordings in all cases. The traces for the blind subject show no detectable electroretinographic responses (Note: [C]shows a drifting baseline.).

Figure 2

Entrained Rest-Activity and Urinary 6-Sulphatoxymelatonin Rhythms in Two Blind Subjects The daily activity rhythm (black) and light (lux) exposure (yellow) patterns of the female (A) and male (B) subjects, recorded at home for 3–4 weeks with wrist actigraphy (Actiwatch-L, Minimitter, New York). Data are double-plotted, with consecutive days plotted next to and beneath each other. The gray bars represent an arbitrary “night” from 23:00–6:00 hr for visual reference. Analysis of actigraphy data indicated that both the female and male subject had sleep onset (mean ± standard deviation [SD] sleep onset = 21:50 ± 1:09 hr and 23:22 ± 0:24 hr, respectively) and sleep offset (8:38 ± 1:29 hr and 6:31 ± 0:26 hr, respectively) times that fell within the range of actigraphically derived sleep times for blind subjects with previously confirmed normally phased circadian sleep and urinary 6-sulphatoxymelatonin rhythms (mean ± 2SD sleep onset = 23:31 ± 2:26 hr, sleep offset = 7:11 ± 2:24 hr) . The urinary 6-sulphatoxymelatonin (aMT6s) rhythm peak time (○) in the male subject confirmed the presence of a normally phased nighttime 24 hr rhythm (mean ± SD = 3:00 ± 1:17 hr) that exhibited a normal phase angle (3:38 hr) with respect to the sleep/wake cycle based on previous studies in entrained blind subjects (mean ±2SD phase angle, sleep onset − aMT6s peak = 4:38 ± 2:28 [3, 14]). The raw urinary data are shown in [C] with the normal peak-time range for the aMT6s rhythms shown in gray (1:42–6:36 hr) .

Figure 3

Short-Wavelength Light Sensitivity for Melatonin Suppression and Enhancement of EEG Alpha Power in a Blind Man The direct effects of exposure to green (555 nm) and blue (460 nm) monochromatic light on the male subject for melatonin suppression (A) and waking-EEG power density (B) as an objective correlate of alertness. Exposure to 555 nm light caused no suppression of melatonin as compared to the corresponding clock time the previous day, whereas exposure to 460 nm light suppressed melatonin (total suppression by AUC = 57%) and maintained the suppression effect throughout the entire 6.5-hr exposure (A). The 460 nm light also caused an elevation of alpha activity (8–10 Hz) in the waking EEG, indicative of a more alert state (B). Only alpha frequencies exhibited a wavelength-dependent difference during the second half of the light exposure (C). These data are consistent with the short-wavelength sensitivity for the acute effects of light in sighted subjects under similar conditions .

Figure 4

Short-Wavelength Light Sensitivity for Pupillary Constriction and Light Detection in a Blind Woman Irradiance-response curves (IRCs) were conducted at eight wavelengths for both eyes (squares indicate left eye, triangles indicate right eye) (A, left panel). Responses are plotted as percentage of maximum response obtained. IRCs were fitted with a four-parameter sigmoid function, with R² values >0.90 in all cases. The resulting action spectrum of pupil responses (A, right panel) provided a poor fit to rod and cone photopigments (rod R² = 0.35; SW cone, MW cone, LW cone R² = 0). An optimum fit to the pupil response to light was provided by an opsin/vitamin A-based template with λ_max 476 nm (R² = 0.89), corresponding closely to the pRGC system. Note: Data shown were not corrected for preretinal lens absorption. When this correction was applied, the λ_max shifted from 476 nm to 480 nm. (B) shows the results of the psychophysical testing in the same subject that indicated conscious perception of light at 481 nm (^∗∗∗p < 0.001) but failure (p > 0.05) to detect light at longer or shorter wavelengths (420, 460, 500, 515, 540, 560, and 580 nm). These results mirror the spectrally tuned response of the pupil, and suggest that the subject's detection and awareness of light also arise from pRGCs. Each histogram represents the percentage of correct responses out of 20 trials for both left and right eyes (360 trials in total).