Significance Melanopsin-containing retinal cells detect bright light and contribute to reflex visual responses such as pupil constriction. Their role in conscious, cortical vision is less understood. Using functional MRI to measure brain activity, we find that melanopsin-directed stimulation reaches the visual cortex in people. Such stimulation also produces a distinct perceptual experience. Our results have clinical importance as melanopsin function may contribute to the discomfort that some people experience from bright light.

Abstract The photopigment melanopsin supports reflexive visual functions in people, such as pupil constriction and circadian photoentrainment. What contribution melanopsin makes to conscious visual perception is less studied. We devised a stimulus that targeted melanopsin separately from the cones using pulsed (3-s) spectral modulations around a photopic background. Pupillometry confirmed that the melanopsin stimulus evokes a response different from that produced by cone stimulation. In each of four subjects, a functional MRI response in area V1 was found. This response scaled with melanopic contrast and was not easily explained by imprecision in the silencing of the cones. Twenty additional subjects then observed melanopsin pulses and provided a structured rating of the perceptual experience. Melanopsin stimulation was described as an unpleasant, blurry, minimal brightening that quickly faded. We conclude that isolated stimulation of melanopsin is likely associated with a response within the cortical visual pathway and with an evoked conscious percept.

Human visual perception under daylight conditions is well described by the combination of signals from the short (S)-, medium (M)-, and long (L)-wavelength cones (1). Melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs) are also active in bright light (Fig. 1A). The ipRGCs have notably prolonged responses to changes in light level and thus signal retinal irradiance in their tonic firing (2). Studies in rodents, nonhuman primates, and people have emphasized the role of the ipRGCs in reflexive, nonimage-forming visual functions that integrate information over tens of seconds to hours, such as circadian photoentrainment, pupil control, and somatosensory discomfort from bright light (3⇓⇓–6).

Fig. 1. Overview and experimental design. (A, Left) The L, M, and S cones, and melanopsin-containing ipRGCs, mediate visual function at daytime light levels. (A, Right) The spectral sensitivities of these photoreceptor classes. (B) Stimulus spectra. Changes between a background (black) and stimulus (red) spectra targeted a given photoreceptor channel. The 400% contrast stimuli are shown. (B, Left) Spectra targeting the L, M, and S cones and thus the postreceptoral luminance channel. The nominal melanopic contrast for this modulation was zero. (B, Right) The corresponding spectra for stimuli targeting melanopsin. The nominal L-, M-, and S-cone contrast of this stimulus was zero. (C) During fMRI scanning, subjects viewed pulsed spectral modulations, produced by a digital spectral integrator, with their pharmacologically dilated right eye. The consensual pupil response of the left eye was recorded in some experiments. (D) Multiple 3-s, pulsed spectral modulations were presented, windowed by a 500-ms half-cosine at onset and offset, and followed by an 11- to 13-s interstimulus interval (ISI). A given experiment presented either a single contrast level, or multiple contrast levels in a counterbalanced order. (E) Spectra were presented on a uniform field of 64° (visual angle) diameter. Subjects fixated the center of a 5° masked region, minimizing macular stimulation. The stimulus spectra had a light-orange hue.

Relatively unexamined is the effect of melanopsin phototransduction upon visual perception, which operates at shorter timescales (7⇓–9). In addition to tonic firing, ipRGCs exhibit transient responses to flashes of light with an onset latency as short as 200 ms (10). Several ipRGC subtypes project to the lateral geniculate nucleus, where they are found to drive both transient and tonic neural responses (2, 11⇓–13). As the geniculate is the starting point of the cortical pathway for visual perception, it is possible that ipRGC activity has a conscious visual perceptual correlate.

Here we examine whether melanopsin-directed stimulation drives responses within human visual cortex and characterize the associated perceptual experience. Our approach uses tailored modulations of the spectral content of a light stimulus, allowing melanopsin to be targeted separately from the cones in visually normal subjects (14, 15). We also studied the converse modulation, which drives the cone-based luminance channel while minimizing melanopsin stimulation. We collected blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) data while subjects viewed brief (3-s) pulses of these spectral modulations. Concurrent infrared pupillometry was used to confirm that our stimuli elicit responses from distinct retinal mechanisms. Finally, we characterized the perceptual experience of selective melanopsin-directed stimulation and examined whether this experience is distinct from that caused by stimulation of the cones.

Discussion Our studies indicate a role for the melanopsin-containing ipRGCs in conscious human vision. We find that high-contrast spectral exchanges designed to isolate melanopsin evoke responses in human visual cortex. Pupil responses to these stimuli are distinct from those produced by luminance contrast, consistent with separation of retinal mechanisms. The cortical response is not easily explained by inadvertent stimulation of the cones and is associated with a distinct perceptual experience. Previous studies in rodents and humans with outer photoreceptor defects have suggested that the visual cortex responds to melanopsin stimulation. Zaidi et al. (19) reported the case of an 87-y-old woman with autosomal-dominant cone–rod dystrophy who was able to correctly report the presence of an intense, 480-nm 10-s light pulse, but not other wavelengths. Similarly, in mice lacking rods and cones, the presentation of a narrowband 447-nm light evoked a hemodynamic (optical imaging) signal change in the rodent visual cortex, with a slightly delayed onset (1 s) and a reduced amplitude compared with the same measurement in a wild-type mouse (11). In our work we measured cortical and perceptual responses to melanopsin-directed stimulation in the intact human visual system. A Cortical Response. The melanopsin-containing ipRGCs have broad projections to subcortical sites (20). Studies in the rodent and primate demonstrate as well projections to the lateral geniculate nucleus, where evoked responses to melanopsin stimulation can be found (2, 11⇓–13). Whether these signals are further transmitted to the visual cortex in normally sighted humans or nonhuman animals has been unknown. We find that pulsed melanopsin stimulation evokes contrast-graded responses within primary visual cortex. Responses to the highest (400%) contrast stimulus extend into adjacent, retinotopically organized visual areas, including ventrally in the vicinity of the peripheral representation for hV4 and VO1 (21); a similar spatial distribution of cortical responses was observed to luminance stimulation. By using a background with reduced short-wavelength light (8), we created substantial melanopic contrast in our stimuli, albeit ∼3.5× less than is available in rodent models with a shifted long-wavelength cone. (12) We found that 100% contrast pulses were required to obtain a measurable cortical response to melanopsin. The contrast response functions for both V1 fMRI amplitude and persistent pupil constriction appeared to be in the linear range and rising even at our maximum, 400% contrast level. A Visual Percept. Consistent with the presence of a V1 neural response, we find that melanopsin-directed stimulation is accompanied by a distinct visual percept. We viewed these stimuli over many hours of experiments and ourselves experienced the onset of the melanopsin spectral pulse as a diffuse, minimal brightening of the visual field. The appearance was curiously unpleasant. The diffuse, even blurry, property of the percept might be related to the broad receptive fields of neurons driven by melanopsin stimulation (22), consistent with the extensive dendritic arbors of the ipRGCs (23). In a prior study, subjects reported that lights appear brighter when melanopsin contrast is added to the stimulation of the cone-based luminance channel (7). We find a conceptually similar effect in our data, as subjects rated pulses of light flux (which contain melanopic contrast) as brighter than pulses with cone contrast alone. The most striking aspect of the percept evoked by the melanopsin pulse is that the brief brightening is then followed by a fading of perception of the stimulus field, on occasion spreading to involve the masked macular region of the stimulus. This was subjectively similar to Troxler fading. This aspect was remarked upon by several of our observers: “[the experience was] like blinding,” and “[the fade] to black that is the noise when your eyes are closed” or “kind of like if you got hit in the head really sharply … flashing lights and fade out.” (SI Appendix, Table S2). The melanopsin-containing ipRGCs send recurrent axon collaterals to the inner plexiform layer where they are positioned to modulate cone signals (24). Consistent with this, melanopic contrast has been shown to attenuate cone-driven electroretinogram responses in the rodent over minutes (12). The prominent and rapid experience of fading for our melanopsin-directed stimulus perhaps reflects the unopposed action of this attenuation mechanism. Our data do not allow us to determine whether one or more of the reported perceptual experiences arising from melanopsin stimulation are a direct consequence of ipRGC signals arriving at visual cortex sites or from the interaction of melanopsin and cone signals at earlier points in the visual pathway. The Challenge of Photoreceptor Isolation. Our conclusions depend upon the successful isolation of targeted photoreceptor channels. Measurements and simulations indicate that the fMRI results are unlikely to be explained by inadvertent cone contrast from known sources of biological variation (SI Appendix, Fig. S5) (18). Nonetheless, we think it prudent to carry forward concern regarding inadvertent cone intrusion and to search for additional means to exclude this possible influence. For example, in the present study we examined in the fMRI data whether there was a difference in the time course of response to luminance and melanopsin-directed stimuli, but did not find convincing evidence of such (SI Appendix, Fig. S3). A time-course dissociation in the fMRI data would have provided further support—similar to that obtained in the pupil data—that our stimuli drive distinct mechanisms. Different temporal profiles of stimulation may afford greater traction on this question in future studies. In our perceptual experiment, the melanopsin stimulus was reported to have a change in hue. This was usually, but not universally, reported as yellow–orange. In this experiment we do not have available an estimate of the amount of reported color change that may be attributable to imperfections in cone silencing. Consequently, we are unable to reject the possibility that small amounts of chromatic splatter produce this percept. Our results are also subject to any systematic deviation of photoreceptor sensitivity from that assumed in the design of our spectral modulations. One example model deviation is the presence of “penumbral” cones that lie in the shadow of blood vessels and thus receive the stimulus spectrum after it has passed through the hemoglobin transmittance function. These photoreceptors can be inadvertently stimulated by a melanopsin-directed modulation, producing a percept of the retinal blood vessels when the spectra are rapidly flickered (≥4 Hz) (17). While it is possible to also silence the penumbral cones in the melanopsin stimulus (14), this markedly reduces available contrast upon melanopsin (below 100%). We circumvented this problem here by windowing the onset of the melanopsin stimulus with a gradual transition (effectively 1 Hz) that removed the penumbral cone percept from our stimulus pulse. We note that these challenges attend our prior study of cortical responses to rapid melanopsin flicker (14). In those experiments, penumbral-cone silent, sinusoidal melanopsin modulations with 16% Michelson contrast were studied. For comparison with the stimuli used in the present study, we can express contrast as the peak of the sinusoid relative to the trough. This yields ∼40% Weber contrast. Given our finding here that roughly 100% Weber contrast was needed to evoke a V1 response, we now regard our prior study as not fully resolving the possibility that rapid modulation of the ipRGCs drives a cortical response. The finding that melanopsin contributes to visual perception at photopic light levels in people challenges the orthodoxy that only three photopigments contribute to daylight vision. Two previous studies using silent substitution methodology reported psychophysical sensitivity in detection of cone-silent spectral modulations at photopic light levels (8, 9). These studies also faced the challenge of photoreceptor isolation, as even small imperfections in the silencing of cones could lead to detection. An inferential strength of the current study is that we measure a graded, suprathreshold visual cortex response to varying contrast levels, which we may compare with the effect of imprecision in cone silencing. Suprathreshold contrast also allowed us to characterize the appearance of the stimulus.

Conclusions Our results suggest that people can “see” with melanopsin. The high-contrast, melanopsin-directed spectral modulation we studied is a distinctly unnatural stimulus but a valuable tool for demonstrating the presence of a melanopic signal in the cortical visual pathway. Many of our subjects found the melanopsin-directed stimulus to be unpleasant to view. We are curious whether variation in the perceptual or cortical response to this stimulus is related to the symptom of photophobia (6). Under naturalistic conditions, it appears that melanopsin adjusts the sensitivity of the cone pathways (12). The interaction of melanopsin and cone signals in human vision is an exciting avenue for investigation, particularly given recent findings of a role for melanopsin in the coarse spatial coding of light intensity (22).

Materials and Methods A digital light synthesis engine (OneLight Spectra) was used to produce spectral modulations that targeted either the melanopsin photopigment or the LMS cones with varying contrast (25%, 50%, 100%, 200%, and 400%) against a rod-saturating background [100–200 candelas/m2 (cd/m2); >3.3 log 10 scotopic trolands (sc td)]. Pulse stimuli (3 s, cosine windowed at onset and offset) were presented within a wide-field, uniform annulus with an outer diameter of 64° and an inner diameter of 5°, minimizing macular stimulation. Stimuli were adjusted for each observer’s nominal age to account for age-specific prereceptoral filtering (SI Appendix, SI Text Online Methods). The quality of photopigment isolation was assessed by combining spectroradiometric measurements of the stimuli with a resampling approach that modeled sources of biological variation in photoreceptor spectral sensitivity (SI Appendix, SI Text Online Methods). Four observers (four men, aged 27 y, 28 y, 32 y, and 46 y, three of whom are authors of this study) viewed the stimuli with their pharmacologically dilated right eye while they underwent fMRI scanning. The consensual pupillary response to the stimuli was measured from the left eye during some scanning sessions, using an infrared eye tracker. Stimulus pulses were jittered in their onset timing and spaced 14–16 s apart. Subjects were asked to detect an occasional, brief (500 ms) dimming of the stimulus field to which they made a button press. This served to monitor subject alertness and provided events that were used to derive a hemodynamic response function (HRF) for each observer. BOLD fMRI data underwent standard preprocessing and were projected to a spherical atlas of cortical surface topology, supporting anatomical definition of the location and organization of retinotopic cortex (SI Appendix, SI Text Online Methods). Because stimuli were presented asynchronously with respect to fMRI acquisitions, the time-series data were fitted with a Fourier basis set to extract the average evoked response to each stimulus type. The resulting evoked response per stimulus type was then fitted with a two-parameter model incorporating the duration of an underlying step of neural activity and the amplitude of this response after convolution by the subject-specific HRF (SI Appendix, SI Text Online Methods). In a separate experiment, conducted outside of the scanner, 20 observers (9 men, 11 women; mean age 27 y, range 20–33 y) viewed the LMS and melanopsin-directed stimuli, as well as pulses of broadband spectral change (light flux) which stimulated both cones and melanopsin. These observers were not involved in the design and conduct of the study and were not informed of the identity of the pulses. They were asked to rate the stimuli along nine perceptual dimensions, given as antonym pairs (SI Appendix, SI Text Online Methods). This research was approved by the University of Pennsylvania Institutional Review Board and conducted in accordance with the principles of the Declaration of Helsinki. All subjects gave written informed consent. All experiments were preregistered in the Open Science Framework. All data and code are available. All raw data are available as packaged and MD5-hashed archives as well as tables detailing the biological variability on FigShare (https://figshare.com/s/0baea6ed50758abbabf4). All code is available in public GitHub repositories (https://github.com/gkaguirrelab/Spitschan_2017_PNAS/). Unthresholded statistical maps from experiments 1 and 2 for each subject are available from NeuroVault (https://neurovault.org/collections/2459/). Detailed methods are described in SI Appendix, SI Text Online Methods.

Acknowledgments We thank Fred Letterio for technical assistance and Andrew S. Olsen for his assistance with data collection. This work was supported by National Institutes of Health Grant R01 EY024681 (to G.K.A. and D.H.B.), Core Grant for Vision Research P30 EY001583, Neuroscience Neuroimaging Center Core Grant P30 NS045839, and Department of Defense Grant W81XWH-15-1-0447 (to G.K.A.).

Footnotes Author contributions: M.S., D.H.B., and G.K.A. conceived the project; M.S. and G.K.A. designed the fMRI experiments; J.R., D.H.B., and G.K.A. designed the perceptual experiment; M.S. and D.H.B. designed the spectral modulations; M.S., A.S.B., J.R., G.F., and G.K.A. collected fMRI data; G.F. collected pupillometry data; J.R. collected perceptual data; M.S., A.S.B., and G.F. analyzed fMRI data; M.S. and G.F. analyzed pupillometry data; G.K.A. implemented temporal models for the fMRI and pupillometry data; J.R., D.H.B., and G.K.A. analyzed perceptual data; M.S. analyzed the effects of biological variability upon photoreceptor contrast; G.K.A. created the figures; and M.S. and G.K.A. wrote the paper with contributions from A.S.B., J.R., G.F., and D.H.B.

Conflict of interest statement: G.K.A., D.H.B., and M.S. are listed as inventors on a patent application filed by the Trustees of the University of Pennsylvania on September 11, 2015 (US Patent Application No. 14/852,001, “Robust Targeting of Photosensitive Molecules”). The authors declare no other competing financial interests.

This article is a PNAS Direct Submission.

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