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. Pupil-lometry confirmed that the melanopsin stimulus drives a retinal mechanism distinct from luminance. 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.
Introduction
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 (Figure 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, non-human primates, and people have emphasized the role of the ipRGCs in reflexive, non-image 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
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.11 As the geniculate is the starting point of the cortical pathway for visual perception, it is possible that ipRGC activity has an explicit visual perceptual correlate.
Here we examine whether isolated melanopsin 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.12,13 We also studied the converse modulation, which drives the cone-based luminance channel while minimizing melanopsin stimulation. We collected blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI) data while subjects viewed brief (three-second) 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.
Results
Four subjects were studied in multiple experiments while they viewed intermittent pulses of spectral contrast directed at either the post-receptoral luminance pathway (LMS, equal contrast on cones) or the melanopsin containing ipRGCs (Figure 1a, 1b). During functional MRI scanning, subjects viewed these stimuli with their pharmacologically dilated right eye; in some experiments the consensual response of the left pupil was also recorded with an infra-red camera (Figure 1c). Different stimuli produced contrast upon the targeted photoreceptors between 25% and 400% (Figure 1d; additional stimulus details in Figure S1). The subject maintained fixation upon a masked central disk (Figure 1e), while spectral changes occurred in the visual periphery against a background that was depleted in short-wavelength light and thus had a light-orange hue (Figure 1f).
V1 cortex responds to melanopsin contrast
We first examined the extent of cortical response to high-contrast spectral pulses. Each subject viewed approximately 200 pulses each of the 400% luminance and melanopsin stimuli. We measured the reliability of the evoked response within subject, and then at a second level across subjects and the two hemispheres. Pulses of luminance contrast that minimized melanopsin stimulation (Figure 2a) produced responses in the early cortical visual areas, generally corresponding to the retinotopic projection of the stimulated portion of the visual field.14 Spectral pulses directed at melanopsin that minimized cone stimulation also evoked responses within the visual cortex (Figure 2b). In subsequent experiments, we examined the evoked responses to luminance and melanopsin stimulation within a region of interest in V1 cortex that lies entirely within the retinotopic projection of the stimulated visual field. The time-series data and evoked responses from within this region for the initial, 400% contrast only experiment can be found in Figure S2.
If the visual cortex encodes information from the ipRGCs, we would expect that the degree of BOLD fMRI response should reflect variation in the degree of melanopsin stimulation, similar to the modulation of cortical response seen to variation in luminance contrast.15 Each of the four observers was studied again, this time with spectral pulses that varied in the degree of contrast upon the LMS or melanopsin channels. Figure 2c shows an example of the data obtained from the V1 region of interest in response to luminance pulses during one scan run for one observer. The time-series was fit with a Fourier basis set that estimated the shape of the BOLD fMRI response evoked by stimuli of each contrast level. Figure 2d presents the time-series data and evoked responses for the four subjects during luminance stimulation. Luminance pulses evoked consistent responses in the V1 region of interest, with a steadily increasing amplitude of evoked response across contrast levels. Variation in melanopic contrast (Figure 2e) produced similar data, with an increasing amplitude of BOLD fMRI response to larger contrasts.
We fit the evoked responses at each contrast level for each subject using an empirical measure of the subject’s hemodynamic response function, along with parameters that controlled the duration of an underlying neural response and the amplitude of the evoked BOLD fMRI signal (Figure S3). We obtained the amplitude of response as a function of contrast for each subject and each stimulus (Figure 3; LMS and melanopsin; grey and blue lines, respectively).
As suggested by the evoked responses in Figure 2, the measured amplitude increased as a function of contrast for both luminance and melanopsin stimulation for all four observers. While we modeled the duration of underlying neural activity, the results did not support the claim of a distinct temporal response to melanopsin stimulation (Figure S4).
While the melanopsin-directed spectral pulses were designed to produce no differential stimulation of the cones, biological variation and inevitable imperfection in device control results in some degree of unwanted cone stimulation (termed “splatter”).12,13,16 We considered the possibility that what appeared to be a visual cortex response to melanopsin contrast was in fact a response to the small amount of cone contrast inadvertently produced by our nominally cone silent spectral pulses.
We obtained spectroradiometric measurements of the stimuli that were actually produced by our device at the time of the BOLD fMRI experiment for each subject. For each of these measurements we calculated the inadvertent contrast that the cones would have experienced within these 400% melanopsin modulations in a biologically typical subject. We took the maximum contrast values calculated for the measurements across subjects, and created a new spectral pulse that was designed to have no melanopsin stimulation, but to have cone contrast equal to this estimate of inadvertent contrast. Scaled versions of this modulation corresponded to logarithmically-spaced larger (2x) and smaller multiples of the “splatter” contrast. We again studied the four subjects with BOLD fMRI while they viewed these stimuli, and measured the amplitude of response as a function of splatter contrast (Figure 3, green line). In all four subjects, the melanopsin response function was larger than the splatter response function. This indicates that the cortical response to melanopsin cannot be explained entirely by imperfection in stimulus generation. We then explored if biological variability could result in a greater degree of inadvertent cone contrast than our analysis of device imprecision alone would suggest. Our characterization of the stimuli in terms of cone contrast relies upon assumed values for several biological variables, including lens density, peak spectral sensitivity of the cone photopigments, their density, and the density of macular pigment. We conducted simulations in which we calculated the degree of inadvertent cone contrast that would have resulted given deviations from our assumptions, following estimated distributions of these biological variables.17 We find that it is very unlikely (approximately one chance in 100,000) that the responses observed in the four subjects could have resulted solely from inadvertent cone contrast (Figure S5).
The spectral sensitivity of the rod photoreceptors overlaps extensively with that of melanopsin. The background used for our melanopsin-directed stimuli was 3.5 log10 scotopic Trolands (scot Td), nominally at or above the rod saturation threshold, found to be 3.0 log10 scot Td (Figure 2 of Adelson 1982)18 or 3.3-3.7 log10 scot Td (Aguilar & Stiles 1954).19 Therefore, we expect in our experiments that there is no, or minimal, time-varying signal contributed by the rods. We attempted in a control experiment to further exclude this possibility by making use of an assumed difference in temporal sensitivity of the rods and melanopsin, but this experiment was uninformative (Figure S6). We return to this topic in the discussion.
A prior functional MRI study that presented a 50% Weber contrast melanopsin modulation did not find responses within the visual cortex, but did observe BOLD fMRI responses within the frontal eye fields.20 The authors speculated that melanopsin stimulation produces changes in alertness that manifest as these cortical responses, although eye movements were not recorded during their study. In our whole brain analysis (Figure 2a, 2b) we find responses within the frontal eye fields for both the luminance and melanopsin pulses at lowered map thresholds (unthresholded maps available from http://neurovault.org/collections/2459/). We considered the possibility that our stimulus pulses might cause subjects to briefly increase or decrease saccadic eye movements. We measured variation in eye position during the 3 s of stimulation and during the interstimulus interval (Figure S7). Subjects consistently reduced eye movements during the luminance and melanopsin stimulation periods as compared to the inter-stimulus-interval. This effect may account for the frontal eye field responses in our data and in the prior report.20 As eye movements alone can evoke responses in visual cortex,21 we considered that a systematic difference in eye movements across contrast levels might confound our finding of a contrast-dependent response in area V1. However, no eye movement difference was seen as a function of contrast level or stimulus type (LMS vs. melanopsin).
Different kinetics of pupil response to melanopic and luminance pulses
We have previously shown using sinusoidal spectral modulations that pupil responses to melanopsin stimulation have different temporal properties as compared to the responses evoked by modulations of luminance.13 In the current study, we recorded pupil responses to pulsed spectral modulations during the presentation of melanopsin and LMS stimulation of varying contrast. We examined these pupil responses for qualitative differences in the time course of the response. Such a demonstration would increase confidence that our stimuli target distinct retinal mechanisms.
The average pupil response was obtained for each contrast level and stimulus type. In the across-subject averages (Figure 4a; individual subject data in Figure S8), an evoked response to LMS stimulation is seen at even the lowest contrast level (25%). As LMS contrast grows, the evoked pupil response becomes larger, with distinct features corresponding to the onset and the offset of the 3 s stimulus pulse. The response to melanopsin contrast (Figure 4b) begins smaller, but also increases with contrast. Unlike the pupil response to LMS contrast, it is difficult to discern an indication of stimulus offset in the extended response to melanopsin stimulation.
We quantified these observations by fitting a temporal model (Figure S9) to the average evoked pupil responses. The model has three temporally distinct components that capture an initial transient constriction of the pupil at stimulus onset, a sustained response that tracks the stimulus profile, and a persistent response as the pupil slowly re-dilates in the seconds after stimulus offset (shown inset in each plot panel in Figure 4a and 4b, and schematically inset left in Figure 4c). The amplitude of each of these components was measured as a function of contrast for the LMS and melanopsin stimuli (Figure 4c; temporal parameter values in Figure S10). The amplitude of both the initial transient and persistent response increase with LMS and melanopsin contrast. The behavior of the sustained component, however, is different for the two types of stimulation. Luminance contrast produces steadily increasing sustained pupil constriction that is time-locked to the profile of the stimulus. In contrast, there is essentially no component of this kind in the melanopsin-driven pupil response. This behavior is in keeping with the temporally low-pass properties of the melanopsin system.13 We verified that the qualitative difference between the pupil response to luminance and melanopsin contrast remained when an alternative fitting procedure that locked the temporal profile of each model component across stimulation conditions was employed.
Melanopsin stimulation evokes a distinct visual percept
We find that a melanopsin-directed spectral pulse evokes a measurable response in the visual cortex. This suggests that people have conscious perceptual awareness of stimulation of the ipRGCs. Prior studies have found that melanopsin contrast contributes to a sensation of brightness, as subjects rate lights that contain melanopsin and luminance contrast as brighter than a light with luminance contrast alone.7 We were curious as to whether the perception of selective melanopsin-directed contrast appears simply as the typical experience of “brightness” conveyed by the luminance channel, or if there is a distinct perceptual experience associated with our melanopsin-directed stimulus.
We recruited 20 subjects and asked them to view 400% contrast pulses of LMS, melanopsin, and a stimulus changing in power by an equal multiplicative factor across all wavelengths, thus stimulating both melanopsin and luminance channels (“light flux”). Subjects were asked to rate nine perceptual qualities of the light pulse, each quality defined by a pair of antonyms (e.g., dim to bright). Subjects were not informed of the different identities of the stimuli, and the order was randomized as described in Online Methods. Subjects were also invited to offer their free-form observations at the end of the study during a debriefing session (summarized in Table S2).
A challenge of such measurements is the psychophysical sensitivity of the human visual system to even small amounts of differential cone contrast.22,23 We implemented additional stimulus calibration measures to further reduce spectral variation due to device instability (see Online Methods). In the measured stimulus spectra, the amount of inadvertent cone contrast in the melanopsin-directed stimulus due to imprecision in stimulus control was small (Figure S11).
Subjects rated each property of each stimulus twice, allowing us to confirm that within-subject reliability was high (across-subject mean Spearman correlation of test-retest reliability = 0.73 ± 0.18 SD). Additionally, there was good subject agreement in the ratings (across-subject mean Spearman correlation of ratings from one left-out subject to mean ratings of all other subjects = 0.53 ± 0.13 SD).
Subjects consistently rated the melanopsin stimulus as perceptually distinct from the LMS or light flux pulses (Table S1). We summarized these measurements by submitting them to a principal components analysis (Figure 5a). The first and second dimensions explained 35% and 19% of the variance in ratings, respectively. Within this space a support vector machine could classify subject responses to melanopsin as distinct from those for LMS or light flux with 92% cross-validated accuracy. A plot of the weights that define the classification dimension (Figure 5b) reveals the primary qualities of melanopsin stimulation. To these subjects, and in our own experience, the onset of the melanopsin contrast appears as a somewhat unpleasant, blurry, minimal brightening of the field. Most notably, however, this percept is fleeting, and rapidly followed by a fading or loss of perception from the stimulus field. Many of the subjects described the melanopsin stimulus pulse as being colored. This was typically with a yellow-orange appearance, although three subjects reported a greenish percept.
The perceptual ratings of the LMS and light flux stimuli were quite similar, with the LMS rated as having more color (again perhaps due to the inadvertent chromatic contrast present in the stimulus; Figure S11) and the light flux as being brighter. Prior studies have found that melanopsin contrast is additive to LMS contrast in the perception of brightness.7 In our data, this would be consistent with higher ratings on the dim-to-bright scale for light flux pulses as compared to LMS. A post-hoc test supported this interpretation (Wilcoxon signed-rank test of dim-to-bright ratings in Light Flux compared to LMS: p=0.0088).
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 and colleagues reported the case of an 87 year-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.24 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 as compared to the same measurement in a wild-type mouse.25 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 sub-cortical sites.26 Studies in the rodent and primate demonstrate as well projections to the lateral geniculate nucleus, where evoked responses to melanopsin stimulation can be found.11,25,27 Whether these signals are further transmitted to the visual cortex in normally sighted humans or non-human 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;28 a similar spatial distribution of cortical responses was observed to luminance stimulation.
By using a background depleted in short-wavelength light,8 we created substantial melanopic contrast in our stimuli, albeit ~3.5x less than is available in rodent models with a shifted long-wavelength cone.27 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 characteristic property of the ipRGCs is their tonic firing to transient stimuli. Our model of the evoked BOLD responses in V1 estimates the underlying duration of neural activity (Figure S3). We observed an increasing duration of neural activity in response to melanopsin stimulation across contrast levels, which was not seen in response to luminance stimulation (Figure S4). We regard this result as tentative, however, principally because a similar, increasing duration of neural response was seen for the “splatter” control modulation.
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,29 consistent with the extensive dendritic arbors of the ipRGCs.30 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.” (Table S2). The melanopsin containing ipRGCs send recurrent axon collaterals to the inner plexiform layer where they are positioned to modulate cone signals.31 Consistent with this, melanopic contrast has been shown to attenuate cone-driven electroretinogram responses in the rodent over minutes.27 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 if 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 functional MRI results are unlikely to be explained by inadvertent cone contrast from known sources of biological variation (Figure S5).17 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 current study we examined in the functional MRI 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 (Figure 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 a 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).16 While it is possible to also silence the penumbral cones in the melanopsin stimulus,12 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 did not explicitly silence rods in our melanopsin-directed stimulus. Our background is at light levels considered to be above rod intrusion, and we have previously demonstrated a pupil response to melanopsin-directed modulation around a background an order of magnitude brighter,13 indicating that the melanopsin system responds at light levels well above rod intrusion. In principle, we could further exclude the possibility of rod intrusion by examining a flickering version of our melanopsin-directed stimulus. In such an experiment we would identify a flicker frequency at which rods could respond (if not saturated) but for which melanopsin might not be expected to do so (e.g., 4-8 Hz). Finding no cortical response to the stimulus would support the contention that the rods are saturated. In practice, this control experiment faces two challenges. First, melanopsin may still respond within this frequency range.10 Second, this stimulus may drive the penumbral cones, producing a percept of the blood vessels and a cortical response.12,16 Modifying the stimulus to silence the penumbral cones would markedly reduce available contrast on both the rods and melanopsin, defeating the purpose of the experiment. Nonetheless, we attempted this control study and obtained uninformative results (Figure S6). An important area for future investigation is the relationship between rod and melanopsin signals in the transition between mesopic and photopic vision.
We note that these challenges attend our prior study of cortical responses to rapid melanopsin flicker.12 In those experiments, penumbral-cone silent, sinusoidal melanopsin modulations with 16% Michelson contrast were studied. For comparison to the stimuli used in the current 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 question of whether melanopsin contributes to visual perception at photopic light levels in people is one of considerable interest, as it 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, supra-threshold visual cortex response to varying contrast levels, which we may compare to the effect of imprecision in cone silencing. Further, presentation of supra-threshold contrast allows for the characterization of the appearance of the stimulus, as was done here.
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 if variation in the perceptual or cortical response to this stimulus is related to the symptom of photophobia.32 Under naturalistic conditions, it appears that melanopsin adjusts the sensitivity of the cone pathways.27 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.29
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 cd/m2; >3.3 log sc td). Pulse stimuli (3s, 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 pre-receptoral filtering (see Online Methods, Visual stimuli). 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 (see Online Methods, Simulation of biological variability causing inadvertent cone contrast).
Four observers (four men; aged 27, 28, 32, 46; three of whom are authors of this study) viewed the stimuli with their pharmacologically dilated right eye while they underwent functional MRI in a 3T Siemens Prisma MRI scanner with a 64-channel headcoil. 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 seconds apart. Subjects were asked to detect an occasional, brief (500 msec) 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 pre-processing and were projected to a spherical atlas of cortical surface topology, supporting anatomical definition of the location and organization of retinotopic cortex (see Online Methods, MRI data acquisition and initial processing). Because stimuli were presented asynchronously with respect to fMRI acquisitions, the time-series data were fit with a Fourier basis set to extract the average evoked response to each stimulus type. The resulting evoked response per stimulus type was then fit 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 (see Online Methods, BOLD fMRI time-series analysis).
In a separate experiment, conducted outside of the scanner, 20 observers (9 men, 11 women; mean age 27, range 20–33) 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 as to the identity of the pulses. They were asked to rate the stimuli along nine perceptual dimensions, given as antonym pairs (see Online Methods, Perceptual rating experiment).
The 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 pre-registered in the Open Science Framework. All data and code are available.
Detailed methods are described in Online Methods.
Acknowledgements
This work was supported by the National Institutes of Health (Grant R01 EY024681 to G.K.A. and D.H.B., Core Grant for Vision Research P30 EY001583, and Neuroscience Neuroimaging Center Core Grant P30 NS045839), the Department of Defense (Grant MR141251 to G.K.A). We thank Fred Letterio for technical assistance, and Andrew S. Olsen for his assistance with data collection.
Footnotes
Competing financial interests: 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 (U.S. Patent Application No. 14/852,001, “Robust Targeting Of Photosensitive Molecules”). The authors declare no other competing financial interests.