Pablo A. Barrionuevo

Assessing Rod, Cone, and Melanopsin Contributions to Human Pupil Flicker Responses

PURPOSE We determined the relative contributions of rods, cones, and melanopsin to pupil responses in humans using temporal sinusoidal stimulation for light levels spanning the low mesopic to photopic range. METHODS A four-primary Ganzfeld photostimulator controlled flicker stimulations at seven light levels (-2.7 to 2 log cd/m(2)) and five frequencies (0.5-8 Hz). Pupil diameter was measured using a high-resolution eye tracker. Three kinds of sinusoidal photoreceptor modulations were generated using silent substitution: rod modulation, cone modulation, and combined rod and cone modulation in phase (experiment 1) or cone phase shifted (experiment 2) from a fixed rod phase. The melanopsin excitation was computed for each condition. A vector sum model was used to estimate the relative contribution of rods, cones, and melanopsin to the pupil response. RESULTS From experiment 1, the pupil frequency response peaked at 1 Hz at two mesopic light levels for the three modulation conditions. Analyzing the rod-cone phase difference for the combined modulations (experiment 2) identified a V-shaped response amplitude with a minimum between 135° and 180°. The pupil response phases increased as cone modulation phase increased. The pupil amplitude increased with increasing light level for cone, and combined (in-phase rod and cone) modulation, but not for the rod modulation. CONCLUSIONS These results demonstrate that cone- and rod-pathway contributions are more predominant than melanopsin contribution to the phasic pupil response. The combined rod, cone, and melanopsin inputs to the phasic state of the pupil light reflex follow linear summation.

A five-primary photostimulator suitable for studying intrinsically photosensitive retinal ganglion cell functions in humans

Intrinsically photosensitive retinal ganglion cells (ipRGCs) can respond to light directly through self-contained photopigment, melanopsin. IpRGCs also receive synaptic inputs from rods and cones. Thus, studying ipRGC functions requires a novel photostimulating method that can account for all of the photoreceptor inputs. Here, we introduced an inexpensive LED-based five-primary photostimulator that can control the excitations of rods, S-, M-, L-cones, and melanopsin-containing ipRGCs in humans at constant background photoreceptor excitation levels, a critical requirement for studying the adaptation behavior of ipRGCs with rod, cone, or melanopsin input. We described the theory and technical aspects (including optics, electronics, software, and calibration) of the five-primary photostimulator. Then we presented two preliminary studies using the photostimulator we have implemented to measure melanopsin-mediated pupil responses and temporal contrast sensitivity function (TCSF). The results showed that the S-cone input to pupil responses was antagonistic to the L-, M- or melanopsin inputs, consistent with an S-OFF and (L + M)-ON response property of primate ipRGCs (Dacey et al., 2005). In addition, the melanopsin-mediated TCSF had a distinctive pattern compared with L + M or S-cone mediated TCSF. Other than controlling individual photoreceptor excitation independently, the five-primary photostimulator has the flexibility in presenting stimuli modulating any combination of photoreceptor excitations, which allows researchers to study the mechanisms by which ipRGCs combine various photoreceptor inputs.

Contributions of rhodopsin, cone opsins, and melanopsin to postreceptoral pathways inferred from natural image statistics.

Visual neural representation is constrained by the statistical properties of the environment. Prior analysis of cone pigment excitations for natural images revealed three principal components corresponding to the major retinogeniculate pathways identified by anatomical and physiological studies in primates. Here, principal component analyses were conducted on the excitations of rhodopsin, cone opsins, and melanopsin for nine hyperspectral images under 21 natural illuminants. The results suggested that rhodopsin and melanopsin may contribute to the three major retinogeniculate pathways. Rhodopsin and melanopsin may provide additional constraints in natural scene statistics, leading to new components that cannot be revealed by analysis based on cone opsin excitations only.

Comparison between an objective and a psychophysical method for the evaluation of intraocular light scattering

In this study we present the comparison of the performance of two systems to measure intraocular scattering. Measurements were made by using a psychophysical system based on a brightness comparison method that provides a glare index and a physical system based on the double-pass technique, which gives an objective scatter index by measuring the optical quality of the eye. Three external diffuser filters that simulated different grades of intraocular scattering were used in subjects with normal vision. The two measured indexes showed a graded rise with increasing level of scattering. The discrimination ability obtained for both systems showed that they were able to distinguish among conditions ranging from normal to early cataracts.

Luminance and chromatic signals interact differently with melanopsin activation to control the pupil light response.

Intrinsically photosensitive retinal ganglion cells (ipRGCs) express the photopigment melanopsin. These cells receive afferent inputs from rods and cones, which provide inputs to the postreceptoral visual pathways. It is unknown, however, how melanopsin activation is integrated with postreceptoral signals to control the pupillary light reflex. This study reports human flicker pupillary responses measured using stimuli generated with a five-primary photostimulator that selectively modulated melanopsin, rod, S-, M-, and L-cone excitations in isolation, or in combination to produce postreceptoral signals. We first analyzed the light adaptation behavior of melanopsin activation and rod and cones signals. Second, we determined how melanopsin is integrated with postreceptoral signals by testing with cone luminance, chromatic blue-yellow, and chromatic red-green stimuli that were processed by magnocellular (MC), koniocellular (KC), and parvocellular (PC) pathways, respectively. A combined rod and melanopsin response was also measured. The relative phase of the postreceptoral signals was varied with respect to the melanopsin phase. The results showed that light adaptation behavior for all conditions was weaker than typical Weber adaptation. Melanopsin activation combined linearly with luminance, S-cone, and rod inputs, suggesting the locus of integration with MC and KC signals was retinal. The melanopsin contribution to phasic pupil responses was lower than luminance contributions, but much higher than S-cone contributions. Chromatic red-green modulation interacted with melanopsin activation nonlinearly as described by a “winner-takes-all” process, suggesting the integration with PC signals might be mediated by a postretinal site.

Estimating photoreceptor excitations from spectral outputs of a personal light exposure measurement device

The intrinsic circadian clock requires photoentrainment to synchronize the 24-hour solar day. Therefore, light stimulation is an important component of chronobiological research. Currently, the chronobiological research field overwhelmingly uses photopic illuminance that is based on the luminous efficiency function, V(λ), to quantify light levels. However, recent discovery of intrinsically photosensitive retinal ganglion cells (ipRGCs), which are activated by self-contained melanopsin photopigment and also by inputs from rods and cones, makes light specification using a one-dimensional unit inadequate. Since the current understanding of how different photoreceptor inputs contribute to the circadian system through ipRGCs is limited, it is recommended to specify light in terms of the excitations of five photoreceptors (S-, M-, L-cones, rods and ipRGCs; Lucas et al., 2014). In the current study, we assessed whether the spectral outputs from a commercially available spectral watch (i.e. Actiwatch Spectrum) could be used to estimate photoreceptor excitations. Based on the color sensor spectral sensitivity functions from a previously published work, as well as from our measurements, we computed spectral outputs in the long-wavelength range (R), middle-wavelength range (G), short-wavelength range (B) and broadband range (W) under 52 CIE illuminants (25 daylight illuminants, 27 fluorescent lights). We also computed the photoreceptor excitations for each illuminant using human photoreceptor spectral sensitivity functions. Linear regression analyses indicated that the Actiwatch spectral outputs could predict photoreceptor excitations reliably, under the assumption of linear responses of the Actiwatch color sensors. In addition, R, G, B outputs could classify illuminant types (fluorescent versus daylight illuminants) satisfactorily. However, the assessment of actual Actiwatch recording under several testing light sources showed that the spectral outputs were subject to great non-linearity, leading to less accurate estimation of photoreceptor excitations. Based on our analyses, we recommend that each spectral watch should be calibrated to measure spectral sensitivity functions and linearization characteristics for each sensor to have an accurate estimation of photoreceptor excitations. The method we provided to estimate photoreceptor excitations from the outputs of spectral watches could be used for chronobiological studies that can tolerate an error in the range of 0.2–0.5 log units. Our method can be easily expanded to incorporate linearization functions to have more accurate estimations.

Veiling luminance as a descriptor of brightness reduction caused by transient glare.

The presence of a glare source in the visual field produces a veiling luminance (L(v)), which generates a brightness reduction that can be expressed in terms of the glare index (V). The relation between the veiling luminance caused by glare and the apparent brightness reduction of a reference target has already been established for steady conditions. In this paper, the relationship is derived for transient glare. First, the relation is tested empirically, and then previous results concerning the effect of transient glare on brightness are summarized and analyzed. From this analysis, a power function relation between L(v) and V is encountered.

Retinal mesopic adaptation model for brightness perception under transient glare

A glare source in the visual field modifies the brightness of a test patch surrounded by a mesopic background. In this study, we investigated the effect of two levels of transient glare on brightness perception for several combinations of mesopic reference test luminances (Lts) and background luminances (Lbs). While brightness perception was affected by Lb, there were no appreciable effects for changes in the Lt. The highest brightness reduction was found for Lbs in the low mesopic range. Considering the main proposal that brightness can be inferred from contrast and the Lb sets the mesopic luminance adaptation, we hypothesized that contrast gain and retinal adaptation mechanisms would act when a transient glare source was present in the visual field. A physiology-based model that adequately fitted the present and previous results was developed.

Assessment of #TheDress With Traditional Color Vision Tests: Perception Differences Are Associated With Blueness

Based on known color vision theories, there is no complete explanation for the perceptual dichotomy of #TheDress in which most people see either white-and-gold (WG) or blue-and-black (BK). We determined whether some standard color vision tests (i.e., color naming, color matching, anomaloscope settings, unique white settings, and color preferences), as well as chronotypes, could provide information on the color perceptions of #TheDress. Fifty-two young observers were tested. Fifteen of the observers (29%) reported the colors as BK, 21 (40%) as WG, and 16 (31%) reported a different combination of colors. Observers who perceived WG required significantly more blue in their unique white settings than those who perceived BK. The BK, blue-and-gold, and WG observer groups had significantly different color preferences for the light cyan chip. Moreland equation anomaloscope matching showed a significant difference between WG and BK observers. In addition, #TheDress color perception categories, color preference outcomes, and unique white settings had a common association. For both the bright and dark regions of #TheDress, the color matching chromaticities formed a continuum, approximately following the daylight chromaticity locus. Color matching to the bright region of #TheDress showed two nearly distinct clusters (WG vs. BK) along the daylight chromaticity locus and there was a clear cutoff for reporting WG versus BK. All results showing a significant difference involved blue percepts, possibly due to interpretations of the illuminant interactions with the dress material. This suggests that variations in attributing blueness to the #TheDress image may be significant variables determining color perception of #TheDress.

Influence of background size, luminance and eccentricity on different adaptation mechanisms

Mechanisms of light adaptation have been traditionally explained with reference to psychophysical experimentation. However, the neural substrata involved in those mechanisms remain to be elucidated. Our study analyzed links between psychophysical measurements and retinal physiological evidence with consideration for the phenomena of rod-cone interactions, photon noise, and spatial summation. Threshold test luminances were obtained with steady background fields at mesopic and photopic light levels (i.e., 0.06-110cd/m(2)) for retinal eccentricities from 0° to 15° using three combinations of background/test field sizes (i.e., 10°/2°, 10°/0.45°, and 1°/0.45°). A two-channel Maxwellian view optical system was employed to eliminate pupil effects on the measured thresholds. A model based on visual mechanisms that were described in the literature was optimized to fit the measured luminance thresholds in all experimental conditions. Our results can be described by a combination of visual mechanisms. We determined how spatial summation changed with eccentricity and how subtractive adaptation changed with eccentricity and background field size. According to our model, photon noise plays a significant role to explain contrast detection thresholds measured with the 1/0.45° background/test size combination at mesopic luminances and at off-axis eccentricities. In these conditions, our data reflect the presence of rod-cone interaction for eccentricities between 6° and 9° and luminances between 0.6 and 5cd/m(2). In spite of the increasing noise effects with eccentricity, results also show that the visual system tends to maintain a constant signal-to-noise ratio in the off-axis detection task over the whole mesopic range.