Andrew J. Zele

The Circadian Response of Intrinsically Photosensitive Retinal Ganglion Cells

Intrinsically photosensitive retinal ganglion cells (ipRGC) signal environmental light level to the central circadian clock and contribute to the pupil light reflex. It is unknown if ipRGC activity is subject to extrinsic (central) or intrinsic (retinal) network-mediated circadian modulation during light entrainment and phase shifting. Eleven younger persons (18–30 years) with no ophthalmological, medical or sleep disorders participated. The activity of the inner (ipRGC) and outer retina (cone photoreceptors) was assessed hourly using the pupil light reflex during a 24 h period of constant environmental illumination (10 lux). Exogenous circadian cues of activity, sleep, posture, caffeine, ambient temperature, caloric intake and ambient illumination were controlled. Dim-light melatonin onset (DLMO) was determined from salivary melatonin assay at hourly intervals, and participant melatonin onset values were set to 14 h to adjust clock time to circadian time. Here we demonstrate in humans that the ipRGC controlled post-illumination pupil response has a circadian rhythm independent of external light cues. This circadian variation precedes melatonin onset and the minimum ipRGC driven pupil response occurs post melatonin onset. Outer retinal photoreceptor contributions to the inner retinal ipRGC driven post-illumination pupil response also show circadian variation whereas direct outer retinal cone inputs to the pupil light reflex do not, indicating that intrinsically photosensitive (melanopsin) retinal ganglion cells mediate this circadian variation.

Intrinsically Photosensitive (Melanopsin) Retinal Ganglion Cell Function in Glaucoma

PURPOSE To determine whether glaucoma alters intrinsically photosensitive retinal ganglion cell (ipRGC) function. METHODS Forty-one patients (25 with glaucoma and 16 healthy age-matched control participants) were tested. Intrinsically photosensitive retinal ganglion cell function was directly measured by the sustained, postillumination pupil response (PIPR). Forty-one eyes of 41 participants were tested with 7°, 10-second, short-wavelength (488 nm; bluish) and long-wavelength (610 nm; reddish) stimuli (14.2 log photons · cm(-2) · s(-1)) presented to the right eye in Maxwellian view, and the consensual pupil response of the left eye was measured by infrared pupillometry. The difference between PIPR amplitude (percentage baseline pupil diameter), net PIPR (percentage change) and kinetics (time in mm · s(-1) to the PIPR plateau) for the blue and red stimuli in patients with early and advanced (moderate/severe) glaucoma was compared to that in age-matched control participants. RESULTS The blue PIPR was significantly smaller between normal participants and patients with advanced glaucoma, as well as between those with early and those with advanced glaucoma (P < 0.05). The kinetics of the red and blue PIPRs were not significantly different between any groups. Normal age-matched participants and patients with early-stage glaucoma were not significantly different on any parameter, and neither was the normal and glaucoma group (advanced and early combined). CONCLUSIONS Persons with moderate and severe glaucoma have a dysfunctional ipRGC-mediated PIPR. Intrinsically photosensitive retinal ganglion cell function measured directly with the PIPR may become a clinical indicator of progressive changes in glaucoma.

Intrinsically photosensitive melanopsin retinal ganglion cell contributions to the pupillary light reflex and circadian rhythm

Recently discovered intrinsically photosensitive melanopsin retinal ganglion cells contribute to the maintenance of pupil diameter, recovery and post‐illumination components of the pupillary light reflex and provide the primary environmental light input to the suprachiasmatic nucleus for photoentrainment of the circadian rhythm. This review summarises recent progress in understanding intrinsically photosensitive ganglion cell histology and physiological properties in the context of their contribution to the pupillary and circadian functions and introduces a clinical framework for using the pupillary light reflex to evaluate inner retinal (intrinsically photosensitive melanopsin ganglion cell) and outer retinal (rod and cone photoreceptor) function in the detection of retinal eye disease.

Measuring Rod and Cone Dynamics in Age-Related Maculopathy

PURPOSE A cathode-ray-tube (CRT) monitor-based technique was used to isolate clinically significant components of dark adaptation. The utility of the technique in identifying adaptation abnormalities in eyes with age-related maculopathy (ARM) is described. METHODS A CRT dark adaptometer was developed to assess cone and rod recovery after photopigment bleach. The following measures were obtained: cone recovery rate (R(c); in decades per minute) and absolute threshold (Tf(c); log candelas per square meter), rod recovery rate (R(r); decades per minute), and rod-cone transition (rod-cone break [RCB], in minutes). These components were isolated by appropriately selecting stimulus size, stimulus location, pigment bleach, and test duration and by coupling the CRT with judiciously selected neutral-density (ND) filters. The protocol was developed by using 5 young observers and was tested on 27 subjects with ARM in the study eye and 22 age-matched control subjects. RESULTS The parameters necessary for effective isolation of cone and early phase rod dark adaptation were a 2.6 ND filter (for a standard CRT monitor, 0.08-80 cd . m(-2) luminance output); a 4 degrees foveated, 200-ms, achromatic spot; approximately 30% pigment bleaching; and a 30-minute test duration. These settings returned obvious rod and cone recovery curves in control and ARM eyes that were compatible with conventional test methods and identified 93% of participants with ARM as having delayed dynamics in at least one of the parameters. Cone recovery dynamics were significantly slower in the ARM group when compared with age-matched control subjects (R(c), 0.99 +/- 0.35 vs. 2.63 +/- 0.61 decades . min(-1), P < 0.0001). Three of the 27 eyes with ARM did not achieve RCB during the allowed duration (30 minutes). The remaining eyes with ARM (n = 24) exhibited a significant delay in rod recovery (R(r)(,) ARM, 0.16 +/- 0.03 vs. controls, 0.22 +/- 0.02 decades . min(-1), P < 0.0001) and the average time to RCB (+/-SD) in the ARM group was significantly longer than in the control subjects (19.12 +/- 5.17 minutes vs. 10.40 +/- 2.49 minutes, P < 0.0001). CONCLUSIONS The CRT dark-adaptation technique described in this article is an effective test for identifying abnormalities in cone and rod recovery. Slowed cone and rod recovery and a delayed RCB were evident in the eyes with ARM. The test method is potentially useful for clinical intervention trials in which ARM progression is monitored.

Rod contributions to color perception: Linear with rod contrast

At mesopic light levels, an incremental change in rod activation causes changes in color appearance. In this study, we investigated how rod mediated changes in color perception varied as a function of the magnitude of the rod contrast. Rod-mediated changes in color appearance were assessed by matching them with cone-mediated color changes. A two-channel four-primary colorimeter allowed independent control of the rods and each of the L-, M- and S-cone photoreceptor types. At all light levels, rod contributions to inferred PC, KC and MC pathway mediated vision were linearly related to the rod incremental contrast. This linear relationship could be described by a model based on primate ganglion cell responses with the assumption that rod signals were conveyed via rod-cone gap junctions at mesopic light levels.

The post‐illumination pupil response of melanopsin‐expressing intrinsically photosensitive retinal ganglion cells in diabetes

Purpose:  This study investigates the clinical utility of the melanopsin‐expressing intrinsically photosensitive retinal ganglion cell (ipRGC) controlled post‐illumination pupil response (PIPR) as a novel technique for documenting inner retinal function in patients with Type II diabetes without diabetic retinopathy.

Vision under mesopic and scotopic illumination

Evidence has accumulated that rod activation under mesopic and scotopic light levels alters visual perception and performance. Here we review the most recent developments in the measurement of rod and cone contributions to mesopic color perception and temporal processing, with a focus on data measured using a four-primary photostimulator method that independently controls rod and cone excitations. We discuss the findings in the context of rod inputs to the three primary retinogeniculate pathways to understand rod contributions to mesopic vision. Additionally, we present evidence that hue perception is possible under scotopic, pure rod-mediated conditions that involves cortical mechanisms.

Melanopsin-expressing intrinsically photosensitive retinal ganglion cells in retinal disease.

&NA; Melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs) are a class of photoreceptors with established roles in non–image-forming processes. Their contributions to image-forming vision may include the estimation of brightness. Animal models have been central for understanding the physiological mechanisms of ipRGC function and there is evidence of conservation of function across species. Intrinsically photosensitive retinal ganglion cells can be divided into five ganglion cell subtypes that show morphological and functional diversity. Research in humans has established that ipRGCs signal environmental irradiance to entrain the central body clock to the solar day for regulating circadian processes and sleep. In addition, ipRGCs mediate the pupil light reflex (PLR), making the PLR a readily accessible behavioral marker of ipRGC activity. Less is known about ipRGC function in retinal and optic nerve disease, with emerging research providing insight into their function in diabetes, retinitis pigmentosa, glaucoma, and hereditary optic neuropathy. We briefly review the anatomical distributions, projections, and basic physiological mechanisms of ipRGCs and their proposed and known functions in animals and humans with and without eye disease. We introduce a paradigm for differentiating inner and outer retinal inputs to the pupillary control pathway in retinal disease and apply this paradigm to patients with age-related macular degeneration (AMD). In these cases of patients with AMD, we provide the initial evidence that ipRGC function is altered and that the dysfunction is more pronounced in advanced disease. Our perspective is that with refined pupillometry paradigms, the PLR can be extended to AMD assessment as a tool for the measurement of inner and outer retinal dysfunction.

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.

Dark-adapted rod suppression of cone flicker detection : Evaluation of receptoral and postreceptoral interactions

Dark-adapted rods in the area surrounding a luminance-modulated field can suppress flicker detection. However, the characteristics of the interaction between rods and each of the cone types are unclear. To address this issue, the effect that dark-adapted rods have on specific classes of receptoral and postreceptoral signals was determined by measuring the critical fusion frequencies (CFF) for receptoral L-, M-, and S-cone and postreceptoral luminance ([L+M+S] and [L+M+S+Rod]) and chromatic ([L/(L+M)]) signals in the presence of different levels of surrounding rod activity. Stimuli were generated with a two-channel photostimulator that has four primaries for a central field and four primaries for the surround, allowing independent control of rod and cone excitation. Measurements were made either with adaptation to the stimulus field after dark adaptation or during a brief period following light adaptation. The results show that dark-adapted rods maximally suppressed the CFF by approximately 6 Hz for L-cone, M-cone, and luminance modulation. Dark-adapted rods, however, did not significantly alter the S-cone CFF. The [L/(L+M)] postreceptoral CFF was slightly suppressed at higher surround illuminances, that is, higher than surround luminances resulting in suppression for L-cone, M-cone, or luminance modulation. We conclude that rod-cone interactions in flicker detection occurred strongly in the magnocellular pathway.