Analysis of retinal light adaptation with the flicker electroretinogram

Chirped flicker optoretinography for in vivo characterization of human photoreceptors' frequency response to light

Flicker electroretinography (ERG) has served as a valuable noninvasive objective tool for investigating retinal physiological function through the measurement of electrical signals originating from retinal neurons in response to temporally modulated light stimulation. Deficits in the response at certain frequencies can be used as effective biomarkers of cone-pathway dysfunction. In this Letter, we present the progress we made on its optical counterpart-photopic flicker optoretinography (f-ORG). Specifically, we focus on the measurement of the response of light-adapted retinal photoreceptors to a flicker stimulus with chirped frequency modulation. In contrast to measurements performed at discrete frequencies, this technique enables a significantly accelerated characterization of photoreceptor outer segment optical path length modulation amplitudes in the nanometer range as a function of stimulus frequency, enabling the acquisition of the characteristic frequency response in less than 2 sec.

Retinal atrophy, inflammation, phagocytic and metabolic disruptions develop in the MerTK-cleavage-resistant mouse model

In the eye, cells from the retinal pigment epithelium (RPE) facing the neurosensory retina exert several functions that are all crucial for long-term survival of photoreceptors (PRs) and vision. Among those, RPE cells phagocytose under a circadian rhythm photoreceptor outer segment (POS) tips that are constantly subjected to light rays and oxidative attacks. The MerTK tyrosine kinase receptor is a key element of this phagocytic machinery required for POS internalization. Recently, we showed that MerTK is subjected to the cleavage of its extracellular domain to finely control its function. In addition, monocytes in retinal blood vessels can migrate inside the inner retina and differentiate into macrophages expressing MerTK, but their role in this context has not been studied yet. We thus investigated the ocular phenotype of MerTK cleavage-resistant (MerTKCR) mice to understand the relevance of this characteristic on retinal homeostasis at the RPE and macrophage levels. MerTKCR retinae appear to develop and function normally, as observed in retinal sections, by electroretinogram recordings and optokinetic behavioral tests. Monitoring of MerTKCR and control mice between the ages of 3 and 18  months showed the development of large degenerative areas in the central retina as early as 4 months when followed monthly by optical coherence tomography (OCT) plus fundus photography (FP)/autofluorescence (AF) detection but not by OCT alone. The degenerative areas were associated with AF, which seems to be due to infiltrated macrophages, as observed by OCT and histology. MerTKCR RPE primary cultures phagocytosed less POS in vitro, while in vivo, the circadian rhythm of POS phagocytosis was deregulated. Mitochondrial function and energy production were reduced in freshly dissected RPE/choroid tissues at all ages, thus showing a metabolic impairment not present in macrophages. RPE anomalies were detected by electron microscopy, including phagosomes retained in the apical area and vacuoles. Altogether, this new mouse model displays a novel phenotype that could prove useful to understanding the interplay between RPE and PRs in inflammatory retinal degenerations and highlights new roles for MerTK in the regulation of the energetic metabolism and the maintenance of the immune privilege in the retina.

Light-adapted flicker optoretinograms captured with a spatio-temporal optical coherence-tomography (STOC-T) system

For many years electroretinography (ERG) has been used for obtaining information about the retinal physiological function. More recently, a new technique called optoretinography (ORG) has been developed. In one form of this technique, the physiological response of retinal photoreceptors to visible light, resulting in a nanometric photoreceptor optical path length change, is measured by phase-sensitive optical coherence tomography (OCT). To date, a limited number of studies with phase-based ORG measured the retinal response to a flickering light stimulation. In this work, we use a spatio-temporal optical coherence tomography (STOC-T) system to capture optoretinograms with a flickering stimulus over a 1.7 × 0.85 mm2 area of a light-adapted retina located between the fovea and the optic nerve. We show that we can detect statistically-significant differences in the photoreceptor optical path length (OPL) modulation amplitudes in response to different flicker frequencies and with better signal to noise ratios (SNRs) than for a dark-adapted eye. We also demonstrate the ability to spatially map such response to a patterned stimulus with light stripes flickering at different frequencies, highlighting the prospect of characterizing the spatially-resolved temporal-frequency response of the retina with ORG.

Cone Photoreceptor Dysfunction in Early-Stage Diabetic Retinopathy: Association Between the Activation Phase of Cone Phototransduction and the Flicker Electroretinogram

Purpose

To define the nature and extent of cone photoreceptor abnormalities in diabetic individuals who have mild or no retinopathy by assessing the activation phase of cone phototransduction and the flicker ERG in these individuals.

Methods

Light-adapted single-flash and flicker ERGs were recorded from 20 diabetic individuals who have no clinically apparent retinopathy (NDR), 20 diabetic individuals who have mild nonproliferative diabetic retinopathy (NPDR), and 20 nondiabetic, age-equivalent controls. A-waves elicited by flashes of different retinal illuminance were fit with a delayed Gaussian model to derive R_mp3 (maximum amplitude of the massed photoreceptor response) and S (phototransduction sensitivity). Fundamental amplitude and phase of ERGs elicited by full-field sinusoidal flicker were obtained across a frequency range of 6 to 100 Hz.

Results

ANVOA indicated that both diabetic groups had significant S losses compared with the controls, whereas mean R_mp3 did not differ significantly among the groups. ANOVA also indicated significantly reduced flicker ERG amplitude for frequencies ≥56 Hz for both diabetic groups compared with the controls. Flicker ERG timing (phase) did not differ significantly among the groups. Log R_mp3 + log S was significantly correlated with the patients' high-frequency (62.5 Hz) flicker ERG amplitude loss (r = 0.69, P < 0.001).

Conclusions

The delayed Gaussian a-wave model is useful for characterizing abnormalities in the activation phase of cone phototransduction and can help explain flicker ERG abnormalities in early-stage diabetic retinopathy. Reduced cone sensitivity and attenuated high-frequency flicker ERGs provide evidence for impaired cone function in these individuals.

Studying the Retinal Source of Photophobia by Facial Electroretinography

PURPOSE

Photophobia is a debilitating clinical condition that disrupts the ability to use vision for everyday tasks in bright lighting conditions. The goal of the study is to develop a methodology to study the neural basis of photophobia and the contribution of the melanopic pathway to its etiology with differential chromatic responses by means of standard electroencephalographic recording equipment.

METHODS

We introduce and validate the approach of recording wavelength-specific electroretinographic (ERG) responses from the face electrodes of the high-density whole-head electroencephalography recording system under light-adapted conditions.

RESULTS

ERGs recorded in this way to whole-field chromatic stimuli exhibit striking differences between the photophobic and non-photophobic groups. The control responses were consistent with photopic intensity in peak time, and in the ordering of peak times as a function of wavelength condition, indicating a predominantly cone source of the signals. The photophobic responses, on the other hand, were substantially slowed relative to controls, with the peak times conforming to a different order as a function of wavelength condition than controls, implying that the cone response has been suppressed and that the responses derived from a different photoreceptor system consistent with mediation by melanopic retinal ganglion cells.

CONCLUSIONS

The results will be important for determining the neural pathways involved in photophobia and potential approaches to its treatment on the basis of this etiology.

The relationship between slow photoresponse recovery rate and temporal resolution of vision

The rate at which photoreceptors recover from excitation is thought to be critical for setting the temporal resolution of vision. Indeed, mutations in RGS9 (regulator of G-protein signaling 9) and R9AP (RGS9 anchor protein) proteins mediating rapid photoresponse recovery impair patients' ability to see moving objects. In this study, we analyzed temporal properties of retinal sensitivity and spatiotemporal aspects of visual behavior in R9AP knock-out mice. Surprisingly, we have found that this knock-out does not affect dim-light vision mediated by rods acting as single-photon counters. Under these conditions, vision was also unaffected in mice overexpressing R9AP in rods, which causes accelerated photoresponse recovery. However, in brighter light, slow photoresponse recovery in rods and cones impaired visual responses to high temporal frequency stimuli, as reported for the daylight vision of human patients. Therefore, the speed of photoresponse recovery can affect temporal resolution and motion detection when photoreceptors integrate signals from multiple photons but not when they act as single-photon counters.

Spatial properties of flicker adaptation

Prolonged viewing of a flickering region reduces sensitivity to a subsequently flickered test patch of identical extent, but the spatial properties of this adaptation are unknown. What happens to the sensitivity to a smaller flickered test patch completely contained in, but inset from, the adapted region? We show that sensitivity to the inset test patch is only slightly affected by adaptation of the larger region. This suggests that neurons that respond to the edges of the smaller test patch are not adapted by the larger flickering region. We then show that an annulus adapter designed specifically to adapt only those edges only slightly reduces sensitivity, demonstrating that neurons that do not adapt to the flickered edges are also involved in detecting flicker. This gives further evidence that flicker detection depends on at least two mechanisms - one sensitive to flickering edges and one sensitive to local flicker, and shows that these mechanisms can operate in isolation.

Harmonic analysis of the cone flicker ERG of rabbit

Harmonic analysis was used to characterize the rabbit flicker ERG elicited by sinusoidally modulated full-field stimuli under light-adapted conditions. The frequency-response function for fundamental amplitude, derived from Fourier analysis of the ERG waveforms, exhibited two limbs, with an amplitude minimum at approximately 30Hz, and a high-frequency region peaking at around 45Hz and extending to more than 100Hz at higher adapting levels. At low frequencies (<20Hz), the fundamental response amplitude was independent of mean luminance (Weber law behavior), whereas the response amplitude at high stimulus frequencies varied nonlinearly with mean luminance. At low frequencies, intravitreal administration of L-AP4, which blocks ON-pathway activity, reduced the fundamental response amplitude and produced a phase shift. On the other hand, PDA, which reduces OFF-pathway activity, had a minimal effect on both the response amplitude and phase at low frequencies. At high frequencies, L-AP4 increased the fundamental response amplitude at low mean luminances, whereas PDA had only a small effect on amplitude and phase. Both pharmacologic agents removed the minimum in the amplitude-frequency function as well as the abrupt change in phase at stimulus frequencies near 30Hz. The results suggest that there is a nonlinear interaction between ON- and OFF-pathway activity over the entire stimulus frequency range examined in this study. These findings provide a basis for formulating protocols to evaluate the effect of pharmacologic agents and/or disease on the cone flicker ERG of rabbit.

Is there an omitted stimulus response in the human cone flicker electroretinogram?

Omitting a stimulus from a train of repetitive stimuli, by either interrupting or terminating the train, can elicit an electrophysiological response that occurs at the time appropriate for the omitted stimulus. This study investigated whether such an omitted stimulus response (OSR) is present in the flicker electroretinogram (ERG) of the human cone system. ERGs were recorded from 11 visually normal subjects in response to full-field sinusoidal flicker trains presented against a rod-desensitizing adapting field at frequencies ranging from 12.5 to 100 Hz. Recordings were synchronized with the onset of the stimulus trains, and the amplitude and relative delay of any additional ERG responses following the offset of the flicker train were analyzed. At stimulus frequencies below 35 Hz, the number of ERG responses always equaled the number of stimulus cycles. However, over the frequency range of 38.5 to 100 Hz, the ERG contained an extra response following flicker train offset. At stimulus frequencies from 38.5 to 62.5 Hz, there was a constant delay between the peak of the extra ERG response and the time at which the next stimulus would have occurred had the flicker train continued. This constant delay is characteristic of an OSR. In addition, an extra ERG response was apparent at these same stimulus frequencies if the flicker train was interrupted by omitting stimulus cycles from the middle of the train. The pattern of ERG findings is consistent with a recently proposed model of the OSR that attributes the phenomenon to a resonant oscillation in retinal bipolar cells.

Activation phase of cone phototransduction and the flicker electroretinogram in retinitis pigmentosa

This study examined the relationship between the activation phase of cone phototransduction and the flicker electroretinogram (ERG) in 15 patients with retinitis pigmentosa (RP) and 12 age-equivalent, visually normal control subjects. Values of Rmp3 (maximum amplitude of P3, the massed cone photoreceptor response) and S (sensitivity of cone phototransduction) were derived from a delayed Gaussian model applied to the leading edge of the ERG a-wave. Fundamental amplitude and phase of the flicker ERG were derived from responses to sinusoidal flicker presented at temporal frequencies ranging from 7.8 to 100 Hz. Patients with RP who had a reduced value of Rmp3 alone had an overall reduction in flicker ERG amplitude with a normal response phase across temporal frequency. Patients with RP who had a reduced value of S, whether or not Rmp3 was reduced, had the greatest amplitude reduction at temporal frequencies above 40 Hz and phase lags across a range of temporal frequencies. At high temporal frequencies, the amplitude reduction of the flicker ERG was predicted by the product of Rmp3 and S for all of the subjects except the three patients with RP who had the lowest fundamental amplitudes. The results indicate that there is a systematic relationship between the derived parameters of the activation phase of cone phototransduction and the characteristics of the flicker ERG in patients with RP, although the phase changes in the flicker ERG were generally greater than predicted by the derived parameters alone.

Visually evoked hemodynamical response and assessment of neurovascular coupling in the optic nerve and retina

The retina and optic nerve are both optically accessible parts of the central nervous system. They represent, therefore, highly valuable tissues for studies of the intrinsic physiological mechanism postulated more than 100 years ago by Roy and Sherrington, by which neural activity is coupled to blood flow and metabolism. This article describes a series of animal and human studies that explored the changes in hemodynamics and oxygenation in the retina and optic nerve in response to increased neural activity, as well as the mechanisms underlying these changes. It starts with a brief review of techniques used to assess changes in neural activity, hemodynamics, metabolism and tissue concentration of various potential mediators and modulators of the coupling. We then review: (a) the characteristics of the flicker-induced hemodynamical response in different regions of the eye, starting with the optic nerve, the region predominantly studied; (b) the effect of varying the stimulus parameters, such as modulation depth, frequency, luminance, color ratio, area of stimulation, site of measurement and others, on this response; (c) data on activity-induced intrinsic reflectance and functional magnetic resonance imaging signals from the optic nerve and retina. The data undeniably demonstrate that visual stimulation is a powerful modulator of retinal and optic nerve blood flow. Exploring the relationship between vasoactivity and metabolic changes on one side and corresponding neural activity changes on the other confirms the existence of a neurovascular/neurometabolic coupling in the neural tissue of the eye fundus and reveals that the mechanism underlying this coupling is complex and multi-factorial. The importance of fully exploiting the potential of the activity-induced vascular changes in the assessment of the pathophysiology of ocular diseases motivated studies aimed at identifying potential mediators and modulators of the functional hyperemia, as well as conditions susceptible to alter this physiological response. Altered hemodynamical responses to flicker were indeed observed during a number of physiological and pharmacological interventions and in a number of clinical conditions, such as essential systemic hypertension, diabetes, ocular hypertension and early open-angle glaucoma. The article concludes with a discussion of key questions that remain to be elucidated to increase our understanding of the physiology of ocular functional hyperemia and establish the importance of assessing the neurovascular coupling in the diagnosis and management of optic nerve and retinal diseases.