Steady-state PERG adaptation: a conspicuous component of response variability with clinical significance

Frequency-dependent dynamics of steady-state visual evoked potentials under sustained flicker stimulation

Steady-state visual evoked potentials (SSVEP) are electroencephalographic signals elicited when the brain is exposed to a visual stimulus with a steady frequency. We analyzed the temporal dynamics of SSVEP during sustained flicker stimulation at 5, 10, 15, 20 and 40 Hz. We found that the amplitudes of the responses were not stable over time. For a 5 Hz stimulus, the responses progressively increased, while, for higher flicker frequencies, the amplitude increased during the first few seconds and often showed a continuous decline afterward. We hypothesize that these two distinct sets of frequency-dependent SSVEP signal properties reflect the contribution of parvocellular and magnocellular visual pathways generating sustained and transient responses, respectively. These results may have important applications for SSVEP signals used in research and brain-computer interface technology and may contribute to a better understanding of the frequency-dependent temporal mechanisms involved in the processing of prolonged periodic visual stimuli.

Compartmental Differences in Macular Retinal Ganglion Cell Function

Purpose

The purpose of this study was to investigate local differences of macular retinal ganglion cell (RGC) function by means of the steady-state pattern electroretinogram (SS-PERG).

Methods

SS-PERGs were recorded in healthy subjects (n = 43) in response to gratings (1.6 c/deg, 15.63 reversals/s, and 98% contrast) presented on an LED display (800 cd/m², 12.5 degrees eccentricity at 30 cm viewing distance) partitioned in triangular sectors (inferior [I]; nasal [N]; superior [S]; and temporal [T]) or concentric regions (central [C] and annulus [A]). For each partition, response amplitude (nV), amplitude adaptation (% change over recording time), phase/latency (deg/ms), and oscillatory potentials (OPs) amplitude (root mean square [RMS] nV) were measured. Data were analyzed with Generalized Estimating Equation (GEE) statistics.

Results

Amplitude differed (P < 0.001) between sectors (I: 254 nV; N: 328 nV; S: 275 nV; T: 264 nV; and N>T, I) as well as concentrically (C: 684 nV; A: 323 nV; and C>A). Latency did not differ between sectors (range = 53–54 ms, P = 0.45) or concentrically (range = 51–51 ms, P = 0.7). Adaptation did not differ (P = 0.66) concentrically (C: −19% and A: −22%) but differed (P = 0.004) between sectors (I: +25% and S: −29%). The OP amplitude did not differ (P = 0.5) between sectors (range = 63–73 nV) as well as concentrically (range = 82–90 nV, P = 0.3).

Conclusions

Amplitude profiles paralleled RGC densities from histological studies. Adaptation profile suggested greater autoregulatory challenge in the inferior retina. Latency profile may reflect axonal conduction time to the optic nerve head assuming a direct relationship between axon length and its size/velocity. Location-independent OPs may reflect preganglionic activity.

Translational Relevance

Normal macular RGC function displays local differences that may be related to local vulnerability in optic nerve disorders.

Modeling Retinal Ganglion Cell Dysfunction in Optic Neuropathies

As in glaucoma and other optic neuropathies cellular dysfunction often precedes cell death, the assessment of retinal ganglion cell (RGC) function represents a key outcome measure for neuroprotective strategies aimed at targeting distressed but still viable cells. RGC dysfunction can be assessed with the pattern electroretinogram (PERG), a sensitive measure of electrical activity of RGCs that is recorded non-invasively in human subjects and mouse models. Here, we offer a conceptual framework based on an intuitive state-transition model used for disease management in patients to identify progressive, potentially reversible stages of RGC dysfunction leading to cell death in mouse models of glaucoma and other optic neuropathies. We provide mathematical equations to describe state-transitions with a set of modifiable parameters that alter the time course and severity of state-transitions, which can be used for hypothesis testing and fitting experimental PERG data. PERG dynamics as a function of physiological stimuli are also used to differentiate phenotypic and altered RGC response dynamics, to assess susceptibility to stressors and to assess reversible dysfunction upon pharmacological treatment.

Adaptation of retinal ganglion cell function during flickering light in the mouse

Rapid dilation of retinal vessels in response to flickering light (functional hyperemia) is a well-known autoregulatory response driven by increased neural activity in the inner retina. Little is known about flicker-induced changes of activity of retinal neurons themselves. We non-invasively investigated flicker-induced changes of retinal ganglion cell (RGC) function in common inbred mouse strains using the pattern electroretinogram (PERG), a sensitive measure of RGC function. Flicker was superimposed on the pattern stimulus at frequencies that did not generate measurable flicker-ERG and alter the PERG response. Transition from flicker at 101 Hz (control) to flicker at 11 Hz (test) at constant mean luminance induced a slow reduction of PERG amplitude to a minimum (39% loss in C57BL/6J mice and 52% loss in DBA/2J mice) 4–5 minutes after 11 Hz flicker onset, followed by a slow recovery to baseline over 20 minutes. Results demonstrate that the magnitude and temporal dynamics of RGC response induced by flicker at 11 Hz can be non-invasively assessed with PERG in the mouse. This allows investigating the functional phenotype of different mouse strains as well as pathological changes in glaucoma and optic nerve disease. The non-contact flicker-PERG method opens the possibility of combined assessment of neural and vascular response dynamics.

Decoding PERG: A neuro-ophthalmic retinal ganglion cell function review

Purpose of review

Currently, the clinical evaluation of neuro-ophthalmologic diseases are mainly focused on identifying stages where structural or functional damage occur. Recognition of retinal ganglion cell (RGC) functional patterns as well as monitoring RGC dysfunction can be performed using steady-state pattern electroretinogram (PERG). The analysis of the amplitude and latency shift aid on providing information on early damage or monitoring of the RGC, allowing for prompt clinical intervention and management modification, potentially changing the natural history of the disease. The purpose of this article is to review the latest findings in PERG, in early manifest glaucoma, non arteritic ischemic optic neuropathy, multiple sclerosis with unilateral recovered optic neuritis and its fellow eyes.

Recent Findings

The steady-state PERG responses provide new and early specific information in neuro-ophthalmic diseases affecting the inner retina.

Summary

Steady state PERG presents specific amplitude and latency outcomes based on the neuro-ophthalmic disease affecting the inner retina, allowing early recognition of changes at the level of RGC and the degree of RGC dysfunction. In addition, PERG alterations may be induced in healthy subjects as well as susceptible eyes using different stress tests such as head down tilting or water drinking tests.