Identification of ERG Contributions From Magno and Parvo Fibers by Means of a Multifocal Paradigm

Abstract

Abstract: : Purpose: The topographic nonlinear analysis of the mfERG provides two new ways to identify and characterize sources of the ERG. 1. By comparing the distributions of the response components with those of known anatomical properties of the retina. 2. By comparing the nonlinear dynamics of response components with known physiological properties of neuron populations. The purpose of this study is to use these tools to identify response contributions from the magno (m) and parvo (p) fibers of the optic nerve fiber layer. Methods: Step 1. Identify the signal contribution from ganglion cell axons known as the Optic Nerve Head Component (ONHC). Step 2. Optimize the stimulation mode for best visibility of the ONHC by varying the temporal stimulation parameters as well as the stimulus intensity, and chromaticity. Step 3. Identify possible contributions to the ONHC from m– and p–fibers. Prediction: The latency increases from the nasal to the temporal retina should be smaller for the contributions from the faster m–fibers. Step 4. Validate the m– and p– components identified in Step 3 by comparing their response dynamics with known properties of the m– and p– channels. Results: The global flash paradigm1 proved best for the identification of contributions from the ONHC. Two global flashes interleaved at 13.3ms and 40 ms after the pseudorandom multifocal stimuli presented at intervals of 67 ms produced the most salient ONHC (Invest. Ophthalmol. Vis. Sci. 39: S973). The contribution from the ONHC often showed two peaks whose latency difference increased with distance from the optic disc as predicted for the m– and p components. The two features became more clearly defined with blue light stimulation. The propagation velocities to the nerve head were estimated as the slopes of linear regression lines through fiber length/latency plots. At 15 – 25 deg eccentricity they were ∼ 50 – 60 cm/s for the faster (m) fibers and ∼ 100 –110 cm/s for the slower (p) fibers. The m– and p–features exhibit different dynamic properties: the m–feature appears with opposite polarity in the two induced components while the p–feature has the same polarity. This suggests that the two components indeed originate from different sources. Conclusions: The technique may permit us to test the hypothesis that m–fibers are more susceptible to early glaucomatous damage. The flat topography with a well defined central minimum seen at the low photopic flash intensities of the blue stimulus (.5 cd*s/m^2) suggests that under these stimulation conditions the induced response component is mostly rod mediated. If this is verified, the blue global flash paradigm may also serve a test for rod function.

Keywords: ganglion cells • neuro-ophthalmology: diagnosis • nerve fiber layer