Optic nerve head component evaluation and intraretinal conduction

The optic nerve head component (ONHC) of the electroretinogram (ERG) is generated by conduction of the nerve impulse through the optic nerve head via the axon of the retinal ganglion cell (RGC). The ONHC can be evaluated by means of its progressive latency increase with increasing distance between the stimulus and the optic nerve head on a multifocal electroretinogram (mfERG). The ONHC is almost never identified in full-field, conventional ERGs, because its peaks are necessarily smoothed out. The ONHC is a pure reflection of RGC activity, and therefore, it shows promise as an indicator of disease of the optic nerve and an objective indicator of glaucoma. Evaluation of the ONHC has also made it possible to calculate intraretinal conduction velocity in the human retina. The average velocity is 0.41 m/sec at 2 to 5 degrees of eccentricity. A slower conduction velocity results in a propagation delay of several milliseconds between the nasal and the temporal macular regions, which is consistent with estimations derived using intraocular comparison of multifocal visual evoked potentials (mfVEPs).