The second harmonic of the electroretinogram to sinusoidal flicker: spatiotemporal properties and clinical application.

The photopic, focal electroretinogram elicited by sinusoidal modulation (8 Hz) of the luminance of a uniform field (flicker electroretinogram) presented in the macular region contains two main components: the first harmonic at the stimulus frequency (1F) and the second harmonic at twice the stimulus frequency (2F). Physiologic and clinical studies have demonstrated that the 1F originates in the outer retina, whereas the 2F has multiple postreceptoral sources, with a contribution of the innermost retina. The 2F, unlike 1F, is selectively altered in amplitude and phase in the early stages of optic neuritis and glaucoma. Both 1F and 2F are altered in maculopathies. An additional property of the focal electroretinogram is that the 2F, unlike 1F, has a very limited summation area (i.e., the function relating 2F amplitude with stimulus area is saturated for central stimulus sizes of the order of the macular region). This overrepresentation of the macular activity suggests that the 2F component of the flicker electroretinogram may provide a specific macular test even for full-field stimuli presented through opacities of the optical media.