Experiments concerning the human C-wave

Origin of the fast oscillation in the electroretinogram of the macaque

To investigate the origin of the fast oscillation, a phenomenon in the electroretinogram evoked with stimulus frequencies of about 8 mHz (a period time of about 2 min), we recorded responses from retina and pigment epithelium in the macaque. Micropipettes were placed in the subretinal space and in the vitreous close to the retina; the reference electrode was in the orbit behind the eye. Thus, simultaneous recordings were obtained of the trans-epithelial, the trans-retinal and the trans-tissue (vitreal) potential. At 10 mHz the trans-retinal and the trans-epithelial responses are of about equal magnitude but of opposite phase, resulting in a small and rather variable vitreal potential. The origin of the fast oscillation evoked with repetitive stimuli lies in subtle differences between retinal and pigment epithelial potentials, in which a pigment epithelial event plays an important role. For single stimuli lasting 60 s again the trans-epithelial and trans-retinal responses were of equal magnitude and opposite polarity. The epithelial responses were found to return more quickly towards the baseline than the retinal responses. In vitreal recordings this causes a trough between the c-wave and the light peak which is referred to as the "trough" fast oscillation. Most of the "trough" fast oscillation is caused by a pigment epithelial event. In view of the complexity of the fast oscillation evoked with repetitive stimuli it might be difficult to relate pathology to specific neuro-epithelial structures.

The amplitude of the c wave in the human ERG as a function of the luminous energy of the stimulus

Variation of the stimulus intensity and duration over a wide range causes a change in amplitude of the c wave. If the energy of the applied stimulus is kept constant, however, stimuli of 0.05-4 s duration effected c waves that were nearly equal in amplitude. As patients consider shorter stimuli with higher intensities to be more comfortable in ERG recording, stimuli of a duration of 2-4 s seem most appropriate for clinical testing.

[Chromatic adaptation produces opposite effects on the on and off responses of the photopic electroretinogram (author's transl)]

The electrical response of the light adapted human eye to onset (a-wave) and offset (d1-wave of the positive off-effect) of monochromatic test stimuli between 448 and 654 nm (Ganzfeld stimulation, duration of test flash 0.3 s) was recorded by means of the electroretinogram in 10 normal observers during steady illumination of various wavelengths of 2.2 log Troland. During exposure to white light the spectral sensitivity of the a-wave and off-effect as determined by a criterion amplitude of 25 micronV were similar, the sensitivity of the off-effect being 0.4 log units lower as compared to the a-wave. Blue adaptation (467 nm) decreased the sensitivity of the a-wave and increased the sensitivity of the off-effect within the short wavelengths region of the spectrum. Red adaptation (630 nm) decreased the sensitivity of the a-wave and increased the sensitivity of the off-effect within longer wavelengths. Green adaptation (530 nm) produced only a small decrease of sensitivity of the a-wave for green test light, the sensitivity of the off-effect remained constant. The opposite changes of the a-wave and off-effect during chromatic adaptation demonstrate different participation of these components in the generation of color mechanisms in the human retina.

The influence of stimulus duration on the human d.c. registered c-wave. A quantitative study

The c-wave of the human ERG was studied at different stimulus durations with a d.c. technique which permitted stable and reproducible recordings. With increasing stimulus lengths the implicit time increased up to a maximum of about 5.5 s. Also the amplitude of the c-wave rose. However, it was influenced by positive and negative off-effects, seen in most volunteers, and at several stimulus lengths superimposed upon the peak of the c-wave. This fact must be considered when developing a standardized method for measuring the c-wave amplitude proper.

ERG of humans without C-wave

Three out of 18 healthy persons investigated with the DC-ERG do not show a c-wave. By varying the adaptation levels and intensities of the stimuli four components in the human ERG can be isolated, the cone and rod late receptor potentials, the positive DC response, and a c-wave in most subjects. It is suggested that investigation of these components reveals a more detailed understanding of the pathophysiologic mechanisms in some retinal diseases.