FIGURE 1. An intracellular recording (left column) is compared to the extracellular ERG (right column) for equivalent stimuli. The ERG is displayed in this figure with negative upwards. (Note that the stimuli are specified as the log of relative intensity, discussed in the main section. The maximum stimulus strength, log 0, is about 20,000 lux in absolute units.) As you can see, the intracellular and extracellular recordings look very similar, but differ in absolute amplitude. For example, an intracellular response of 40 mV corresponds to an ERG of less than 2 mV. This figure indicates that at least in waveform, the ERG is a reasonably good representation of intracellular events in the photoreceptors.
From: Stieve, H., M. Bruns and H. Gaube (1978) Simultaneous recording by extra- and intracellular electrodes of light responses in the crayfish retina. Vision Research 18: 621-628.
FIGURE 2. A comparison of the relative response amplitudes of the ERG and intracellular recordings to a range of stimulus intensities. In this case, the response amplitudes are expressed as relative responses by dividing each absolute response amplitude by the maximum response that could be elicited, and expressing the fraction as a percent. (The maximum response is 100%.) In panel (a), intracellular responses are compared to the stimulus intensity. In panel (b), the extracellular ERG is plotted. The two recording methods give similar results.
