Electrophysiology of Natural and Artificial Vision - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 371

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Figure 20.5. Representative eERG waveforms recorded in three animals. Each waveform
is the average of 100-300 responses to a biphasic square pulse stimulus (100A each
phase) delivered at approximately 3-second intervals. Animals were pigmented P23H
rats, 6-12 weeks of age. Retinal stimulating array electrodes were 500m diameter gold
electrodes (cf. Figure 20.4). Major response components (see Figure 20.2) are consistently
present across animals.

SNR was achieved by averaging several hundred responses to the same stimulus.
The peak amplitude of about

10V is consistent with the area of the retina
subtended by the electrode pair. The lower trace in Figure 20.2 plots the mean
eERG response recorded with these electrodes in six animals (different from
those in Figure 20.5), and represents the typical waveform. Comparison with
the light-induced ERG above reveals obvious differences between the responses
to light and the electrical stimulation. The cellular basis for these differences is
explored below, but first a number of technical considerations should be noted.
When delivering a large number of repetitive stimuli, several questions must
be answered. At what frequency should the stimuli be delivered? Does the retina
adapt or fatigue in response to the frequent stimuli? A typical interval between
light flashes when performing a typical single-flash ERG experiment is 1-2
minutes, to allow the animal to dark-adapt between stimuli. Light adaptation
occurs at several levels in the visual pathway, and so it may be assumed that
adaptation to electrical stimulation may also occur. Are there any other physio-
logical changes that might occur during the tens of minutes required to deliver
hundreds of repeated stimuli? All of these factors must be considered within the
constraint of the practical duration for an in vivo experiment, during which an
animal must be kept anesthetized, or a patient able to maintain concentration
and avoid excessive discomfort.
Two other issues require special consideration when recording the response to
electrical stimulation. These are the determination of threshold and saturation ,

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