Electrophysiology of Natural and Artificial Vision - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 378

35

Baseline

30

25

20

Aspartate

15

L-AP4

10

5

NMDA

0

-5

-10

-200 0 200 400 600

Time (ms)

Figure 20.8. Summary of pharmacological dissection results. Each waveform is the
average response recorded in two animals, except the Baseline response, which is the
average across all six animals. The lower three waveforms were recorded in the presence
of the glutamate agonist indicated. See text for interpretation.

in interpreting results. For example, our results show that in the presence of
aspartate (analog/agonist which binds to all post-receptor glutamate receptors),
the eERG response to subretinal stimulation using bipolar electrodes is abolished.
One interpretation of this result is that the stimulus field subtends only the
photoreceptor layer, and the photoreceptors themselves do not contribute signif-
icantly to the recorded response. Thus, the recorded response is comprised of
contributions of cells post-synaptic to the photoreceptors. An alternate interpre-
tation is that the stimulus field extends deep into the retina and subtends all of the
retinal cell types above the electrode. We still conclude that the photoreceptors
do not contribute strongly to the eERG, but we cannot rule out the possibility
that the second- and third-order neurons, which are all maximally hyperpolarized
or depolarized, are unable to respond to the stimulus in the presence of aspartate.
Agonists can be useful in identifying cellular origins of eERG components, but
antagonists are required to determine the depth of the stimulus field.

Summary

Invasive and non-invasive recording, and pharmacological dissection, have been
used for decades to teach us a great deal about the response of the retina, both
healthy and diseased, to natural light stimuli. We have described a number of
these historical techniques and suggested ways that they may be adapted to
teach us about the response of the retina to electrical stimulation. Early results
demonstrating eERG recording, and the use of pharmacological dissection to
understand this response, have been presented. While the example of eERG
waveforms provided represent typical responses, these were obtained under a