Electrophysiology of Natural and Artificial Vision - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 357

conceptually, thegeneral development of retinal prostheses from a functional point
of view. Two primary challenges are to understand the relationship between
neural activity in the diseased retina and the formation of images in the mind's
eye, and the relationship between microelectrode-associated currents and neural
activity in the diseased retina. We know almost nothing about the former, and
very little about the latter.
The objectives of this chapter are to summarize techniques that have been used
extensively during the past several decades to learn about the response of the
retina to natural, photic stimuli, and to then describe how these techniques can be
modified to understand the response of the retina to electrical stimulation. Results
from two of these modified techniques, applied to understand the response of
the retina to subretinal stimulating electrodes, are presented. The scope of this
chapter is limited to in vivo electrophysiological techniques, or those methods
which can be used to measure the responses of the retina when it is in as natural
a state as possible, in the intact subject (animal or human). Psychophysical tests,
in which a patient reports his or her perception of a stimulus, are also excluded
due to the complexity of applying these approaches in animal studies (in which
case the perception must be inferred from a behavioral response).
With knowledge of how the retina responds to artificial electrical stimulation
gained from animal studies, and knowledge of the perceptions of humans for
given electrical stimuli, we may be able to infer the activity in the human
retina, understand the relationship between activity in the diseased retina and
perceptions, and then make effective decisions to improve the design of the
prosthesis.
Electrophysiology of Natural Vision
The light response of the retina has been measured by a number of electro-
physiological methods, each involving one or more recording electrodes which
transduce the ion-based potential differences within the body to electron-based
potential differences in metal conductors. The measured potentials are either
membrane potentials or field potentials. Membrane potentials are measured by
placing one electrode inside a neuron, and measuring the potential difference
relative to a second electrode outside the neuron. Due to the high mechanical
precision and stability required to place the intracellular electrode without
damaging the target neuron, these approaches are rarely used in vivo, and will
not be covered here. It should be noted that membrane potential can also be
measured using a membrane-bound dye that changes its fluorescent emission
properties in a manner proportional to the potential across the membrane to
which it is bound; for a number of reasons, these potentiometric probes, or
voltage-sensitive dyes, are best suited to in vitro applications when single-cell
resolution is the goal.
Local field potentials (LFPs) result from the combined contribution of all
nearby neurons, each of which is associated with an electric field due to its