Electrophysiology of Natural and Artificial Vision - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 372

two fundamental characteristics of any response. In a stochastic, binary system,
such as a spiking neuron, threshold is typically defined as the stimulus strength
required to elicit a just-measurable response (i.e. a single action potential) 50% of
the time. For a signal that is continuous, such as the eERG amplitude, threshold
can be defined as a signal that is some criterion amount above the baseline noise
level (e.g. 3 the RMS noise, or 3 standard deviations above the mean noise

level). However, for small signals, the noise level is inversely proportional to
the square root of the number of responses averaged, and the threshold is thus a
function of the recording protocol, and becomes difficult to assess.
Saturation is defined as the maximum response of the system being measured.
A plot of response amplitude vs. stimulus intensity for neural systems often takes
the form of a saturating exponential function. As the stimulus strength increases,
eventually a maximum response is elicited, and further increases in stimulus
strength result in no further increase in response amplitude. In the healthy retina,
the saturated ERG response occurs when the number of photons delivered to the
eye is sufficient to result in the closing of all of the cGMP-gated cation channels
that mediate the photoreceptor dark current. This happens at about 1000 photons
absorbed per photoreceptor, and doubling the number of photons delivered to
the eye does not further increase the response.
The special consideration when dealing with electrical stimuli is that of
stimulus field containment and recruitment of cells. When the potential applied
to a stimulating electrode increases, the field potentials around the electrode also
increase. If you assume that there is a threshold value of field potential that
results in activation of a retinal neuron, then as the field potential at a given
point in space increases, it becomes more likely that a neuron in that location
will reach threshold and contribute to the recorded response. The region of the
retina that is brought above threshold by direct effect of the delivered electrical
stimulus (i.e. not via synaptic connections) is termed the stimulus field . The
number of neurons contributing to the response (due to both direct stimulation
and synaptic connections) then becomes a function of the stimulus strength,
which is only limited by the potentials that cause electrode or tissue damage.
The plot of eERG amplitude vs. stimulus strength is not well fit by a simple
saturating exponential, because as neurons near the electrode become saturated
at high stimulus strengths, neurons farther away from the electrode are just
reaching threshold. This issue of cell recruitment also illustrates the need for
electrode designs that create consistent stimulus fields over a range of stimulus
strengths, to avoid recruitment of neurons outside of the desired stimulus field
when using stronger stimuli.

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Adaptating the Paired-flash ERG

Adapting the pfERG technique to electrical stimulation depends on the ability
to establish two critical stimulus amplitudes. The lower stimulus amplitude is
defined as that which elicits a response just above threshold (analogous to the
first flash in the pfERG protocol). The upper stimulus amplitude is one in which
the stimulus field is the same as that for the low stimulus (attained through