Retinal Prosthesis Dissertation - Student Home Pages

electrical pulse generated is called in biological terminology, an action potential, and
is shown in Figure 21.
Membrane potential (mv)
+30
0
-70
0 1 2 3 4 5
Time(ms)
Figure 24 Action Potential
3.3 Natural limit for pulse duration
The biological process of producing an action potential delivered from the retina to
the optic nerve fibres creates the pulse. [10, 11, 48]As can be seen from Figure 21;
although the actual pulse occurs within 2ms there is a 2ms refractory period before
another pulse can be produced. The pulse rate varies between five to fifty pulses/s
where 5 is the eye `resting’ (if all fibres of the nerve receive the same signal) and 50
where the optic nerve fibre is responding to a full intensity signal. At this maximum
throughput a spike plus its interval to next spike will take 1000 50 = 20ms. At a
nominal minimum throughput of five spikes a spike plus its interval to next spike
would be 10005 = 200ms.
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