AAWSAP DIRD, Technological Approaches to Controlling External Devices, March 2010

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The signal transmission down the axon of a neuron is an all-or-nothing process. When
the cell body is stimulated above threshold, the axon transmits the same action
potential at the same speed and in the same direction, regardless of the extent above
threshold or duration of the input. Action potentials have durations of 1-10
milliseconds. Input signals can result in transmission of multiple action potentials, and
thus the frequency and number of neuronal firings do vary with the input. Neurons
require some time to reset between firings, nominally the duration of the pulse for that
axon, yielding a typical maximum firing rate of between 100 hertz (Hz) and 1 kilohertz
(kHz).
It is instructional at this point to contrast this mechanism with propagation of signals
through a physical electrical circuit, the planned external portion of our BMI. In a
copper wire, electrons carry the signal. Electrons drift along the signal path, but the
signal itself moves as a compression wave rather than a transverse wave as in the
biological system. Going back to our garden hose example, consider the hose now filled
with small marbles: inserting a marble at one end will move each marble in the hose
just a little, but very rapidly the last marble in line will pop out of the far end of the
hose. As with the biological system, the signal is propagated to the far end of the hose
by local actors rather than physical motion of a single ion or electron moving the whole
distance. The underlying physics governing the signal transmission makes metallic and
semimetallic circuits about a million times faster than the biological system.
This difference in the carriers and underlying mechanisms of signal transmission
between biological and physical circuits has so far prevented the invention of a direct
connection between the two disparate systems. Instead, both noninvasive and invasive
direct-detection technologies rely on placing physical sensors or transmitters in close
proximity to the neurons of interest and utilizing classical electrodynamics to govern
signal jump between the systems. Additionally, given the maximum typical firing rate
for neurons of 1 kHz, sampling of action potentials at a few kHz will be fast enough to
detect firing of any individual neuron, though super-sampling above 10 kHz can be used
to reduce noise. Higher sampling frequencies also may be required if multiple neurons
are monitored and quantitative information about their relative firing sequence is
desired . Frequencies of a 1 kHz or below are sufficient to stimulate action potentials,
and again, higher system frequency may be required for multiple neuron sequential
stimulation. Finally, higher frequencies may be required if monitoring or stimulation of
some aspect of signal transmission other than action potentials is sought, such as
monitoring single ion channels. 1 There is currently no evidence that transduction
frequencies above 100 kHz have any advantage in BMis, thus the main challenge is the
connection dynamics, since even this ultra-maximum frequency is easily attainable with
current electronic manufacturing technology.
The human brain doesn't process information as a traditional computer does.
Information is moved around through pathways and at certain neurons it is allowed or
not allowed to pass based on excitory or inhibitory dendritic signals arriving before
triggering of action potentials. Local groups of neurons can act nearly coherently, for
example in volition of motor action like a hand movement. Detecting such coherent
firing at nodes around the brain is robust both noninvasively and invasively, though
1 Technologies that monitor single ion channels on a neuronal membrane are important for research on neural
function and neurotransmitter action. However, these devices are ultra-sensitive to physical motion, and have so
far not proven useful in large scale information transfer required in BMis.
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