Brain–Computer Interfaces - Index of

Intracortical BCIs: A Brief History of Neural Timing 205
Similarly, the implanted microelectrode recording systems can detect the exact
firing patterns of the individual neurons within the brain. These detailed firing patterns
are rich with complex information, such as how much you need to activate
each specific muscle to make a given movement or the detailed position, velocity,
and acceleration of the limb about each joint. This level of detail about what a person
is thinking or trying to do, so far, has eluded researchers using non-invasive
recording methods.
Finally, to carry this metaphor even further, let us compare the technical challenges
with measuring the room’s temperature versus detecting the velocity of each
individual molecule. It would be impractical to install sensors at every possible location
within the room for the purpose of measuring the velocity of every molecule.
There would be no room left for the people or for the molecules of air to travel.
Similarly, you cannot put enough microelectrodes in the brain to detect the activity
of every neuron in a given area. The electrodes would take up all the space leaving
no room for the neurons. Microelectrode arrays sample only a very small fraction
of the available neurons. And, although complex information can be extracted from
this sparse sampling of neurons, this information is inherently just an estimate of
the true information encoded in the detailed firing patterns of one’s brain. In the
next section, we will describe the imperfect process by which the firing patterns
of individual neurons are recorded using intracortical microelectrode technologies.
Then, in the rest of this chapter, we will discuss how researchers have been able to
use even a single neuron to a few hundreds of neurons to very effectively control
different BCI systems.
3 Neurons, Electricity, and Spikes
As you are reading this sentence, the neurons in your brain are receiving signals
from photoreceptors in your eyes that tell the brain where light is falling on your
retina. This information gets passed from the eyes to the brain via electrical impulses
called “action potentials” that travel along neurons running from the eyes to the
brain’s visual processing center. These electrical impulses then get transmitted to
many other neurons in the brain through cell-to-cell connections called synapses.
Each neuron in the brain often receives synaptic inputs from many different neurons
and also passes on its own synaptic signals to many different neurons. Your brain
is made up of about a hundred billion neurons that synapse together to form complex
networks with specific branching and connection patterns. Amazingly, these
networks can transform the electrical impulses, generated by the light and dark
patterns falling on your retina, into electrical impulses that encode your symbolic
understanding of words on the page, and (hopefully) into electrical impulse patterns
that encode your understanding of the more abstract concepts this book is trying to
get across.
At the heart of perceiving light and dark, or understanding a word, or abstractly
thinking about how we think – is the action potential. The action potential is an