Brain–Computer Interfaces - Index of

216 D.M. Taylor and M.E. Stetner
eventual human testing of intracortically-controlled whole arm and hand prosthetics
within the next few years.
Human testing of intracortical BCIs has already been going on since 1998.
Dr. Phil Kennedy and colleagues received the first approval from the US Food
and Drug Administration (FDA) to implant intracortical electrodes in individuals
with “locked-in syndrome” who are unable to move. These early implant systems
recorded only a handful of neurons, but locked-in individuals were able to use the
firing patterns of these neurons to select letters in a typing program [18].
In the Spring of 2004, Cyberkinetics Inc. received FDA approval to test 96channel
intracortical microelectrode arrays in people with high-level spinal cord
injuries. They later also received approval to evaluate this intracortical technology
in people with locked-in syndrome. The results of their initial testing showed that
the firing activity of motor cortex neurons was still modulated with thoughts of
movement even years after a spinal cord injury. These researchers also showed that
the study participants could effectively use these intracortical signals to control a
computer mouse, which the person would then use for various tasks such as sending
emails or controlling their TV [19]. The study participants were easily able to control
the mouse without much concentration, thus enabling them to talk freely and
move their head around while using their brain-controlled computer mouse. The US
National Institutes of Health and the Department of Veteran’s Affairs continue to
strongly fund research efforts to develop intracortical BCI technologies for severely
paralyzed individuals.
7 What Else Can We Get from Intracortical Microelectrodes?
So far, the studies described in this chapter have focused on decoding information
from the firing rates of neurons, which are usually re-calculated each time the braincontrolled
device is updated (e.g. rates calculated by counting the number of times
a neuron fires over about a 30–100 millisecond window). However, useful information
may also be encoded in the fine temporal patterns of action potentials within
those short time windows, as well as by the synchronous firing between pairs of
neurons. Studies have shown that these two aspects of neural firing are modulated
independently while planning and executing movements [20].
Over the course of planning and executing a movement, the timing of spikes
across neurons changes back and forth between being synchronized (i.e. spikes from
different neurons occur at the same time more often than would be expected by
chance) and being independent (i.e. two neurons fire at the same time about as often
as you would expect by chance). What these changes in synchronous firing represent
or encode is a much debated topic in neuroscience. A study by Oram and colleagues
suggested that the changes in spike timing synchronization do not encode any directional
information that isn’t already encoded in the firing rates [21]. Instead, spikes
become synchronized when a movement is being planned. The level of synchrony
is highest at the end of movement planning just before the arm starts to move [22].