Brain–Computer Interfaces - Index of

Brain Signals for Brain–Computer Interfaces 39
[e.g. 102]. Early studies showed that the firing rates of individual cortical neurons
could be operantly conditioned [103–105]. Monkeys were able to increase or
decrease single-neuron firing rates when rewarded for doing so. While such control
was typically initially associated with specific muscle activity, the muscle activity
tended to drop out as conditioning continued. However, it remains possible that
undetected muscle activity and positional feedback from the activated muscles contribute
to this control of single-neuron activity in the motor cortex. It also remains
unclear to what extent the patterns of neuronal activity would differ if the animal
were not capable of movement (due, for example, to a spinal cord injury).
Firing rates of motor cortex neurons correlate in various ways with muscle activity
and with movement parameters [106, 107]. Neuronal firing rates may be related
to movement velocity, acceleration, torque, and/or direction. In general, onset of
a specific movement coincides with or follows an increase in the firing rates of
neurons with preference for that movement and/or a decrease in the firing rates
of neurons with preference for an opposite movement. Most recent research into
the BCI use of such neuronal activity has employed the strategy of first defining
the neuronal activity associated with standardized limb movements, then using this
activity to control simultaneous comparable cursor movements, and finally showing
that the neuronal activity alone can continue to control cursor movements
accurately in the absence of continued close correlations with limb movements
[108–111].
In addition to real or imagined movements, other cognitive activities may modulate
cortical cells. Motor planning modulates firing rates of single neurons in
posterior parietal areas [e.g., 99, 112]. Both a specific visual stimulus and imaginative
recall of that stimulus activate neurons in sensory association areas [113].
Andersen and colleagues have used firing rates of single neurons in parietal areas of
the monkey cortex to decode goal-based target selection [99] or to predict direction
and amplitude of intended saccades [114]. The degree to which these signals will be
useful in BCI applications is as yet unclear.
Two groups have used single-neuron activity for BCI operation in humans with
severe disabilities. Kennedy and colleagues implanted neurotrophic cone electrodes
[115] and found that humans could modulate neuronal firing rates to control switch
closure or one-dimensional cursor movement [116]. Donoghue, Hochberg and their
colleagues have implanted multielectrode arrays in the hand area of primary motor
cortex in several severely disabled individuals. A participant could successfully
control the position of a computer cursor through a linear filter that related spiking
in motor cortex to cursor position [93]. Subsequent analyses of data from two
people revealed that neuronal firing was related to cursor velocity and position
[Fig. 2d; 117].
3 Requirements for Continued Progress
At present, it is clear that all three classes of electrophysiological signals – EEG,
ECoG, and intracortical (i.e., single neurons/LFPs) – have promise for BCI uses. At
the same time, it is also clear that each method is still in a relatively early stage of
development and that substantial further work is essential.