Defense Intelligence Reference Document Technological Approaches To Controlling

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noninvasive techniques currently cannot resolve firing sequences of individual neurons
within such groups.
Living neurons in an active tissue fire in the rest state. Changes in the frequency of
these firings imply that a given neuron is currently involved in the processing of
information. Bulk changes in local field potential oscillations imply that several neurons
are active. This is the signal seen in the noninvasive direct-measurement techniques of
electroencephalography (EEG) and magnetoencephalography (MEG).
Movement as simple as an eye blink involves signal communication through a million
neurons. Several locales in the nervous system thus offer the possibility of placement of
an invasive monitoring device for detecting electrical activity associated with an eye
blink or other signal of interest. Many other factors will influence the placement
decision, but considering only catching the signal as an action potential, using any
neuron along the complete pathway is equally as good, assuming detection of individual
neuronal firing can be accomplished.
Considering cortical placement of detection devices, the most common area of activity
under study is the outer covering of gray matter of the cerebral cortex, the neocortex.
The neocortex is about 2.5 mm thick in humans and follows the ridges and fissures of
the brain (gyri and sucli, respectively, or gyrus and sulcus, if singular). The neocortex is
roughly divided into six layers and different cortical probes can concentrate on activity
in different layers, or just measure the combined activity on the surface of the cortex,
or the combined activity that is detectable noninvasively through the layers of tissue
and bone of the cranium.
Detecting single firings of individual neurons is a difficult process because the signals
are weak to start with, and not isolated from the rest of the electrical activity within the
brain. Large groups of coherent neurons, perhaps a few thousand to tens of thousands
all firing at once in relation to an external event, are the most studied of single firing
signals. 2 One of the most studied signals is the so-called P300 which occurs in many
cognitive tasks (References 2-4).
P300 is a term used to indicate a pulse signal from a large group of neurons that
appears about 300 milliseconds after a stimulus event. Peaks that occur between 200
and 400 milliseconds are commonly grouped into the P300 category. The "P" stands for
a positive measured voltage, while "N" signals are negative. 3 The P300 signal has been
shown to be influenced by top-down executive function and is therefore a prime
candidate for a trainable interface (Reference 5). Earlier signals, like PS0 and N100, are
of a greater interest for differential diagnosis of pathology in current research.
The preceding discussion is greatly simplified version of the electrical dynamics of
neuronal firing and does not include differences between axon and dendrite signals, or
the transmission of signals across the synapse. Complete discussion of underlying
electrical signals in the nervous system can be found in (References 6, 7).
Indirect Neuronal Signals - The BOLD Effect
2 Single firing signals here mean a single peak of combined electrical activity relative to an event. This is not
necessarily the same as single firings of each neuron.
'The literature is not consistent with plotting P signals up, and N signals down; however, P peaks are always in the
opposite direction of N peaks.
3
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