Defense Intelligence Reference Document Technological Approaches To Controlling

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The brain activity mentioned above is a complex chain of ionic motion within the central
nervous system. Ionic production, release, and movement within and between cells all
consume energy. This energy is supplied by the conversion of blood-borne
oxyhemoglobin to deoxyhemoglobin. The rate of oxygen consumption in a localized
volume varies based on local neural activity. The circulatory system compensates for
changes in energy demand by increasing or decreasing both the flow rate and volume
of blood, regionally and locally. Local energy demand, expressed in the capillary beds,
will alter the rate at which oxygen is metabolized, called the cerebral rate of oxygen
metabolism, abbreviated CMRO2. When brain activity increases in a region, the
circulatory response, called the hemodynamic response, will be increases in flow and
volume, while the local areas increase CMRO2.
The hemodynamic response consistently provides an excess of oxygen over what is
required, and this results in some oxyhemoglobin traveling through the capillary bed
and local venous structure without being converted into deoxyhemoglobin. Oxy- and
deoxyhemoglobin have different magnetic susceptibilities, and different infrared
spectra. The hemodynamic response, by changing the net ratio of oxy- to
deoxyhemoglobin in the local venous structure, thus changes the local magnetic
susceptibility and local infrared resonance spectra around focused brain activity. This
complex chain reaction is called the Blood Oxygen Level Dependent, or BOLD effect
(Reference 8). The BOLD effect leads to a method to indirectly measure the local brain
activity by monitoring the hemodynamic response using Magnetic Resonance Imaging
(MRI) or Near Infrared Spectroscopy (NIRS).
The BOLD response to any event peaks about 4-6 seconds a~er the event occurs,
limiting the applications for which monitoring these signals and their associated delay
may be useful, given the BMI operating parameters defined above Furthermore, person
to person variation in distributed signals show significant differences in regions
activated (Reference 9), though there is evidence that these inter-subject variations are
stable intra-subject over time (Reference 10).
Control of External Devices
In general, there are two types of BMI design called open-loop and closed-loop. Open
loop designs, those either detecting activity or providing stimulation, are important for
BMI research and application other than the practical control of an external device.
Closed-loop systems include both output signals from the brain to effect a change in the
state of a device, and a stimulation channel, which can be used as feedback. In some
system designs, the feedback can be via visual cues such as watching a display
(Reference 11). Open-loop systems are important for BMI research and have
application in sense replacement such as artificial hearing, or when extensive output
signal processing is combined with external device state feedback (see remote robot
operation below). For learning and fine, efficient control of external devices without
significant external processing, closed-loop systems are typically needed.
Several components constitute a general BMI system for control of an external device
(Figure 2). As mentioned in the previous section, the interfaces where the information
signal makes the jump between biological and physical pathways are the most
challenging part of the system. These connections can be placed to stimulate or read
peripheral or cerebral neurons. The processing step could be as simple as amplification
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