AAWSAP DIRD, Technological Approaches to Controlling External Devices, March 2010

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cerebral-spinal fluid (CSF), and internal tissue structures of various other organs. Unlike
x-ray based technologies, MRI scans can be optimized to contrast any of the many
parts of the physics signal: total density of protons, water content, lipid content, and
even particle motion in advanced techniques involving diffusion or spin labeling. Using
such scans sequences that take several minutes, one can construct very high resolution
images of gray and white matter structure for comparison with, and also mapping onto
a "standard brain" template.
In addition to electron screening, macroscopic susceptibility will also change the local
response to RF stimulation. The presence of even a small amount of metal, say as small
as a hairpin, will greatly distort the reconstructed images. Indeed, the usual effect is a
shift of frequency completely outside the sensitivity of the machine RF receiver in what
is known as "drop-out." Smaller changes in local susceptibility, like produced in the
BOLD effect, are measureable.
A series of fast scan sequences, typically collecting an entire brain volume at a
resolution of 3 mm3 in 2 seconds, that are calibrated to optimize detection of the BOLD
signal will show the dynamics of brain function under the specific internal or applied
conditions; this is known as a functional MRI, or simply fMRI. 11 The major advantages
of fMRI are unmatched 3D spatial resolution, compared to other noninvasive imaging
methods, and complete skull penetration, making it the only imaging modality to
unambiguously detect limbic activations important for determining emotionally-laden
neuropsychological states. The main disadvantage for BMI is that the BOLD signal is
detected several seconds after the neuronal firing takes place, making fMRI
inappropriate for many naturalistic applications.
A long term prospect, likely in the 20- to 40-year timeframe, is that combined low-field
MRI and MEG technology could detect neuronal firing deep in the brain and with high
temporal accuracy. Initial experiments indicate some level of feasibility, but there is
substantial development work required in room temperature low field magnetic field
detection devices, such as atomic magnetometers, and signal processing algorithms to
sift through the substantial electromagnetic background (References 32, 33).
NIRS
Near-infrared spectroscopy is an additional technology to monitor the BOLD effect
noninvasively. Studies have shown it correlates well with the fMRI signal in animal
models, although with reduced coverage and lower resolution (Reference 34). This
lowered resolution greatly affects the accuracy of the technique, with recent work
involving single trials and a decision attaining only 80 percent accuracy (Reference 35).
Invasive Technologies
The most prolific invasive BMI for use in humans is the cochlear implant (Reference
36), a sensory neuroprosthesis designed to aid in hearing for deaf individuals. This
device, under continued development and refinement for more than 30 years, consists
of a microphone, sound processor, and a receiver that is attached to an array of
11 Specifically this is T2* Echo-Planar Imaging, also called BOLD EPI, Gradient Echo EPI, or BOLD fMRI. This
approach is used in well over 95 percent of published functional studies, though there are more advanced
techniques that concentrate on smaller portions of the hemodynamic signal. For example, Spin-Echo EPI will
provide a higher localization within the gray matter, but the cost is a loss of 90 percent of the signal amplitude.
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