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Biologically Inspired Robotics
• Gamma: greater than 30 Hz (simultaneous processes)
It is important to note that voltages drop with the fourth power of the radius,
so neural activities from deep sources are more difficult to register than
activities from neurons near the skull (Johnson and Guy 1972).
In addition, the bioelectrical signals in the head reflect activation of the
head musculature, eye movements, interference from nearby electric devices,
and changing conductivity in the electrodes due to the subject’s movements
or physicochemical reactions at the electrode sites. All of these activities that
are not directly related to the current cognitive processing of the subject are
considered noise.
Bioelectrical signals were analyzed by using common signal processing
techniques.
10.2.2 Measurement of Blood Information of the Brain
In order to measure blood information of the brain using optical data,
Lambert–Beer’s law was used.
According to Lambert–Beer’s law, there is a logarithm relationship between
the incident light beams I in and the transmitted light beam I out , absorption coefficient ε, the distance the light travels d, and concentration of the solution C.
I
ε
out = I
− Cd
in 10
(10.1)
The light absorbance Abs for liquids is defined as:
⎛ I
g
in ⎞
Abs = − lo 10
= ε
⎜
⎟
Cd
(10.2)
⎝ I ⎠
out
Lambert–Beer’s law, however, usually does not apply for very high concentrations or if the material is highly scattering. When light is applied to blood
it is attenuated by both absorption and scattering phenomena. Furthermore,
when light is used on the head, the light emitter and receiver are not facing
each other. Therefore, a modified version of Lambert–Beer’s law is necessary.
The modified Lambert–Beer’s law is as follows:
Abs = εC d + S
(10.3)
is the average length of the path followed by the light and S is a component affected by scattering.
When a light beam is applied to a living organism, its intensity also varies according to the organic tissues under the target region and blood flow.