Biomedical Engineering Reference
In-Depth Information
/mm
2
the normalized resistance is about 0.1 M
and the normalized capacitance about
0.1 nF/mm
2
.
The resistive component of the impedance drops sharply with increased current, which
results in an extremely nonlinear response. For this reason the electrodes are usually
stimulated with constant current rather than constant voltage. Changes in the resistance
also alter the effective time constant of the electrode, and this can be a problem for high-
frequency stimulation.
The dynamic range of electrotactile stimulation is determined by ratio of the pain
threshold level to the sensation threshold level, P/S (for voltage or current). This can vary
from a factor of 2 (6 dB) to about 10 (20 dB) at best. This range is very limited compared
with the other senses. For example, the ear has a dynamic range of 120 dB and the eye 70 dB.
Dynamic range measurements made by different researchers vary by a large factor
because, as are most measurements on human subjects, the thresholds are so subjective.
Though the threshold for stimulation is determined with good accuracy, there is no def-
inition for the threshold of
pain
. In addition, pain thresholds vary with a whole raft of
psychological factors. Furthermore, skin condition and the position of electrodes also
have a large effect on the dynamic range and comfort of electrical stimulation.
For pulsed stimulation, the sensation threshold increases as the pulse width decreases,
suggesting that the threshold is determined by the total charge (current
×
duration), as
seen in Figure 7-33.
A number of different institutions worldwide are researching various aspects of elec-
trotactile excitation. One of these is the Kajimoto laboratory, which has developed a 32
16
element electrotactile display with an electrode spacing of 3 mm. This display is designed
to be mounted on the forehead for of ease of installation—it is held in place by a sweatband.
Visual images captured by the camera are converted to tactile information through
two processes. The first is spatial outline extraction to enhance edges. The second is a
temporal band-pass filtering to enhance time-varying information. In essence, the forehead
recognition sensory system (FSRS) imitates functions performed in the retina and visual
cortex to facilitate image cognition.
As with many other pieces of new technology, the electrotactile ideas developed within
the Kajimoto laboratories have been spun off and are being developed for production as
×
FIGURE 7-33
Electrotactile
sensation threshold
as a function of (a)
current and (b)
charge. [Adapted
from (Kaczmarek,
Webster et al.,
1991).]
