MEMS in the Nervous System 77
Figure 4.5. A cochlear implant, complete with CMOS circuitry. Image from Bhatti and
Wise, IEEE Journal of Solid State Circuits, Vol 41, Iss 12, pp 2965-2973. ©2005 IEEE
the ear is functioning properly, the basilar membrane vibrates. Each location on
the membrane corresponds to a certain frequency. The auditory nerve interprets
these vibrations of a specific area of the membrane as specific frequencies of sound.
This frequency-location relationship of the membrane is called tonotopic mapping.
Because of this tonotopic mapping, stimulation at different points in the cochlea
is interpreted by the auditory cortex as sound of different frequencies.
72
The
University of Michigan has created a 2.5 μm thick boron-doped silicon cochlear
microelectrode that is 8 mm long and has 32 iridium electrodes on it; this electrode
also possesses on-chip circuitry to stimulate and record inside the cochlea (see
Fig. 4.5).
73
A cochlear implant only works when the cochlea is intact and the auditory
nerve from it is functional. If this is not the case, a cochlear implant will not
restore hearing, so one must move further down the auditory processing pathway
to restore hearing. Another area of interest for auditory prosthetics is the auditory
cortex. Early experiments exploring the cat auditory cortex were performed
using single tungsten electrodes.
74
Experimenters have shown that two-shank
16-electrode Michigan probes implanted in the inferior colliculus central nucleus
can elicit activity in the auditory cortex (A1) when stimulated.
75
Brainstem im-
plants can be placed directly into the auditory cochlear nucleus, which also has
a tonotopic mapping.
76
Experiments have shown that rats trained to respond
to different auditory signals can respond to microelectrode stimulation in the
auditory cortex,
77
and a human experiment using a polyurethane hybrid density
electrode, a recording device with both high impedance Teflon-coated platinum-
iridium microwires and low impedance electroencephalogram (EEG) electrodes,
78
has shown that human auditory cortex shows a tonotopic mapping as well.
79
4.2.8 Spinal Cord Electrodes
Spinal cord injury (SCI) affects a staggering number of otherwise healthy young
individuals each year. Patients with SCI are often left with permanent motor and
sensory deficits. A spinal cord lesion interrupts the signal transduction from the
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