Biomedical Engineering Reference
In-Depth Information
when SIDNE is present, the functionality of the prosthesis obviously will be
degraded and/or unstable.
We have observed persisting but ultimately reversible SIDNE in the feline
cochlear nucleus and in the cerebral cortex after prolonged microstimulation [6,
9, 12], so it may be a general manifestation of the response of neurons to highly
localized protracted stimulation. We investigated SIDNE in the sensorimotor
cortex of the cat [9], a site which, while not identical to the primate visual
cortex, is a convenient model because many of its neurons project into the
corticospinal tract (the “pyramidal tract”), a bundle of axons that traverses from
the cortex into the spinal cord and is sufficiently compact to be accessible
with a single recording microelectrode. Arrays of 16 iridium microelectrodes
similar to Figure 16.2a were implanted chronically into the sensorimotor cortex
of adult cats for at least 40 days, and a recording electrode was implanted into
the pyramidal tract (Figure 16.7a). Neuronal responses characteristic of single
pyramidal tract axons (“unit-like responses” or ULRs) were recorded during
ICMS (Figure 16.7b). Each trace was generated by averaging the response to
2048 consecutive intracortical pulses. The negative peak of the ULRs is indicated
by
∗
. The graph's abscissa is the latency after the start of the 150s/phase
biphasic stimulus pulse. The number near the right edge of each averaged trace
signifies the amplitude of the 150s intracortical stimulus pulse. The threshold of
this ULR is 8 A. When the intracortical microelectrodes were not pulsed except
to determine the electrical threshold of the ULRs, the electrical threshold of the
ULRs was very stable for at least 7 hours (Figure 16.7c). Note that the threshold
of most of the ULRs was below 12 A (1.8 nC, with the 150 s stimulus pulses
used in the study). The abscissa is the ULR's initial threshold and the ordinate
is the threshold after 7 hours, during which the microelectrodes were not pulsed.
The broken lines represents a change of one stimulus level, and is the limit of
accuracy for the determination of the unit's threshold. The numbers adjacent
to some circles indicates multiple ULRs at those coordinates. As discussed
above, eight hours of continuous pulsing of the intracortical microelectrodes
at 4 nC/ph and 50 pulses per second did not induce histologically detectable
neural damage, and when these stimulus parameters were applied for 7 hours to
only 1 of the 16 microelectrodes in the intracortical array, there was elevation
of the electrical threshold of only 1 of 18 ULRs evoked from these pulsed
microelectrodes (Figure 16.7d). The data were acquired from 5 cats in which
only the single intracortical microelectrode from which the ULRs were evoked
was pulsed continuously for 7 hours, at 265A (4 nC/ph) and at 50 Hz. The
threshold of all but one of the ULRs was unchanged. However, when all 16
microelectrodes were pulsed for 7 hours at 4 nC/ph, the threshold of most of
the ULRs became markedly elevated (Figures 16.8a, b), and the SIDNE was
perhaps even more severe when the 16 microelectrodes were pulsed sequentially
(at 50 Hz per microelectrode) (compare Figures 16.8a and 16.8b).
We postulate that reversible SIDNE is caused by the prolonged high-rate
neuronal activity that is induced by the stimulation, and when the stimulus
amplitude is sufficiently high so as to produce significant overlap in the regions
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