Biomedical Engineering Reference
In-Depth Information
In regard to vision replacement, direct cortical stimulation would seem to be the best
interface as it does not interfere with other senses. However, the huge interfacing problems
and invasive nature of the approach, together with the low-resolution results obtained until
now, probably make it a long-term research item.
As an alternative, when considering interference a vibrotactile or electrotactile skin-
stimulating system would seem to be a good choice because much of the skin plays only
a subordinate role as a communication channel under normal conditions. Unfortunately,
that same consideration might indirectly be its major disadvantage, because there is no
strong evolutionary reason to expect the touch sensation to provide a high-bandwidth
communication channel. This restriction applies not only to the skin interface but also all
the way up to the cognitive brain centers.
Experiments in the 1960s conducted by Paul Bach-Y-Rita using a 20
20 grid of
1 mm electrotactile stimulators attached to the subject's back and activated by a video
camera showed that if the subjects had control of the camera's pan-and-zoom facility they
were soon able to discern lines and edges (Bach-y-Rita 1969). With some practice they
could even recognize depth, shadows, and shapes and even individual faces. This is an
amazing result given the poor resolution of both the camera and the electrotactile stimu-
lator. More importantly, the initial tactile sensations were soon forgotten, and perceived
objects appeared to exist in three-dimensional space. However, this heightened perception
capability occurred only if subjects were moving the camera themselves, and measure-
ments made using static cameras failed to produce any real perception. It was as if the
perception was founded in the sensory-motor loop as a whole and not in the static transfer
of an image to the electrotactile array.
It has been argued that reasonable resolution can be achieved by stimulating small areas
of skin but that the limited sensory bandwidth available would lead to severe information
loss when stimulating a large matrix of skin positions. Experiments have yet to show that
this is always the case, but it does suggest that using tactile substitution may not be the
ultimate path to high-fidelity sensory substitution.
Knowing the importance of bandwidth in communicating data—in this case detailed
environmental information—an alternative would be to exploit the capabilities of the hu-
man hearing system. Although it cannot be claimed that this would produce a perfect
solution, it is known that the human hearing system is capable of processing and inter-
preting extremely complicated and rapidly changing acoustic patterns, such as speech or
music in a noisy environment. The available effective bandwidth, of the order of 10 kHz,
corresponds to a channel capacity of many thousands of bits per second.
As with the tactile option, restrictions may be imposed by the mechanics of the
cochlea or with information encoding in the neural architecture. However, in spite of
these uncertainties, the known capabilities of the human hearing system in learning and
understanding complicated acoustical patterns provide a strong motivation for developing
auditory sensory substitution systems.
The following sections address some of the issues involved with both tactile and
auditory systems.
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7.7.1 Auditory Substitution
One of the major problems with auditory sensory substitution is the masking effect, in
which sound cues normally used by a blind person are swamped by feedback from the
prosthesis. Under laboratory conditions, the interference of an auditory prosthesis with
