Biomedical Engineering Reference
In-Depth Information
additional features of maintaining and utilizing results from earlier experiments
(31). Adams (1) specializes the learning aspect to improvement in the quality of
motor performance with respect to accuracy, speed, and minimal energy,
whereas skill is more than the ability to perform and often peaks in individual
different proficiencies. Mechanisms different from these are "habituation" or
"sensitization," which represent the change in response amplitudes to repeti-
tively given identical stimuli. The amplitudes may increase (sensitization) or
decrease (habituation) due to an increasing (sensitization) or decreasing (ha-
bituation) mismatch between stimulation and an updated representation of the
outside world.
Adaptation, habituation, and sensitization may be summarized as non-
associative procedures. Correspondingly, other types of learning involve the
formation of association among stimuli or among stimuli and actions (e.g. (20)).
Results of associative processes can be studied experimentally by classical and
instrumental conditioning. In instrumental conditioning the subject learns rela-
tions among actions and their outcome. This is different from classical condi-
tioning, in which the subject learns relations among stimuli. Two stimuli—the
conditioning stimulus (CS) and the unconditioned stimulus (US)—are paired
with each other, so that the CS comes to evoke a conditioned response (CR),
which is similar to the unconditioned response (UR) elicited by the US. An ef-
fective protocol is delay conditioning, in which the CS is preceding US onset
and coterminates with the US. Classical conditioning of the eyeblink reflex is
one of the most studied experimental approaches of simple associative learning
in mammals (e.g. (11)). In a typical protocol, tone is the CS and corneal air puff
the unconditioned stimulus. The unconditioned stimulus elicits closure of the
eyelid in humans as well as extension across the cornea of the nictitating mem-
brane (the internal eyelid) in rabbits. With repetitive presentation of paired CS-
US (tone-air puff), subjects learn to blink in response to the tone prior to onset
of the air puff or to the tone alone. Animal and human lesion studies could
show that elemental delay conditioning of eyeblink is dependent on the cerebel-
lum but independent of the cerebral cortex and hippocampus. In more complex
forms of conditioning other brain regions are engaged. For example, in trace
conditioning, the CS starts and ends before the US starts, and the subjects must
hold offline information about the CS before US onset. In addition to the cere-
bellum, trace conditioning is dependent on other brain regions including the
hippocampus.
"Despite continuing work on the structure and function of the cerebellum,
there is still no consensus as to what it does and how it does it" (67). The cere-
bellar cortex appears as a structure similar to a rectangular lattice of high preci-
sion. Together with its extraordinary double innervation via a low-frequency
system (climbing fiber system) of high transmission probability and a high-
frequency system characterized by an enormous amount of convergence (mossy
fiber-granule cell system), scientists have been inspired to numerous and even
