Biomedical Engineering Reference
In-Depth Information
taneous activity in the developing spinal cord, also applicable to other develop-
ing systems. Other types of changes following possibly similar rules are seen in
tumor growth and other negative aspects of nervous systems; a novel model for
that aspect is presented in previous chapter 5.1 by Reeke.
Learning based on plastic changes is a multilevel, multifaceted phenome-
non, and its analysis depends on the complexity of the learning situation. In a
visuospatial delayed-response task, a monkey must retain the previously given
information about the position of a raisin in one of two food wells, both then
covered with identical pieces of cardboard. A screen is then lowered for a "de-
lay" of approximately 10 s and then raised again. To obtain the reward, the
monkey must choose the position at which the food had been placed prior to the
delay. The marine polychete worm
Nereis pelagica
contracts in response to a
sudden vibration or a shadow. Both behavioral reactions are typical examples of
learning. The analysis of very simple learning in a very simple nervous system is
attractive from the point of dissecting and understanding it. The analysis of
learning in complex systems may be more "interesting" and relevant to learning
in humans. The examples given differ markedly in their complexity, and their
analyses thus require completely different experimental approaches.
The general meaning of learning is associated with acquisition and retention
of either facts or behaviors that were not present in the organism beforehand.
The type of learning is closely related to the corresponding memory. The mem-
ory can be classified (66) into two main categories, one of which, the declarative
explicit memory, and the other, the non-declarative implicit memory. The ex-
plicit memory refers to recollection of facts and events, and requires the integ-
rity of the medial temporal lobe. The implicit memory is related to the ability to
perform procedures (procedural memory) and seems to be independent of the
medial temporal lobe. The corresponding location within the brain is a matter of
an ongoing debate, although data have been accumulated providing evidence
that the cerebellum is involved. This chapter focuses on the implicit memory
and the corresponding procedures for acquisition of motor behavior.
Although the category "motor learning" is defined loosely only (8), Ito (31)
provided helpful interpretations: "motor learning" implies both "adaption" and
"learning"—processes that are not always clearly distinct from each other,
whereas "learning" implies more than "adaptation." Both procedures must be
able to adjust parameters to maintain optimal control performance under chang-
ing circumstances with the adjustment as a progressive process and sequential
exploration of optimal conditions. During repetitive trials an adaptive system
will always make the same sequential exploration, whereas in a learning system
exploration will be improved from trial to trial such that the system will achieve
the optimal point faster and with more accuracy than in the previous trial (31).
Adaptation utilizes preceding experience within an ongoing experiment,
whereas learning is based on experiences acquired in preceding experiments.
Consequently, learning is based not only upon adaptive mechanisms but also on
