The planarian has many specialized receptors for detecting light, chemicals,
vibrations, water currents, temperature gradients, pressure variations,
electrical currents, gravity, and magnetic currents. Ironically,
little is known about these receptors and this activity will focus on planaria's
reaction to electrical current and its effect on learning.
Until John B. Watson, a pioneer of the behaviorist
revolution (1920), the study of memory was rarely objective.
Man's memory as compared to that of animals always seemed to be endowed
with more than just mental properties. It was Watson who substituted
this unobjective, supernatural-like characteristic of the mind with science.
Therefore, the study of animal
behavior began with the onset of the behaviorist revolution.
Planaria (Genus: Dugesia)
Planaria are flatworms of the order Platyhelminthes,
and are a common organism for study in high school biology classrooms.
They live in fresh water and can be maintained easily in culture in classrooms
or in freshwater aquaria.
Movements of planaria are as follows:
1. Major movements:
(a) a smooth or gliding
motion (thought to be accomplished by the rhythmic movement of
cilia on the animal's ventral or bottom surface.)
(b) a "humping" or "inching"
movement which is usually referred to as"amoeboid"
movement and is accomplished by muscular contractions. (This is noted mainly
when the planarian has been disturbed by its environment).
2. Minor movements:
(c) some "minor" movements
as a brief lifting of the head, a sudden contraction of its
anterior or posterior end, a twitching of its auricles.
(d) "specialized" movements
involved in eating and reproductive activities.
(e) when under serious stress,
they roll over on their sides displaying a rhythmic
convulsive activity. (James V. Mc Connell, 1967)
Training of Planaria
Classical conditioning of planaria was first reported
by R. Thompson and J.V. McConnell in 1955. The planaria became a
fairly common model for studying conditioning and memory in the 1960's.
In 1967, Richard Block and McConnell published an article in Nature, that
reported results of classical conditioning in brown planaira (Dugesia
dorotocephala). In their study,
they paired an electrical shock (conditioning stimuli) with a flash
of light (unconditioned stimuli). When the shock would occur, the
worms would contract and turn at the anterior end. The experiment
was controlled by a group that received no electric shock, and by a group
that received light and shock at random. After training, the planaria
were given stimulus by light alone without electric shock. The planaria
would exhibit the same type of response that was originally caused by the
electric shock. Block and McConnell followed this with extinction trials,
that is they "reverse trained" the planaria to forget their earlier learning.
Studies of this kind fell out of favor in the early
1970's, as some researchers, notably Allan Jacobson, Sheldon Horowitz,
Clifford Fried, John Hullett, and M.J. Homzie, argued that the training
was not true classical conditioning, but rather that the planaria exhibited
"pseudo-conditioning" and "sensitization." For the purposes of this
exercise, we will not explore the scientific debate as to how the learning
occurs, but use the planaria as a model for learning in relation to animal
behavior.
- The predecessors to Operant Conditioning were first introduced
by Pavlov's Classical Conditioning and E. L. Thorndike's Law of Effect..
When experimenting with dogs Pavlov demonstrated that a neutral stimulus
(such as a bell) prompts no response until it becomes paired with an unconditioned
stimulus (such as meat). Eventually, the "bell" gains the power of
that stimulus, "the meat", to cause a response (such as salivation).
- Behavorist, E.L. Thorndike, in 1966, continued along these
lines in his study of cats in boxes from which they had to escape to get
food. He developed "The Law of Effect" that states that a favorable
effect (behaviors that led to their successful escape in order to obtain
food) is more likely to be repeated in similar situations.
Conversely, behavior which resulted in unsatisfying change in the environment
or ineffective results would decrease in time.
- Finally, B. F. Skinner, like Thornkike, also focused on the relation
between behavior and its consequences. Skinner's box consisted
of a bar, easily accessible for the enclosed rats to press, a food and
water dispenser, and the absence of outside stimuli. This establishes a
careful scientific study of behavior in a controlled environment.
His results demonstrated that the use of pleasant and unpleasant consequences
to change behavior is often referred to as "operant conditioning".
- It is "operant conditioning" which our team has employed in their
training of planaria. In this way the planaria will be taught
to use locomotive behavior and go in the "right direction" whenever they
are placed in the environment of a "T-maze". (Slavin, 1997)
Biochemical Memory
As work on planaria continued in the 1960's, scientists
began to look for the mechanism behind this phenomenon. It was discovered
by McConnell et. al. that if the planaria that had been taught a
specific task were chopped into small pieces (to inhibit regeneration)
and were then fed to untrained planaria, the cannibal planaria learned
the task more quickly. In 1966,
Jacobson, Fried, and Horowitz, published results indicating that it
was possible to remove RNA from trained planaria and inject it into untrained
planaria which subsequently learned a conditioned task more quickly.
This indicated that RNA may have been the important biomolecule for transfer
of memory. Furthermore, results indicated that this would occur regardless
of whether the RNA was extracted from the anterior or posterior of the
trained animal. In 1968, further work was reported in Nature by W.C
Corning and Simon Freed in which the ratio of specific activity between
DNA and RNA was measured at different times
in the training process. It appeared that RNA was very active
in the middle of the training process, and was equally active as DNA in
the beginning and the end of the training process.