52
M. J. WELLS
To have dealt adequately with all this would have led to an indefinite
expansion of the review. The result is a compromise and suffers from
the defects inherent in compromises. On the one hand a more detailed
criticism of the experiments would have been desirable. On the other a
further expansion of the physiological background and behaviour of the
animals would have made it easier to assess the status of learning in the
various phyla.
Admitting all this, and agreed that the present survey pursues none
of these topics in depth, it seems that the following generalities can
usefully be made about the study of learning in invertebrate animals :
The phenomenon called " habituation '' by vertebrate workers
seems to be universal. Animals do not continue to respond to irrelevant
stimuli and in general there seems to be a neural mechanism for ensuring that the response stops before either the sense organs or the muscular system is fatigued. Escape responses to simple stimuli disappear
within three or four trials in a wide variety of organisms, and return only
after a matter or minutes or hours. Such rapid failures of the response
occur, however, only when very simple stimuli are used ; in any more
complex situation habituation is liable to be much slower. One is led
to suspect mechanisms for ensuring that even the simplest animals
remain unusually wary of complex stimulation and that many of them
may be protected by highly specific and virtually unhabituatable
responses to particular patterns of stimuli. Systematic studies of the
interacting effects of simple stimuli in habituation could well be used to
discover facts about the neural organization of some lower animals,
just as discrimination training experiments have been used to study
sensory integration elsewhere.
Most invertebrate animals continue to show rhythms of activity
under constant laboratory conditions. The pacemakers for these
cycles are sometimes known, and are very commonly not immediately
related to the highest parts of the CNS. It would be remarkable to
find that learning proceeds equally readily at all phases of these
cycles, some of which are of short (minutes rather than hours) duration.
Their effect must clearly be taken into account before changes in
behaviour associated with training experiments can reliably be classed
as the result of the animals' experience. Undirectional responses
(animal " learns " to escape more rapidly or to respond to a stimulus
that did not elicit a response before) are particularly dangerous as
indicators of learning for this reason, and even in discrimination
experiments some control is necessary to ensure that the chance of a
choice in a particular direction does not fluctuate in time. Very little
has been done to quantify the effect of such cycles on learning by
1.
2.
M. J. WELLS
To have dealt adequately with all this would have led to an indefinite
expansion of the review. The result is a compromise and suffers from
the defects inherent in compromises. On the one hand a more detailed
criticism of the experiments would have been desirable. On the other a
further expansion of the physiological background and behaviour of the
animals would have made it easier to assess the status of learning in the
various phyla.
Admitting all this, and agreed that the present survey pursues none
of these topics in depth, it seems that the following generalities can
usefully be made about the study of learning in invertebrate animals :
The phenomenon called " habituation '' by vertebrate workers
seems to be universal. Animals do not continue to respond to irrelevant
stimuli and in general there seems to be a neural mechanism for ensuring that the response stops before either the sense organs or the muscular system is fatigued. Escape responses to simple stimuli disappear
within three or four trials in a wide variety of organisms, and return only
after a matter or minutes or hours. Such rapid failures of the response
occur, however, only when very simple stimuli are used ; in any more
complex situation habituation is liable to be much slower. One is led
to suspect mechanisms for ensuring that even the simplest animals
remain unusually wary of complex stimulation and that many of them
may be protected by highly specific and virtually unhabituatable
responses to particular patterns of stimuli. Systematic studies of the
interacting effects of simple stimuli in habituation could well be used to
discover facts about the neural organization of some lower animals,
just as discrimination training experiments have been used to study
sensory integration elsewhere.
Most invertebrate animals continue to show rhythms of activity
under constant laboratory conditions. The pacemakers for these
cycles are sometimes known, and are very commonly not immediately
related to the highest parts of the CNS. It would be remarkable to
find that learning proceeds equally readily at all phases of these
cycles, some of which are of short (minutes rather than hours) duration.
Their effect must clearly be taken into account before changes in
behaviour associated with training experiments can reliably be classed
as the result of the animals' experience. Undirectional responses
(animal " learns " to escape more rapidly or to respond to a stimulus
that did not elicit a response before) are particularly dangerous as
indicators of learning for this reason, and even in discrimination
experiments some control is necessary to ensure that the chance of a
choice in a particular direction does not fluctuate in time. Very little
has been done to quantify the effect of such cycles on learning by
1.
2.