LEBRNING BY MARINE INVERTEBRATES
15
difFering in weight. This is rather a surprising result, since octopuses
quite obviously compensate by increasing muscle tension to support
objects they pick up. Any observer, watching octopuses handling the
test objects in experiments designed to train them to distinguish
weights, can tell, simply by watching the animal, which object it is
handling. An arm grasping a heavy object is passively extended as it
takes the weight and contracts to take the strain before passing the
object to the mouth or rejecting it in the normal way. The animals
seem incapable of learning that one of the two test objects necessitates
their doing this (Wells, 1961).
The results of texture, size and shape discrimination experiments
all imply that octopuses never take into account the spatial distribution
of the sense organs in contact with objects they handle. Together with
the weight discrimination experiments they suggest that octopuses
cannot use information about the position or movement of parts of
their own bodies in learning. The tactile discriminations that they fail
to make are those that would require integration of proprioceptive
inputs with information from more superficial receptors. This finding is
consistent with the consequences of statocysts removal in visual
experiments. Once retinal orientation is upset the animal seems
incapable of orientation discrimination, and indeed behaves as if the
retinae were still correctly orientated : the implication is again that the
animal cannot take its own bodily position into account (see 11, A, 2,
p. 7). It seems that in octopuses at least proprioceptive inputs are
utilized for local adjustments to movement and that this class of
information never penetrates to levels of the central nervous system
concerned in learning. It can be argued that a hierarchic decentralized
control of bodily movement is inevitable in animals where movement is
unrestricted by joints, and that a similar state of affairs with failure to
learn to recognize proprioceptive inputs may be expected in other
animals with hydrostatic skeletons (Wells, 1963b, 1965b). It should be
noted that a failure to use information from muscular stretch receptors is
characteristic of vertebrates too (see Merton, 1964) and that the octopod
failure to learn from the position of parts of its own body is associated
with an absence of anything comparable with the vertebrate deep
pressure, joint or tendon receptors (Wells, 1964b).
B. Brain lesions and learning in cephabpods
Since it was found that octopuses can so readily be trained to make
visual and tactile discriminations, a considerable number of experiments
has been made to identify the parts of the brain responsible. The
brains of cephalopods are divided into about thirty anatomically
15
difFering in weight. This is rather a surprising result, since octopuses
quite obviously compensate by increasing muscle tension to support
objects they pick up. Any observer, watching octopuses handling the
test objects in experiments designed to train them to distinguish
weights, can tell, simply by watching the animal, which object it is
handling. An arm grasping a heavy object is passively extended as it
takes the weight and contracts to take the strain before passing the
object to the mouth or rejecting it in the normal way. The animals
seem incapable of learning that one of the two test objects necessitates
their doing this (Wells, 1961).
The results of texture, size and shape discrimination experiments
all imply that octopuses never take into account the spatial distribution
of the sense organs in contact with objects they handle. Together with
the weight discrimination experiments they suggest that octopuses
cannot use information about the position or movement of parts of
their own bodies in learning. The tactile discriminations that they fail
to make are those that would require integration of proprioceptive
inputs with information from more superficial receptors. This finding is
consistent with the consequences of statocysts removal in visual
experiments. Once retinal orientation is upset the animal seems
incapable of orientation discrimination, and indeed behaves as if the
retinae were still correctly orientated : the implication is again that the
animal cannot take its own bodily position into account (see 11, A, 2,
p. 7). It seems that in octopuses at least proprioceptive inputs are
utilized for local adjustments to movement and that this class of
information never penetrates to levels of the central nervous system
concerned in learning. It can be argued that a hierarchic decentralized
control of bodily movement is inevitable in animals where movement is
unrestricted by joints, and that a similar state of affairs with failure to
learn to recognize proprioceptive inputs may be expected in other
animals with hydrostatic skeletons (Wells, 1963b, 1965b). It should be
noted that a failure to use information from muscular stretch receptors is
characteristic of vertebrates too (see Merton, 1964) and that the octopod
failure to learn from the position of parts of its own body is associated
with an absence of anything comparable with the vertebrate deep
pressure, joint or tendon receptors (Wells, 1964b).
B. Brain lesions and learning in cephabpods
Since it was found that octopuses can so readily be trained to make
visual and tactile discriminations, a considerable number of experiments
has been made to identify the parts of the brain responsible. The
brains of cephalopods are divided into about thirty anatomically