NERVOUS SYSTEM AND BEHAVIOUR 461 



possessing sensory, motor and associative functions. When the vertical 

 lobe, or the medial superior frontal lobe is removed, or when the tract 

 between the two is severed, the memory preventing the attack is lost and 

 cannot be re-acquired. A fleeting memory lasting a few minutes is set up 

 when the situation is presented repeatedly at very short intervals. Partial 

 removal of the vertical lobe system does not interrupt the memory trace ; 

 the ability to discriminate figures is reduced, however, in proportion to the 

 amount of vertical lobe tissue removed. Following lesions to the lateral 

 parts of the superior frontal lobe, an octopus no longer attacks crabs 

 unless they are placed close to the animal. This kind of injury produces 

 functional unbalance within the c.n.s. such that a centre responsible for 

 inhibiting an attack on distant objects assumes control. 



The vertical lobes and optic lobes of the octopus are concerned with 

 establishing memories (the memory trace). In the optic lobes some form of 

 interaction takes place between afferent impulses from different regions of 

 the retina and from tactile receptors in the arms, whereby transitory 

 associations are set up. Similar associations appear to be set up in succeed- 

 ing lobes (superior frontal and vertical), and since there are circular path- 

 ways between the centres, the pattern which originates in the optic lobes can 

 be returned to these structures. It has been suggested that the vertical lobe 

 system, by the return presentation of an association to the optic lobes, 

 acts so as to prolong memories in the latter, thereby evoking changes of a 

 more permanent nature (13, 14, 121a, I2\b, 133). 



The behaviour of fish is varied and complex, involving many different 

 kinds of activities, e.g. schooling, migration, nuptial display, nest-building, 

 guarding of eggs, etc. Training experiments reveal that fish learn readily. 

 Conditioning to particular stimuli and forming of associations obviously 

 play an important part in the lives of many species, e.g. in enabling the 

 animal to locate some particular feature in its environment and find its 

 way about. 



Many studies of conditioned reflexes in fishes have been concerned with 

 determining powers of sensory discrimination, and have not been designed 

 specifically to throw light on processes of learning. As an example may be 

 cited experiments in which a wrasse (Crenilabrus) was conditioned to 

 respond to sound, with food as the unconditioned stimulus. Positive 

 responses appeared after twenty trials with food, and the association was 

 well established in thirty-two trials (16). In specifically designed learning 

 experiments, rock wrasse (Ctenolabrus) have learnt to swim around ob- 

 structions to reach food. Individual fish showed much diversity of be- 

 haviour in the maze, and some animals were evidently more capable of 

 profiting by experience than others. Learning was enhanced by accentua- 

 ting the sensory clues, which produced stronger imprinting on the memory 

 trace (107). Second-order learning (i.e. second-order conditioned reflex) 

 has proved possible in the goldfish. The fish were trained initially with an 

 optic stimulus and food as a reward, and an olfactory stimulus was then 

 associated with the conditioned optic stimulus (16#, 95). 



