Studies of the conditioned reflex in the lower vertebrates. II. Early development of the defensive conditioned reactions in fish larvae

Tuge, Hideomi; Ochiai, H.

Science Reports of the Tohoku University, Series 4. Biology 22(1): 21-28


ISSN/ISBN: 0040-8786
Accession: 025547126

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A defensive conditioned reflex was established in the larvae of both Lebistes reticulatus and Oryzias latipes in early stages of development by combining an unconditioned stimulus (electric current) and a conditioned stimulus (light) Lebistes is an ovoviviparous and Oryzias an oviparous animal. In Lebistes, larvae two days old are likely to begin to obtain the ability to form a defensive conditioned reflex, while, in those after the third day of their birth, the conditioned connection is established without exception by several repetitions of reinforcement. In larvae about six days old, the intensity of the conditioned reflex seems to almost equal that in an adult guppy. At about the same stage a conditioned reflex which has been established appears to remain till the following day. In Lebistes, the defensive conditioned reflex which has been established in the early stages of the larvae was extinguished by application of conditioned stimuli without reinforcement. It was also extinguished when the larvae were left alone. In both cases, it required many applications of conditioned stimuli, and took considerable time to extinguish the conditioned reflex completely. The results demonstrate the existence of an internal inhibition process even in the early stages of the larvae. In Oryzias latipes, the defensive conditioned reflex was established for the first time in larvae about 9 days old. The genesis of the defensive conditioned reflex as such is substantially similar to that observed in the larvae of Lebistes.