Repeated Presentation Experiments and the Function of the Vertical Lobe in Octopus

Abstract

When presented with a small object that it has not previously encountered, an octopus passes the object to its mouth and attempts to eat it. Inedible objects are rejected after a period of minutes, and when the same object is repeatedly presented at short intervals, the animal rapidly learns to examine and reject it without passing it to the mouth. That the animal is learning to recognize a particular object and not merely becoming fatigued or generally less reactive can be demonstrated by changing the object; a new object is almost always taken, even if the animal has not taken the original for many trials. After a break in training of the order of hours, the animal may accept the same object when it is presented again, but relearns not to take it in fewer trials than in the original training. The performance of two animals tested after removal of the optic lobes was in all respects comparable to that of three controls (two unoperated and one with the optic nerves cut). Six animals with the vertical lobe of the brain removed were also tested. The performance of these animals differed from that of five controls in two respects: they took six times as long to examine and reject the objects in the first trial, and they showed relatively poor retention following breaks in training. The first of these differences is particularly important because it shows that removal of the vertical lobe has effects other than increasing the rate of fading of memories between trials. The significance of this in the interpretation of the function of the vertical lobe from discrimination experiments is discussed.