SPATIAL SUMMATION IN THE PHOTOTACTIC BEHAVIOR OF NAUTILUS-POMPILIUS

Phototactic behaviour in Nautilus pompilius was studied for stimuli of two different sizes, the larger one having 4 times the area of the sm aller one. When the stimuli were of equal intensity, the larger was ch osen more often. As its intensity was reduced compared to the smaller stimulus this preference decreased, and eventually reversed. The two s timuli were equally preferred when the intensity of the large one was about 0.5 log units less than that of the small one, at which point th e total radiant flux from each stimulus was approximately the same. It is argued that summation must occur within the Nautilus visual system over visual angles of 13(d)egrees or more, and that this is in agreem ent with previous data suggesting that in this animal sensitivity is i ncreased at the expense of resolution.