Visual acuity in Octopus pallidus and Octopus australis

Specimens of Octopus pallidus and 0. australis were trained to discriminate vertical from horizontal rectangles, vertical from horizontal gratings, and vertical and horizontal gratings from uniform grey. In the discriminations that involved gratings a conditional simultaneous discrimination procedure was used, in which the two stimuli to be discriminated were presented stationary at the two ends of the tank, and a moving white disc was shown in front of each of them. Attacks on a disc were then rewarded or punished depending on the background against which it was shown. Animals rapidly reached performance levels of better than 80 % correct responses on all discriminations. With one specimen of 0. pallidus and three of 0. australis when progressively finer gratings were used the discrimination broke down with stripe widths between 4*4 ' and 9 7 ' , showing that for both species the minimum separable visual acuity is less than 9-7'. The behaviour of the two species is very similar to that of 0. vulgaris, except that they accept less food per day, so fewer trials could be given