Short Communication the Visual Acuity of Octopuses for Gratings of Different Orientations

A recent study has shown that the minimum separable angle of visual acuity of Octopus pallidus and O. australis lies between 4-4' and 9-7' when vertical and horizontal gratings are used as test objects (Muntz & Gwyther, 1988). In view of other observations the horizontal rectangular shape of octopus pupils, the pronounced astigmatism of the lens of Eledone cirrhosa (Sroczynski & Muntz, 1985), the difficulty that O. vulgaris has in discriminating between oblique rectangles (Sutherland, 1957), and the vertically and horizontally arranged rectilinear organization of both retina and neurones in the optic lobe of O. vulgaris (Young, 1960) it seemed worthwhile testing the effect of grating orientation on visual acuity in more detail, and in particular extending the measurements to include oblique gratings oriented at 45° with respect to gravity. The animals were trained to discriminate gratings of different orientations from uniform grey stimuli, using the conditional simultaneous discrimination procedure described by Muntz & Gwyther (1988). The two stimuli to be discriminated were presented stationary at the two ends of the tank, and a moving 3-7cm diameter disc shown in front of each of them. Attacks were rewarded when made to the disc presented in conjunction with a grating. Two minor modifications were made to the previous procedure. First, it was found unnecessary to use electric shock as punishment, with the result that the level of attack improved, especially with the more difficult discriminations. Second, circular backgrounds (grating or grey) of diameter 13-0 cm were employed instead of square ones, so that the overall appearance of the gratings was not altered by their orientation. Gratings with stripes subtending 52-9', 15-4', 7 1 ' and 5-3' at the centre of the tank were used, in all cases paired with the uniform grey stimulus. The subjects were three male O. pallidus. They were initially trained using the 52-9' grating alone. About five rewarded trials were needed before the animal learnt to attack with little or no hesitation. Discrimination between this grating and the grey stimulus was then introduced, and was learnt after two or three unrewarded attacks towards the grey stimulus. Only the results of trials following this training period are included in the data. In this part of the experiment only the three larger gratings were used, and during each experimental session, gratings of