Visual Discrimination of Shape by Octopus. Squares and Triangles

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Abstract
An experiment was performed upon visual discrimination of shape by the octopus in order to test two predictions derived from a theory of visual discrimination of orientation and shape. Two groups of octopuses were trained to discriminate between a square and a triangle and between a diamond and a triangle. It was found that octopuses discriminate more readily between a square with base horizontal and a triangle than between a diamond and a triangle. Transfer tests showed that: (1) For the octopus, an upright and an inverted triangle have more equivalence than a diamond and a square with base horizontal. (2) Octopuses do not discriminate between the figures used by analysing only differences in one part of the figures (e.g. bases or tops). (3) Having learned the initial discrimination, octopuses transfer to both larger and smaller figures. (4) A pentagon has more equivalence to a square or diamond than to a triangle. (5) A circle is not treated as equivalent to a square. The results are taken to be in agreement with the theory that octopuses analyse the vertical and lateral extents of figures, and that shape discrimination is achieved by analysing the changes in the rates of change in the firing of neurons representing the vertical and lateral extents of the shapes. The results are shown to differ from those obtained with birds and rats, but to agree with results found for higher mammals where these are available for comparison.

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