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Some evidence for colour-blindness in Octopus.

1. Groups of O. vulgaris were trained successively to make brightness and hue discriminations with painted vertical rectangles. These were of approximately the same physical intensity and were equally preferred. 2. Using one-cue discrimination training we found that ( a ) Seven octopuses that discriminated between dark blue and light blue did not discriminate between dark blue and dark red shapes. ( b ) Nine octopuses that discriminated between light red and black did not discriminate between light red and light green shapes. ( c ) Fifteen octopuses that discriminated between medium green and light green did not discriminate between medium green and medium blue shapes. All these octopuses later discriminated between black and white shapes. 3. Another group of octopuses was trained with two cues, one or both of which were relevant. The cues used were brightness (dark or light) and hue (blue or green). ( a ) Group 1 ( N = 14) discriminated between dark and light shapes where hue was irrelevant. ( b ) Group 2 ( N = 14) discriminated between dark blue and light green shapes. ( c ) Group 3 ( N = 14) failed to discriminate between blue and green shapes where brightness was irrelevant. Overall the scores of Groups 1 and 2 did not differ significantly. 4. Thus under experimental conditions where octopuses rapidly learned to make brightness discriminations they did not learn discriminations based on hue alone; even if colour is perceived, it seems a relatively unimportant cue for this animal. 5. Another group of octopuses ( N = 10) was tested, in a special apparatus, for nystagmus and optomotor responses. 6. Such responses were always given when stripes of different brightness moved relative to the retina, but when stripes of the same subjective brightness but different hue moved across the eyes they rarely occurred. Such occurrences were no more frequent than the responses given to a plain grey background. 7. Similar results were obtained under conditions of illumination ranging from less than 1 lux to 7.5 x 10 4 lux. 8. The absence of visual response to moving coloured stripes suggests that the failure of octopuses to learn a hue discrimination may be the result of an inherent incapacity at the retinal level. Together with other findings, which are discussed, this strongly suggests that O. vulgaris is colour-blind.