The authors of the new work created special cameras and divided them into two screens. Each screen showed a different image, for example, five dots on one and three on the
the other.
The fish could then choose one of their sides and be rewarded or not. Over time, they learned to associate the visual stimuli on the screens with the desired result.
Further the task became more difficult: first the authors showed figures of different colors, then they had to memorize them in 5 seconds. Then a gate was opened and the fish had to choose between two doors with screens: one with an extra piece and one with one less piece.
Blue meant addition and yellow meant subtraction. If the image had three blue pieces, they had to add one to three and choose an image with four pieces. Then they got a reward.
If two of the figures in the original image were yellow, the fish were only rewarded in the test area if they swam to an image showing one figure – this was similar to the subtraction operation.
Tests were conducted with eight mbuna zebras and the same number of stingrays. As a result, six mbuna zebras and three stingrays learned to count: they successfully answered addition questions 78% and 94% of the time, respectively. Subtraction was more difficult: the percentage of correct answers dropped to 69% and 89%, respectively.
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