How do fish make a decision when faced with conflicting signals?

My newest paper called ‘Fish use colour to learn compound visual stimuli‘ has been published in the journal Animal Behaviour.

The aim of this work was to understand how fish respond when two pieces of visual information that they previously learned is conflicting. For example, there is a psychology experiment called the Stroop Test in which observers are presented with the name of a colour printed in either the same colour or a different colour and subjects are asked to read the word aloud. You can try this yourself with the following words:


When people are asked to read the words composed of incompatible colours (e.g. BLACK, BLUE) there is little change in how quickly they can answer. However, when people are asked to say the colour when the word is incompatible, they take much longer to answer (Stroop 1953, Macloud 1991). This suggests that when two pieces of visual information are in conflict, we rely on some information more than others. In this case, people rely on the word information more than the colour information, at least in this particular experiment. Stroop suggested that this occurred because subjects have more practice with one task (i.e. reading words) than the other (i.e. identifying colours).

We wondered if fish also pay more attention to one type of information than another. To test this, we used Picasso triggerfish (Rhinecanthus aculeatus) and trained them to discriminate between two circles that had both pattern (stripes vs cross) and colour (blue vs yellow) information.

Newport et al 2017 figure

We then changed the circles so that the patterns and colours were switched and therefore in conflict…

Newport et al 2017 figure 2

What we found is that the fish trained to choose the blue stripes, chose the blue cross when we changed the circles. We saw a similar pattern when the fish were trained to yellow crosses as they chose yellow stripes when we tested them.

This showed that the fish made their decision based on the colour of the circle and not the pattern.

Experiments like this are important because they help us understand what fish can see and what they think about the things that they see. This tells us a lot about how fish brains work and how their brains are different from the human brain. In this case, we learned that, just like humans, the fish brain can process colour and pattern information separately. It was equally possible that the fish wouldn’t be able to make a decision between the new circles because they weren’t identical to what the fish had previously learned. So even though the fish brain is a lot smaller than the human one, and made up of different structures, in this case, the two brains solved a problem the same way.

This experiment also shows us how important colour is to this species of fish. Picasso triggerfish live on extremely colourful coral reefs  and our experiments show that the fish likely use this colour information to tell objects apart.