Study reveals how fish detect fast-moving prey
The workings of the part of the brain that helps zebrafish track prey are uncovered in a new study published in the journal Science. The research was partly funded by a Marie Curie Outgoing International Fellowship from the EU. The findings also shed light on how we process certain visual information. 'We are particularly sensitive to high-contrast, moving objects that fill only a small portion of our visual field,' explained Ehud Isacoff of Lawrence Berkeley National Laboratory and the University of California at Berkeley in the US. 'When you stand next to a busy road and track cars going by, the coordination of the motor control in the eyes that allows you to visually track cars is very important.' Similarly, the zebrafish needs to be able to detect and track tiny, fast-moving objects that could be a tasty morsel. Key to the fishes' ability to do this is a structure in the brain called the optic tectum which receives visual data from the eye and filters it before sending signals to the parts of the brain responsible for motion. To see how the optic tectum works, the scientists created genetically engineered fish in which the neurons in the optic tectum light up when they are activated. By using fast microscopy, the researchers could observe individual neurons blinking on and off as they transmitted signals. The researchers showed the fish films featuring thin, moving black bars that mimic the size and speed of the fish's typical prey; this triggered intense activity in the output part of the optic tectum, as signals were sent to the regions of the brain involved in chasing down prey. However, when the fish viewed films with large flashes of light and dark that covered most of their field of view, the output neurons of the optic tectum remained relatively quiet. 'We identified a difference in the optic tectum's output between visual information that covers the whole visual field versus a small object moving across it,' commented Professor Isacoff. The next challenge for the team was to determine how the optic tectum works. The structure consists of two layers. The top layer, which receives inputs from the retina, comprises both excitatory and inhibitory neurons. When the retina detects large objects, large numbers of cells in this input layer, including numerous inhibitory neurons, are activated. These inhibitory neurons drown out the signal, so that when it reaches the inner, output layer of the optic tectum, it is very faint. 'The inhibition is so dominant that it kills the signal,' said Professor Isacoff. However, when a tiny object crosses the fish's visual field, very few inhibitory neurons are activated and the signal makes it through to the output part of the optic tectum largely unimpeded. Professor Isacoff added: 'We know that the inhibitory neurons are the key to this process because if we interfere with their function the animal loses the ability to hunt.' The researchers point out that the optic tectum has an equivalent in the mammalian brain in the form of the superior colliculus. The study is therefore relevant to our understanding of how humans detect and track small objects.
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