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EDITOR’S CHOICE IN NEUROSCIENCE

Despite their vacant stares, fish are surprisingly brainy. They can quickly learn tasks, pilot vehicles, and may even be able to count. Some fish are able to pick up on complex tasks more quickly than others, and researchers previously attributed these individuals’ smarts to bulkier brains. But research published July 13 in Proceedings of the Royal Society B finds that some individual differences in cognitive ability among fish of the same species may stem from relative size differences between specific brain regions, not just brain size overall (289:20220844, 2022).

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Zegni Triki, a biologist at Stockholm University, had an inkling that brain region size might influence task performance. For example, she knew that among wild gobies, another family of small fish, species that dwell in craggy rocks have bigger telencephalons, while sand-dwelling species have larger optic lobes. The telencephalon is a brain region associated with cognitive skills including memory and decision making, while optic lobes are brain areas that process visual information.

So Triki selected guppies (Poecilia reticulata) with large or small telencephalons and optic lobes. She bred the two groups separately over three generations, then measured how well each group performed at two tasks. First, guppies learned to discriminate which of two differently colored wells contained food. The second task added a twist. Once the fish had learned the first task, the colors were reversed. “That means, once your animals learn an association successfully, you make them unlearn it,” Triki says. The researchers measured the time it took the guppies to adjust—cognitive flexibility that Triki says represents “quite a difficult task for animals.”

She then compared the brains of the two groups and found that, while the average brain size remained the same between the two, fish with larger optic lobes excelled at the initial color discrimination task. Triki says this makes sense since the area “is mainly used for visual information processing.”

(Left) The optic lobes are thought to be involved in visual processing. In this study, researchers found that guppies with larger optic lobes more quickly learned a visual discrimination task—identifying which color well contained food. (Right) The fish telencephalon is thought to be involved in spatial learning, memory, and inhibitory control. Here, the researchers found that a larger telencephalon might enhance the fish’s cognitive flexibility, allowing them to more quickly associate food with a new color after the researchers switched it.
The optic lobes are thought to be involved in visual processing (left). In this study, researchers found that guppies with larger optic lobes more quickly learned a visual discrimination task—identifying which color well contained food. The fish telencephalon is thought to be involved in spatial learning, memory, and inhibitory control (right). Here, the researchers found that a larger telencephalon might enhance the fish’s cognitive flexibility, allowing them to more quickly associate food with a new color after the researchers switched it. WEB | PDF
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Meanwhile, fish with larger telencephalons fared better at the second task. That came as more of a surprise, Triki says, adding that it’s the first evidence that the telencephalon is involved in cognitive flexibility.

B. Wren Patton, a graduate student in marine biology at Pennsylvania State University who was not involved in the research, says that the study “was really clean . . . they did a really good job being very precise with their descriptions.”

Patton says that she appreciated that the study focused on brain regions, rather than the brain as a whole. She also says that the artificial selection in the study was a “really interesting aspect of the story’’ and she’d like to see the researchers breed the fish for a few more generations before testing. The experimental design “make[s] sense in the field of animal behavior,” she adds. “If you want to make comparisons between what these parts of the brain are really driving . . . [in terms of ] the behavioral and cognitive capabilities of the individual, you really need exactly this kind of design.”

Z. Triki et al., “Brain morphology correlates of learning and cognitive flexibility in a fish species (Poecilia reticulata),” Proc R Soc B, 289:20220844, 2022.