For many animals, it pays to have a split mind. A brain with multiple lobes helps humans make more efficient decisions, and it allows birds to spot predators quickly. But a new study in fish uncovers possible hidden costs to a divided brain.
Scientists once thought that only humans had a split in brain functions, called lateralization, with some tasks performed on the left side and others on the right. More recently, studies in primates, birds, and fish have suggested that asymmetric brains are common in many vertebrates and that more lateralized brains are more efficient. In a recent experiment, for example, parrots with more brain lateralization were more successful at finding seeds hidden among pebbles. But despite this cognitive boost, scientists have observed a great range of brain lateralization among fish and birds. Some of these animals can have almost completely symmetrical brains. So are there disadvantages to lateralization?
To find out, Marco Dadda, a psychobiologist at the University of Padova in Italy, and his colleagues focused on goldbelly topminnows (Girardinus falcatus), a species known for its lateralized brain. For 4 years, the scientists bred three minnow groups to have different dominant sides of their brains. They determined the stronger half by watching which way the fish turned to avoid a predator: Turning right 80% of the time meant the right eye--and thus the left brain hemisphere--was dominant, whereas showing no turn preference pointed to a nonlateralized brain.
The researchers then gave the animals some tests. In one experiment, the minnows had to decide between two groups, or shoals, of fellow minnows; one was visible only to the left eye, and another visible only by the right. Fish tend to prefer shoals with many friends or those with fish similar to their own size so as not to stand out to predators. Dadda and his team tested both shoal features in two different trials. Although 60% of the nonlateralized fish picked the optimal shoal features in both trials, only 34% of the right or left dominant minnows made the preferred choice both times, the researchers report online today in the Proceedings of the Royal Society B. Instead, these fish more often picked the group from their dominant side.
Dadda believes that fish with more symmetric brains can judge information from both visual fields equally. (In people, the problem of brain asymmetry is lessened by the significant overlap in the two visual fields.)
The study looks at a neglected aspect of brain lateralization, says cognitive neuroscientist Michael Corballis of the University of Auckland in New Zealand. "People working on asymmetry in nonhuman animals have been so determined to show that asymmetry is not uniquely human that they have neglected the advantages of symmetry," he says.