Does brain asymmetry allow efficient performance of simultaneous tasks?

Rogers (2000, Brain and Language, 73, 236–253) suggested that the ubiquity of cerebral lateralization could be explained by the advantages it provides to individuals in coping with two simultaneous tasks, because in lateralized individuals each task could be carried out by a different hemisphere. To test this hypothesis, we compared lateralized and nonlateralized fish, obtained through selective breeding, in a situation requiring the sharing of attention between two simultaneous tasks, prey capture and predator vigilance. Food-deprived individual Girardinus falcatus had to enter a compartment adjacent to the home tank to capture live brine shrimps either in the presence or in the absence of a live predator placed at some distance. When no hazard was present, lateralized and nonlateralized fish did not differ in rate of prey capture. However, when the predator was visible, lateralized fish were twice as fast at catching shrimps as were nonlateralized fish. A fine analysis of fish movements revealed that lateralized fish tended to monitor the predator with one eye and to use the other eye for catching prey, whereas nonlateralized fish swapped between tasks, using each eye for both functions.