Crows Read Moving Dots the Way Monkeys Do, in Half the Time

For decades, the standard map of how brains choose a direction has been drawn from monkeys watching drifting dots. Now two carrion crows have run the identical test, and their brains answered the same question in about half the time.

In the random-dot-motion task, a field of white specks on a black screen drifts in one direction, and the subject must judge whether motion is present and where it points. In primates, neurons in the neocortex accumulate that visual evidence until their firing rate crosses a threshold and the animal commits. Crows have no neocortex, which left open how their brains handle the same problem. "This was the big question," said Andreas Nieder, a neurobiologist at the University of Tübingen. "How [would] a differently organized avian brain encode decisions?"

Nieder and colleague Philipp Schmidbauer trained two hand-raised carrion crows on the task, seating them in front of a touch-sensitive monitor showing white dots on a black background. Some share of dots drifted together left, right, up or down, and the birds reported the direction by pecking a target beside the display, earning a mealworm for a correct answer. Difficulty scaled with coherence: 81 percent of dots moved together at the easy end, 3 percent at the hard end, with a zero percent condition in which no correct answer existed. Nieder said the team kept the apparatus identical to the primate version. "We intentionally wanted to have exactly the same random-dot motion task," he said. "The only difference was that the crows had to peck to indicate their decision, whereas monkeys were indicating by making an eye movement."

The crows arrived at their choices using the same mathematical logic primates use, the researchers found, and did so twice as fast as monkeys in the same test. Brain recordings taken while the birds worked showed activity tracking the accumulation of visual evidence toward a decision, a process primates carry out in neocortex, which birds lack. The result shows a brain organized without a neocortex can still reach a comparable decision process.

The work by Nieder and Schmidbauer was published alongside their report of the task performance and neural recordings from the two crows. No further experiments with other bird species have been announced. The University of Tübingen has not scheduled a public lecture on the findings.