BioEd Online

Sudden movements and sounds can trigger a battle between neurons in the brain, and the winners get to decide where an animal will look, according to new research.

Working with barn owls, neuroscientists at Stanford University in California found that neurons in the midbrain, which acts as a relay for sensory information, engage in a 'winner takes all' battle with one another. To the victors go the owl's gaze and attention. The findings, presented at the annual meeting of the Society for Neuroscience in Chicago, Illinois, could explain how the brain decides where to look in an emergency¹_,².

The senses detect everything around them, but the brain can only focus on one part of the world at a time. In everyday life, the conscious mind decides what this should be. But when confronted with a loud noise or fast-moving object, the urge to look becomes automatic. That response makes sense, says principal investigator Eric Knudsen. "When there are features in the environment signalling something that could be life-or-death, we'd want to know about it," he says.

Let's get ready to rumble

What is less clear is how this head-turning decision happens on a cellular level. To find out, Knudsen's team used barn owls, which have neatly arranged midbrains that are easy to study. Research focused on a circuit in the midbrain known as the isthmotectal network, which receives sensory information and tells the eyes where to look.

To prevent the animals from controlling their own gaze, the researchers began by sedating them with nitrous oxide. The scientists then presented a fast-approaching dot in the owls' visual field, positioning the dot so that it would stimulate a particular region of the isthmotectal network. A subset of neurons in the region responded to the dot by firing at a high rate, which — had the animal been awake — might have directed its eyes and attention towards the potentially menacing dot. They found the same result when they recorded from another region of the network.

The authors then presented a second looming dot, this time outside the field of view of the neurons they were studying. The neurons began slowing down. The faster the competing dot approached, the slower the neurons became, and when the competing dot moved faster than the first dot, the neurons switched to a low-firing rate. That switch suggests that they had lost the battle with other neurons in the region that had been signalling the competing dot.

The authors found similar results when the competing stimulus was an increasingly loud auditory tone: at a certain decibel level the neurons responding to the dot switched from high to low activity, suggesting that the sound had become more important.

Intriguingly, the neurons in the network fired in bursts, primarily at 30–40 hertz. This frequency is meaningful for attention — similar frequencies has been seen in primates when they attend to stimuli³.

Competing for attention

The findings suggest that neurons in the isthmotectal network engage in a constant battle to decide which ones are sensing the most important stimulus. Usually nothing in the environment will be arresting enough to trigger the system, but when something does come along — a flash of light, a shotgun's crack — the responding neurons will switch on, sending a strong signal that usurps the animal's attention.

Neuroscientist Richard Krauzlis at the Salk Institute for Biological Studies in La Jolla, California, thinks the switch-like behaviour is intriguing. "This could be part of a mechanism for helping to select the relevant stimulus, and this is a very interesting possibility," he says. Further studies are needed to prove that neurons in the network actually compete, he says, and that neurons in awake animals behave in the same way.