The barn owl hunts down its prey with a mental map tuned to the location of sounds--say, of a scurrying mouse. But the owl's brain uses input from the eyes to keep its auditory map up to date. Now, researchers have for the first time shown how visual information chimes in to keep the map grounded in reality.
The owl's auditory map room is a brain area called the external nucleus of the inferior colliculus (ICX), as a group led by Stanford's Eric Knudsen discovered in the early 1980s. It usually works wonders, but any change in the owl's hearing--for instance, as a result of aging--can misalign the map. In January, Knudsen's team discovered that a brain area called the optic tectum (OT) sends visual signals that tweak the map: When the team members destroyed the connections between the OT and ICX, the map ceased adjusting. But strangely, they were never able to register a response in ICX neurons to visual stimuli.
Postdoc Yoram Gutfreund wondered if the flow of information was somehow blocked in the anesthetized owls on which the experiments were done. So he treated their OTs with a chemical that can wake up neural pathways. As a result, electrodes in the ICX began to pick up neural responses to flashes of light, he reports in the 30 August issue of Science. The responses were specific; just as each ICX neuron registers sounds from a particular location, each also selectively responded to visual cues from the same location. That meant that the visual and auditory maps in ICX are aligned.
Gutfreund next tricked the owls' brains to make the maps seem misaligned. Through headphones, he delivered sounds to the owls' ears that sounded as though they came from a place that was slightly offset from the location of the light. The ICX neurons responded much more strongly to the light than they had when the maps were aligned. That, says Gutfreund, is what one would expect for an instructive signal: It points out an error to be corrected.
With these findings, a "circle is closed," says neuroscientist Mark Konishi of the California Institute of Technology in Pasadena: The researchers know where the visual information comes from and how it alters the map.