A simple structure in the brain, called the thalamus, contains a mental searchlight that filters what people pay attention to, experiments on monkeys have confirmed.
Kerry McAlonan and colleagues at the National Eye Institute in Bethesda, Maryland, say that this suggestion was made way back in 1984 by the co-discoverer of DNA Francis Crick.
The thalamus was once thought to be only a highway that connects the eyes to the brain's outermost layer and main site of consciousness, the cortex, which is responsible for housing the attention steering mechanisms that sort out all this sensory input.
During the course of the study, the researchers trained three macaque monkeys to pay attention to rectangular spots of light, each about the size of a thumb held up at arm's length.
Their results show a quick surge of activity in the part of the thalamus that relays information to the cortex and, a split second later, a drop in activity in the thalamic reticular nucleus (TRN), a satellite structure known to turn off this superhighway of sensory information during sleep.
The researchers think that when one pays attention, the TRN glances at the "images" coming through the thalamus and selectively turns on and off relays to pass on only the bits that deserve attention.
"If the thalamus is the gateway to the cortex, the TRN is the gatekeeper," New Scientist magazine quoted McAlonan's colleague Robert Wurtz, co-author on the paper, as saying.
Sabine Kastner of Princeton University, who has studied the structure in humans, said that the feedback loop emancipates the thalamus from its slavery to the conscious cortex.
"We're going to have to rewrite all of the textbooks," she says.
A research article on the study has been published in the journal Nature.