Beneath our awareness, the brain lets certain kinds of stimuli automatically capture our attention by lowering the priority of the rest.
Jordana Cepelewicz | NAUTILUS
Last week, Quanta reported on the filtering mechanisms that allow us to focus our attention on stimuli of interest—that let us tune out the music in a room to listen to a nearby conversation, or disregard greens, blues and yellows in a crowd when searching for a friend wearing red. That kind of processing, which involves the suppression of some sensory data to highlight signals that are more relevant, is directed by a goal.
But other processes operate well below this level of awareness, filtering out information that the brain deprioritizes without our willing it, or even knowing it. In these cases, our focus is directed not by a goal but rather by particular properties of the stimuli, like their brightness or motion—properties that we’re seemingly hard-wired to consider important. “It makes sense from an evolutionary perspective,” said Duje Tadin, a neuroscientist at the University of Rochester. “If something is moving, it’s often fairly important to your survival.”
Scientists have long known that our sensory processing must automatically screen out extraneous inputs—otherwise, we couldn’t experience the world as we do. When we look at our surroundings, for instance, our perceived field of view holds steady or moves smoothly with our gaze. But the eye is also constantly making small movements, or saccades; our visual system has to subtract that background jitter from what we see.
“Automatic suppressive types of mechanisms take place … through large swaths of the brain,” said Richard Krauzlis, a neuroscientist at the National Eye Institute at the National Institutes of Health in Maryland. “Basically all over the place.”
And automatic background subtraction, it turns out, can also manifest in intriguing, unexpected ways. Take a counterintuitive finding that Tadin and his colleagues made in 2003: We’re good at perceiving the movements of small objects, but if those objects are simply made bigger, we find it much more difficult to detect their motion.