We simply can't become conscious of and remember everything that we see. Multiple representations of objects in our visual field are constantly competing with each other for our brain's limited visual processing capacity. What's more, they mutually cancel each other out; visual clutter actually suppresses the brain's responsiveness. So how does the brain cope?
Based on previous studies in monkeys, National Institute of Mental Health neuroscientists Drs. Sabine Kastner, Peter De Weerd, Robert Desimone and Leslie Ungerleider hypothesized that by focusing its attention on just one stimulus, the brain cancels out the suppressive influence of nearby stimuli. In this way, it enhances information processing of the desired stimulus.
To test their hunch, the researchers designed a series of human experiments using fMRI (functional Magnetic Resonance Imaging), which tracks oxygenated blood to get snapshots of how active various parts of the brain are at any given moment; the more active areas use more oxygen. They report on their findings in the Oct 2 Science.
The first experiment sought to demonstrate the brain-dampening effects of visual clutter. Subjects in the MRI scanner focused both their eyes and their attention on the lower left corner of a computer screen, while colorfully patterned square-shaped images flashed in the upper right quadrant. The images appeared under two conditions: one at a time, or four simultaneously. As expected, when presented simultaneously, the stimuli evoked weaker responses in the brain than when presented sequentially. This confirmed that multiple stimuli do, in fact, suppress each other.
Also as predicted from the monkey studies, the amount of this mutual suppression progressively increased along a circuit that processes object vision, which runs forward and downward from the back of the brain. Neurons at the beginning of this circuit "see" only a very small portion of the visual field, while neurons near the end respond to almost all of what the eyes see. So neurons at the beginning of the circuit simply couldn't view the multiple stimuli and were thus spared the suppressive effects. Spacing the images farther apart also decreased suppressive interactions.
To rule out any possibility that the suppression effect might have been caused simply by the longer overall exposure to the sequentially presented images, the presentation rate was held constant in a second control experiment. One image was presented just above the center line in the top half of the visual field under two conditions: alone, and in the presence of three images just below it in the top part of the lower visual field. Since the brain processes upper and lower visual field representations in spatially separated regions, such nearby stimuli may competitively interact, but activate different brain areas. Thus, it was possible to see the effect of visual clutter on the brain's response to the single image. Again, as expected, response in the outer reaches of the object vision circuit was significantly stronger for the image presented by itself.
In a third -- and most critical -- experiment, subjects were tested under two conditions: unattended vs. attended. In the unattended condition, as in the first experiment, they fixated their eyes and focused their attention on the lower left corner. In the attended condition, they continued to fixate their eyes on this corner, but directed their attention instead to counting the occurrences of a particular target stimulus (indicated by its presentation before each scan) within a nearby image. Again, four images were presented under sequential and simultaneous conditions.
"We can attend to things we're not necessarily looking at," explained Ungerleider.
As predicted, focused attention enhanced the end-of-circuit responses to simultaneously presented stimuli more strongly than to sequentially presented stimuli. "Thus, the suppressive interactions were partially canceled-out by attention," note the researchers. Since the amount of the attention effect increased in tandem with the amount of suppressive interactions -- especially near the end of the circuit -- the researchers found support for the idea that spatially directed attention enhances the brain's processing ability by quenching suppression caused by nearby stimuli. Modulation of suppression in this circuit may thus be a mechanism by which attention filters out unwanted information, suggest the researchers.
"This work raises the standards of brain-imaging research well above the routine inventories of brain activations that are the standard fare of the field," observe MIT researchers Drs. Nancy Kanwisher and Paul Downing in an accompanying "Perspective."
The National Institute of Mental Health (NIMH) is a component of the National Institutes of Health, an agency of the U.S. Department of Health and Human Services.