The Exclusion Method: How the Brain Decides What We See

How does attention work? What kinds of stimuli does the brain allow to automatically capture our focus, lowering the priority of everything else? And why do moving objects grab our gaze, no matter their size?

An osprey can focus on a barely visible trout even against distracting sensory information, like the flow of a river. This ability is partly due to automatic filtering mechanisms built into the brain.

We know that we have filtering systems that let us focus on attention-grabbing stimuli or, conversely, ignore irrelevant information when searching for something specific (for example, when looking for a person in red in a crowd, we automatically filter out other colors). The essence of this filtering is to suppress some of the incoming data to highlight more relevant signals and achieve our goal.

However, certain processes in the brain operate on an even deeper level, filtering information when we are completely unaware of it. In these cases, our attention is not directed at a goal, but at specific properties of stimuli (such as brightness or movement)—properties that our brains seem to be hardwired to consider fundamentally important. Neurobiologist Duje Tadin from the University of Rochester explains:

“This is due to evolution. If something moves, it usually matters for our survival.”

Scientists have long known that our signaling system is designed to automatically filter out extraneous input; otherwise, we couldn’t perceive the world as we do now. For example, when we look at our surroundings, the perceived image remains stable or smoothly follows our gaze. However, our eyes are constantly making tiny movements (saccades), so our visual system has to correct for this background lag in our vision. Richard Krauzlis, a neurobiologist at the National Institutes of Health in Maryland, says:

“Automatic suppression mechanisms occupy large areas of the brain.”

At the same time, automatic filtering of background information can show up in unexpected ways. Back in 2003, Tadin and his colleagues found that we perceive the movement of small objects very well. However, if these objects are made larger, it becomes harder for us to distinguish their movement.

In an article for Nature Communications, Tadin’s team offered an interesting explanation: the brain is tuned to detect objects that are important for us to see, and these are usually small. For example, for a hunting hawk, a mouse suddenly darting across a field is more important than the swaying grass and trees around it. So, as the researchers discovered, the brain suppresses information about background movements—and as a result, it’s harder for us to perceive the movement of larger objects, since the brain also treats them as background.

Researchers confirmed these findings through a training experiment with older adults. Previous studies had shown no significant difference in how attentively older people observed the movement of small versus large objects. So Tadin and his colleagues hypothesized that older adults would have trouble detecting small moving objects against a moving background—and this was confirmed. However, after several weeks of training, participants became much better at distinguishing such movements. Still, as the researchers found, the training did not improve their ability to detect these moving objects.

According to Tadin, these results highlight that people’s sensitivity to large moving objects is lower “because this strategy is specifically used by our brain to make small moving objects stand out more against the background.”

This is the same strategy (carried out by a different mechanism) that the brain uses in voluntary attention processes: it discards information that is distracting or less useful to highlight more relevant input. Tadin notes:

“Before attention even gets to work, the information has already been trimmed.”

To perceive movement, this reduction must happen automatically—as quickly as possible.

“Attention can do the same thing, but much more intelligently, flexibly, and with less effort.”

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