In the past, we've covered evidence that the human brain can process information from visual inputs without letting the conscious portion of the brain know what it's up to. It's been proposed that this filtering activity is needed to prevent people from getting overwhelmed by sensory input. But some new research suggests that the filtering comes at a cost—in some cases, the mental price can be so high that it actually interferes with what you are trying to pay attention to.

The authors of a new study set up a task where subjects had to focus on a series of characters displayed at the center of their field of vision while small dots moved at the periphery. Those dots either tracked a "coherent" path (meaning they moved along a specific trajectory) or moved randomly. Previous studies had shown that the human brain can track the motion if the dots seem to be following a trajectory; otherwise, their motion doesn't register. The research tracked whether the ability of the brain to "see" a dot's motion interfered with character recognition, noting that "One would naturally assume that the degree of an invisible stimulus's influence is generally weaker than that of a visible stimulus." That assumption turns out to be badly wrong.

If a non-moving dot was viewed as a baseline, equivalent character recognition success was obtained when the dot's path was greater than 20 percent coherent. Below 20 percent, however, success rates dropped, reaching a low point at about 5 percent coherence. In other words, as the motion of the dots became impossible to detect, the task performance plunged. Making the dots harder to see by decreasing their brightness generated a similar result curve, but shifted to where the worst performance came at 30 percent coherence, again suggesting that the worst performance came when the dot's motion was essentially invisible.

Using functional MRI, the authors also followed the activity of two areas of the brain: the one responsible for tracking motion, and the one responsible for suppressing distractions. The area of the brain that tracks motion is most active when the motion was below the threshold for detection. Once the motion became detectable, however, the distraction-killing area kicked in, and the motion tracking area was suppressed.

The authors propose that even when the motion of an object isn't coherent, the visual system expends a lot of effort trying to track it. This, in turn, distracts the visual system from the primary task. Once the brain gets a handle on the motion—when it perceives the motion as coherent—it's able to make the distraction handling system aware of it, at which point it can be ignored. In other words, something has to attract your brain's attention before it can decide to ignore it. All of this, of course, happens without you being consciously aware of it.