If you have not seen this video of students passing basketballs around, watch it now before reading further.

The now famous video, from Simons 1999, is a demonstration of inattentional blindness. There is no trick here, just a demonstration of normal brain functioning. When we are focusing our attention on one type of stimuli we can filter out “distracting” stimuli that doesn’t fit the parameters. In the basketball example you were instructed to pay attention to the students in white, so your brain flagged the students in black as distracting information. The gorilla, which is also black, was therefore filtered out as well (for about 40% of subjects).

Interestingly, often people take pride in having noticed the gorilla, but this is not necessarily a manifestation of having better attention. In fact noticing the gorilla, if anything, might mean you are more distractable and have worse attention (it also can be mostly random chance). The brain is supposed to filter out extraneous stimulation, otherwise we would not be able to function. Those suffering from traumatic brain injury, for example, often complain that they cannot filter out distracting sensory stimuli, so they find noisy or busy environments (like crowds) very uncomfortable. They may not be able to focus on work unless they are in a distraction-free environment. It’s impairing.

In other words – in the experimental setup of the students passing around the basketballs, not noticing the gorilla was an active step of filtration by the brain, not a failure to notice the gorilla. Such inattentional blindness is now experimentally well-established. Actually (interesting story) the first experiments demonstrating inattentional blindness were in 1959, but they were accidental. Paranormal researcher Tony Cornell published a couple of experiments regarding people noticing a subject wearing a ghost costume, and found that surprisingly few people did. He interpreted the results incorrectly, as evidence that a person in a sheet lacked the psi phenomena of a “real” ghost, but now we can look back and clearly see inattentional blindness at work.

There are a number of deeper neuroscientific questions about inattentional blindness. For example, exactly how does our brain focus its attention? If you are looking for blue marbles in a barrel filled with marbles of every color, where in the brain is the blue “flag” being applied? If you have ever played a video game for a long time where you are searching for and collecting something specific, you may have noticed that this flag might linger. You start to notice similar things in the real world, or continue to notice them in the video game long after the task has been completed. It’s as if your brain has flagged the item as important, so you notice it, and the flag has not been updated.

Further it seems that noticing one set of parameters comes at the expense of others. You notice the blue marbles partly by filtering out the red, green, yellow, and purple ones, or perhaps entirely by filtering out non-blue marbles. There is good reason to suspect the latter, because our brains filter out most of the sensory stimuli that comes our way. Our brains filter out anything not deemed important, which is most stuff. A strong filter already exists, so altering attention can simply be a matter of tweaking the filter.

This happens in the context of the phenomenon of “attention”, meaning that the brain can change not only what it is focused on, but how. You can consciously alter your filters, paying attention to anything happening in your environment, or focusing down on one tiny detail at the expense of everything else. This is why, in these experiments, generally people are directly told or indirectly influenced to focus their attention on something that is not the distracting stimuli. The more they are focused, the more likely the unexpected stimuli will be filtered out.

Here is another specific question – at what point in the sensory processing stream is the unwanted or distracting sensory information filtered out? That question is the focus of a recent study that was the trigger for this post. The researchers trained mice to respond to a tactile stimuli on the whisker on one side, but not the other, by licking. They would get a reward for which strongly reinforced the behavior. So called “expert” mice would consistently respond to stimuli on one side, but ignore stimuli on the other.

They then imaged the brain function of the mice while they stimulated their whiskers. They did so using a technique of “in-vivo widefield Ca²⁺ imaging”. They use transgenic mice who have calcium channels altered with a bioluminescent protein. When neurons are active they release calcium which binds to the receptors and causes them to light up. Cameras attached to the outside of the mice heads can see the bioluminescence. The wide field imaging systems can look at multiple brain regions at once. This is a way to image mouse brain activity in real time with high spatiotemporal resolution (better than fMRI).

The researcher found that when they stimulated the target whisker, the one the mice were supposed to respond to, the primary somatosensory cortex light up first and then this signal strongly propagated to other brain regions. This makes sense, the “primary” sensory cortex is the one that gets the signal first, does basic processing and then sends the information to secondary brain regions for higher-level processing. When the distracting whisker was stimulated the primary sensory cortex lit up as usual, but this signal was not strongly propagated to other brain regions. It was significantly attenuated.

So based on this result it seems that the filter for information flagged by the brain as distracting is in the primary sensory cortex, which decides somehow which information to send to the rest of the brain and which to not. Whether or not this is the identical mechanism in all cases of inattentional blindness remains to be seen, but it’s a start. If so this would also mean that we have no awareness of stimuli processed in the primary sensory cortex, and that conscious awareness only comes when that information is shared with other brain regions.

This is important to recognize. Getting back to the gorilla, it is not as if you noticed the gorilla and then ignored him or interpreted the stimuli incorrectly. It’s likely your brain filtered out the gorilla at the first cortical step, and you never had any conscious awareness of his existence. You were literally blind to the gorilla.

Of course, any complex neuroscientific question like this is not answered fully with one narrow study. This adds an interesting piece to the puzzle, but many more are necessary to build a picture of what is going on.