Can I have your attention please?

What can you see right now? You probably think you are seeing these words, the icons around the website and even the room you’re reading this in. But actually, you’re only attending to a tiny part of the scene in front of you (hopefully these words), and this is all that is reaches our awareness.

Although we perceive a rich, detailed world all around us, we can only process a few things at a time: we all suffer from ‘inattentional blindness’.  You will probably have had the experience of not hearing what someone is saying to you when you’re concentrating on the TV.  Many of you will have also seen the video in which you are asked to count how many basketball passes a team in white makes. If you haven’t, watch it now before reading on:

You’re so busy counting the team’s passes that you tend not to notice is that a gorilla walks across the screen halfway through. When this Invisible Gorilla video was first shown to participants by Simons and Chabris in 1991, only half the participants noticed the gorilla.  Countless examples of people’s failure to perceive obvious events have been shown.

This occurs because there is a limit to brain processing: there is a ‘bottleneck’ somewhere in the system at which point unimportant signals (things you’re not attending to) are filtered out and not processed further.  This selective attention sounds like something evolution should have fixed so you don’t ignore something vital. But the beauty of this system is vital things aren’t ignored because these are the items that get past the bottleneck.  The things that are attended to are likely to be what is most important for survival (or for keeping up with the plot of your favourite TV show), so it is good we preferentially process these and don’t waste energy on things that don’t matter.  If the gorilla had been looking threatening or been bright red, you wouldn’t have missed it, but it was unimportant to your task so you didn’t see it.

However, though useful generally, this inattentional blindness is dangerous in some situations, such as driving.  If you’re concentrating on doing something else like talking on the mobile phone, you might not notice a danger in the road.  You may have seen the road safety advert warning to look out for cyclists, which used a variation of the invisible gorilla experiment to show the unreliability of our attention system.

So our attentional deficit can be detrimental.  But this is nothing compared to extreme attentional deficits caused by brain damage. For example, after a stroke affecting the right side of the parietal cortex (a brain area that integrates various sensory signals) some people develop hemispatial neglect. People with neglect are just not aware of the contralesional side of things (the side opposite to the brain damage, so normally the left).  For example, they’ll only write on the right hand side of a page, put make up on only the right side of their face and eat from the right of a plate (a deficit that can also be seen in dogs).  This isn’t a visual problem: their eyesight is fine, and some patients imagine only the right side of things, seeing only the right side of any dreams, hallucinations or even memories they have. They just seem unable to attend or orient to the left.

However, it is important to ask what they can’t see the left of. Some patients can’t see any of the objects to the left in a room, whereas others can see all objects in a room but can’t see the left hand side of any of the individual objects.  And ‘left’ can be very subjective for patients.  Amazingly, some patients may correct an inverted or slanted object and ignore the left side of the corrected image.  For example, if shown a mirror image of a map of the world, some ignore the western hemisphere, despite the fact it is on the right, because they have corrected the map of the world in their mind. There is clearly some processing of the left hand side going on, but what is reaching consciousness becomes distorted.

It is almost unimaginable what it must be like to have attentional deficits like hemispatial neglect, but they are fascinating disorders that can tell us how weird and wonderful the brain is and the devastating consequences when something goes wrong. However, even without brain damage our mind is not infallible. We only consciously perceive a tiny proportion of what is in the world, and though useful for efficient processing, this inattentional blindness is not without consequences.