The visual cortex is a fantastical portion of the brain, and one which is enlarged in our species to the extent that we can use our eyes to make sense of the world around us in amazing detail. We notice colours, shapes, motion, direction, and even three dimensions, allowing the brain to generate a coherent percept of the visual scene. Many primates like ourselves are extremely visual animals, and rely on eyes not only to inform about the world around us, but to warn of impending danger or alert about opportunity.

Despite the amazing resolution and sophistication the visual system has, what could be argued as one of its most interesting features is a mechanism of noise filtration in which the brain effectively ignores irrelevant information it receives, even resulting in features in the environment being completely deleted from the scene a person sees. One of the most familiar examples of this is that you can’t see your own nose when you look at a scene. The position of the nose means it should take a commanding, even blocking position in the visual field, and prevent us seeing objects in front of it. However, we never see the dark shadow of our nose when we look around. This is because the brain filters out the stimulus. Instead, it seems the scene is ‘filled in’ where the nose should be with what the brain ‘expects’ to see- the nose is there all the time, but rarely provides anything informative, so can usefully be ignored.

Another interesting way in which what we don’t see can reveal more about how visual processing works than what we do see comes from the phenomenon of ‘inattentional blindness’. One would imagine, that when a person is concentrating intensely on a task which involves vision, that they would be more observant. It seems, the opposite is the case, and they are in fact much more likely to miss obvious features in a scene presented right in front of their eyes. A famous example is what happens when subjects are shown a video of a basketball match, and are asked to count the number of passes that happen during a game sequence. During play, a person dressed in a gorilla costume crosses the shot. When asked to report on what they saw, a 1999 study showed subjects could report the number of passes observed, yet, incredibly did not report seeing the gorilla if asked whether they noticed anything unusual about the video. In fact, people appear flummoxed when they are told the gorilla featured, and are astounded when they watch the video back, knowing that it will appear.

More recently, in a study (published 17th July, 2013) at Brigham and Women’s Hospital, researchers have probed the ‘inattentional blindness’ phenomenon further. It seems that even experts in a field, when carrying out a routine task in their line of work, cannot notice the obvious, or even the absurd!

In the investigation, 24 radiologists were asked to examine scans of patients’ lungs and search for nodules which were perhaps indicative of pathology (namely, carcinoma). They examined five scans with an average of ten nodules each, and on the last scan, an image of a gorilla, 48 times larger than the included nodules also appeared on the lung surface. Despite its obvious presence, the great majority (83%) of radiologists tested did not report seeing the gorilla.

Of course, it could be argued that this is down to radiologists being over-worked, or highly stressed, or, simply, incompetent. This is perhaps a ‘blinkered’ view of the situation, but one that must be considered. Alternatively, and in my opinion, far more intriguingly, this study possibly indicates something more eye-opening (excuse the pun!). Perhaps what we perceive is a lot more influenced by what we are searching for, and what we expect to see than we might think, and that, far from seeing an objective representation of the world around us, the visual cortex unconsciously makes decisions to include or exclude things in the environment that it regards as more or less important to achieving a task. Even when an observer is experienced, or even expert at a task, spotting what is obvious might be a challenge for them, if it is very far from what they expect.

The idea that the brain lets us see a perhaps biased view of reality, and we see only what we expect to see is perhaps a sinister one, but it is perhaps a more potent mechanism of sensory noise filtration than was thought. In evolutionary terms, it is a waste of energy to pay attention to things which provide no relevant information, and so, more successful individuals are those who can selectively filter stimuli which provide information to enhance survival.

What is clear, is that being attentive certainly does not equate to being observant. Further, no matter how experienced you are, and no matter how hard you are concentrating, being unable to spot what is right in front of you is harder than you might think! Perhaps, concentration and the ability to observe the unexpected are negatively correlated. It might be useful to ask whether those with attention deficit disorders, who are unable to focus on a task, are more likely to notice the gorilla. This is entirely speculative, but it is without doubt something worth investigating.

Citation: 'If you're not looking for it, you probably won't see it: Brigham and Women's Hospital study examines inattentional blindness in expert observers'- Press Release, 19th July 2013.