You board the train, find a seat and open the latest bestseller by your favourite author. The couple sitting opposite are having a conversation, and the driver announces that there will be a short delay to your journey, but you are so engrossed in your book that you are unaware of these sounds. In fact, you have become almost completely oblivious to your surroundings, and you fail to notice that the train is approaching your stop. You reach the end of a paragraph and, looking up from your book, see the train pulling out of the station…

Everyday experiences like this show us that focused attention has a significant effect on how we perceive the world and, therefore, on what enters into our conscious awareness. This has also been confirmed in the lab, a particularly striking example being the "Invisible Gorilla" experiment, by psychologists Dan Simons of the University of Illinois and Chris Chabris of Union College, New York.

First performed in 1999, this demonstrates a phenomenon called inattentional blindness, whereby focused attention causes a failure to see something that might otherwise be glaringly obvious.

"I was curious whether people could miss an unexpected object that was fully visible," says Simons. "We chose a gorilla because we wanted something dramatic, something that if people missed it, they would be surprised by how visible it was ... but we weren't sure if the effect depended on the unexpected object being hard to see. As it turns out, people missed the fully visible unexpected object far more than we expected."

More recently, Simons and Chabris investigated inattentional blindness in a real-world situation. They were inspired by the case of Kenneth Conley, a police officer in Boston who ran right past a vicious beating while chasing a suspect, and claimed not to have seen it. During his trial, the jurors assumed Conley must have been lying, and he was convicted of perjury and obstruction of justice.

Simons and Chabris simulated the scenario to test Conley's claim. They asked a group of participants to follow a jogger through a park, and staged a fight along the route. Some of the participants were also asked to watch the jogger closely and to count how many times he reached up and touched his hat.

Their findings were just as remarkable as those of the invisible gorilla experiment: at night, just one third of the participants said they had noticed the fight, while just over one half noticed it when the experiment was conducted during the day. Crucially, the researchers found that participants were far less likely to notice the fight when they were focusing their attention on how many times the jogger touched his hat.



There's plenty of other evidence for these phenomena, but most of it shows how attention affects visual perception. Another new study now establishes the auditory equivalent, referred to as inattentional deafness. The research, carried out by Nillie Lavie of the Institute of Cognitive Neuroscience in London and her PhD student James MacDonald, now at the University of Oxford, shows that focusing one's attention on a demanding visual task can lead to a failure to hear an otherwise obvious sound.

Lavie and MacDonald showed their study participants cross shapes on a computer screen. Each cross had one green and one blue arm, and one of the arms was slightly longer than the other. The participants were asked to indicate either which arm was blue, or which arm was longest, using the keyboard. This second task was slightly more difficult than the first, because the participants had to pay closer attention to see the subtle difference between the length of the cross arms.

The aim of the study was to investigate how manipulating what is called perceptual load can affect awareness of an unexpected sound. The participants wore headphones throughout, which, they were told, would help them concentrate on the task. During some trials, an audible tone was played through the headphones, either amidst white noise or on its own, and afterwards the participants were asked if they had noticed it. The researchers found that they were far less likely to hear the tone while performing the more difficult visual task, even when it was played on its own.

"Perceptual load corresponds to the amount of information that needs to be processed in a task," Lavie explains, "and the extent to which processing the task information fills up the senses. It corresponds roughly to task difficulty, because a more difficult task is likely to load more on the senses." This may not always be the case, however: "You may be engaged in a computerised task that is not too difficult but loads up your vision with many different visuals on the screen."

These findings have obvious and important implications for everyday life. Texting on your mobile phone while crossing the road, for example, might deafen your ears to the sound of an approaching car. Similarly, focusing your attention on the sat nav or on a passing billboard might make you unaware of the sound of a car horn or cyclist's bell. In their paper, Lavie and MacDonald note Transport for London's Cycle safety campaign, which alerts drivers to the dangers of inattentional blindness, and suggest that it should be extended to raise awareness of inattentional deafness as well.

"The question of whether a certain sound or sight is noticed depends on the strength of the signal versus the level of 'noise' around," says Lavie. "If you put a glaringly obvious signal among lots of noise, people may still fail to notice it if their attention is occupied with a high information load. This suggests it is possible that if a car horn was played among other noises in conditions of high visual load ... [it] would not be noticed either."

Although inattentional blindness and deafness can often be unwanted consequences of focused attention, they also have their benefits. They can, for example, enable us to avoid distraction by ignoring irrelevant sights and sounds, such as pop-up ads or noisy building work near the office. "A lot of my research concerns these positive implications," says Lavie, "which should be beneficial for learning and for greater productivity in the work place."

As well as having practical implications, studies of inattentional blindness and deafness provide insights into the brain mechanisms underlying attention. Lavie and MacDonald's results suggest that attention has a limited capacity that is shared between vision and hearing. Further evidence for this idea comes from a 2005 study which used a variation of the invisible gorilla method. Participants engaged in a complex visual search task not only failed to see a woman entering the scene, but also failed to hear her scratching her fingernails down a chalkboard.

It follows that attentional capacity may also be shared between other senses, such as smell and touch. If so, then tasks in which participants are required to discriminate between different smells or textures could also use up the capacity, leading to inattentional blindness or deafness. Lavie thinks that these possibilities are interesting directions for future research, and points out other studies showing that focusing attention on a complex visual task can diminish the sensation of pain.

"I see no reason why the same principles wouldn't apply to other [sensory] modalities as well," says Simons. "Once you use up cognitive capacity, you don't tend to notice unexpected events, regardless of the modality." But as far as he is concerned, our failure to notice otherwise obvious sights or sounds is not the most important issue. "The problem," he says, "is with our mistaken intuitions about what we will notice."

As part of the invisible gorilla study, he and Chabris conducted a national survey in which 1,500 American adults were asked about their intuitions. The results, published last week, show that more than three quarters of respondents firmly believed that they would notice something unexpected when it enters their field of view, even when they are paying attention to something else. Inattentional blindness and deafness show that this belief is false, and clearly demonstrate what Simons and Chabris call "the illusion of attention."

References

MacDonald, JSP and Lavie, N (2011) Visual perceptual load induces inattentional deafness. Attention, Perception, and Psychophysics 10.3758/s13414-011-0144-4

Chabris, CF et al. (2011). You do not talk about Fight Club if you do not notice Fight Club: Inattentional blindness for a simulated real-world

assault. i-Perception dx.doi.org/10.1068/i043

Simons, DJ and Chabris, CF (1999) Gorillas in our midst: sustained inattentional blindness for dynamic events. Perception 10.1068/p2952

Simons, D., & Chabris, C (2011) What people believe about how memory works: a representative survey of the US population. PLoS ONE 10.1371/journal.pone.0022757