Can a gorilla go completely unnoticed in the midst of a crowd of human beings? Yes, as shown by an experiment carried out in 1999 by the psychologists Chabris and Simons. Blame it on a phenomenon called “inattention blindness”.
In the summer of 1999, in a Harvard University classroom, Christopher F. Chabris and Daniel J. Simons of the University of Illinois were about to perform a small experiment on their students in psychology. Chabris asks them: “Please watch carefully the video that I am going to show you now (see On the Web). You will see two basketball teams, one in a black jersey, the other in a white jersey, playing a game. Focus on the players in white and count all their passes. The students then do their best: the ball flying from one side of the screen to the other is difficult to follow with their eyes. But most of them feel that they managed to count all the passes.
50% of subjects do not see the gorilla
At the end of the screening, Chabris asks them questions: “And what did you think of the gorilla? The young people do not answer, perplexed. At that time, the two teachers were studying a phenomenon already known in neuropsychology, but for which there are still many gray areas: inattentive blindness. Watching the video again, the students are indeed amazed to see that an individual disguised as a gorilla arrives in the middle of the field halfway through the film, stops, repeatedly hits his chest, before exiting. the scene on the other side. This gorilla test, with its many variations, is one of the most famous psychology experiments of recent years. According to Chabris and Simons, who continued their experiments with more subjects of varying ages, about half of the observers do not notice the animal, although its appearance is unusual to say the least. How is it possible ?
Forgetting your child in the car …
Inattentive blindness is the inability of a subject to notice an unexpected stimulus that arises in his visual field, due to the fact that he is already busy with a task that requires all his attention. A variant of this phenomenon is the cognitive tunnel: a person is absorbed by a manual action or their thoughts, and not paying enough attention to their surroundings. According to some neuroscientists, cognitive tunnels are the cause of many accidents, such as abandoned children in vehicles, due to cognitive overload that prevents the brain from processing all information.
In 1998, Arien Mack and Irvin Rock, psychologists at the University of California at Berkeley, had already used the term inattentive blindness before Simons and Chabris. And the gorilla test is just a more advanced version of an experiment conducted in 1975 by Ulric Neisser, an American cognitive psychologist. The latter then proposed a cognitive model — Human Information Processing — where the brain produces information and receives it from outside, before actively selecting what is relevant for the pursuit of its goals.
According to this model, then supplemented and deepened by other researchers, external stimuli, particular or unexpected, must capture our attention and then reach our consciousness. There are therefore two stages. Inattentive blindness occurs when something interferes with moving from one stage to the next. Three situations are possible. In the first, the stimulus is too weak: it has been perceived, but it does not reach consciousness. In the second case, our implicit memory, the memory of analogous situations, precedes conscious perception and interferes with the actual stimulus. This is what happens when we perform routine tasks, automatically, when the presence of an unusual element should prompt us to act differently. Third possibility: an unexpected stimulus is sometimes not perceived at all, as in the case of the gorilla test.
Intentions, needs and expectations influence what we perceive
Psychologists have always wondered if our intentions, needs and expectations influence what we perceive. An important question to understand how we explore and interact with the environment, but also to determine how much we can trust our memory, especially when it comes to reproducing an event that we have witnessed. As early as 1890, psychologist William James announced that the capacity of our consciousness is limited because we are unable to pay attention to everything that is happening around us. James distinguished, long before it was possible to study the areas of the brain involved in attention, an exogenous attention, triggered by an intense scent or a visual stimulus that detaches itself from its surroundings, like a woman dressed in red surrounded by men in black, and endogenous attention, produced by focusing on a given stimulus. In the first case, the phenomenon starts from the bottom: the stimulus is strong enough to take precedence over the others and to attract our attention. In the second case, the process starts from the top: we are the author.
In 1985, a team from the American Institute of Mental Health, led by Robert Desimone, discovered that in the visual cortex of monkeys, in particular in the area V4 dedicated to color perception, certain neurons were activated more. when the animal stared at a colored object than when it was content to see it without concentrating on it. The attention we pay to a stimulus therefore influences how we perceive it or how we identify its shape, color, etc.
Neurosciences have solved another difficulty, which partly explains inattentive blindness: how do you distinguish an object as a whole? Distinct regions of the cortex perceive its color, shape and size. In order to be aware of this, all of this information must be integrated. Christoph van der Malsburg, from the University of the Ruhr, Germany, assumes that there is a synchronized activity of all the neurons which perceive the same stimulus: the cells then form a kind of “assembly”, the importance of which is has been demonstrated by other researchers. Nobel laureate Francis Crick and neuroscientist Christof Koch argued in the early 1990s that only signals that came from assemblies of neurons were powerful enough to reach consciousness: all others were seen only in fragmentary fashion.
Our expectations also affect awareness. When a person focuses on certain sounds in music, the neurons associated with the perception of these pitches are more active than others. It is thanks to this mechanism that we succeed in following a conversation in a very noisy restaurant: what we expect to hear — the response of our neighbor to our question, for example — influences what the we actually hear and allows us to partially exclude other sounds. The phenomenon of inattention therefore manifests itself with the other senses.
Another process explains inattention blindness. An individual who has to perform two cognitive tasks — for example identifying green circles and black crosses that appear randomly among letters — tends to not see the second stimulus if it appears too soon after the first. Thus, if the black crosses arrive between 200 and 300 milliseconds after the green circles, they do not reach the subject’s consciousness, although they have been seen. This interruption of attention is very important in situations of attentional overload and effectively “cancels out” part of perceived reality.
Brain synchronization
What are the areas of the brain responsible for inattentive blindness? Although environmental awareness requires the activation of many regions, only a few of them are involved in this phenomenon. According to some studies, it is a network that encompasses part of the frontal cortex and the parietal cortices up to the amygdala. This last structure, a crossroads of emotions, explains why our emotions modify our perceptions. And it is likely that the frontal cortex and the amygdala either facilitate or inhibit the synchronization mechanisms of neurons. If a stimulus matches what you expect to see, the input signals to this network are reinforced. Conversely, a “jarring” stimulus associated with predictable events is often suppressed, as is the case in the gorilla test.
Neuronal synchronization therefore makes it possible to put some order in the information that the brain receives; if these all reached the consciousness, it would quickly cause a short circuit of the system. The gorilla test shows that the brain is not passive vis-à-vis its environment: it selects, chooses and reinforces the elements it wants to perceive.
