A simple piece of tangled string lying in the corner of a room catches the eye, makes the muscles tighten and the heart beat faster of those who suffer from arachnophobia, an exaggerated fear of spiders. As strong as it is uncontrollable, this sensation invades the body every time people notice some being or object in the environment that reminds them, however vaguely, of spiders with their long legs and generally reserved behavior. Were it not for the terror that it instills, it would be a natural reaction to any circumstance that places survival at risk and prepares the body for fight or flight. A recent discovery by the Brazilian neuroscientist Luiz Pessoa helps one to understand why, in cases of phobia or other disturbances associated with high levels of anxiety, such as generalized anxiety or post-traumatic stress, objects to which we should be indifferent act as a type of magnet that we are unable to take our eyes off.
In these situations, a deep, almond-shaped region in the brain – the amygdala, which is responsible for processing emotions such as fear and aversion – becomes more active and increases the functioning level of the visual cortex, where images are decoded. In other words, it is not the entangled piece of string that draws the attention of the arachnophobe, but it is the person who becomes more aware of looking for anything that resembles a spider. “This apparently desirable effect [being aware of danger] may in fact be transformed into mental distress, because it occupies a substantial part of the cerebral processing that keeps the individual from focusing his or her attention on other activities,” explains Pessoa, head of the Cognition and Emotion Laboratory at the University of Indiana, in the United States.
Pessoa and two researchers from his laboratory, Seung-Lark Lim and Srikanth Padmala, identified this role of the amygdala in experiments with 30 healthy individuals. In a series of tests, they presented each participant with an ultrafast sequence of photos, comprising three types of image: shuffled black and white squares; a face; and a scene (a house or a building). Each battery of tests lasted two seconds, during which time the images appeared in random order, for only 100 milliseconds. At the end of each sequence, the person watching had to identify the face (Andy, Bill or Chad) and what type of building was shown. Detail: the image of one of the three faces always appeared between 200 and 500 milliseconds before the photo of the scene.
Pessoa and his team in Indiana already knew from work done by other groups that the participants’ tendency in this type of test is almost always to identify the first target-image – in this case the face – and not to register the second. Jane Raymond, Kimron Shapiro and Karen Arnell, the Canadian researchers who described this phenomenon in 1992, called it “attention blink,” a type of lack of attention or momentary blindness – the person sees the image, but does not register it; it is as if they had blinked.
Therefore, the researchers from Indiana decided to make the test more sophisticated in order to investigate how emotion influences behavior. Instead of just presenting the sequence of images while they recorded brain activity using a nuclear magnetic resonance device, they included a new phase: in an initial awareness-building stage, the participants were given a very slight electric shock, the strength of which they themselves controlled, every time the photo of the house or building appeared (the second target-image). “Because it’s aversive, the shock adds an emotional component to the experiment,” explains Pessoa.
The participants who got the electric discharge, associated with the image of the house during the awareness-building exercise, identified it 72% of the times it was shown to them, while they only saw the building in 62% of the cases. Those who got a shock when they saw the building started identifying it more frequently than they noticed the house, according to an article, published on-line in September in Proceedings of the National Academy of Sciences (PNAS). It was already known that people have a better memory and more visual perception of images that have some emotional content. What it was necessary to discover, however, was why this happened.
By monitoring the functioning of the brain during the tests, Pessoa’s team saw that the emotional stimulus increased activity levels of the amygdala – especially the right amygdala – and of the visual cortex. “There was a subtle increase in the functioning of the amygdala and the visual cortex,” explains the Brazilian neuroscientist, who has been living in the United States for 10 years. “Though small, this increase was measurable and strong enough to modify behavior.”
Signs that this processing of images with an emotional content goes first through the amygdala led Pessoa’s group to conclude that the stronger activation of this almond-shaped region of the brain amplifies the functioning of the visual cortex.
A more active visual cortex, in turn, favors identification of the visual signs of danger in the environment. Maintained throughout the evolution of various species – including human beings – this neuronal circuit possibly favored survival under adverse circumstances, comments the Brazilian researcher. In some cases, however, this protective system may turn against the person it should protect. “As the amygdala is more active in people with anxiety problems, like fear of spiders,” comments Pessoa, “it’s probable that they more easily identify images in the environment that would go unnoticed by others.”
LIM, S. L. et al. Segregating the significant from the mundane on a moment-to-moment basis via direct and indirect amygdala contributions. PNAS. In press.