Photo: Museum of Fine arts of Lyon
A series of paintings by Claude Monet depicting the cathedral of Rouen. Each version is lit very differently depending on whether the artist has painted the cathedral in the evening or in the morning, or even at a different time of the year.
The position of the light source, the perspective, the constancy of the nature of the objects despite the color changes, the constraints in perception related to the distance at which we admire in a work are all clues that the brain uses to make us understand the scene that is offered to us. The figurative painters had obviously understood these principles, because, by their technical work, they make us live an experience that is very vivid in the visual world. “Their paintings are a testimony to the key of the way in which our brain decodes the visual world “, summed up neuroscientist Peggy Gérardin Group cortical architecture, coding and perception of the national Institute of health and medical research (Inserm) and the University of Lyon in France.
When we look at a scene, the light it emits stimulates the retina at the back of the eye before it reaches the brain in the form of electrical pulses that encode the visual information. The latter contains various clues that will help the brain to deduce what we see. “The brain must then accomplish a lot of work to understand this visual information. According to the model of the predictive coding developed by neuroscientists, the brain relies on internal representations that allow it to quickly make assumptions, predictions [what he sees] that he continuously compares it with the sensory information coming in. These comparisons may sometimes generate errors of prediction if the internal representation is not in line with the external stimulation, ” says Ms. Gérardin. “It is often said that one must see it to believe it, but in fact it is rather the reverse that is happening. We tend to see what we believe, what our brain has deduced, that is to say that there is a kind of anticipation on what we see. We’re constantly making inferences about what we see. And this is so not only in vision but also in audition and several other sensory aspects. “
“Visual illusions are a reflection of our internal representations and points of view that the brain imposes on us. Several are due to the fact that we see what we believe in the beginning, ” noted neuroscientist.
The brain develops its predictions from various visual cues as well as with certain rules to discern the invariants in a changing environment.
The constancy colorful
The colours of our environment are constantly changing whether from morning to evening, or depending on the weather and even the season. However, we recognize that it is the same landscape, the same monument or the same object. The example par excellence of this phenomenon in painting is the series of paintings by Claude Monet depicting the cathedral of Rouen. Each version is lit very differently depending on whether the artist has painted the cathedral in the evening or in the morning, or even at a different time of the year. “This series shows us that despite bright and colorful changing, our brain applies a degree of consistency in our way of seeing things so that we are not influenced by the effect of light on the color of the object. As well, we will see as very different from an orange that is illuminated by a blue light like that of day or by a more yellow light like that of the evening, and yet there does not arise the question of whether it is an orange because we have a kind of internal representation of the orange, ” says Ms. Gérardin.
The assumption of the light
Our brain has built that the light was coming mostly from above, as light from the sun comes from above, ” says dr. Gérardin who is interested in the brain regions that deal with this assumption of light from above to infer the forms. A visual illusion typical example of this a priori that raises the brain is the fact that we see the same objects with concave or convex depending on the position of the light source. For example, hieroglyphs hanging on the picture rails of the Musée des Beaux-arts in Lyon, where the lighting comes from the ceiling appear concave to the visitors. If one rotates the picture of the plate of hieroglyphics so that its top and the lighting come from the bottom, the hieroglyphs appear to us then convex.
Modeling of the face
Photo: Museum of Fine arts of Lyon
From the Renaissance the development of linear perspective made it possible to represent even more clearly the third dimension on a table in two dimensions.
The clues provided by the light are also very important in our perception of faces. If we get to see the faces in a painting in three dimensions, it is thanks to the shadow effects generated by the light reaching the faces. “We have a priori very strong on the fact that a face must always be three-dimensional. A flat face has no meaning, this is why we don’t get to see the inside of a mask as it really is, that is to say concave, because our brain does not understand it and that he absolutely wants to see a face in three dimensions in the right direction, ” says Ms. Gérardin.
Effect of depth
The convergence of our two eyes, each of which detects a two-dimensional image, participates in the effect of depth that we perceive. But it is not a question of the fundamental index. The light and perspective are two other cues used by the brain to estimate the depth of a scene or an object, ” says the neuroscientist. “Knowing that the light comes from above, the way in which it is distributed in the scene is a very important cue for the brain. Curiously, an object or scene appear to be more depth and more volume when they are illuminated from above but from the left rather than the right. The brain interprets it as being more in-depth than if they were illuminated by the right. The painted portraits from the Renaissance are mostly lit by the top left hand corner, which probably means that the painters had understood this trick. “
From the Renaissance the development of linear perspective made it possible to represent even more clearly the third dimension on a table in two dimensions. “It took geniuses to achieve this great step forward,” stresses Peggy Gérardin.
The direction of the shadows is not at all important for the brain, says neuroscientist. “There are paintings where the shadows are physically impossible, and yet that we do not interfere when we contemplate the work because we don’t get to see spontaneously that there was an error. The fact not to pay attention to any inconsistencies physical means that the brain performs a processing representative, he attends only some of the clues that he has learned to recognize as being relevant to decipher a scene. It is precisely these relevant indices which are of interest to neuroscientists. “
Constraints in perception
The constraint of the distance in relation to the work is also very important because it will affect the way the brain will understand what is in front of him.
For each point in the visual field we have a representation of this point in the brain that passes through the retina. The cells of the retina distinguish contrasts, like the black-and-white, which is the strongest, but also the blue-yellow and red-green. The brain tries to organize these contrasts in order to know if it is the same object or different objects. Certain cerebral areas include the points that are more or less the same color, ” says Ms. Gérardin before making a reference to the pointillists, who illustrate well this type of treatment. “Their table consists of small dots with different contrasts. When one looks at the painting from a distance, our brain combines the colors between them, while when one approaches the work, one realizes that these are small points different from each other yet in a coherent way, forming a whole at a certain distance. “The constraint of perceptual distance is therefore very important for the brain that will attach to the details of short-distance and contrasts the more coarse, allowing us to see the overall shape at a greater distance.
The techniques used by painters since the Renaissance bear witness to their vast knowledge of our visual perception, and for this reason, they represent a great source of discovery for neuroscientists.