The achievement of information on the environment through a sensory pathway is called perception. In the case of the human sight, the brain recognizes instantly a familiar object,
from any angle and any distance.
A book looks like a book, no matter if it is positioned vertically, horizontally or an edge. Still, the size and shape the book can form on the human retina vary a lot. For example, if we keep a closed book near the eye, the produced image gradually reduces, being narrower in the rear part than the front part. But we can see its rectangular shape, as the brain adapts automatically to varied images produced by the book. This phenomenon is called perceptive constant. The process could be associated with the inferior temporal lobes of the cortex, as the area has been found to be involved in hallucinations.
The fact that we automatically perceive shapes and forms is proved by the drawings of the toddlers, devoid of perspective: they do not draw what they see but what they know to be there. A book will be drawn rectangular, no matter its angle. To perceive whole objects, no matter how they are presented, training is required. To make three-dimensional images, the brain needs information about the distances where the objects are found. These data are achieved through varied meanings, called visual indicators.
One of the most important indicators is the binocular disparity, the difference of sight between the two eyes. Movement parallax is another depth indicator. If the head is moved from one side to the other, the nearby objects move more in the image projected on the retina, than the remote ones. Interposition is also important. If an object partially shadows another, we will know it is closer to us.
Perspective indicators also offer information. Most natural surfaces (pastures, sand dunes, trees) are textured. The texture disposal, like the gradual movement of the sands as they come off, delivers a "reference frame" allowing us to perceive the distance where any object in the frame is located.
In man-made environments, the convergence of the parallel lines, as they come off (like railway lines), is what gives us the sensation of depth.
Perceptive indicators are usually efficient, even when ambiguous. If railways form a convergence on the retina image, we see them as parallel convergent lines. But the same image on the retina can be formed by the contour of a church steeple, for example. We are rarely conscious of these alternative interpretations, but brain mechanisms make the proper interpretation.
Interpretations frequently chosen by the perception mechanisms of the brain are those of right angles and right lines, and this is the level where illusions can appear, by creating models meant to give the same retinal images like familiar objects, but on very different forms.
The same sensory model can deduce two different things, equally valid, like the drawing representing a cup and two human faces at the same time. The two images alternate in our perception, and we cannot exert any conscious control, even if the information does not vary.
Once the brain formed a preliminary interpretation of the information offered by the senses, this will determine the way the entire sensory model will be interpreted. What happens is that the brain chooses to interpret an object relying not only on the information delivered by its sensory system, but also by what surrounds the objects. We will perceive the object the way we know it from experience.
In other cases, identical shapes are perceived as different because the brain selects the most probable variants, rejecting the others that do not have much logic to it.
Perceptive learning and long experience in making subtle perceptive distinctions allow an expert to make a correct discreetness. For example, an experimented ornithologist can recognize a bird species by one glance, while a beginner may be incapable of recognizing it or sighting it at all.
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