Sensory and Motor System

Sensory and Motor System

DOI: 10.4018/978-1-5225-4834-8.ch005


When we cannot see an object, it is often possible to detect its presence, to identify its origin and even to receive a message from it, precisely because we can hear it. In addition to having the ability to detect and locate sound, we are also able to perceive and interpret its nuances. We can immediately distinguish a dog's bark, the voice of a certain friend, a sea wave breaking. Because humans can produce a wide variety of sounds as well as listen to them, spoken language and their reception by the auditory system have become an extremely important medium of communication. Hearing in humans has evolved beyond the strictly utilitarian functions of communication and survival; in a manner analogous to artists who use visual means, musicians exploit the sensations and emotions caused by sound.
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Vision is a remarkable sense that allows us to detect things as tiny and close as a mosquito perched on the tip of our nose, or as immense and distant as a galaxy in the confines of the universe. Sensitivity to light enables animals, including humans, to detect prey, predators, and partners of the opposite sex. The success of the visual process requires that the light reflected by distant objects be in relation to the individual who sees and the environment that surrounds it; That identification of objects occurs based on their sizes, shapes, colors and previous experience; That the movements of the objects are detected and, finally, that the recognition of the objects is possible in the whole range of lighting conditions normally experienced by the individual in his habitat.

In objects, being absorbed, scattered, reflected and diverted. Due to the nature of electromagnetic waves and their interactions with the environment, it is possible for the visual system to extract information about the world. This is a huge task, which requires a lot of neural machinery. The progressive specialization of vision during vertebrae evolution, however, has brought surprising rewards. It allowed the emergence of new forms of communication, giving rise to encephalic mechanisms capable of predicting trajectories of objects and events in time and space and providing new forms of mental images and abstractions that culminated in the creation of the art world. The importance of vision must be duly certified by the fact that about half of the human cerebral cortex is involved with the analysis of the visual world.

The visual system of mammals begins in the eye. At the back of the eye is the retina that contains photoreceptors specialized in converting light energy into neural activity. The rest of the eye acts like a photographic camera, forming clear images of the world on the retina. Like a high-quality 35mm camera, the eye automatically adjusts to lighting differences and automatically focuses on objects of interest. The eye, however, has some features not yet available on cameras, such as the ability to follow moving objects (by eye movements) and to keep their transparent surfaces clean (through tears and blinking).

The axons of the retinal neurons meet in bundle in the optic nerves, which distribute the visual information, in the form of action potentials, to several encephalic structures that perform various functions. Some targets of the optic nerves are involved in the regulation of biological rhythms, synchronized with the light-dark daily cycle; Others are involved in the control of the eye's optic position o. The first synaptic station in the pathway that serves visual perception occurs, however, in a group of dorsal thalamus cells called the geniculate nucleus or NCL, from which the information ascends to the cerebral cortex, where it will be interpreted and remembered.

The Properties of The Light

The visual system uses light to form images of the world around us. We shall briefly review the physical properties of light and its interactions with the environment.


Light is the electromagnetic radiation that is visible to our eyes. Electromagnetic radiation can be described as a wave of energy, and as such has a wavelength, which is the distance between successive “peaks” or “valleys”, a frequency, which is the number of waves per second, and an amplitude, which is the difference in height between the top of the peak and the end of the valley of the wave.

The energy content of an electromagnetic radiation is proportional to its frequency. High frequency (i.e., short wavelength) radiations have higher energy content; Examples are gamma rays and X-rays, with wavelengths of less than 10-9 m (<1nm). On the other hand, radiations emitted at low frequencies (i.e., long wavelengths) have lower energy; Examples are radio waves and radar waves, which have wavelengths greater than 1 mm. Only a small part of the electromagnetic spectrum is detectable by our visual system; The visible light consists of wavelengths between 400 and 700 nm. As was first demonstrated by Isaac Newton, at the beginning of the eighteenth century, the mixture of sun-emitted wavelengths in this range is perceived by humans as 'white', whereas light of a single wavelength is perceived as A “warm” color, such as red or orange, consists of light with longer wavelengths and thus has less energy than a “cold” color, such as blue or violet. Undoubtedly, colors receive in the brain, themselves, their subjective attributes based on our experiences.

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