What are the senses?

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Humans process information using at least five sensory modalities: sight, sound, taste, smell, and the body senses, which include touch, temperature, balance, and pain. Because people’s sensation and perception of external stimuli define their world, knowledge of these processes is relevant to every aspect of daily life.
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Introduction

Humans have five sense organs: the eyes, the ears, the taste buds, the nasal mucosa, and the skin. Each sense organ is specialized to intercept a particular kind of environmental energy and then to convert that energy into a message that the brain can interpret. Together, these two processes are called sensation.

The first step of sensation, the interception of external energy, is done by the part of the sense organ that is in direct contact with the environment. Each sense organ has a specialized shape and structure designed to intercept a particular form of energy. The second step, conversion of the captured energy into signals the brain can understand, is done by cells inside the sense organ called receptors. Receptors are structures to which physicists and engineers refer as transducers: They convert one form of energy into another. Artificial transducers are common. Hydroelectric plants, for example, intercept flowing water and convert it to electricity; then appliances convert the electricity into heat, moving parts, sound, or light displays. Receptors are biological transducers that convert environmental energy intercepted by the sense organ into neural signals. These signals are then sent to the brain, where they are interpreted through a process called perception.

The eye, the best understood of all the sense organs, consists of a lens that focuses light (a kind of electromagnetic energy) through a small hole (the pupil) onto a sheet of cells (the retina). The retina contains the eye’s receptor cells: the rods, which are sensitive to all wavelengths of light in the visible spectrum, and three kinds of cones, which are sensitive to those wavelengths that the brain perceives as blue, green, and yellow.

The ear funnels air pressure waves onto the tympanic membrane (more commonly known as the eardrum), where vibrations are transmitted to the inner ear. In the inner ear, receptors called hair cells are stimulated by different frequency vibrations; they then send signals to the brain which are interpreted as different pitches and harmonics.

Taste buds are small bumps on the tongue and parts of the throat that are continuously bathed in liquid. Receptors in the taste buds intercept any chemicals that have been dissolved in the liquid. Molecules of different shapes trigger messages from different receptors. Humans have several kinds of taste receptors that send signals the brain interprets as bitter, at least two kinds of receptors that send signals interpreted as sweet, and one kind of receptor each that sends signals interpreted as salty and sour.

The nasal mucosa, the organ responsible for one's sense of smell, is a layer of cells lining parts of the nasal passageways and throat; it intercepts chemicals directly from inhaled air. Apparently, cells in the nasal mucosa can produce receptor cells (called olfactory receptors) throughout life. This way, people can develop the capacity to smell “new” chemicals that they could not smell before. New olfactory receptors seem to be created in response to exposure to novel chemicals, analogous to the production of antibodies when the immune system is exposed to foreign material. Because of this ability to create new olfactory receptors, it is not possible to list and categorize all the different types of smells.

The skin is the largest sense organ in the human body; its sense, touch, actually consists of several different senses, collectively referred to as the cutaneous senses. Receptors called mechanoreceptors are triggered by mechanical movements of the skin and send signals that the brain interprets as vibration, light or deep pressure, and stretching. Thermoreceptors intercept heat passing in or out of the body through the skin; their signals are interpreted by the brain as warmth and cold, respectively. Receptors that are triggered when skin cells are damaged are called nociceptors; their signals to the brain are interpreted as pain.

Animal Senses

Some animals have sense organs that humans do not and can thereby sense and perceive stimuli that humans cannot. Many birds and probably a variety of marine creatures can detect variations in the earth’s magnetic field; some fish and invertebrates can detect electrical fields. Other animals have sense organs similar to, but more sensitive than, those of humans; they can intercept a broader range of energy or detect it at lower levels. Insects can see ultraviolet light, while pit vipers can sense infrared light. Elephants can hear infrasound, and mice can hear ultrasound. The olfactory sensitivity of most animals far surpasses that of humans. Because of differences in sensory apparatus, each animal experiences a different sensory reality; this is termed each animal’s Umwelt.

Bioengineering Uses

One application of the knowledge of sensory modalities is in the field of bioengineering. Knowing that sense organs are biological transducers allows the possibility of replacing damaged or nonfunctional sense organs with artificial transducers, the same way artificial limbs replace missing ones. Today’s most advanced artificial limbs can be connected directly to nerves that send information from the motor (movement) areas of the brain; thus, a person can direct movement of the artificial limb with neural messages via thoughts. Similarly, bioengineers are researching the use of small sensors that can be set up to send electrical signals directly to a person’s sensory nerves or the sensory cortex of the brain. Researchers have already developed the first version of a hearing aid to help people who have nerve deafness in the inner ear but whose auditory processing centers in the brain are still intact.

Human Factors Engineering

Another field that applies the findings of experimental sensory psychologists is called human factors engineering. People who design complicated instrument panels (for example, in jet cockpits or nuclear reactors) must have an understanding of what kinds of stimuli will elicit attention, what will be irritating, and what will fade unnoticed into the background. Using knowledge of how sound is transmitted and how the human brain perceives sound, human factors engineers have designed police and ambulance sirens that make one type of sound while the vehicle is moving quickly (the air-raid-type wailing sound) and another while the vehicle is moving slowly, as through a crowded intersection (alternating pulses of different pitches). These two types of sounds maximize the likelihood that the siren will be noticed in the different environmental settings. Research by human factors engineers has also prompted many communities to change the color of fire engines from red to yellow; since red is difficult to see in twilight and darkness and bright yellow can be seen well at all times of day, yellow makes a better warning color.

Research by human factors engineers and environmental psychologists is also used to improve commercial products and other aspects of day-to-day living, answering questions such as: How loud should the music be in a dentist’s waiting office? What color packaging will attract the most buyers to a product? How much salt does a potato chip need? How much light is necessary to maximize production in a factory? Will noise in a domed stadium cause damage in the fans’ ears? Research on sensation and perception is applied in almost every setting imaginable.

Influencing Animal Behavior

Knowledge of sensation and perception can also be used to influence the behavior of other animals. Since people visit zoos during the daytime, nocturnal animals are often housed in areas bathed in only red light. Most nocturnal animals are colorblind, and since red light by itself is so difficult to see, the animals are tricked into perceiving that it is nighttime and become active for the viewers. Knowing that vultures have an exceptionally good sense of smell and that they are attracted to the scent of rotting meat allowed scientists to find an invisible but dangerous leak in a long, geographically isolated pipeline; after adding the aroma of rotting meat into the pipeline fuel, they simply waited to see where the vultures started circling—and knew where they would find the leak.

Learning Theory

The knowledge that sensation and perception differ across species has also influenced the biggest and perhaps most important field in all of psychology: learning theory. The so-called laws of learning were derived from observations of animals during the acquisition of associations between two previously unassociated stimuli, between a stimulus and a response, or between a behavior and a consequent change in the environment. These laws were originally thought to generalize equally to all species and all stimuli. This belief, along with the prevailing Zeitgeist that held that learning was the basis of all behavior, led to the assumption that studies of any animal could serve as a sufficient model for discovering the principles guiding human learning and behavior. It is now known that such is not the case.

Although laws of learning do generalize nicely in the acquisition of associations between biologically neutral stimuli, each animal’s sensory apparatus is designed specifically to sense those stimuli that are relevant for its lifestyle, and how it perceives those stimuli will also be related to its lifestyle. Therefore, the meaning of a particular stimulus may be different for different species, so results from studies on one animal cannot be generalized to another; neither can results from studies using one stimulus or stimulus modality be generalized to another.

Finally, it is important to note that scientific inquiry itself is dependent on human understanding of the human senses. Scientific method is based on the philosophy of empiricism, which states that knowledge must be obtained by direct experience using the physical senses (or extensions of them). In short, all scientific data are collected through the physical senses; thus, the entirety of scientific knowledge is ultimately based on, and limited by, human understanding of, and the limitations of, the human senses.

Evolution of Study

In the late nineteenth and early twentieth centuries, Wilhelm Wundt, often considered the founder of scientific psychology, aspired to study the most fundamental units (or structures) of the mind. Wundt and other European psychologists (called structuralists) focused much of their attention on the description of mental responses to external stimuli—in other words, on sensation and perception. Around the same time, educational philosopher William James developed functionalism in the United States. Functionalists avoided questions about what was happening in the mind and brain and focused on questions about why people respond the way they do to different stimuli.

Today, both the structuralist and the functionalist methodologies have been replaced, but the fundamental questions they addressed remain. Psychologists who study sensation and perception still conduct research into how sense organs and the brain work together to produce perceptions (proximate studies) and why people and other animals have their own particular Umwelts (ultimate studies). Results from proximate and ultimate studies typically lead to different kinds of insights about the human condition. Proximate studies lead to solutions for real-world problems, while studies of ultimate functions provide enlightenment about the evolution of human nature and humans’ place in the world; they help identify what stimuli were important throughout human evolutionary history.

For example, the human ear is fine-tuned so that its greatest sensitivity is in the frequency range that matches sounds produced by the human voice. Clearly, this reflects the importance of communication—and, in turn, cooperation—throughout human evolution. More specifically, hearing sensitivity peaks nearer to the frequencies produced by female voices than male voices. This suggests that human language capacity may have evolved out of mother-infant interactions rather than from the need for communication in some other activity, such as hunting.

Stimuli Adaptations for Survival

Knowing what kinds and intensities of stimuli the human sense organs can detect suggests what stimuli have been important for human survival; furthermore, the way the brain perceives those stimuli says something about their role. Most stimuli that are perceived positively are, in fact, good for people; food tastes and smells “good” because without some kind of psychological inducement to eat, people would not survive. Stimuli that are perceived negatively are those that people need to avoid; the fact that rotting foods smell “bad” is the brain’s way of keeping one from eating something that might make one sick. To give an example from another sensory modality, most adults find the sound of a crying baby bothersome; to stop the sound, they address the needs of the infant. Cooing and laughing are rewards that reinforce good parenting.

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