Chapter 4: Sensation and Perception Chapter Summary

I. UNDERSTANDING SENSATION

A. Processing – Transduction, or the conversion of physical stimuli into neural impulses, occurs at the receptors in the sense organs. Each sensory modality is specialized to code its stimuli into unique sets of neural impulses that the brain interprets as light, touch, and so on. Since people receive much more information at the sensory receptors than can possibly ever be processed, it is necessary to select only the information that is important, a process known as sensory reduction. Reduction is performed chiefly by an area of the brain called the reticular activating system. Here, incoming sensory information is filtered and analyzed before going to the brain where the coded information will evoke a unique sensation.

B. Thresholds - The absolute threshold is the smallest magnitude of a stimulus a subject can detect. The difference threshold is the smallest change in the magnitude of a stimulus that a subject can detect. Thresholds can be affected by the process of sensory adaptation.

C. Adaptation - Sensory adaptation allows people to operate efficiently in a wide range of stimulus intensities by decreasing the sensitivity to constant, unchanging stimuli. The Gate Theory of Pain is presented as an explanation of why touch and pressure can reduce pain.

II. HOW WE SEE AND HEAR

A. Vision - The physical stimulus for vision is light, a form of energy that is part of the electromagnetic spectrum. The wavelength of light determines its hue, or color; and the amplitude, or height, of the light wave determines its brightness. The function of the eye is to capture light and focus it on the visual receptors that convert light energy into neural impulses. The major parts of the eye include the cornea, the pupil, the iris, the lens, and the retina. All light enters the eye through the cornea which protects the eye and bends incoming light rays toward the lens. Light then enters through an opening called the pupil. The pupil is surrounded by the iris which dilates or constricts to vary the amount of light entering the eye. Behind the pupil and iris is the lens, a clear elastic structure that can change its shape to focus an image on the retina at the back of the eye. The lens thins to focus light on the retina from distant objects and bulges to focus light from near objects. The retina is the back layer of the eye that contains the visual receptor cells. The visual receptors, called photoreceptors, are the rods and cones. The rods are very sensitive to light and enable individuals to see at night. The cones are specialized for bright light conditions and enable individuals to see close and fine detail. Nearsighted can be caused by a longer than normal eyeball or a too sharply curved cornea. Farsightedness is caused by a shorter than normal eyeball. Both conditions are easily remedied with corrective lenses.

B. Hearing - The sense of hearing, known as audition, detects sound waves, which result from rapid changes in air pressure caused by vibrating objects. The frequency of sound waves is measured in cycles per second and is sensed as the pitch of the sound. The amplitude of the sound is measured in decibels and is sensed as the loudness of the sound. The ear is the sense organ specialized for receiving and converting sound information. The structures of the ear include the pinna, the external visible part of the ear; the eardrum, or tympanic membrane, that vibrates when hit by sound waves; the ossicles—the malleus, incus, and stapes—that transmit the sound vibrations through the middle ear; the oval window, the membrane separating the middle ear from the inner ear; and the cochlea, the structure that forms the inner ear. The major structures of the cochlea are the basilar membrane and the auditory receptor cells (hair cells). Place and Frequency theories explain how we hear higher-pitched and lower-pitched sounds respectively.

III. OUR OTHER SENSES

A. Smell and Taste - Olfaction (the sense of smell) and gustation (the sense of taste) are called the chemical senses and are closely interrelated. The receptors for olfaction are in the olfactory epithelium located at the top of the nasal cavity. According to the lock-and-key theory, humans can smell various odors because each three-dimensional odor molecule fits into only one type of receptor. The role of pheromones in attraction continues to be investigated… Taste receptors are located on the tongue and are sensitive to the four major tastes: salty, sweet, sour, and bitter.

B. The Body Senses- The body senses include the skin senses, the vestibular sense, and the kinesthetic sense. The skin senses, which include pressure, temperature, and pain, not only protect the internal organs but also provide basic survival information. The vestibular sense is the sense of balance. Located in the inner ear, the vestibular apparatus is composed of the semicircular canals and the vestibular sacs. The semicircular canals, three arching structures in the inner ear, located above and attached to the entrance to the cochlea, provide the brain with balance information about the rotation of the head. The vestibular sacs contain hair cells sensitive to the tilt of the head and provide the brain with this information. The kinesthetic sense provides the brain with information about bodily posture and orientation, as well as bodily movement. The kinesthetic receptors are spread throughout the body in muscles, joints, and tendons.

IV. UNDERSTANDING PERCEPTION - The chapter continues with an overview of perception. Illusions have been used by psychologists to study the process of perception because illusions represent situations in which sensory information is interpreted improperly. This improper interpretation can give researchers insights into how perceptual systems process sensory information. Perception is the process of selecting, organizing, and interpreting sensory data into a usable mental representation of the world.

A. Selection- The first step in perception is selection, the process that allows people to choose which of the billions of separate sensory messages will eventually be processed. Thus, selective attention allows individuals to direct attention to the most important or critical aspect of the environment at any one time. During the selection process, feature detectors distinguish between various sensory inputs. Feature detectors are specialized cells in the brain that distinguish between different sensory inputs. Early deprivation may lead to problems with feature detectors. The selection process is particularly sensitive to change in the environment. Stimuli that remain the same can cause perceptual habituation, in which the brain ignores the constant stimuli.

B. Organization - The second process of perception, the organization of sensory data into meaningful perceptions, involves: form, constancy, depth, and color.

1. Form Perception - The school of Gestalt psychology explored the principles of form perception including figure and ground, proximity, continuity, closure, similarity. Gender and Cultural Diversity: Are the Gestalt Laws Universally True? - Using women from the former USSR as subjects, Luria (1976) concludes that the Gestalt principles are only valid for people who have been schooled in geometrical concepts, not for uneducated people who perceive shapes in an object-oriented world.

2. Perceptual Constancies - The process of perceptual organization is also seen in the perceptual constancies: size constancy, shape constancy, color constancy, and brightness constancy. Size constancy is the process whereby the perceived size of an object remains the same even when the image of the object on the retina may change size due to changes in distance. Shape constancy occurs when an object is perceived to have the same shape even when the image of the object on the retina changes shape. Color constancy is the process whereby colors appear to have the same relative color in varying conditions of illumination. Brightness constancy is the process whereby colors appear to have the same relative brightness in varying conditions of illumination.

3, Depth Perception- Another part of the process of organization is the depth perception of size and distance, for which there are two major types of cues: binocular and monocular. Binocular cues, which require two eyes, include retinal disparity and convergence. Monocular cues, which require only one eye and Figure 4.23 demonstrates six monocular cues. Additionally, accommodation and motion parallax are discussed.

4. Color Perception - The perception of color is another process of perceptual organization. Color perception is explained by a combination of two traditional color theories: the trichromatic theory and the opponent-process theory. The trichromatic theory proposes that three kinds of color systems are maximally sensitive to blue, green, and red. The opponent-process theory proposes that, indeed, three color systems exist, but that each is sensitive to two opposing colors—blue and yellow, red and green, and black and white. These systems work in an on-off fashion (for example, a person may see blue or yellow, but not both at the same time). It appears that the trichromatic system operates at the level of the retina while the opponent-process system occurs at the level of the brain. Some people cannot perceive color normally.

C. Interpretation - The final stage of perception is interpretation and is influenced by several factors including perceptual adaptation, perceptual set, individual motivation, frame of reference, and bottom-up or top-down processing.

Case Study: Helen Keller’s Triumph and Advice – The inspiring story of Helen Keller is highlighted. Left blinded and deaf at nineteen months from a fever, Helen Keller overcomes tremendous limitations to become a famous author and lecturer advising all to never take the gift of senses for granted.

Research Highlight: Is There Scientific Evidence for Subliminal Perception and ESP? - Subliminal stimuli, which are stimuli presented below the threshold of awareness do occur, but the effect on behavior is uncertain and certainly not as effective as attention-getting stimuli. Extrasensory perception (ESP), another possible way of organizing sensory information, is the ability to perceive things through senses that go beyond the “known” senses. Critics condemn its scientific validity because it lacks experimental control and replicability. Recent, better controlled, studies report contradictory or “fragile” results lacking in stability and replicability. ESP’s undiminished popularity can be attributed to our need to believe, and to people’s difficulty with evaluating complex scientific information. Critical Thinking/Active Learning: Problems With Believing in Extrasensory Perception - Using so-called examples of ESP, students are given a chance to practice recognizing four types of faulty reasoning—fallacy of positive instances, innumeracy, willingness to suspend disbelief, and the “vividness” problem.