CONCEPT I: Stimuli, Sensations, and Perceptions

Sensation is the process whereby stimulation of the receptor cells in the sense organs results in a neural impulse to the brain. Perception is the brain's interpretation of this sensory information. Different species have different sensory capabilities as do different members of the same species. Sensory difference can be biologically based, or can result from different experiences and motivations. Information flowing from the sense organs to the brain is called bottom-up or data-driven processing; knowledge stored in the brain which directs your attention and influences your perception is called top-down or conceptually driven processing.

Psychophysics is the study of the relationship between physical stimuli and our subjective experience of them.

A stimulus is any form of energy to which an organism can respond. The organism's response is called a sensation and can be affected by both the quality, or kind, of sensation it produces and the quantity, or amount, of stimulation.

The sensory threshold is defined as the lowest intensity stimulus to which a person will respond 50 percent of the time under ideal conditions. Sensory capacity can be limited by several factors, including the presence of background noise, lack of prior information about the stimulus, and motivation to ignore weak stimuli. Signal detection theory attempts to separate these various influences.

Just noticeable difference is the amount of change required for the difference in the stimulus to be perceived. Weber's law postulates that a just noticeable difference in sensation (the smallest detectable change in stimulation) is always a constant proportion of the initial stimulus intensity. It does not hold under all circumstances, but it is a useful approximation. All sensory systems display adaptation, the reduction of sensitivity due to constant stimulation, although the degree of adaptation varies from one sense to another.

CONCEPT II:The Human Senses

Humans possess more senses than the basic five—sight, sound, smell, taste, touch. Each sense depends on a particular sense organ (or receptor) to change stimulation into sensation.

Electromagnetic energy, or light, behaves as if it were composed of subatomic particles, called photons. The wavelength of light determines color; the intensity determines brightness.

Light enters the eye through the transparent cornea and goes through a hole in the iris called the pupil and through the lens, whose shape is changed by ciliary muscles in order to focus the image on the retina. The outermost layer of the retina contains two types of receptor cells, rods> and cones. These receptors channel their impulses through the bipolar cells and then through the ganglion cells to the optic nerve, which carries visual information to the brain for interpretation.

Cones, cells that function in bright light and are responsible for color vision, are concentrated in the fovea, a depression at the center back of the eye, so foveal vision provides the sharpest detail. Rods are denser in the areas farther away from the center of the fovea, sense black and white, and work in dim light. Both rods and cones sense light by means of chemical reactions in which a pigment, such as rhodopsin in the rods, is bleached out. Consequently, when one comes from a light to a dark area, vision is diminished until the pigment is replenished. Night blindness occurs when rods become insensitive to dim light, and can sometimes result from an inability to store Vitamin A in the body.

Fibers from the optic nerve form several pathways to the brain, the most important of which goes through the thalamus to the primary visual cortex at the back of the brain. Hubel and Wiesel's research indicates that each of the neurons in the visual cortex is activated only by certain types of stimuli, such as a horizontal line. This suggests that these feature detector cells actually are involved in an early step in the processing of visual stimuli. More recent research suggests that beyond the primary visual cortex in the secondary visual cortex, nerve impulses feed into at least three separate processing systems: one each for analyzing shapes; colors; and movement, location, and depth.

The trichromatic theory of color holds that there are three types of cones (red, blue, and green), each maximally sensitive to a different wavelength (color) of light. The opponent-process theory explains that the three types of cones are linked to form three opponent systems in the brain: red-green, yellow-blue, and light-dark. It explains the occurrence of afterimages and color blindness. Both theories have validity: as the trichromatic theory suggests, there are three different kinds of cones. Beyond the cones, in the ganglion cells and neural network, cells operate as opponent-process units by increasing or decreasing their rates of firing depending on the color being sensed.

People who have cones containing all three kinds of iodopin and see all colors are called trichromats. People with the most common color blindness are called red-green dichromats and have difficulty distinguishing red from green because they lack either the red or green visual pigment. Less common are blue-yellow dichromats and this form of color blindness is more common in males than females. Monochromatic people see no color because their visual information is transmitted only by one kind of iodopsin. Recent evidence suggests that the range of defects in color vision is quite diverse.

Sound waves are produced by vibrating molecules and are converted to neural impulses in the ear. Their frequency determines pitch; their intensity, or amplitude (measured in decibels), determines loudness. The human ear is sensitive to frequencies ranging from 20 hertz (Hz) to 20,000 Hz, and human voices range from 100 to 3,500 Hz. Normal conversation occurs at about 60 decibels; sounds above 120 decibels are painful and can cause hearing loss.

The pinna, or outer ear, funnels sound into the auditory canal, which amplifies the sound. A thin membrane called the eardrum seals off the end of this passage and vibrates in harmony with the sound waves. Three small bones - the hammer, anvil, and stirrup - amplify and conduct this information from the eardrum to the oval window, by which point the original message is amplified up to ninety times. Conduction deafness occurs when the ear's ability to amplify waves malfunctions. The oval window transmits the message to the cochlea, the organ of hearing. In the cochlea, hair cells (receptors) are positioned between the basilar membrane and the tectorial membrane. These membranes rub against the hair cells, triggering a neural impulse that travels via the auditory nerve to the brain. Neural deafness occurs when there is damage to the basilar membrane or the hair cells. In this case, cochlear implants, which involve placing microelectrodes in the cochlea, can restore some hearing. The place theory of hearing argues that the pitch we perceive depends on which part of the basilar membrane is most displaced. However, it cannot explain all phenomena; for example, low-frequency sounds vibrate the membrane uniformly. The frequency theory and the volley principle suggest that loudness is perceived according to how often the fibers of the auditory nerve fire. Both theories must be used to explain how we hear: the place theory probably explains how we hear high-pitched sounds whereas the frequency theory provides a better explanation for low-pitched sounds.

Several types of receptors embedded in the skin are probably responsible for the skin sensations. Meissner's corpuscles are thought to be pressure sensitive, as are receptors around the roots of hair cells. Merkel disks and Ruffini endings seem to be involved with the sensation of steady pressure. Pacinian corpuscles are very sensitive to light pressure. Pain is probably detected by free nerve endings, but it is also influenced by external conditions such as anxiety. The gate control theory suggests that pain messages can be blocked in the spinal cord if there is high activation of A fibers relative to C fibers. This theory may explain the effects of acupuncture.

Olfaction (smell) requires that vaporized molecules of a substance enter the nasal passages, which are lined with the olfactory membranes containing the receptor cells. These receptor cells detect the chemical stimuli, sending nerve impulses along the olfactory bulbs to other regions in the brain. Our sense of taste is restricted to four basic categories: sweet and salty, which are sensed on the front of the tongue; sour, sensed on the sides; and bitter, sensed on the back. Taste is sensed by the taste buds located in the papillae (or bumps) of the tongue. Each of the four basic taste categories may operate independently of the others, or different tastes may be based on different patterns of neural activity.

Our sensory systems are interconnected, especially taste and smell, and all are influenced by our expectations and experiences.

CONCEPT III: Perceiving a Complex World

Perception is the process of giving order and meaning to sensations. There are two views of perception. The direct perspective view specifies that sensations contain sufficient information to be structured automatically into a meaningful whole. It is, therefore, related to bottom-up processing. The indirect perspective, or constructivist view, holds that sensory information must be supplemented with information from memory. In this view, mental representations, called schemas, are used to compare with sensory information and give it meaning. This process is top-down. There are three major areas of study of perceptual processes: form perception, perceptual constancy, and depth perception. Gestalt psychologists, primarily interested in form perception, suggested that perceptions are more than the sensations that give rise to them. For example, we sometimes "fill in" missing aspects of a picture, a phenomenon called subjective contour. In similar fashion, we also tend to group together things that are close (proximity), things that form a continuous pattern (continuity), and things that resemble one another (similarity). These perceptual sets are examples of the principle of perceptual grouping. Modern psychologists believe Gestalt psychologists underestimated the significance of prior knowledge in shaping our perceptions, a criticism made clear by our ability to see figure and ground without confusion. The principle of figure-ground refers to the separation of a visual scene into regions that represent objects (the figure) and regions that represent space between objects (the ground). Perceptual constancy is our ability to perceive objects as having certain constant properties, even though the sensations they produce vary. Depth perception, which arises from several sources, is the ability to tell how far away an object is. Binocular disparity aids in depth perception, since each eye sees an object at a slightly different angle. Motion parallax refers to the fact that as we move, movement of objects in our field of vision is relative to their distance from us. Other monocular (or single-eye) cues to depth include partial overlap (where one object blocks the view of an object behind it), linear perspective (produced by the apparent convergence of lines), relative size (where more distant objects appear smaller), texture gradient(in which near objects appear larger and coarser), relative closeness to the horizon (objects closer to the horizon appear more distant), and light and shadow (where shadowed objects appear more distant). A perceptual illusion is a perception not in accord with the true characteristics of an object or event. The Ames room illusion and other illusions are caused by our being fooled as we process information. The empiricist view argues that perceptual processes are largely the result of learning. The nativist view suggests that some perceptual processes are the result of inherited biological factors. Empiricist views are linked to the indirect or constructivist perspective whereas the nativist view is tied to the direct perspective. There is evidence to support both views. Most psychologists today take an interactionist perspective and believe that both heredity and environment are involved. For example, when newborn kittens are deprived of the simultaneous use of both of their eyes, they do not develop the binocular neurons that play a role in normal depth perception. Perceptual sets, or expectations, involve contexts that establish expectations about what we think we should perceive. Generalized knowledge in the form of schemas influences expectations and thus perception. Computer models are being developed to recognize specific patterns (like addresses on envelopes) and to help us to understand how human perception works. Models must be able to detect the features that characterize an object, and then use symbolic processing to interpret this information. In humans, the failure to recognize objects is called agnosia, and is related to brain damage. Researchers have long been interested in the effectiveness of subliminal perception, the brain's ability to register a stimulus even though it is not consciously perceived. Although subliminal stimuli have been shown to have an effect on performance in experimental tasks, there is virtually no evidence that subliminal messages can get people to do things they ordinarily would not do.