CHAPTER 5: SENSATION
Sensing the World: Some Basic Principles
To study sensation is to study an ageless question: How does the world out there get represented in here,
inside our heads? Put another way, how are the external stimuli that strike our bodies transformed into
messages that our brains comprehend?
Thresholds
Each species comes equipped with sensitivities that enable it to survive and thrive. We sense only a portion of
the sea of energy that surrounds us, but to this portion we are exquisitely sensitive. Our absolute threshold for
any stimulus is the minimum stimulation necessary for us to detect it 50 percent of the time. Signal detection
researchers report that our individual absolute thresholds vary with our psychological state.
Experiments reveal that we can process some information from stimuli too weak to recognize. But the restricted
conditions under which this occurs would not enable unscrupulous opportunists to exploit us with subliminal
messages.
To survive and thrive, an organism must have difference thresholds low enough to detect minute changes in
important stimuli. In humans, a difference threshold (also called a just noticeable difference, or jnd) increases
in proportion to the size of the stimulus—a principle known as Weber’s law.
Sensory Adaptation
Sensory adaptation refers to our ability to adapt to unchanging stimuli. For example, when we smell an odor in
a room we’ve just entered and remain in that room for a period of time, the odor will no longer be easily
detected. The phenomenon of sensory adaptation focuses our attention on informative changes in stimulation
by diminishing our sensitivity to constant or routine odors, sounds, and touches.
Vision
Each sense receives stimulation, transduces it into neural signals, and sends these neural messages to the
brain. We have glimpsed how this happens with vision.
The Stimulus Input: Light Energy
The energies we experience as visible light are a thin slice from the broad spectrum of electromagnetic
radiation. The hue and brightness we perceive in a light depend on the wavelength and intensity.
The Eye
After entering the eye and being focused by a camera-like lens, light waves strike the retina. The retina’s lightsensitive rods and color-sensitive cones convert the light energy into neural impulses, which are coded by the
retina before traveling along the optic nerve to the brain.
Visual Information Processing
In the cortex, individual neurons called feature detectors, respond to specific features of a visual stimulus, and
their information is pooled for interpretation by higher-level brain cells. Sub-dimensions of vision (color,
movement, depth, and form) are processed separately and simultaneously, illustrating the brain’s capacity for
parallel processing. The visual pathway faithfully represents retinal stimulation, but the brain’s representation
incorporates our assumptions, interests, and expectations.
Color Vision
Research on how we see color supports two nineteenth-century theories. First, as the Young-Helmholtz
trichromatic (three-color) theory suggests, the retina contains three types of cones. Each is most sensitive to
the wavelengths of one of the three primary colors of light (red, green, or blue). Second, as opponent-process
theory maintains, the nervous system codes the color-related information from the cones into pairs of opponent
colors, as demonstrated by the phenomenon of afterimages and as confirmed by measuring opponent processes
within visual neurons of the thalamus. The phenomenon of color constancy under varying illumination shows
that our brains construct our experience of color.
Hearing
The Stimulus Input: Sound Waves
The pressure waves we experience as sound vary in frequency and amplitude, and correspondingly in perceived
pitch and loudness.
The Ear
Through a mechanical chain of events, sound waves traveling through the auditory canal cause minuscule
vibrations in the eardrum. Transmitted via the bones of the middle ear to the fluid-filled cochlea, these
vibrations create movement in tiny hair cells, triggering neural messages to the brain.
Research on how we hear pitch supports both the place theory, which best explains the sensation of highpitched sounds, and frequency theory, which best explains the sensation of low-pitched sounds. We localize
sound by detecting minute differences in the intensity and timing of the sounds received by each ear.
Hearing Loss and Deaf Culture
Hearing losses linked to conduction and nerve disorders can be caused by prolonged exposure to loud noise and
by diseases and age-related disorders. Those who live with hearing loss face social challenges. Cochlear
implants can enable some hearing for deaf children and most adults. But Deaf Culture advocates, noting that
Sign is a complete language, question the enhancement. Additionally, deafness can lead to sensory
compensation where other senses are enhanced. Advocates feel that this furthers their view that deafness is not
a disability.
Other Important Senses
Touch
Our sense of touch is actually four senses—pressure, warmth, cold, and pain—that combine to produce other
sensations, such as "hot." One theory of pain is that a "gate" in the spinal cord either opens to permit pain
signals traveling up small nerve fibers to reach the brain, or closes to prevent their passage. Because pain is
both a physiological and a psychological phenomenon, it often can be controlled through a combination of
physical and psychological treatments.
Taste
Taste, a chemical sense, is likewise a composite of five basic sensations—sweet, sour, salty, bitter, and
umami—and of the aromas that interact with information from the taste buds. The influence of smell on our
sense of taste is an example of sensory interaction.
Smell
Like taste, smell is a chemical sense, but there are no basic sensations for smell, as there are for touch and
taste. Unlike the retina’s receptor cells that sense color by breaking it into component parts, the 5 million
olfactory receptor cells with their 1000 different receptor proteins recognize individual odor molecules. Some
odors trigger a combination of receptors. Like other stimuli, odors can spontaneously evoke memories and
feelings.
Body Position and Movement
Finally, our effective functioning requires a kinesthetic sense, which notifies the brain of the position and
movement of body parts, and a sense of equilibrium, which monitors the position and movement of the whole
body.