How do we feel? Identify the key structures involved in sensing and perceiving touch and describe their role in feeling.

Overview: This purpose of the week 3 discussion board is to examine sensation and perception. Answer prompt 1. Then select and answer one prompt from prompts 2-5. Refer to Chapter 4 to answer the prompts.
Instructions: Respond to prompts in paragraph form (200-400 words) by Thursday’s class. Reply to the main discussion for your initial post. Then, reply to 2 students in ~50 words.
Prompt 1: Describe 3 topics from Chapters 4 that interests you.
Prompt 2: How do we see? Identify the key structures involved in sensing and perceiving light and describe their role in vision.
Prompt 3: How do we hear? Identify the key structures involved in sensing and perceiving sound waves and describe their role in hearing.
Prompt 4: How do we feel? Identify the key structures involved in sensing and perceiving touch and describe their role in feeling.
Prompt 5: How do we smell? Identify the key structures involved in sensing and perceiving chemical stimuli and describe their role in smelling.
Chapter Overview

A. The Processes and Purposes of Sensation and Perception
Ophthalmology is the study of the structure, function, and diseases of the eye.
Audiology is the science of hearing.
Neurology is the study of the nervous system.
Understanding sensation and perception requires comprehending the physical properties of the objects of our perception—light, sound, the texture of material things, and so on.
Bottom-up and Top-down Processing
Bottom-up processing occurs when information from the external environment is registered and sent up to the brain for interpretation. This type of processing occurs with first-time information, meaning information that a person is not familiar with.
Top-down processing occurs with higher levels of cognitive processing. Such processing occurs with information that a person is already familiar with.
The Purposes of Sensation and Perception
From an evolutionary perspective, the purpose of sensation and perception is adaptation that improves a species’ chances for survival.
B. Sensory Receptors and the Brain
Sensory receptors are cells that detect and transmit information to sensory (afferent) nerves in the brain.
Sensory receptors set off action potentials in sensory neurons, which then carry the information to the central nervous system.
The sense organs and sensory receptors are of three classes, based on the type of energy being transmitted. Photoreception involves the detection of light. Mechanoreception is the detection of pressure, vibrations, and movement perceived as touch, hearing, and equilibrium. Chemoreception is the detection of chemical stimuli, in the form of smell and taste.
C. Thresholds
Absolute Threshold
An absolute threshold is the minimum amount of energy needed for a person to detect something. If a stimulus falls below the threshold, the person does not experience it.
Under ideal circumstances, a person’s absolute thresholds are very low. The problem is that there is so much noise around people that they cannot detect half of the stimuli confronting them. Noise is the term given to irrelevant, competing stimuli.
Difference Threshold
Difference threshold refers to the difference that must exist between two stimuli before the difference between them can be detected.
The difference threshold is also referred to as the just noticeable difference.
Weber’s law expresses the idea that two stimuli must differ by a constant minimum percentage, rather than a constant amount, to be perceived as different.
Subliminal Perception
Subliminal perception is what occurs when people are affected by information that is presented below their level of awareness.
Signal detection theory focuses on decision making about stimuli while in the presence of uncertainty. It depends on individual and contextual variations such as fatigue, expectancy, and the urgency of the moment.
D. Perceiving Sensory Stimuli
Attention
Selective attention involves focusing in on a specific aspect of an experience while ignoring the others.
Attention is not only selective but is also shiftable, meaning that a person can be focusing his or her attention on one thing, but then switch attention to something else.
The cocktail party effect is an example of selective attention where one has the ability to focus on one voice among many in a crowded airline terminal or noisy restaurant.
Inattentional blindness refers to the failure to detect unexpected events when attention is engaged by a task.
Culture, Attention and Perception
Culture also influences the kinds of stimuli that are missed in inattentional blindness.
Perceptual Set
A perceptual set is the predisposition to perceive something in a particular way.
E. Sensory Adaptation
Sensory adaptation occurs when there is a change in the responsiveness of the sensory system based on the average level of the surrounding stimulation.
2. The Visual System
A. The Visual Stimulus and the Eye
Light
Light is a form of electromagnetic energy. Light travels through space in waves.
The wavelength of light is the distance from the peak of one wave to the peak of the next.
Amplitude, the height of the wave, is associated with the brightness of a visual stimulus.
Purity is the degree to which wavelengths are mixed in light.
Saturation is the richness of a visual stimulus.
The Structure of the Eye
The eye is set up like a camera in that it is able to take a picture of the world.
The sclera is the white outer part of the eye that gives the eye its shape and protects the eye from injury.
The iris is the colored part of the eye.

The pupil, which appears black, is the opening in the center of the iris. The iris contains muscles that allow the pupil to grow larger or smaller, depending on how much light is being let into the eye.
The cornea is the clear membrane on the outer part of the eye. The curved surface of the cornea bends light on the surface of the eye in order to focus it to the back of the eye.
The lens is transparent and somewhat flexible. When a person is looking at an object far away, the lens has a relatively flat shape. However, when a person is looking at a closer object, more bending of the light is needed, which is done by the lens.
The retina is the light-sensitive surface that records what a person sees and then converts it to a neural impulse for processing in the brain. The retina is the important mechanism for sight.
Rods are sensitive to light and allow a person to see at night.
Cones are used for color perception and are not very helpful at night.
The fovea is in the center of the retina. It contains only cones and is important to many visual tasks.
Rods and cones transduce light into electrochemical impulses that go to the bipolar cells and then move on to ganglion cells. The axons of the ganglion cells make up the optic nerve.
The optic nerve carries visual information to the brain.
The blind spot on the retina contains neither rods nor cones.
B. Visual Processing in the Brain
The optic nerve carries information about light from the eye to the brain. Stimuli in the left visual field travel to the right side of the brain and vice versa.
The optic chiasm is the point at which the nerve fibers divide and visual information crosses over to the opposite side of the brain.
The Visual Cortex
The visual cortex is located in the occipital lobe of the brain. It functions in vision.
Feature detectors are neurons in the brain’s visual system that respond to particular features of a stimulus.
Parallel Processing
Parallel processing is the simultaneous distribution of information across different neural pathways. Such processing helps information move rapidly through the brain.
Binding
Binding is the bringing together and integration of what is processed by different neural pathways or cells.
Through binding, a person can integrate information about the various parts of an object. For example, if one sees a chair, through binding he or she not only sees just the chair but also the size, color, motion, and other properties of the chair.
C. Color Vision
Color is a pattern of neural responses that generate color vision. The process of color perception starts in the retina, the eyes’ film.
The trichromatic theory states that color perception is produced by three types of receptors.
The term color blind refers to the ability to see some colors but not others. Color blindness depends on which of the three kinds of cones (green, red, and blue) is inoperative.
Afterimages are sensations that remain after a stimulus is removed.
The opponent-process theory states that cells in the visual system respond to red-green and blue-yellow colors. A given cell might be excited by red and inhibited by green, whereas another cell might be excited by blue and inhibited by yellow.
D. Perceiving Shape, Depth, Motion, and Constancy
To perceive a visual stimulus, the fragments of information that the eye sends to the visual cortex must be organized and interpreted.
Shape
The figure–ground relationship occurs when a person organizes the perceptual field into stimuli that stand out (the figure) and those that are left over (the ground).
Gestalt psychology explains how people naturally organize their perceptions according to certain patterns.
Depth Perception
Depth perception is the ability to see objects in three dimensions.
Binocular cues are depth cues that depend on the combination of the images in the left and right eyes and on the way the two eyes work together. For example, if a person holds a hand over one eye and focuses on an object and then switches to cover the other eye, the switching back and forth between the eyes will cause the object to seem to jump back and forth. The brain uses these differences in views to determine depth and distance.
Convergence occurs when two eyes focus on an object. If the object is near us, our eyes converge, or almost move together, almost crossing. If the object is far away, we can focus on it without pulling our eyes together. This muscle movement gives information to the brain about distance or depth.
Monocular cues are depth cues that are available from the image in just one eye, whether the left or the right. Some examples of monocular cues are familiar size, height in the field of view, linear perspective, overlap, shading, and texture gradient.
Motion Perception
In some animals, motion perception is a means of survival.
Apparent movement occurs when an object is stationary but it is perceived as moving. An example of apparent motion is watching an Imax movie.
Perceptual Constancy
Perceptual constancy refers to the recognition of objects as remaining stationary and unchanging even though the sensory input about them is changing. There are three types of perceptual constancy: size constancy, shape constancy, and color constancy.
3. The Auditory System
A. The Nature of Sound and How We Experience It
Sound waves are vibrations in the air that are processed by the auditory system.
Wavelength determines the frequency of the sound wave or the number of cycles that pass through a particular point in a given time.
Pitch is the perceptual interpretation of the frequency of a sound. High-frequency sounds are perceived as having high pitch, such as with a soprano voice, low-frequency sounds a low pitch, as with a bass voice.
Amplitude, measured in decibels, is the amount of pressure produced by sound waves relative to a standard.
Loudness is the perception of sound wave’s amplitude.
Complex sounds are frequencies that are blended.
Timbre is the perceptual quality of the sound.
B. Structures and Functions of the Ear
The ear transmits a high-fidelity version of sounds in the world to the brain for analysis and interpretation.
Outer Ear
The outer ear contains the pinna, the outer visible part of the ear. It also contains the external auditory canal.
Middle Ear
After passing through the pinna, sound waves go through the auditory canal to the middle ear. Sound travels through the eardrum, hammer, anvil, and stirrup to the inner ear.
The eardrum vibrates in response to sound.
The hammer, anvil, and stirrup are three small bones in the middle ear. When they vibrate, sound waves are transmitted to the inner ear.
Inner Ear
The inner ear contains the oval window, the cochlea, and the basilar membrane.
The oval window sends sound waves to the cochlea.
The cochlea is a tubular fluid-filled coil that is similar to a snail’s shell.
The basilar membrane lines the inner wall of the cochlea. It is comprised of tiny hair cells called cilia that are sensory receptors. Any movement against the tectorial membrane (a jelly-like flap) generates impulses that are interpreted by the brain.
The purpose of the inner ear is to transduce sound waves into neural impulses that are then sent to the brain for interpretation.
In humans and other mammals, there are hair cells that line the basilar membrane. These are the sensory receptors of the ear. These hair cells can be damaged from too much loud noise.
Cochlear implants are devices that were specifically developed to replace damaged hair cells. A cochlear implant —a small electronic device that is surgically implanted in the ear and head—allows deaf or profoundly hard-of-hearing individuals to detect sound.
C. Theories of Hearing
Place theory is the phenomenon of displacement that occurs when each frequency produces vibrations at a particular spot on the basilar membrane. A high-frequency sound stimulates a specific part of the basilar membrane; however, a low-frequency sound causes a large portion of the basilar membrane to be displaced.
Frequency theory states that the perception of a sound’s frequency depends on how often the auditory nerve fires. High-frequency sounds cause the nerve to fire more than low-frequency sounds.
The volley principle comes into play when a cluster of nerve cells fire neural impulses in rapid succession to produce a volley of impulses.
D. Auditory Processing in the Brain
Once information is picked up from the environment it must move on to the brain for interpretation.
Information about sound moves from the hair cells of the inner ear to the auditory nerve, which then carries the neural impulses to the brain.
E. Localizing Sound
A sound coming from the left reaches the left ear faster than it reaches the right ear.
The sound reaching the closer ear is more intense, because it did not have to travel as far but also because the other ear is in the sound shadow of the listener’s head.
Differences in the timing of a sound and its intensity help a person locate a sound.
4. Other Senses
A. The Skin Senses
The cutaneous senses are the receptors on the skin for temperature, touch, and pain.
Touch
In touch, a person is detecting mechanical energy, or pressure against the skin.
Sensory fibers from receptors in the skin enter the spinal cord, from which the information travels to the brain stem. At the brain stem most fibers from the body cross over to the opposite side of the brain. The information then goes on to the thalamus, which projects a map of the body’s somatosensory areas of the parietal lobes in the cerebral cortex.
Temperature

Thermoreceptors, located under the skin, respond to changes in temperature at or near the skin. They provide input to keep the body’s temperature at a steady 98.6 degrees Fahrenheit. There are both warm and cold thermoreceptors.
Pain
Pain is the sensation that warns of damage to the body. It works quickly to tell the motor system of the brain that it must act quickly to eliminate or minimize the damage.
There are pain receptors throughout the body, which differ in the type of physical stimuli to which they respond.
Pain receptors have a higher threshold for firing than do either temperature or touch receptors. They react mainly to physical stimuli that distort them or they react to chemical stimuli that irritate them into action. Prostaglandins are secreted from inflamed joints or sore muscles.
In the fast pathway, fibers connect directly to the thalamus and then to the motor and sensory areas of the cerebral cortex. This pathway reacts to sharp, localized pain such as when a person cuts himself or herself.
In the slow pathway, pain information travels through the limbic system.
Scientists think the brain turns pain signals on and off. This switching is probably the result of a chemical process involving endorphins being released mainly in the slow pathway.
Culture and ethnic contexts can determine the degree to which an individual experiences pain.
B. The Chemical Senses
Smell (which detects airborne chemicals) and taste (which detects chemicals in saliva) are responsible for processing chemicals in the environment. Smell and taste are often stimulated at the same time.
Taste
Papillae, the small bumps on a person’s tongue that contain taste buds, are the receptors for taste. The information picked up by taste bud receptors travels to the brain for interpretation.
The four different taste qualities that a person can perceive are sweet, sour, bitter, and salty. Different regions of the tongue are more sensitive to certain tastes than are other areas. The tip of the tongue is more sensitive to sweet and salty, the sides more sensitive to sour, and the rear more sensitive to bitterness.
Many neuroscientists think that taste is more complicated than the four categories just listed. Taste fibers from the taste buds to the brain have been shown to respond to a range of chemicals such as salty and sour.
A new taste is being explored called umami, a Japanese word for delicious or yummy. Umami detects tastes like soy sauce, hearty meat broths, mozzarella and parmesan cheese, anchovies, and mushrooms.
Smell
Humans use the sense of smell to decide what to eat and as odor trackers.
The olfactory epithelium lines the roof of the nasal cavity and contains a sheet of receptor cells for smell. These receptor cells are covered with millions of hair-like antennae.
The neural pathway first goes to the olfactory areas of the temporal lobes and then to various brain regions, especially the limbic system.
Smell may play a role in mating behavior.
C. The Kinesthetic and Vestibular Senses
The kinesthetic senses provide information about movement, posture, and orientation. The kinesthetic senses are located in muscle fibers and joints throughout the body. When a person stretches or moves, these fibers signal the state of the muscle.
The vestibular sense provides information about balance and movement. The vestibular sense tells a person whether his or her head is tilted, moving, slowing down, or speeding up.
Proprioceptive feedback is information about the position of limbs and body parts in relation to other body parts.
The semicircular canals, located in the inner ear, contain the sensory receptors that detect head motion.
The brain pathway for the vestibular sense starts in the auditory nerve. Most of the axons of the vestibular nerve connect with the medulla, although some go directly to the cerebellum.Reply

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