Part 5 Sensation and Perception

Part 5 Sensation and Perception

Answers to Before You Go On Questions

Module 24

1. What is sensory transduction?Sensation and perception are experienced differently in each of our sensory modalities, but our senses also share some common processes. Each of the senses has a set of specialized cells called sensory receptor cells that convert a specific form of environmental stimuli into neural impulses. This conversion is called sensory transduction.

2. What are absolute and difference thresholds?An absolute threshold is considered to be the minimal stimulus necessary for detection by an individual through our sensory receptors. This threshold varies from person to person, but in most cases it is surprisingly small. The difference threshold (or just noticeable difference) is the minimal difference between two stimuli necessary for us to detect a difference between the two. For example, if you are listening to a radio, how much do you have to turn the radio up before you notice that it is louder? The amount depends on how loud the radio is in the first place. When sensory systems are working optimally, the difference threshold is also remarkably small.

3. Compare and contrast bottom-up and top-down processing. Perception can occur through bottom-up processing, which begins with the physical stimuli from the environment, and proceeds through transduction of those stimuli into neural impulses that move onto successively more complex brain regions. For example, when you look at the face of your best friend, your eyes convert light energy into neural impulses, which travel into the brain’s visual regions. This information forms the basis for sensing the visual stimulus and, ultimately, its perception. Equally important, however, is top-down processing, which involves previously acquired knowledge. When you look at your best friend’s face, for example, brain regions that store information about what faces look like, particularly those that are familiar to us, can help you perceive and recognize the specific visual stimulus, in this case, your best friend.

Module 25

1. What five tastes have specific receptors? There are five major kinds of taste receptors; each responds to a specific taste in our food. The basic tastes are (1) sweet, (2) sour, (3) bitter, (4) salty, and (5) umami (the taste of monosodium glutamate (MSG)).

2. Which parts of the brain are involved in sensing and perceiving odours?Olfactory receptor neurons transduce information from odorant molecules that enter the nose. This information is carried by the olfactory nerve into the brain where it synapses in the olfactory bulb (located at the base of the front of the brain, beneath the frontal lobes). Olfactory information is then sent to regions of the cerebral cortex that are important for recognizing and discriminating among odours, including the piriform cortex (Wilson, 2001).

3. What are supertasters?Supertasters (25% of the population) are a grouping of human beings based on their taste sensitivity—in this case, their ability to detect and respond negatively to a specific bitter substance (Bartoshuk et al., 1996). Supertasters are repulsed by the bitter chemical. The functional difference between differing levels of tasters is the result of variations in the concentration of taste buds on the tongue. Women make up a higher proportion of supertasters than do men.

4. How are smell and taste involved with migraines and epileptic seizures?Some people experience hallucinations called auras either before or during migraine headaches or epileptic seizures. Auras can involve any of the sensory systems, including experiencing strong, often unpleasant, smells or tastes. The involvement of different senses indicates which brain circuits are compromised in these conditions. For example, if a person’s seizure is preceded by strong olfactory hallucinations, it’s likely that his or her olfactory pathways are initiating the seizure, or at least participating in its generation.

Module 26

1. List the different types of tactile receptors in the skin and the primary functions of each. The tactile senses rely on a variety of receptors located in different parts of the skin, including free nerve endings (located mostly near the surface of the skin, these function to detect touch, pressure, pain, and temperature), Meissner’s corpuscles (transduce information about sensitive touch and are found in the hairless regions of the body, such as the fingertips, lips, and palms), Merkel’s discs (transduce information about light to moderate pressure against the skin), and Ruffini’s end-organs (located deep in the skin, they register heavy pressure and movement of the joints).

2. Compare and contrast slow and fast pain pathways.Pain information travels to the brain via two different types of pain fibres. One system, called the fast pathway, uses myelinated axons that carry signals faster than unmyelinated axons. Messages about sharp, localized pain travel along the fast pathway directly up the spinal cord to the thalamus and to areas of the somatosensory cortex. Pain information received via the fast pathway helps us to respond quickly with a withdrawal reflex, such as pulling a hand away after touching a hot stove. Slow pathways, in contrast, use unmyelinated axons; these inputs communicate with brain regions involved in processing emotions. Pain we perceive via the slow pathway is more often burning pain than sharp pain.

3. Why do children so often enjoy getting tickled?The reaction we have to tickling is a result of activation of somatosensory pathways in an uneven, uncontrollable, and unexpected manner. Not only are our sensory systems organized to detect change, but they are most tuned to stimuli that are unexpected and surprising. When you move your body and produce tactile sensations, these stimuli are less noticeable to you than are sensations produced by another individual.

4. What are some possible explanations for individual differences in pain sensitivity?Although it is believed that learning plays some role, groups of people also differ in the actual sensation and perception of pain as a result of physical differences in their sensory systems. Research suggests that women may have about twice as many pain receptors in their facial skin than men. This suggests a physical cause for at least some of the differences in pain sensitivity. Ronald Melzack and Patrick Wall (1982) theorize that some patterns of neural activity can actually create a “gate” that prevents messages from reaching parts of the brain where they are perceived as pain (gate control theory of pain).Individual differences in gating mechanisms may result in the wide range of pain sensitivity across people (Romanelli & Esposito, 2004).

Module 27

1. What happens in the ear to transduce sound waves into neural signals?First, sound waves enter the outer ear and, at its deepest part, deflect the ear drum or tympanic membrane. Vibrations of the tympanic membrane set in motion a series of three tiny bones, or ossicles, called maleus (hammer), incus (anvil), and stapes (stirrup). The stapes, which is the last bone in the chain, hits the oval window, a membrane separating the ossicles and the inner ear. Deflection of the oval window causes a wave to form in the fluid-filled cochlea of the inner ear. When fluid moves in the cochlea, it deflects the basilar membrane that runs down the middle of the cochlea. The basilar membrane is covered with rows of hair cells, the auditory sensory receptors. Movement of the basilar membrane bends the hair cells that transduce the “fluid sound wave” into electrical activity. Finally, the hair cells communicate with nerves in the cochlea that, in turn, send the neural impulses to the brain.

2. What is a tonotopic map? Part of the primary auditory cortex is organized in a tonotopic map (a representation in the auditory cortex of different sound frequencies). That is, information transmitted from different parts of the cochlea (sound waves of different frequency and, hence, sounds of different pitch) is projected to specific parts of the auditory cortex, so that our cortex maps the different pitches of sounds we hear. Auditory information from one ear is sent to the auditory cortex areas on both sides of the brain.

3. What are sensitive periods and how are they important for hearing? Sensitive periods exist for the development of both language and music learning (Knudson, 2004).We acquire certain abilities during sensitive periods of development much more easily than we do after the sensitive period has ended. The tonotopic map in the primary auditory cortex of the brain is organized during such a sensitive period of development. In humans, the auditory brain is set up to acquire information about speaking and music most readily relatively early in life, during the preschool years. This is why it is more difficult, but by no means impossible, for us to learn additional languages or certain music skills after we mature.

4. What is tinnitus?About one in every 200 people is affected by tinnitus, or ringing in the ear. Tinnitus has multiple causes, some of which are related to abnormalities in the ear itself (Lanting et al., 2009). Most people are able to cope with the noise, but some find it too loud and distracting to ignore.

Module 28

1. What are rods and cones?Two major classes of visual receptors, or photoreceptors, exist in the retina, called the rods and the cones. Rodsare photoreceptors responsive to levels of light and dark (thus, to motion), and they predominate. There are over 100 million rods in the human retina, and they are highly sensitive to small amounts of light and are critical for functioning in dim light or at night. However, rods give the perceiver no colour sensation. Cones, on the hand, are colour receptors, because they respond to light of different wavelengths. The cones are located in the foveal area of the retina and are much fewer in number, with only about 4.5 to 6 million per human retina (Curcio et al., 1990).

2. What are the two theories of colour vision and how do they work together? The two theories discussed in the text are the Young-Helmholtz trichromatic theory (there are three distinct types of cone cells responsive to different colours of light that provide us with information about the colours we are seeing at the level of the retina) and the opponent process theory(cells further up in the visual system indicate one sensation by firing rapidly and another by firing slowly). The two theories can be used together to create a successive physiological two-stage model of colour vision. In stage one, the cones in the retina respond to three primary wavelengths and send signals to the visual cortex. Along the way, in stage two, the signal is processed by opponent process cells. The result?Colour.

3. What do the “what” and “where” pathways in the brain do?The pathways that process information about complex visual stimuli can be roughly divided into the “what” and the “where” pathways. That is, the regions that process visual information to help us determine what is the identity of an object (is it an apple, a car, or a house?) are different from those where we process the visual information to figure out where in space the object is located (is the apple on the table, under the table, or behind the table?). The “what” pathway involves axons that travel from the occipital cortex to the temporal cortex.The “where” pathway involves axons that travel from the occipital cortex to the parietal cortex.

4. What are the two major types of depth perception cues and what is the difference between them?We distinguish depth or distance either by using monocular (one eye) or binocular (two eye) cues. Retinal disparity (the slight difference in images processed by the retinas of each eye) and convergence (the inward movement of the eyes to view objects close to oneself) are examples of binocular cues. Retinal disparity provides us with a binocular cue of depth. Monocular cues include all the sorts of things you can learn should you wish to draw or paint in ways that express depth or distance in two-dimensional sketches or paintings.

5. What is strabismus, how is it treated, and what can happen if it is not treated promptly? Strabismus occurs when people do not naturally develop coordinated movement of both eyes. It affects about 2 percent of the population. To avoid seeing double images, children with strabismus will rely on the visual information from one eye while ignoring information from the other. Strabismus is commonly treated by having the child wear a patch over the stronger eye, thus forcing the child to use the weaker one, or by surgery. If children are treated early in life, their normal binocular vision can be preserved. If strabismus remains uncorrected past the age of about six years, however, it will eventually lead to a loss of visual abilities in the weaker eye, or amblyopia. Amblyopia can be a permanent condition that results from abnormal development of the brain’s visual cortex.

6. Where is our vestibular sense located and how does it work?Our vestibular sense is located in the semicircular canals of our inner ear. The fluid in the canals shifts around as we move, and in so doing moves tiny hair cells that transduce that movement into neural signals that indicate whether our body is moving, standing up, lying down, or falling. When our vestibular information matches what we see with our eyes we have no problems. But when we are on a boat or a carnival ride these two sources of information may not match up, which can make us feel ill.

Answers to Your Brain and Behaviour Questions

1. Explain how multiple senses (vision, smell, taste, touch) are involved in our experience of eating.Vision is involved in determining whether the food appears to be edible/appetizing. Information from photoreceptors in the eyes is sent to the thalamus, then the visual cortex in the occipital lobes. The experiences of taste and smell combine to produce flavour (as revealed by the dampening of food’s taste when we have a cold). Information from the tongue (e.g., about a food’s texture) is sent to the somatosensory cortex.
2. Explain how the taste of food may be enhanced if we close our eyes.When our eyes are open, some neural circuits show lessened activity. Closing our eyes may “turn up the volume” of these circuits and enhance our experience of taste and smell.
3. Which areas of the brain are responsible for integrating information about various components of eating (e.g., taste, smell, texture)? Association areas of the neocortex integrate the information received by the senses to produce our overall experience of eating.

Comer/Psychology Around Us, Second Canadian Edition