Biopsychology, Sensory Systems PDF

Chapter 7 Sensory Systems, Perception, and Attention How You Know the World Barry Diomede/Alamy Stock Photo Chapter Overview and Learning Objectives Principles of Sensory LO 7....

Chapter 7 Sensory Systems, Perception, and Attention How You Know the World Barry Diomede/Alamy Stock Photo Chapter Overview and Learning Objectives Principles of Sensory LO 7.1 Name and define the three types of sensory cortex. System Organization LO 7.2 In the context of sensory system organization, explain what is meant by each of the following terms: hierarchical organization, functional segregation, and parallel processing. Summarize the ­current model of sensory system organization. Auditory System LO 7.3 Explain the relationship between the physical and perceptual dimensions of sound. LO 7.4 Describe the components of the human ear, and explain how sound is processed within its various structures. LO 7.5 Describe the major pathways that lead from the ear to the ­primary auditory cortex. 183 M07_PINE1933_11_GE_C07.indd 183 22/01/2021 11:00  184 Chapter 7 LO 7.6 Describe the organization of auditory cortex. LO 7.7 Describe the effects of damage to the auditory system. Somatosensory System: LO 7.8 Name some of the cutaneous receptors and explain the Touch and Pain ­functional significance of fast versus slow receptor adaptation. LO 7.9 Describe the two major somatosensory pathways. LO 7.10 Describe the cortical somatosensory areas and their somatotopic layout. LO 7.11 Name the areas of association cortex that somatosensory signals are sent to, and describe the functional properties of one of those areas. LO 7.12 Describe the two major types of somatosensory agnosia. LO 7.13 Describe the rubber-hand illusion and its neural mechanisms. LO 7.14 Explain why the perception of pain is said to be paradoxical. LO 7.15 Define neuropathic pain and describe some of its putative neural mechanisms. Chemical Senses: LO 7.16 Describe two adaptive roles for the chemical senses. Smell and Taste LO 7.17 Describe the olfactory system. LO 7.18 Describe the gustatory system. LO 7.19 Explain the potential effects of brain damage on the chemical senses. Perception LO 7.20 Use examples to illustrate the role of experience in perception. LO 7.21 Explain perceptual decision making, using some examples of phantom percepts to illustrate. LO 7.22 Explain the binding problem and describe two potential ­solutions to it. Selective Attention LO 7.23 Describe the two characteristics of selective attention and explain what is meant by exogenous versus endogenous attention. LO 7.24 Describe the phenomenon of change blindness. LO 7.25 Describe the neural mechanisms of attention. LO 7.26 Describe the disorder of attention known as simultanagnosia. Two chapters in this text focus on the five human extero- Although we focus on the five human exteroceptive ceptive sensory systems: Chapter 6 and this one. Whereas senses in this book, it is important to realize that there Chapter 6 introduced the visual system, this chapter focuses are other sorts of exteroceptive senses that we don’t have. on the remaining four of our five exteroceptive sensory For example, many species can sense the earth’s magnetic systems (sensory systems that detect stimuli outside of field and use that information to navigate (see Nordmann, our bodies): the auditory (hearing), somatosensory (touch), Hochstoeger, & Keays, 2017). In addition, sharks, electric ­olfactory (smell), and gustatory (taste) systems. fish, and many amphibians detect minute electrical signals, M07_PINE1933_11_GE_C07.indd 184 22/01/2021 11:00  Sensory Systems, Perception, and Attention 185 and use them for navigation, hunting, and communication secondary, and association. The primary sensory cortex of (see Bellono, Leitch, & Julius, 2017). a system is the area of sensory cortex that receives most of In addition to covering the mechanisms of various sorts of its input directly from the thalamic relay nuclei of that sys- sensation (the process of detecting the presence of stimuli), this tem. For example, as you learned in Chapter 6, the primary chapter also discusses mechanisms of p ­ erception: the higher- visual cortex is the area of the cerebral cortex that receives order process of integrating, recognizing, and interpreting most of its input from the lateral geniculate nucleus of the ­patterns of sensations. Although you will encounter examples thalamus. The secondary sensory cortex of a system com- of perception in the second, third, and fourth modules, the prises the areas of the sensory cortex that receive most of their topic of perception is the focus of the fifth module. The chapter input from the primary sensory cortex of that system or from ends with an overview of the mechanisms of attention: how other areas of secondary sensory cortex of the same system. our brains manage to attend to a select few sensory stimuli ­Association cortex is any area of cortex that receives input despite being continuously bombarded by thousands of them. from more than one sensory system. Most input to areas of Before you begin the first module of this chapter, con- association cortex comes via areas of secondary sensory cortex. sider the following case (Williams, 1970). As you read the The interactions among these three types of sensory chapter, think about this patient, the nature of his deficit, cortex and among other sensory structures are character- and the likely location of his brain damage. By the time ized by three major principles: hierarchical organization, you have reached the final module of this chapter, you will functional segregation, and parallel processing. better understand this patient’s problem. Features of Sensory System The Case of the Man Who Could Organization See Only One Thing at a Time LO 7.2 In the context of sensory system organization, A 68-year-old patient was referred because he had difficulty find- explain what is meant by each of the following ing his way around—even around his own home. The patient terms: hierarchical organization, functional attributed his problems to his “inability to see properly.” It was segregation, and parallel processing. Summarize found that if two objects (e.g., two pencils) were held in front of the current model of sensory system him at the same time, he could see only one of them, whether organization. they were held side by side, one above the other, or even one partially behind the other. Pictures of single objects or faces Sensory systems are characterized by hierarchical could be identified, even when quite complex; but if a picture ­organization. A hierarchy is a system whose members can included two objects, only one object could be identified at a be assigned to specific levels or ranks in relation to one time—he would perceive the first object, after which it would be another. For example, an army is a hierarchical system replaced by a perception of the second object, which would then because all soldiers are ranked with respect to their author- be replaced by a perception of the first object, and so on. If the ity. In the same way, sensory structures are organized in a patient was shown overlapping drawings (i.e., one drawn on top hierarchy on the basis of the specificity and complexity of of another), he would see one but deny the existence of the other. their function. As one moves through a sensory system from receptors, to thalamic nuclei, to primary sensory cortex, to secondary sensory cortex, to association cortex, one finds neurons that respond optimally to stimuli of greater and Principles of Sensory greater specificity and complexity. Each level of a sensory hierarchy receives much of its input from lower levels and System Organization adds another layer of analysis before passing it on up the hierarchy (see Rees, Kreiman, & Koch, 2002). The visual system is by far the most thoroughly studied The hierarchical organization of sensory systems is sensory system. As a result, it is also the best understood. apparent from a comparison of the effects of damage to vari- However, as more has been discovered about the other sen- ous levels: The higher the level of damage, the more specific sory systems, it has become apparent that each is organized and complex the deficit. For example, destruction of a sen- like the visual system in fundamental ways. sory system’s receptors produces a complete loss of ability to perceive in that sensory modality (e.g., total blindness or Types of Sensory Areas of Cortex deafness); in contrast, destruction of an area of association or secondary sensory cortex typically produces complex and LO 7.1 Name and define the three types of sensory cortex. specific sensory deficits, while leaving fundamental sensory The sensory areas of the cortex are, by convention, consid- abilities intact. Dr. P., the man who mistook his wife for a hat ered to be of three fundamentally different types: primary, (Sacks, 1985), displayed such a pattern of deficits. M07_PINE1933_11_GE_C07.indd 185 22/01/2021 11:00  186 Chapter 7 FUNCTIONAL SEGREGATION. It was once assumed that Case of the Man Who Mistook the primary, secondary, and association areas of a sensory His Wife for a Hat* system were each functionally homogeneous. That is, it was assumed that all areas of cortex at any given level of a sen- Dr. P. was a highly respected musician and teacher—a charm- sory hierarchy acted together to perform the same function. ing and intelligent man. He had been referred to the eminent However, research has shown that functional segregation, neurologist Oliver Sacks for help with a vision problem. At least, rather than functional homogeneity, characterizes the orga- as Dr. P. explained to the neurologist, other people seemed to nization of sensory systems. It is now clear that each of the think that he had a vision problem, and he did admit that he three levels of cerebral cortex—primary, secondary, and sometimes made odd errors. Dr. Sacks tested Dr. P.’s vision and found his visual acuity association—in each sensory system contains functionally to be excellent—Dr. P. could easily spot a pin on the floor. The distinct areas that specialize in different kinds of analysis. first sign of a problem appeared when Dr. P. needed to put his PARALLEL PROCESSING. It was once believed that the shoe back on following a standard reflex test. Gazing at his foot, different levels of a sensory hierarchy were connected in a he asked Sacks if it was his shoe. serial fashion. In a serial system, information flows among Continuing the examination, Dr. Sacks showed Dr. P. a the components over just one pathway, like a string through glove and asked him what it was. Taking the glove and puzzling over it, Dr. P. could only guess that it was a container divided a strand of beads. However, we now know that sensory sys- into five compartments for some reason. Even when Sacks tems are parallel systems in which information flows through asked whether the glove might fit on some part of the body, the components over multiple pathways (see Lleras et al., Dr. P. displayed no signs of recognition. 2017). Parallel systems feature parallel processing—the At that point, Dr. P. seemed to conclude that the examina- simultaneous analysis of a signal in different ways by the tion was over and, from the expression on his face, that he had multiple parallel pathways of a neural network. done rather well. Preparing to leave, he turned and grasped SUMMARY MODEL OF SENSORY SYSTEM ORGANIZA- his wife’s head and tried to put it on his own. Apparently, he thought it was his hat. TION. Figure 7.1 summarizes the information in this mod- Mrs. P. showed little surprise. That kind of thing happened ule by illustrating how thinking about the organization of a lot. sensory systems has changed. In the 1960s, sensory systems were believed to be hierarchical, functionally homogeneous, Figure 7.1 Two models of sensory system organization: The former model was hierarchical, functionally homogeneous, and serial; the current model, which is more consistent with the evidence, is hierarchical, functionally segregated, and parallel. Not shown in the current model are the many descending pathways—one means by which higher levels of sensory systems can influence sensory input. Former Model Current Model Hierarchical Hierarchical Functionally Homogeneous Functionally Segregated Serial Parallel Association Cortex Secondary Sensory Cortex Primary Sensory Cortex Thalamus Receptors *Based on The Man Who Mistook His Wife for a Hat and Other Clinical Tales by Oliver Sacks. Copyright © 1970, 1981, 1983, 1984, 1986 by Oliver Sacks. M07_PINE1933_11_GE_C07.indd 186 22/01/2021 11:00  Sensory Systems, Perception, and Attention 187 in sequence at the auditory system, the Figure 7.2 The relation between the physical and perceptual dimensions of sound. somatosensory system, and the chemical sensory systems (smell and taste). Physical Physical Perceptual Dimension Stimulus Dimension Amplitude Loudness Auditory System The function of the auditory system is the High Low perception of sound. Sounds are vibrations of air molecules that stimulate the auditory Frequency Pitch system; humans hear only those mole­cular vibrations between about 20 and 20,000 hertz Low High (cycles per second). Complexity Timbre Physical and Perceptual Pure Rich Dimensions of Sound LO 7.3 Explain the relationship between the physical and Figure 7.3 The breaking down of a sound—in this case, the sound of a perceptual dimensions of clarinet—into its component sine waves by Fourier analysis. When added sound. together, the component sine waves produce the complex sound wave. Figure 7.2 illustrates how sounds are com- monly recorded in the form of waves and the relation between the physical dimensions of sound vibrations and our perceptions of them. The amplitude, frequency, and complexity When of the molecular vibrations are most closely added linked to perceptions of loudness, pitch, and together, these sine timbre, respectively. waves Pure tones (sine wave vibrations) exist only produce in laboratories and sound recording studios; this clarinet in real life, sound is always associated with sound. complex patterns of vibrations. For example,­ Figure 7.3 illustrates the complex sound wave associated with one note of a clarinet. The figure also illustrates that any complex Waveform of a clarinet sound sound wave can be broken down mathemati- cally into a series of sine waves of various frequencies and amplitudes; these compo- nent sine waves produce the original sound and serial. However, subsequent research has established when they are added together. ­Fourier ­analysis is the that sensory systems are hierarchical, functionally segre- mathematical procedure for breaking down complex gated, and parallel (see Rauschecker, 2015). waves into their component sine waves. One theory of Not shown in Figure 7.1 are the many neurons that audition is that the auditory system performs a Fourier-like descend through the sensory hierarchies. Although sen- analysis of complex sounds in terms of their component sory systems carry information from lower to higher sine waves. levels of their respective hierarchies, they also conduct For any pure tone, there is a close relationship between information in the opposite direction (from higher to the frequency of the tone and its perceived pitch; however, lower levels). These are known as top-down signals (see the relation between the frequencies that make up natural Bressler & Richter, 2015; Marques et al., 2018; Ruff, 2013). sounds (which are always composed of a mixture of Now that you have an understanding of the general frequencies) and their perceived pitch is complex (see principles of sensory system organization, let’s take a look Bidelman & Grall, 2014): The pitch of such sounds is M07_PINE1933_11_GE_C07.indd 187 22/01/2021 11:00  188 Chapter 7 related to their fundamental frequency: the frequency that fundamental frequency (i.e., the highest common divisor) is the highest common divisor (a number that divides of 200, 300, and 400 Hz. This important aspect of pitch another number) for the various component frequencies. perception is referred to as the missing fundamental (see For example, a sound that is a mixture of 100, 200, and Oxenham, 2018). 300 Hz frequencies normally has a pitch related to 100 Hz because 100 Hz is the highest common divisor of the The Ear three components. An extremely important characteristic of pitch perception is the fact that the pitch of a complex LO 7.4 Describe the components of the human ear, sound may not be directly related to the frequency of any and explain how sound is processed within its of the sound’s components (see Lau & Werner, 2014). For various structures. example, a mixture of pure tones with frequencies of 200, The ear is illustrated in Figure 7.4. Sound waves travel 300, and 400 Hz would be perceived as having the same from the outer ear down the auditory canal and cause pitch as a pure tone of 100 Hz—because 100 Hz is the t ympanic membrane (the eardrum) to vibrate. the ­ Figure 7.4 Anatomy of the ear. Auditory Semicircular nerve canals Ossicles Cochlea (unwound) Round window Oval Tympanic window membrane Cross Section of Cochlea Tectorial membrane Organ Hair cells of Corti Basilar membrane Auditory nerve M07_PINE1933_11_GE_C07.indd 188 22/01/2021 11:00  Sensory Systems, Perception, and Attention 189 These vibrations are then transferred to the three ossicles— Middlebrooks, 2013; Christison-Lagay & Cohen, 2014; the small bones of the middle ear: the malleus (the hammer), Christison-Lagay, Gifford, & Cohen, 2015). For example, the incus (the anvil), and the stapes (the stirrup). The vibrations you hear the speech of the person standing next to you as a of the stapes trigger vibrations of the membrane called the separate sequence of sounds, despite the fact that it contains oval window, which in turn transfers the vibrations to many of the same component frequencies coming from the fluid of the snail-shaped cochlea (kokhlos means “land other sources. The mechanism underlying this important snail”). The cochlea is a long, coiled tube with an internal ability has yet to be identified, but one theory is that it is due structure running almost to its tip. This internal structure is to the synchronous relationship over time of the frequency the auditory receptor organ, the organ of Corti. elements of each sound source (see ­Oxenham, 2018). Each pressure change at the oval window travels along Figure 7.4 also shows the semicircular canals—the the organ of Corti as a wave. The organ of Corti is composed receptive organs of the vestibular system. The vestibular of several membranes; we will focus on two of them: the system carries information about the direction and intensity basilar membrane and the tectorial membrane. The audi- of head movements, which helps us maintain our balance tory receptors, the hair cells, are mounted in the basilar (see Brandt & Dieterich, 2017; Gu, 2018). membrane, and the tectorial membrane rests on the hair cells. Accordingly, a deflection of the organ of Corti at any From the Ear to the Primary point along its length produces a shearing force on the hair cells at the same point. This force stimulates the hair cells, Auditory Cortex which in turn increase firing in axons of the auditory nerve LO 7.5 Describe the major pathways that lead from the (see Wu et al., 2017)—a branch of the auditory-vestibular ear to the primary auditory cortex. nerve (one of the 12 cranial nerves). The vibrations of the There is no major auditory pathway to the cortex compa- cochlear fluid are ultimately dissipated by the round window, rable to the visual system’s retina-geniculate-striate path- an elastic membrane in the cochlear wall. way. Instead, there is a network of auditory pathways, The cochlea is remarkably sensitive (see Hudspeth, some of which are illustrated in Figure 7.5. The axons of 2014). Humans can hear differences in pure tones that dif- each auditory nerve synapse in the ipsilateral cochlear nuclei, fer in frequency by only 0.2 percent. The major principle from which many projections lead to the superior olives of cochlear coding is that different frequencies produce on both sides of the brain stem at the same level. The axons maximal stimulation of hair cells at different points along of the olivary neurons project via the lateral lemniscus to the the basilar membrane—with higher frequencies producing inferior colliculi, where they synapse on neurons that greater activation closer to the windows and lower frequen- project to the medial geniculate nuclei of the thalamus, cies producing greater activation at the tip of the basilar which in turn project to the primary auditory cortex. Notice membrane. Thus, the many component frequencies that that signals from each ear are combined at a very low compose each complex sound activate hair cells at many level (in the superior olives) and are transmitted to both different points along the basilar membrane, and the many ipsilateral and contralateral auditory cortex. signals created by a single complex sound are carried out The subcortical pathways of the auditory system are of the ear by many different auditory neurons. Like the inherently complex, and they have many more synapses cochlea, most other structures of the auditory system are than the other senses (see Jasmin, Lima, & Scott, 2019; arrayed according to frequency. Thus, in the same way that Wang, 2018). Some researchers believe that the complex the organization of the visual system is largely retinotopic, subcortical organization of the auditory system is related the organization of the auditory system is largely tonotopic to the complexity of the analyses that the auditory system (see Schreiner & Polley, 2014). has to perform (see Wang, 2018). This brings us to the major unsolved mystery of audi- tory processing. Imagine yourself in a complex acoustic environment such as a party. The music is playing; people Auditory Cortex are dancing, eating, and drinking; and numerous conver- LO 7.6 Describe the organization of auditory cortex. sations are going on around you. Because the component frequencies in each individual sound activate many sites Recent progress in the study of human auditory cortex has along your basilar membrane, the number of sites simul- resulted from the convergence of functional brain-imaging taneously activated at any one time by the party noises is studies in humans and invasive neural recording studies in enormous. But somehow your auditory system manages monkeys (see Saenz & Langers, 2014). Still, primate audi- to sort these individual frequency messages into sepa- tory cortex is far from being well understood—for example, rate categories and combine them so that you hear each our understanding of it lags far behind our current under- source of complex sounds independently (see Bremen & standing of the visual cortex. M07_PINE1933_11_GE_C07.indd 189 22/01/2021 11:00  190 Chapter 7 identified. First, like the primary visual Figure 7.5 Some of the pathways of the auditory system that lead from one ear to the cortex. cortex, the primary auditory cortex is orga- nized in functional columns (see Mizrahi, Shalev, & Nelken, 2014): All of the neu- rons encountered during a vertical micro- electrode penetration of primary auditory ­cortex (i.e., a penetration at right angles to the cortical layers) tend to respond opti- mally to sounds in the same frequency Lateral Forebrain fissure range. Second, like the cochlea, auditory cortex has a tonotopic organization (see Primary auditory Jasmin, Lima, & Scott, 2019; Schreiner & cortex Polley, 2014): Each area of auditory cor- Medial geniculate tex appears to have a gradient of frequen- nucleus (thalamus) cies from low to high along its length. Third, auditory cortex is also organized Inferior colliculus according to the temporal components (tectum) of sound; that is, variations in the ampli- tude of particular sound frequencies over Midbrain time. For example, our auditory environ- ments almost never consist of sounds that do not vary in their intensity over Lateral lemniscus time. It seems that auditory cortex is sensitive to such fluctuations. This third organizing principle of auditory cortex Cochlear nuclei is known as p ­ eriodotopy (see Brewer & Barton, 2016). Hindbrain WHAT SOUNDS SHOULD BE USED TO STUDY AUDITORY CORTEX? Why has research on auditory cortex lagged behind research on visual cortex? There are sev- eral reasons, but a major one is a lack of Superior Auditory clear understanding of the dimensions olives nerve Cochlea along which auditory cortex evaluates In primates, the primary auditory cortex, Figure 7.6 General location of the primary auditory cortex and areas which receives the majority of its input from the of secondary auditory cortex. Most auditory cortex is hidden from view medial geniculate nucleus, is located in the tem- in the lateral fissure. poral lobe, hidden from view within the lateral Primary fissure (see Figure 7.6). Primate primary auditory auditory cortex comprises three adjacent areas (see Moerel, cortex De Martino, & Formisano, 2014): Together these three areas are referred to as the core region. Sur- Secondary auditory rounding the core region is a band—often called cortex the belt—of areas of secondary auditory cortex. Areas of secondary auditory cortex outside the belt are called parabelt areas (Jasmin, Lima, & Scott, 2019). In total, there seem to be about 13 separate areas of auditory cortex in primates (see Brewer & Lateral Barton, 2016). fissure ORGANIZATION OF PRIMATE AUDITORY ­CORTEX. Three important principles of orga- nization of primary auditory cortex have been M07_PINE1933_11_GE_C07.indd 190 22/01/2021 11:00  Sensory Systems, Perception, and Attention 191 sound (Sharpee, Atencio, & Schreiner, 2011). Figure 7.7 The hypothesized anterior and posterior auditory You may recall that research on the visual cor- pathways. tex did not start to progress rapidly until it was discovered that most visual neurons respond to Anterior Posterior auditory auditory contrast. There is clear evidence of a hierarchi- pathway pathway cal organization in auditory cortex—the neural responses of secondary auditory cortex tend Primary to be more complex and varied than those of auditory ­primary auditory cortex (see Jasmin, Lima, & cortex Scott, 2019). Many neurons in auditory cortex respond only weakly to simple stimuli such as pure tones, which have been widely employed in electrophysiological studies of auditory cortex. Secondary This practice is changing, however, partly in auditory cortex response to the discovery that natural sounds, in general, are better at eliciting responses from neurons in mammalian auditory cortex (see ­G ervain & ­G effen, 2019; Kopp-Scheinpflug, Sinclair, & ­Linden, 2019). WHAT ANALYSES DOES THE AUDITORY C ­ ORTEX AUDITORY–VISUAL INTERACTIONS. Sensory systems ­PERFORM? We now know that calculations by the audi- have traditionally been assumed to interact in association tory cortex produce signals that are not faithful representa- cortex. Indeed, as you have already learned, association cortex tion of sounds (see Tsunada et al., 2016; Wang, 2018). More is usually defined as areas of cortex where such interactions, specifically, auditory cortex is now known to integrate or associations, take place. Much of the research on sensory information about the current perceptions and behaviors system interactions has focused on interactions between the of an animal in order to produce auditory signals that are auditory and visual systems, particularly on those that occur relevant to the animal’s current situation (see Kuchibhotla in the posterior parietal cortex (see Brang et al., 2013; Cohen, & Bathellier, 2018; Lima, Krishnan, & Scott, 2016; Schneider 2009). In one study of monkeys (Mullette-Gillman, Cohen, & Mooney, 2018). & Groh, 2005), some posterior parietal neurons were found One example of an output signal from auditory cortex to have visual receptive fields, some were found to have that is particularly relevant to an animal’s current situa- auditory receptive fields, and some were found to have both. tion is the creation of representations of auditory objects. Functional brain imaging is widely used to investigate For example, it is believed that the auditory cortex can sensory system interactions. One advantage of functional brain take the complex mixture of frequencies produced by imaging is that it does not focus on any one part of the brain; a piano and convert it into a sound representation that it records activity throughout the brain. Functional brain- allows us to say “That’s the sound of a piano!” (see imaging studies have confirmed that sensory interactions Angeloni & Geffen, 2018; Kuchibhotla & Bathellier, 2018; do occur in association cortex, but more importantly, they Tsunada et al., 2016). have repeatedly found evidence of sensory interactions at the lowest level of the sensory cortex hierarchy, in areas of TWO STREAMS OF AUDITORY CORTEX. Thinking primary sensory cortex (see Man et al., 2013; Smith & Goodale, about the general organization of auditory cortex has 2015). This discovery is changing how we think about the been inspired by research on visual cortex. Researchers interaction of sensory systems: Sensory system interaction is have proposed that, just as there are two main cortical not merely tagged on after unimodal (involving one system) streams of visual analysis (dorsal and ventral), there are analyses are complete; sensory system interactions seem to be two main cortical streams of auditory analysis. Auditory an early and integral part of sensory processing. signals are ultimately conducted to two large areas of association cortex: prefrontal cortex and posterior parietal WHERE DOES THE PERCEPTION OF PITCH OCCUR? cortex. There is good evidence that the anterior auditory Recent research has answered one fundamental ques- pathway is more involved in identifying sounds (what), tion about auditory cortex: Where does the perception of whereas the posterior auditory pathway is more involved in pitch likely occur? This seemed like a simple question to locating sounds (where)—see Jasmin, Lima, & Scott (2019) answer because most areas of auditory cortex have a clear and van der Heijden et al. (2019). These pathways are tonotopic organization. However, when experimenters illustrated in Figure 7.7. used sound stimuli in which frequency and pitch were M07_PINE1933_11_GE_C07.indd 191 22/01/2021 11:00  192 Chapter 7 different—for example, by using the missing fundamental 2016). Total deafness is rare, occurring in only 1 percent of technique—most auditory neurons responded to changes hearing-impaired individuals. in frequency rather than pitch. This information led Severe hearing problems typically result from damage Bendor and Wang (2005) to probe primary and second- to the inner ear or the middle ear or to the nerves leading ary areas of monkey auditory cortex with microelectrodes from them rather than from more central damage. There are to assess the responses of individual neurons to missing two common classes of hearing impairments: those associ- fundamental stimuli. They discovered one small area just ated with damage to the ossicles (conductive deafness) and anterior to primary auditory cortex that contained many those associated with damage to the cochlea or auditory neurons that responded to pitch rather than frequency, nerve (nerve deafness). The major cause of nerve deafness regardless of the quality of the sound. The same small is a loss of hair cell receptors (see Wallis, 2018; Wong & area also contained neurons that responded to frequency, Ryan, 2015). and Bendor and Wang suggested that this area was likely If only part of the cochlea is damaged, individuals the place where frequencies of sound were converted to may have nerve deafness for some frequencies but not the perception of pitch. A comparable pitch area has been others. For example, age-related hearing loss features a identified by fMRI studies in a similar location in the specific deficit in hearing high frequencies. That is why human brain. elderly people often have difficulty distinguishing “s,” “f,” and “t” sounds: They can hear people speaking to them but often have difficulty understanding what people are Effects of Damage to the Auditory saying. Often, relatives and friends do not realize that System much of the confusion displayed by the elderly stems from difficulty discriminating sounds (see Wingfield, Tun, LO 7.7 Describe the effects of damage to the auditory & McCoy, 2005). Unfortunately, hearing aids often do not system. help with the processing of speech (see Lesica, 2018; Peelle The study of damage to the auditory system is important & Wingfield, 2016). for two reasons. First, it provides information about how Hearing loss is sometimes associated with tinnitus the auditory system works. Second, it can serve as a source (ringing of the ears). When only one ear is damaged, of information about the causes and treatment of clinical the ringing is perceived as coming from that ear; how- deafness. ever, cutting the nerve from the ringing ear has no effect on the ringing. This suggests that neuroplastic changes AUDITORY CORTEX DAMAGE. Following bilateral to the auditory system resulting from deafness are the lesions to the primary auditory cortex, there is often a cause of tinnitus (see Eggermont & Tass, 2015; Elgoyhen complete loss of hearing, which presumably results from et al., 2015; Shore, R ­ oberts, & Langguth, 2016; Sedley the shock of the lesion because hearing recovers in the et al., 2016). ensuing weeks. The major permanent effects are loss of the ability to process the structural aspects of sounds— an ability that is necessary for the processing of speech Journal Prompt 7.1 sounds. Accordingly, patients with bilateral auditory cor- tex lesions are often said to be “word deaf” (see Jasmin, Why do you think tinnitus is associated with deafness? [Hint: Sensory neurons don’t stop firing in the absence Lima, & Scott, 2019). of sensory input.] Consistent with the definitions of the two cortical auditory pathways, patients with damage to the anterior auditory cortex pathway (the what pathway) have trouble identifying sounds. Whereas, patients with damage to Some people with nerve deafness benefit from the posterior auditory cortex pathway (the where path- cochlear implants (see Figure 7.8). Cochlear implants bypass way) have difficulty localizing sounds (see Jasmin, Lima, damage to the auditory hair cells by converting sounds & Scott, 2019). picked up by a microphone on the patient’s ear to elec- trical signals, which are then carried into the cochlea by DEAFNESS IN HUMANS. Deafness is one of the most a bundle of electrodes. These signals excite the auditory prevalent human disabilities: An estimated 360 million nerve. Although cochlear implants can provide major ben- people currently suffer from disabling hearing impair- efits, they do not restore normal hearing. The sooner a ments (Lesica, 2018). Hearing impairment affects more person receives a cochlear implant after becoming deaf, than one’s ability to detect sounds: it can lead to feelings of the more likely they are to benefit, because disuse leads social isolation and has been associated with an increased to alterations of the auditory neural pathways (see Kral & risk for dementia (see Lesica, 2018; Peelle & Wingfield, Sharma, 2012). M07_PINE1933_11_GE_C07.indd 192 22/01/2021 11:00  Sensory Systems, Perception, and Attention 193 Figure 7.8 Cochlear implant: The surgical implantation is shown on the left, and a child with an implant is shown on the right. AJPhoto/Science Source Gene J. Puskar/AP Images Scan Your Brain Before we go on to discuss the other sensory systems, pause 7. The three smallest bones in the human body are the and scan your brain to check your knowledge of what you have malleus, the incus, and the _______. learned in this chapter so far. Fill in the following blanks with 8. The _______ are auditory receptors located in the the most appropriate terms. The correct answers are provided cochlea on the basilar membrane, and they increase at the end of the exercise. Before proceeding, review material firing in axons of the auditory nerve. related to your errors and omissions. 9. Axons from olivary neurons project to the _______ _______ via the lateral lemniscus. 1. An area of the cortex that receives input from multiple 10. The cochlea and the primary auditory cortex are both sensory systems and integrates sensory information is organized _______ on the bases of sound frequencies. called the _______ cortex. 11. Damage to the ossicles is associated with _______ 2. Sensation is the process of detecting the presence of deafness, while damage to the cochlea is associated stimuli, and the higher-order process called _______ with _______ deafness. allows the interpretation of sensory patterns. 12. The anterior auditory pathway is more involved in 3. The simultaneous analysis of signals, also known as i­dentifying sounds (what), whereas the _______ auditory _______ processing, allows for information to flow pathway is more involved in locating sounds (where). through multiple pathways at the same time. 4. In the 1960s, the sensory organization was believed to be hierarchical, _______ _______, and serial. 5. The frequency of sound vibrations is linked to ­perceptions of _______. (11) conductive, nerve, (12) posterior. 6. Sound waves travel from the external environment to (8) hair cells, (9) inferior colliculi, (10) tonotopically, the outer ear, and through the auditory canal where they (4) functionally homogeneous, (5) pitch, (6) tympanic, (7) stapes, reach the _______ membrane. Scan Your Brain answers: (1) association, (2) perception, (3) parallel, M07_PINE1933_11_GE_C07.indd 193 22/01/2021 11:00  194 Chapter 7 Figure 7.9 Four cutaneous receptors that occur in Somatosensory System: human skin. Ruffini Merkel’s Free Pacinian Touch and Pain ending disks nerve ending corpuscles Sensations from your body are referred to as somatosensa- tions. The system that mediates these bodily sensations—the somatosensory system—is composed of three separate but interacting systems: (1) an exteroceptive system, which senses external stimuli that are applied to the skin; (2) a propriocep- tive system, which monitors information about the position of the body that comes from receptors in the muscles, joints, and organs of balance; and (3) an interoceptive system, which pro- vides general information about conditions within the body (e.g., temperature and blood pressure). This module deals almost exclusively with the exteroceptive system, which itself comprises three somewhat distinct divisions: a divi- sion for perceiving mechanical stimuli (touch), one for thermal stimuli (temperature), and one for nociceptive stimuli (pain). Cutaneous Receptors LO 7.8 Name some of the cutaneous receptors and explain the functional significance of fast versus slow receptor adaptation. There are many kinds of receptors in the skin (see Owens & Sweat Lumpkin, 2014; Zimmerman, Bai, & Ginty, 2014). Figure 7.9 Artery Vein gland Fat illustrates four of them. The simplest cutaneous receptors are the free nerve endings (neuron endings with no specialized function. However, in general, the various receptors tend structures on them), which are particularly sensitive to tem- to function in the same way: Stimuli applied to the skin perature change and pain. The largest and deepest cutane- deform or change the chemistry of the receptor, and this ous receptors are the onion-like Pacinian corpuscles; because in turn changes the permeability of the receptor cell mem- they adapt rapidly, they respond to sudden displacements brane to various ions (see Delmas, Hao, & Rodat-Despoix, of the skin but not to constant pressure. In contrast, Merkel’s 2011). The result is a neural signal. disks and Ruffini endings both adapt slowly and respond to Initially, it was assumed that each type of receptor gradual skin indentation and skin stretch, respectively. located in the skin (see Figure 7.9) mediates a different To appreciate the functional significance of fast and tactile sensation (e.g., touch, pain, heat), but this has not slow receptor adaptation, consider what happens when a proven to be the case. Each tactile sensation appears to be constant pressure is applied to the skin. The pressure evokes produced by the interaction of multiple receptor mecha- a burst of firing in all receptors, which corresponds to the nisms, and each receptor mechanism appears to contrib- sensation of being touched; however, after a few hundred ute to multiple sensations (see Hollins, 2010; Lumpkin & milliseconds, only the slowly adapting receptors remain Caterina, 2007; McGlone & Reilly, 2010). In addition, skin active, and the quality of the sensation changes. In fact, cells that surround particular receptors also seem to play a you are often totally unaware of constant skin pressure; role in the quality of the sensations produced by that recep- for example, you are usually unaware of the feeling of your tor (see Zimmerman, Bai, & Ginty, 2014). Indeed, new forms clothes against your body until you focus attention on it. of tactile sensation are still being discovered (see McGlone, As a consequence, when you try to engage in stereognosis Wessberg, & Olausson, 2014; Ran, Hoon, & Chen, 2016). (identifying objects by touch), you manipulate the object in your hands so that the pattern of stimulation continually Two Major Somatosensory Pathways changes. Having some receptors that adapt quickly and some that adapt slowly provides information about both LO 7.9 Describe the two major somatosensory the dynamic and static qualities of tactual stimuli. pathways. The structure and physiology of each type of somato- Somatosensory information ascends from each side of the sensory receptor seem to be specialized for a different body to the human cortex over several pathways, but there M07_PINE1933_11_GE_C07.indd 194 22/01/2021 11:00  Sensory Systems, Perception, and Attention 195 are two major ones: the dorsal-column medial-lemniscus enter the spinal cord via a dorsal root, ascend ipsilaterally system and the anterolateral system. The dorsal-column in the dorsal columns, and synapse in the dorsal column medial-lemniscus system tends to carry information about nuclei of the medulla. The axons of dorsal column nuclei touch and proprioception, and the anterolateral system neurons decussate (cross over to the other side of the brain) tends to carry information about pain and temperature (see and then ascend in the medial lemniscus to the contra- lateral ventral posterior nucleus of the thalamus. The Ran et al., 2016). The key words in the preceding sentence are “tends to”: The separation of function in the two pathways isventral posterior nuclei also receive input via the three far from complete. Accordingly, lesions of the dorsal-column branches of the trigeminal nerve, which carry somatosen- medial-lemniscus system do not eliminate touch perception sory information from the contralateral areas of the face. or proprioception, and lesions of the anterolateral system doMost neurons of the ventral posterior nucleus project to not eliminate perception of pain or temperature. the primary somatosensory cortex (SI); others project to the The dorsal-column medial-lemniscus system is illus- secondary somatosensory cortex (SII) or the posterior pari- trated in Figure 7.10. The sensory neurons of this system etal cortex. Neuroscience trivia buffs will almost certainly want to add to their collection the fact that the dorsal column neurons that originate Figure 7.10 The dorsal-column medial-lemniscus system. The pathways from only one side of the body are shown. in the toes are the longest neurons in the human body. The anterolateral system is illustrated Somatosensory in Figure 7.11. Most dorsal root neurons of cortex the anterolateral system synapse as soon as they enter the spinal cord. The axons of most of the second-order neurons decussate but then ascend to the brain in the contra- Forebrain lateral anterolateral portion of the spinal Ventral posterior cord; however, some do not decussate but nucleus (thalamus) ascend ipsilaterally. The anterolateral sys- tem comprises three different tracts: the spinothalamic tract, the spinoreticular tract, and the spinotectal tract. The three branches of the trigeminal nerve carry pain and tem- Medial lemniscus perature information from the face to the same thalamic sites. The pain and tempera- Trigeminal ture information that reaches the thalamus nucleus is then distributed to somatosensory cortex and other parts of the brain. If both ascending somatosensory Hindbrain Three branches paths are completely transected by a spi- of the trigeminal nerve nal injury, the patient can feel no body sensation from below the level of the cut. Dorsal column Clearly, when it comes to spinal injuries, nuclei lower is better. Dorsal column Cortical Areas of Somatosensation LO 7.10 Describe the cortical somatosensory areas and their Spinal Dorsal root somatotopic layout. cord In 1937, Penfield and his colleagues mapped the primary somatosensory cor- Sensory neuron tex of patients during neurosurgery (see from the skin Figure 7.12). Penfield applied electrical stimulation to various sites on the cortical M07_PINE1933_11_GE_C07.indd 195 22/01/2021 11:00  196 Chapter 7 somatotopic map is commonly referred Figure 7.11 The anterolateral system. The pathways from only one side of the body are shown. to as the ­somatosensory homunculus (homunculus means “little man”). Notice in Figure 7.12 that the somatosensory homunculus is dis- torted; the greatest proportion of SI is dedicated to receiving input from the Thalamic nuclei parts of the body we use to make tactile (ventral posterior, Forebrain discriminations (e.g., hands, lips, and intralaminar, parafascicular, tongue). In contrast, only small areas etc.) of SI receive input from large areas of the body, such as the back, that are not usually used to make somatosensory discriminations. The Check It Out dem- Tectum onstration on page 197 allows you to experience the impact this organization Midbrain has on your ability to perceive touches. A second somatotopically organized area, SII, lies just ventral to SI in the post- central gyrus, and much of it extends into the lateral fissure. SII receives most of its input from SI and is thus regarded as secondary somatosensory cortex. In contrast to SI, whose input is largely contralateral, SII receives substantial Hindbrain Three branches input from both sides of the body. Much of trigeminal of the output of SI and SII goes to the nerve association cortex of the posterior parietal lobe (see McGlone & Reilly, 2010). Spinothalamic Studies of the responses of single tract neurons in primary somatosensory Spinotectal cortex found evidence for columnar tract ­organization—-similar to that in visual Spinoreticular and auditory cortex. Each neuron in a tract particular column of primary somato- sensory cortex had a receptive field on the same part of the body and responded most robustly to the same type of tac- Spinal cord tile stimuli (e.g., light touch or heat). Moreover, single-neuron recordings suggested that primary somatosensory- cortex is composed of four functional strips, each with a similar, but separate, somatotopic organization. Each strip of primary somatosensory cortex is most surface, and the patients, who were fully conscious under a sensitive to a different kind of somato- local anesthetic, described what they felt. When stimulation sensory input (e.g., to light touch or pressure). Thus, if one was applied to the postcentral gyrus, the patients reported were to record from neurons across the four strips, one somatosensory sensations in various parts of their bodies. would find neurons that “preferred” four different kinds of When Penfield mapped the relation between each site of tactile stimulation, all to the same part of the body. stimulation and the part of the body in which the sensation Reminiscent of the developments in the study of visual was felt, he discovered that the human primary somato- and auditory cortex, it has been proposed that two streams sensory cortex (SI) is somatotopic—organized according of analysis proceed from SI: a dorsal stream that projects to to a map of the body surface (see Chen et al., 2015). This posterior parietal cortex and participates in multisensory M07_PINE1933_11_GE_C07.indd 196 22/01/2021 11:00  Sensory Systems, Perception, and Attention 197 Figure 7.12 The locations of human primary somatosensory cortex (SI) and one area of secondary somatosensory cortex (SII) with the Check It Out conventional portrayal of the somatosensory homunculus. Something has always confused us about this portrayal of the somatosensory Touching a Back homunculus: The body is upside down, while the face is right side up. It now appears that this conventional portrayal is wrong. The results Because only a small portion of human primary of an fMRI study suggest that the face representation is also inverted. somatosensory cortex receives input from the (Based on Servos et al., 1999.) entire back, people have difficulty recognizing objects that touch their backs. You may not have r Shoulde Head noticed this tactile deficiency—unless, of course, Neck Trunk you often try to identify objects by feeling them Leg Wri rm Elbow Hip with your back. You will need one thing to dem- a Litt d st Fore Han Rin le Th dex le Mi g onstrate the recognition deficiencies of the human In dd b back: a friend. Touch your friend on the back with um one, two, or three fingers, and ask your friend how e Fa e Foot many fingers he or she feels. When using two or Ey s lip No ce r three fingers, be sure they touch the back simul- Toes pe Up ip er l taneously because temporal cues invalidate this Genitals Low test of tactile discrimination. Repeat the test many times, adjusting the distance between the touches on each trial. Record the results. What you should Teeth begin to notice is that the back is incapable of dis- criminating between separate touches unless the Tongue distance between the touches is considerable. In Pharynx contrast, fingertips can distinguish the number of simultaneous touches even when the touches are Abdomen very close. Central Primary fissure Secondary somatosensory somatosensory cortex (SI) cortex (SII) Based on Servos, P., Engel, S. A., Gati, J., & Menon, R. (1999). fMRI evidence for an inverted face representation in human somatosensory cortex. Neuroreport, 10(7), 1393–1395. Steven J. Barnes integration and direction of attention, and a ventral stream that projects to SII and participates in the perception of (1970) assessed the somatosensory abilities of patients with objects’ shapes (Yau, Connor, & Hsiao, 2013). epilepsy before and after a unilateral excision that included EFFECTS OF DAMAGE TO THE PRIMARY SOMATO- SI. Following the surgery, the patients displayed two minor SENSORY CORTEX. The effects of damage to the contralateral deficits: a reduced ability to detect light touch primary somatosensory cortex are often remarkably mild—­ and a reduced ability to identify objects by touch (i.e., a defi- presumably because the somatosensory system features cit in stereognosis). These deficits were bilateral only in those numerous parallel pathways. Corkin, Milner, and Rasmussen cases in which the unilateral lesion encroached on SII. M07_PINE1933_11_GE_C07.indd 197 22/01/2021 11:00  198 Chapter 7 Somatosensory System and only the left side of the body, and it is usually associated with extensive damage to the right temporal and posterior Association Cortex parietal lobe (Feinberg et al., 2010). The case of Aunt Betty LO 7.11 Name the areas of association cortex that (Klawans, 1990) is an example. somatosensory signals are sent to, and describe the functional properties of one of those areas. The Case of Aunt Betty, Somatosensory signals are ultimately conducted to the highest level of the sensory hierarchy, to areas of association Who Lost Half of Her Body* cortex in prefrontal and posterior parietal cortex. Aunt Betty was my patient. She wasn’t really my aunt, she was Posterior parietal cortex contains bimodal neurons (neu- my mother’s best friend. rons that respond to activation of two different sensory sys- As we walked to her hospital room, one of the ­medical tems); some of these respond to both somatosensory and ­s tudents described the case. “Left hemiplegia [left-side visual stimuli. The visual and somatosensory receptive fields ­paralysis], following a right-hemisphere stroke.” I was told. of each neuron are spatially related; for example, if a neu- Aunt Betty was lying on her back with her head and eyes ron has a somatosensory receptive field centered in the left turned to the right. “Betty,” I called out. hand, its visual field is adjacent to the left hand (see Crochet, I approached her bed from the left, but Aunt Betty did not Lee, & Peterson, 2019). Remarkably, as the left hand moves, turn her head or even her eyes to look toward me. the visual receptive field of the neuron moves with it. The “Hal,” she called out. “Where are you?” existence of these bimodal neurons motivated the following I turned her head gently toward me, and we talked. It was clear that she had no speech problems, no memory interesting case study by Schendel and Robertson (2004). loss, and no confusion. She was as sharp as ever. But her eyes still looked to the right, as if the left side of her world did not exist. The Case of W.M., Who Reduced I held her right hand in front of her eyes. “What’s this?” His Scotoma with His Hand I asked. “My hand, of course,” she said with an intonation that W.M. suffered a stroke in his right posterior cerebral artery. The ­suggested what she thought of my question. stroke affected a large area of his right occipital and parietal “Well then, what’s this?” I said, as I held up her limp left lobes and left him with severe left hemianopsia (a condition in hand where she could see it. which a scotoma covers half the visual field). When tested with “A hand.” his left hand in his lap, W.M. detected 97.8 percent of the stimuli “Whose hand?” presented in his right visual field and only 13.6 percent of those “Your hand, I guess,” she replied. She seemed puzzled. presented in his left visual field. However, when he was tested I placed her hand back on the bed. with his left hand extended into his left visual field, his ability to “Why have you come to the hospital?” I asked. detect stimuli in his left visual field improved significantly. Further “To see you,” she replied hesitantly. I could tell that she analysis showed that this general improvement resulted from didn’t know why. W.M.’s greatly improved ability to see those objects in the left Aunt Betty was in trouble. visual field that were near his left hand. Remarkably, this area of improved performance around his left hand was expanded even further when he held a tennis racket in his extended left hand. As in the case of Aunt Betty, asomatognosia is often