Sensory Systemspreview.kingborn.net/528000/9e5399f3923643baa8514cdaa3cdeb63.pdf · Sensovy Systems ANATOMY AND PHYSIOLOGY Aage R. Maller School of Human Development Callier Center

Sensory Systems

ANATOMY AND PHYSIOLOGY

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Sensovy Systems

ANATOMY AND PHYSIOLOGY

Aage R. Maller School of Human Development

Callier Center for Communication Disorders Tne University of Texas at Dallas

Dallas, Texas

ACADEMIC PRESS An imprint of Elsevier Science

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General Introduction

Acknavledgmen ts

Preface

CONTENTS

1 Basic Psychophysics Abbreviations Abstract

I. Introduction 11. Threshold of Detection

111. Discrimination Between Different Stimuli A. Factors That Affect Detection of Faint Stimuli

A. Perception of Stimulus Strength B. Discrimination of Small Differences Between Stimuli C. Temporal Resolution D. Spatial Resolution References

2 Anatomy and Physiology of Sensory Organs Abbreviations Abstract

xi

xv

xvil

1 2 2 3 4

18 19 19 20 24

30

33 34

V

vi Contents

I. Introduction II. Anatomy of Sensory Organs

A. The Ear B. The Eye C. Skin D. The Chemical Senses

III. Sensory Receptors A. Anatomy of Sensory Receptors B. Innervation of Sensory Cells

IV. Conduction of the Physical Stimulus to the Receptors A. Improvement of Transmission to the Receptors B. Modification of the Physical Stimulus

V. Physiology of Sensory Receptors A. General Principles for Sensory Transduction B. Information Processing at the Receptor Level C. Amplitude Compression D. Selectivity of Receptors E. Receptive Fields References

35 37 37 40 42 42

43 44 51 56 56 57 63 64 68 69 70 71 72

3 Sensory Nervous Systems

Abbreviations Abstract

Anatomy Information Processing in Ascending Sensory Pathways

I. Introduction II. Anatomy of Sensory Nervous Systems

A. Ascending Sensory Pathways B. Descending Systems C. The Thalamus is the Gateway to the Cerebral Cortex D. Anatomy of Sensory Cerebral Cortices E. Corpus Callosum

III. Information Processing in the Sensory Nervous System A. Information Processing in the Classical Ascending

System B. Processing of Object and Spatial Properties of

Sensory Stimuli C. Processing of Spatial Information

IV. Neural Control of Sensory Processing in Ascending Sensory Pathways A. Arousal B. Attention

75 76 76 78 8O 81 81 97 99

102 107 108

109

116 127

140 141 142

Contents vii

C. Control of Ascending Neural Activity by Descending Pathways V. Processing of Information in the Nonclassical Pathways

A. Developmental Aspects of Classical and Nonclassical Pathways

B. Processing of Information in Descending Nonclassical Systems

VI. How is the Neural Code of Sensory information Interpreted? A. Coding in Single Nerve Cells B. Coherent Firing of Groups of Neurons C. Maps D. Where is the Anatomical Location for Interpretation of

Sensory Information? VII. Sensory Information Can Reach Nonsensory

Regions of the CNS A. Motor Systems B. Autonomic Reactions to Sensory Stimulation C. Emotional Reactions to Sensory Stimuli

VIII. Processing of Information in the Sensory Nervous System is Dynamic A. Neural Plasticity B. Plastic Changes in the Nervous System May Cause

Symptoms and Signs of Disease References

143

145

146

147

148 148 149 149

152

153 153 154 155

164 165

170

172

4 Somatosensory System


Classical Somatosensory System Nonclassical Somatosensory System Pain

I. Introduction II. Anatomy of the Somatosensory System

A. Classical Somatosensory System B. Nonclassical System C. Descending Somatosensory Pathways

III. Physiology of the Somatosensory System A. The Classical Somatosensory System B. Physiology of the Nonclassical System C. Function of Descending Systems D. Pain E. Itch References

185 186 186 188 189

189 190 190 205 217

220 221 238 241 241 262

263

viii Contents

5 Heating


Conduction of Sounds to the Receptors Receptors Classical Auditory Pathways Physiology of the Classical Auditory System Nonclassical Pathways Physiology of the Nonclassical Pathways

I. Introduction II. The Ear

A. Anatomy B. Physiology of the Ear

III. The Auditory Nervous System A. Anatomy of the Classical Ascending Auditory Pathways B. Physiology of the Classical Auditory Nervous System C. Descending Systems

IV. The Nonclassical Ascending Pathways A. Anatomy of the Nonclassical Ascending Pathway B. Physiology of the Nonclassical System

V. Neural Plasticity A. Neural Plasticity Can Alter the Threshold and

Perception of Sounds B. Neural Plasticity from Overstimulation References

271 272 272 273 274 274 276 276

276 277 278 284

305 3O5 316 346

347 347 353 357

358 359 359

6 Vision


The Eye Visual Nervous System

I. Introduction II. The Eye

A. Anatomy B. Physiology of the Eye

III. The Visual Nervous System A. Anatomy of the Classical Ascending Visual Nervous System B. Physiology of the Visual Nervous System

IV. Nonclassical Visual Pathways A. Anatomy of Nonclassical Visual Pathways B. Physiology of the Nonclassical System References

373 374 374 374

376 376 377 38O

387 388 4OO 416 416 419

421

Contents ix

7 Chemica l Senses" Olfact ion and Gus ta t ion


Taste Olfaction

I. Introduction II. Anatomy

A. Receptors B. The Media that Conduct the Stimulus to the Receptors C. Innervation of Receptors D. Gustatory and Olfactory Neural Pathways

III. Physiology of the Chemical Senses A. Receptors B. Coding of Information in the Gustatory and

Olfactory Nervous Systems References

425 425 426 426

427 428 429 431 432 435

443 443

448

449

Index 451


GENERAL INTRODUCTION

This book is written for students of physiology, anatomy, and behavioral sciences, and for all individuals who want to understand some of the most fascinating wonders in bioIogy. Ren~ Descartes, (1596-1650) summarizes his line of reasoning in the famous phrase, 'I think, therefore I am.' This is true, but without sensory systems there would not be an intellect. While many people are fascinated by our technological achievements in such areas as computers and communication systems, the efficiency and complexity of sensory systems far exceeds even the most sophisticated man-made systems. Sensory systems not only interpret physical stimuli such as those camed by sound, light, and odors but they also provide input to our emotional brain either consciously or unconsciously. Understanding the function of sensory systems is important for many professions. This book discusses many aspects of sensory systems and their interaction with many other parts of the brain. Sensory Systems: Anatomy and Physiology provides a "joy of understanding" of some truly fascinating biological systems and can be appreciated by all individuals with an interest in living things.

Sensory Systems: Anatomy and Physiology not only presents facts regarding the anatomy and function of sensory systems but it also provides interpretation and synthesis of our present understanding of the organization and function of these complex systems. The book covers areas that have not been extensively represented in other books such as the function of nonclassical sensory pathways and the input to the emotional brain. The book discusses parallel processing (processing of the same information in different populations of nerve cells) and stream segregation (processing of different kinds of information in different populations of nerve cells).

Scientists try to relate neural activity in the central nervous system (CNS) to sensory stimulation in order to understand how sensory stimuli reach

Xli General Introduction

our consciousness and how they become interpreted. This problem has been approached in two different ways: the first approach attempts to understand how individual neurons work and the second approach attempts to understand how different parts of the CNS are connected into systems and how such systems process information (systems approach). A modem analogy to these two approaches would be similar to studying how transistors work and how computers work. This book concerns a systems approach to understand how sensory systems work.

Sensory systems are also important for our general well-being. Sensory systems control basic bodily functions such as what we eat and how much. Whether we are going to be overweight or anorexic depends to some extent on our sensory systems. Under normal circumstances and during diseases, sensory systems are important for our mood. The connections to the limbic system are important in that respect. Sensory systems also provide input to other regions of the brain such as the motor systems, the cerebellum, and to the core of the brainstem (reticular formation) that control the degree of wakefulness.

This book covers the anatomy and function of the five senses: hearing, vision, somesthesia, taste, and olfaction. Pain is also included as a part of somesthesia. The book emphasizes the similarities between the function of receptors and between the ways that these different senses process very different physical stimuli. Specifically, the book discusses the information processing that occurs in the sensory organs and the nervous system.

Proprioception is not included in this book because activation of proprio-ception does not reach consciousness. The feeling of mental fatigue or the sensations of hunger and thirst are not included either because the feeling of fatigue, hunger, and thirst are caused by some internal processes and although they reach consciousness they are very different from the sensory systems that communicate information from the environment. Why include pain in this book? Although pain is not always caused by external events, the close association with the somatosensory system and the fact that it often communicates information from the environment to the conscious mind justifies inclusion.

Sensory Systems: Anatomy and Physiology emphasizes the similarities between the different sensory systems and their function. Therefore the different components of sensory systems are first discussed together in order to emphasize the similarities and differences rather than the more conventional way of treating the different sensory systems separately.

Why do we want to know about sensory systems and why write a book devoted to them? First of all, sensory systems are some of the most intriguing systems of our body. Exploring sensory systems is now more fascinating than ever before because technological advances have provided excellent tools

General Introduction Xlll

for studying the function of sense organs and in particular to study the function of the nervous system.

Sensory Systems: Anatomy and Physiology is directed to physiologists who study sensory systems, to health professionals who are involved in diagnosis of disorders of sensory systems, and to any interested person who wants a broad understanding of how sensory systems function.

Chapter 1 provides a brief introduction to psychophysics. Chapter 2 is devoted to the anatomy and general function of sense organs. Chapter 3 discusses the anatomy and physiology of the ascending and descending sensory pathways. The anatomy and physiology of classical and nonclassical sensory systems are described and parallel processing and stream segregation important for processing sensory information are discussed. The different connections from sensory systems to limbic structures and other nonsensory parts of the CNS and their functional importance are also discussed in this chapter. Neural plasticity and its importance for development of sensory systems are discussed extensively in Chapter 3. How external and internal events can cause changes in the function of the nervous system by "revering" parts of the brain is described. Such changes can compensate for losses caused by injuries or diseases such as stroke. Plastic changes can also cause symptoms and signs of diseases that can manifest by chronic pain, hyperactivity, hypersensitivity, distension of sensory input, and emotional reactions to stimuli that normally do not elicit such reactions. Thus, there is both "good" and "bad" neural plasticity. Chapters 4 through 7 provide detailed descriptions of the anatomy and physiology of each of the five senses beginning wdth somesthesia (Chapter 4), followed by hearing (Chapter 5), vision (Chapter 6), and the chemical senses, taste and olfaction (Chapter 7).


ACKNOWLEDGMENTS

I have had help from many people in writing this book, especially Dr. George Gerken for his valuable comments on early versions of the manuscripts for many of the chapters of this book. Dr. Steve Lomber's suggestions about the chapter on vision were most valuable. Jan Nordmark commented on certain parts of the manuscripts. I also would like to thank many of my students at the University of Texas at Dallas for their valuable comments, and a special thanks to Pritesh Pandya for his comments on earlier versions of all the chapters. Many of my students at the School of Human Development, University of Texas at Dallas have provided valuable feedback and comments on earlier versions of the manuscript, as well as Dr. Karen Pawlowski. Phillip Gilley helped with the graphs and Karen Schweitzer typed the many revisions of the manuscripts.

I also want to thank Hilary Rowe and Cindy Minor at Academic Press in San Diego for their excellent work on the book, as well as Paul Gottehrer, Project Manager, for his dedicated work and professionalism. His copyeditor, Sarah Nicely Fortener meticulously copyedited this specialized book without changing its style or meaning. Debby Richer worked on the artwork and supervised the redrawing of many of the illustrations. It has been a real pleasure to work with these professionals.

Without the support from the School of Human Development at the University of Texas at Dallas I would not have been able to write this book.

Last but not least I want to thank my wife, Margareta B. MoUer, M.D., D. Med. Sci. not only for her patience with my occupation on this book and her encouragement during my writing of this book, but also for her comments and suggestions regarding earlier versions of the manuscripts.


PREFACE

The purpose of Sensory Systems: Anatomy and Physiology is to provide a comprehensive understanding of the anatomy and function of sensory systems. This book provides a systems approach to sensory systems and covers aspects of sensory systems not commonly found in textbooks such as the anatomy and function of nonclassical (nonspecific) sensory systems, parallel processing, stream segregation, and neural plasticity. The role of sensory input to nonsensory parts of the brain such as the Umbic system (the emotional brain) and the physiology of various forms of pain are topics discussed extensively.

The book is written for all students of life sciences, for scientists w ho want a broad and comprehensive coverage of sensory systems, and for healthcare professionals dependent on sensory systems in one way or another, such as in restoring function after diseases that have impaired normal function of one or more of our senses. The book is based on a course I teach in the School of Human Development at the University of Texas at Dallas.

Sensory Systems: Anatomy and Physiology is suitable for anyone who wants to learn about the function of biological systems. I hope this book will encourage students to choose biology in one form or another for their career, be it clinical medicine, biomedical research, or other forms of life sciences.

I have enjoyed writing this book very much and hope the reader will have an equal enjoyment in acquiring insight to truly fascinating biological systems.

Aage R. Moller Dallas, Texas

May, 2002


CHAPTER 1 Basic Psychophysics

ABBREVIATIONS

CFF: Critical fusion frequency CNS: Central nervous system

dB: Decibel, one tenth of a logarithmic unit HL: Hearing level. (Sound level above the normal threshold of hearing) Hz: Hertz, (frequency in cycles per second)

MAF: Minimal audible field MAP: Minimal audible pressure msec: Millisecond

nm: Nanometer, 10~^ millimeter SL: Sensation level. (Sound levels an individual person's threshold)

TTS: Temporary threshold shift jim: Micrometers, (10~^ meter or 1/1000 millimeter) jiS: Microsecond

2 Chapter 1: Basic Psychophysics

ABSTRACT

1. Threshold of detection and discrimination of specific features of sensory stimuU are assessed through psychophysics studies.

2. Detection of stimuli is dependent on: a. The intensity of the stimulus that reaches the receptor b. The sensitivity of the receptors for the stimulus in question c. The duration of the stimulation (temporal integration) d. Number of receptors that are stimulated (spatial integration) d. Background stimulation (masking) e. Prior stimulation (adaptation and fatigue) f. Attention

3. Discrimination of stimuli depends on temporal and spatial resolution of the sense.

4. Qualities of the stimuli that can be discriminated include: a. Intensity of stimuli b. Temporal properties c. Spatial properties d. Small difference in quaUties (difference limen) such as intensity, visual

contrast, frequency of sounds, concentration of odors and taste

I. INTRODUCTION

Psychophysics is a branch of psychology that deals with the relationship between a physical stimulus and the resulting sensation. Detecting the presence of a stimulus was probably the primary advantage of the evolution of such senses as vision and hearing when vertebrate species began to adapt to terrestrial life. Detecting odors was also important for many species. Early in the development of species, discrimination between different kinds of stimuli was of less importance. Much later in the evolution of terrestrial vertebrates it was still important to be able to detect the faintest sound or the weakest light, but the ability to discriminate between different stimuli became increasingly important as vertebrate species developed sensory systems adapted to these needs.

In this chapter, we will discuss such basic properties as the threshold of detection and the perception of strength and their relation to the physical properties of stimulation. The purpose is to provide the reader with a general overview of basic psychophysics in order to support the perspective on the main theme of this book, namely that of the anatomy and physiology of sensory systems. For more details on psychophysics, the readers are referred to standard texts on the subject.

II. Threshold of Detection

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FIGURE 1.1 Threshold of hearing in a human subject. Circles indicate the ampUtude of the dis-placement of the tympanic membrane at threshold (in cm);' " the soUd line is a curve fit to the data points.

II. THRESHOLD OF DETECTION

In psychophysics the threshold of detection of a physical stimulus refers to the minimum amount of the stimulus that is needed for the sensory system to elicit a behavioral response. The threshold varies between sensory systems, between animal species, and as a function of the properties of the stimulus.

The sensitivity of sensory systems is enormous and it surpasses most of our technical systems. Eyes of insects such as night moths, which are active when light is very weak, can detect one or two light quanta, and even our own eyes are very sensitive and can detect approximately 15 light quanta. *' ^ The threshold of hearing is no less impressive than that of vision. In the most sensitive frequency range the amplitude of vibration of the eardrum at the threshold of hearing is less than 10~^ cm (1/100 of one millimicron or nanometer [nm] or 10^ micrometers [jim]), corresponding to less than 1% of the diameter of a hydrogen molecule (Fig. 1.1)." ^

* A light quantum is the smallest amount of hght that can be produced. It refers to the quantum theory of light production, which assumes that light is both a particle and a wave.


Under the best circumstances, the sensitivity of the auditory system in humans is near its theoretical limits set by Brownian motion of the cochlear fluid. The sensitivity of the human ear is similar to that of other animal species, although the cat has an approximately 10-dB-lower threshold than humans between 1 kHz and 7 kHz and 20 dB or more between 10 and 20 kHz. Some animals can hear in a wider range of frequencies than humans. ^ For example the cat and the common laboratory rat hear up to 50 kHz, and some species of the flying bat, whales, and dolphins hear up to or above 100 kHz.

The sensitivity of the vibration of the skin is much less impressive, but nonetheless the stimulus required to elicit a conscious response is small. The greatest sensitivity for sinusoidal vibration applied to the palm is approximately 0.2 |im of skin displacement in humans. It is slightly higher for the monkey when the threshold is defined as a 50% correct response rate.^° The sense of the skin is highly complex, involving several stimulus modalities, such as touch, pain, and thermal stimuli. The threshold of sensitivity in each of these modalities varies according to region of the body and species.

The sensitivity of the olfactory system also varies widely among species of mammals.^^ The sensitivity of the olfactory system in some mammals (for instance, dogs) is very high and only a few molecules of a substance are required for detection. The nose of humans is less sensitive, but the sensitivity of the human sense of smell varies within wide ranges for different odors. It also varies widely among different individuals and decreases with age. ^

A. FACTORS THAT AFFECT DETECTION OF FAINT STIMULI

The sensitivity to a stimulus depends on many factors besides the type of the physical stimulus, such as the frequency of sounds or vibration, the wavelength of light, or the kind of taste and odor. Prior stimulation can decrease the sensitivity of sensory systems because of adaptation or fatigue. Simultaneous presentation of another stimulus can also increase the threshold to stimulation known as masking. Additionally, the threshold of detection decreases as the duration of the stimulus is increased up to a certain duration. This is known as temporal integration. Stimulation of a large number of receptors may decrease the threshold because oi spatial integration. Finally, detection of faint stimuli as determined experimentally depends on the criteria for detection and how and where the stimulus is measured.

1. Type of Stimulation

The frequency range of hearing in different species of terrestrial verte-brates differs. Humans can hear in the frequency range from approximately


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2 - Binaural M.A.F., 0° Azimuth

100

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FIGURE 1.2 Threshold curves of hearing in three different situations: (1) monaural minimum audible pressure (MAP) determined using a calibrated earphone; (2) binaural minimum audible field (MAF) determined using a loudspeaker as sound source placed in front of the observer; and (3) similar as (2) but with the sound coming from a random azimuth. "^

20 to 20,000 Hz (Fig. 1.1). The upper limit decreases with age. Some animals have hearing at higher frequencies than do humans. For example, the cat hears up to approximately 50 kHz, approximately the same as the rat. The flying bat, dolphins, and whales can hear sounds above 100 kHz.

The threshold of sensory systems depends on the type of stimulation, and the sensitivity of the ear is not uniform over the audible frequency range. The highest sensitivity of the human ear is in the frequency range from 500 to 6000 Hz (Fig. 1.1). The absolute value of the threshold depends on how the sounds are presented: monaural (sound to one ear only), binaural (sound to both ears at the same time) by earphones or in a free field at 0 degrees azimuth (angle in the horizontal plane) or at random incidence (Fig. 1.2). ^

when measured in the ear canal, near the tympanic membrane, or at the entrance of the ear canal, hearing thresholds are usually referred to as the minimal audible pressure (MAP). If the threshold is referred to the sound pressure at the place where the person is located (without the person being present), yet other threshold values will be obtained. When the sound is measured in the place where the test subject is to be placed during the test, the threshold is known as the minimal audible field (MAF). Typically, the threshold for a MAP response is lower than that for a MAF response for sounds coming from a direction in front of the person being tested. The threshold for


sounds that reach both ears is approximately 3 dB lower than when sounds reach only one ear when referred to the sound pressure at the entrance of the ear canal. However, when placed in a free sound field, the effect of the head on the sound pressure at the entrance of the ear canal is in most situations greater than that. The reason is the acoustic effects of the head on the sound pressure that reaches the entrance of the ear canal. Depending on the direction to the sound source, the head functions either as a baffle (that increases the sound, mostly for high frequencies) or as a shadow to the ear on the opposite side of where the sound source is located, which generally makes the sound at the ear canal lower than that measured in the place of the person in question.

Like the ear, the sensitivity of the eye depends on the stimulus, and the sensitivity of the eye is not uniform over the range of wavelengths to which it responds (Fig. 1.3). The range of vision is similar for most mammals but, for instance, insects can see ultraviolet light, which is outside the visible range for mammals. Rods have their best sensitivity at wavelengths of approximately 500 nm, corresponding to green, bluish light (Fig. 1.4). The color or wavelength of light to which the human eye is most sensitive therefore depends on its state of adaptation (see below). Daylight in the middle of the day has a broad spectrum with considerable energy emitted in the range of wavelengths from 420 to 700 nm, (i.e., from violet to deep red). The light-adapted eye using cones (photopic vision) is most sensitive to light of a wavelength of approximately 555 nm, corresponding to green light. The sensitivity of the eye also depends on

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Wavelength (nm)

FIGURE 1.3 The sensitivity of the dark-adapted eye, peripheral to the fovea, expressed in the number of light quanta Qeft scale) and in log units (right scale) required to be detected as a function of the w^avelength (in nm) of the test light.


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the location on the retina where the light is projected because the density of photoreceptors on the retina is not uniform (see Chapters 6, Fig. 6.1).

Mechanoreceptors in the skin are sensitive to deformation of the skin, and different types of mechanoreceptors have different sensitivity to such stimuli. The sensitivity to sinusoidal vibration is greatest around 200 Hz for some receptors known as Pacinian corpuscles (Fig. 1.5). Some receptors are most sensitive to rapid change in deformation of the skin compared with steady deformation of the skin, while in others the opposite is true.

While the olfactory receptors respond to a wide range of different odors, taste is limited to four categories, namely sweet, sour, salty, and bitter (and possibly a fifth, monosodium glutamate). The olfactory system has different sensitivity to different odors, and likewise the taste sense has different sensitivity to the four or five different substances to which taste receptors are sensitive.

2. Adaptation

The dependence of the threshold of sensory systems on prior stimulation is known as adaptation. Prior exposure decreases sensitivity of sensory receptors. For vision, the highest sensitivity is achieved when no light has


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FIGURE 1.5 Threshold to mechanical vibration delivered to the skin of the hand for an experienced human observer. The ampUtude that gave a response in 50% of the trials is shown as a function of the frequency of sinusoidal vibration."^^

reached the eye for some time (the dark adapted state of the eye) (Fig. 1.6). Exposure to Ught reduces the sensitivity of the eye to an extent and for a period of time that depends on the exposure. In that state the sensitivity of the eye is determined by the sensitivity of the rods (scotopic vision).

Prior stimulation also affects the sensitivity of the ear, and exposure to loud sounds causes temporary threshold shift (TTS); exposure to even louder sounds causes permanent damage to the ear known as permanent threshold shift (PTS). The decrease in sensitivity that is caused by overexposure is also sometimes referred to as (auditory) fatigue. The reduction in the sensitivity of the ear depends on the intensity of the sound and its duration and frequency (spectrum). The TTS is largest approximately Yi octave (1400 Hz) above the frequency of the tone that caused the fatigue (1000 Hz) (Fig. 1.7); it accelerates as the intensity of the fatiguing sound is increased. In a similar way, prior stimulation of the chemical senses, olfaction and taste, affects the sensitivity of these senses, and prior stimulation of the skin causes a reduction in sensitivity.

3. Masking

The threshold of a test stimulus can be elevated by the concomitant presence of another stimulus of the same type, thereby masking the perception of the test


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FIGURE 1.6 Dark-adaptation curves showing the relative intensity of light to reach threshold as a function of time after exposure to bright light. The dotted curve was obtained in a totally color-blind person assumed to have only rods. The dashed curve was obtained in response to red light illuminating the fovea, and the continuous curve was obtained using white light illuminating the extra foveal regions of the retina in a person with normal vision." ^

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Simultaneous

700 800 900

Masker frequency Hz

T 1 1 . 1 1 1

1000 1100 1200 1300

FIGURE 1.8 Comparison between simultaneous and forward masking of a weak (10-dB sensation level [SL]) 1000-Hz tone as a function of the frequency of the masker/''

Stimulus. Masking is pronounced in hearing, where it has been studied extensively. It is well-known how stimulus parameters affect a sound's ability to mask another sound.

The efficiency of a sound in masking another sound depends on its intensity and its frequency relative to the test sound. In general, low-frequency sounds are more effective in masking high-frequency sounds rather than vice versa. Different sounds mask each other according to the width of the critical hand.* Two sounds that are separated in frequency by less than one critical band are most efficient in masking each other, while sounds that are separated in frequency by more than one critical band are less effective in masking each other.

Stimuli do not have to occur simultaneously to mask each other. A stimulus that occurs just before another (second) stimulus, but does not overlap it in time, may change the threshold of the second stimulus. This temporal effect in masking is referred to as forward masking. Forward masking is less efficient than simultaneous masking (Fig. 1.8). A masker can also affect the threshold of

* Critical band is the band of frequencies over which the auditory system integrates the energy of a broadband sound. It is also a measure of the ability of one sound to mask another sound.

II. Threshold of Detection 11

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FIGURE 1.9 Stimulation of two points on the skin; masking of one stimulus by the other as a function of the temporal interval between the two stimuli.^

sensory systems to a stimulus that is presented after the masker, and this is known as backward masking. Backward masking is usually less efficient than forward masking, and its effect occurs during a shorter interval of time relative to the test stimulus.

For stimulation of the skin (touch), stimuli are most effective in masking each other if they occur within 100 msec relative to each other^ (Fig. 1.9), but a masker can affect the threshold of a test sound that is presented as much as 1.2 sec prior to the test stimulus.

4. Temporal Integration

Temporal integration implies that energy is summed over a certain period of time. The result is that the threshold is lower for stimuli that last longer up to a duration, known as the integration time. For instance, the threshold of hearing decreases as the duration of the stimulus is increased up to approximately 300 msec" '" ^ (Fig. 1.10), but the relationship between duration of sounds and the threshold is not an exact power function. Also, the temporal integration depends on the frequency of the sounds and it is different above threshold, where it is generally shorter. Temporal integration of mechanical stimulation of the skin is similar to that of sound in hearing, and it is different for different types of skin receptors.

Olfactory sensory organs integrate stimulus input over time; consequently, the sensitivity to short puffs of odors is less than it is to longer exposures when

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Sensory Systemspreview.kingborn.net/528000/9e5399f3923643baa8514cdaa3cdeb63.pdf · Sensovy Systems ANATOMY AND PHYSIOLOGY Aage R. Maller School of Human Development Callier Center