Nonvisual Sensation and Perception - Chapter 7

Earbuds and Hearing Loss & Sonic Weapons

• Earbuds and Hearing Loss:

  • Certain hearing loss results from cumulative noise exposure.

  • The volume of devices like iPhones can exceed 120 dB, which can cause hearing loss in as little as 75 minutes.

  • Experts recommend the 60/60 rule: 60% of maximum volume for an hour per day.

  • Adaptation to loudness causes people to increase the volume over time.

  • If the listener can't hear anything else, the sound is too loud.

  • Symptoms of hearing loss include ringing, muffled sounds, and needing higher volumes on electronic devices.

  • Using headphones instead of earbuds, limiting listening time, and capping maximum volume can help preserve hearing.

• Sonic Weapons:

  • Sonic weapons can disrupt, injure, or kill using sound.

  • The LRAD (long-range acoustic device) can cause permanent damage from 1,000 feet away, emitting a sound louder than a jet engine at 150 dB.

  • Any sufficiently loud frequency can destroy the eardrum and cause pain and disorientation.

  • Sonic weapons typically use high frequencies or infrasound.

  • Sonic blasts of 19,000 to 20,000 Hz disperse teens but don't affect older individuals due to age-related hearing loss.

  • Infrasound weapons emit frequencies around 7 Hz, penetrating concrete and armor, causing neurological symptoms like a mild head injury after 15 minutes of exposure.

Chapter 7 Outline: Nonvisual Sensation and Perception

• Audition

  • Sound as a Stimulus

  • The Structure and Function of the Auditory System

  • Auditory Perception

  • Hearing Disorders

• The Body Senses

  • The Vestibular System

  • Touch

  • Pain

• The Chemical Senses

  • Olfaction

  • Gustation

  • Synaesthesia

Learning Objectives

• L01: Identify the major features of sound as a stimulus.

• L02: Trace the process of audition from the outer ear to the cerebral cortex.

• L03: Explain the perception of pitch, loudness, and the location of sounds.

• L04: Describe the structures and functions of systems responsible for the perception of body position and movement, touch, and pain.

• L05: Describe the structures and pathways responsible for olfaction and gustation.

• L06: Identify the key features of synaesthesia.

Audition: An Overview

• Audition is the sense of hearing.

• Helen Keller noted that deafness separates a person from people more than blindness separates a person from things.

• Audition helps us identify objects and determine their location in relation to our bodies.

• Our auditory world is uniquely human, with a limited range of sound we can sense.

• Dogs can hear higher-pitched sounds than humans.

• Bats use high-pitched vocalizations for echolocation.

Sound as a Stimulus

• Philosophical Question: If a tree falls in the forest and nobody is around to hear it, does it make a sound?

  • Neuroscientist's Answer: Yes, the falling tree produces a physical sound stimulus, but no, sound as a perceptual experience requires hearing.

• Physical sound begins with the movement of an object.

• Movement creates waves of vibration through collisions between adjacent molecules, producing bands of high and low pressure.

• Sound cannot occur in a vacuum because it requires jostling between molecules.

• Air is the usual medium for humans, but sound can also travel through liquids and solids.

• Sound interacts with the environment:

  • Fabrics absorb sound waves.

  • Echolocation is used by bats and can be improved in humans with practice.

• Sound energy is described in waves, similar to electromagnetic energy.

Wave Characteristics

• Amplitude (intensity):

  • Height of a wave.

  • Perceived as loudness.

  • High-amplitude waves: Loud sounds.

  • Low-amplitude waves: Soft sounds.

  • Related to sound intensity, defined as the amount of energy passing through a specific area within a specific time.
    *Intensity is expressed in decibels (dB).

• Frequency (wavelength):

  • Number of cycles per second.

  • Determines pitch.

  • Long wavelengths: Low pitch.

  • Short wavelengths: High pitch.

  • Measured in Hertz (Hz).

• Timbre (complexity):

  • Distinct quality or uniqueness of a sound.

  • The "color" or "quality" of tones, even with equal pitch and loudness.

  • Pure tones have a single frequency.

  • Complex tones consist of multiple frequencies.

Types of Sound Waves

• Pure Tone:

  • The simplest type of sound wave with a single frequency (e.g., tuning fork).

• Complex Tones:

  • Combinations of multiple waves.

  • Characterized by timbre.

  • The same note played by different instruments sounds different due to timbre.

• Noise:

  • Waves that do not regularly repeat themselves.

  • Perceived as opposed to identifiable tones.

Intensity and Loudness

• Humans can perceive a wide range of intensities, over 10 billion times, from quietest sounds to a jet engine at takeoff.

• Decibel Scale:

  • A logarithmic scale is used to manage the wide range of intensities.

  • A sound of 30 dB is 10 times more intense than a sound of 20 dB.

  • A sound of 40 dB is 100 times more intense than a sound of 20 dB.

• Threshold for Hearing:

  • Set at 0 dB, equivalent to a mosquito flying 3m away.

• Common Sound Intensities:

  • Whisper: 20 dB

  • Maximum iPhone loudness: 120 dB

  • Pain threshold: 130 dB, can cause permanent damage.

Frequency and Pitch

• Frequency is the number of cycles per second, measured in Hertz (Hz).

• Human hearing ranges from approximately 20 Hz to 20,000 Hz.

• Infrasound: Frequencies below human hearing (used by elephants).

• Ultrasound: Frequencies above human hearing (used for cleaning and medical imaging).

The Structure of the Auditory System

• The ear is divided into three parts: the outer, middle, and inner ear.

• Outer Ear:

  • Includes the pinna and the auditory canal.

  • Pinna collects and focuses sounds and helps locate the source of sound.

  • Auditory canal channels sound to the tympanic membrane (eardrum), about 3 cm long and 7 mm wide.

• Middle Ear:

  • Tympanic membrane (eardrum) separates outer and middle ear.

  • Oval window separates middle and inner ear.

  • Ossicles (malleus, incus, stapes) bridge the middle ear, transferring sound energy to the inner ear.

    • These bones help to recover 23 dB out of the 30 dB that would be otherwise lost.

• Inner Ear:

  • Contains fluid-filled cavities embedded in the temporal bone of the skull.

  • Includes the vestibular system and the cochlea.

  • Cochlea (snail in Greek): Contains receptor cells that respond to vibrations, about 32 mm long and 2 mm in diameter

    • The cochlea is divided into three chambers: vestibular canal, tympanic canal, and cochlear duct.

    • Vestibular and tympanic canals contain perilymph, similar to cerebrospinal fluid.

    • Cochlear duct contains endolymph, rich in potassium and low in sodium.

    • Reissner’s membrane separates the vestibular canal and cochlear duct.

    • Basilar membrane separates the tympanic canal and the cochlear duct.

    • Oval window covers the vestibular canal.

    • Round window covers the tympanic canal and acts as an "escape valve."
      *The Organ of Corti transduces vibrations into neural messages.

• Organ of Corti

  • Consists of rows of hair cells, resting on the basilar membrane.

  • Tectorial (roof) membrane is attached to the cochlear duct on one side.

  • The basilar membrane is five times wider at its apex than at its base and 100 times stiffer at its base than at its apex.

  • Sound vibrations cause a wavelike motion in the basilar membrane.
    *High-frequency sounds produce peak vibration near the base.
    *Low-frequency sounds produce peak vibration closer to the apex.

• Hair Cells:

  • About 15,500 hair cells in each human inner ear. Cilia on the tips of the hair cells.

• Inner Hair Cells:

  • Around 3,500, act as the actual auditory receptors found near the junction of the tectorial membrane and cochlear duct.

• Outer Hair Cells:

  • Around 12,000, amplify sound. Only 5% of auditory nerve fibers connect with outer hair cells. Approximately 95 % connect with the inner hair cells.

  • Movement of cilia in endolymph hyperpolarizes and depolarizes the hair cells, with a resting potential around -70mV.

• Neural Response:

  • Opening and closing of mechanically gated potassium channels located in the tips of the cilia.

• Potassium and Depolarization:

  • Endolymph has a higher concentration of potassium, so opening channels causes potassium to rush in resulting in depolarization due to diffusion and electrostatic pressure.

Central Auditory Pathways

• Spiral Ganglion Neurons:

  • Bipolar

  • Connect hair cells of the cochlea with the brain. Cell bodies are located in the cochlea.

• Auditory Nerve (Cranial Nerve VIII):

  • Fibers project to the dorsal and ventral cochlear nuclei of the medulla.

• Superior Olive:

  • Axons from the ventral cochlear nuclei synapse in the superior olive in the pons.

• Lateral Lemniscus:

  • The superior olive forms connections via the lateral lemniscus with the inferior colliculus.

• Inferior Colliculus:

  • Neurons project to the medial geniculate nucleus (MGN) of the thalamus.

• Medial Geniculate Nucleus (MGN):

  • Receives auditory information and input from the reticular formation of the brainstem and adjusts hearing sensitivity according to the state of arousal.

• Primary Auditory Cortex (A1):

  • Located in the temporal lobe, receives projections from the MGN.

Auditory Cortex

• Primary Auditory Cortex (A1): Organized in columns that respond to single frequencies.
  Lower frequencies produce a response in rostral columns. Higher Frequencies produce a response in caudal columns.
  Input from both ears produce a stronger response than input received by a single ear. The opposite can hold true as well.

• Secondary Auditory Cortex: Areas surrounding A1 activated by complex stimuli, processing the quality of a sound (“what”) and its location (“where”).

Auditory Perception

Pitch Perception

• The high or low quality of a sound associated with frequency.

• Tonotopic Organization:

• Neurons responding to similar frequencies are located next to each other.

  • Georg von Békésy’s Place Theory:
    Peak of the wave traveling along the basilar membrane correlates with the sound's frequency. Well for sound above 4,000 Hz.

  • Temporal Theory:
    Pattern of neural firing matches actual frequency of a sound. For frequencies below 4,000 Hz.

Loudness Perception

• The decibel level of a sound wave and its perceived loudness are related but not the same thing. Doubling of loudness happens with each 10 dB increase in stimulus intensity.

• Equal Loudness Contours:

• Ability to detect loudness varies with the frequency of a sound. constructed using a model 1,000Hz tone

• Auditory Neurons Response

• Amplitude Range
  A single neuron can respond to range of about 40 dB, while humans can perceive a range of 130 dB.
  Sounds that last longer are usually perceived as louder because of temporal summation.

Localization of Sounds

• Our primary way of localizing sound in the horizontal plane is comparing arrival times at each ear.
  Differences in arrival time are quite small -> between 0 msec for sounds that are either straight ahead or behind you to 0.6 msec for sounds coming from a point perpendicular to your head on either side.
  Distinction between arrival times is made by neurons called binaural neurons

• Localization of Sound Intensity

• Can also localize sound by assessing the differences in the intensities of sound reaching each ear. This system only works for high frequency sounds.

• Role of Pinna and Vision

  • The pinna of the ear is essential for localizing the elevation of sounds in the vertical plane (above or below).

  • Sound localization also involves vision

Hearing Disorders

• Hearing loss affects nearly 1.5 billion people, or nearly 20% of the global population.
  The inability to hear sounds less than 25 dB is considered a mild hearing loss.
  Deafness refers to having very little or no perception of sound.
  Age-related hearing loss results from a variety of factors, including poor circulation to the inner ear or the cumulative effects of a lifetime of exposure to loud noise.
  Conduction loss, resulting from problems in the outer or middle ear, can result from a buildup of wax in the ear canal, infections of the middle ear, or a disease known as otosclerosis.

• Central Hearing Loss
  Hearing loss can also occur due to damage to the inner ear, the auditory pathways, or the auditory cortex.
  Cochlear prosthetics (cochlear implants) can treat damaged inner ears. Electrode arrays are threaded through the round window of the cochlea toward the apex of the basilar membrane.

The Body Senses

• The somatosensory system provides information about the position and movement of our bodies and about touch, skin temperature, and pain.

The Vestibular System

• The vestibular system provides information about the position and movements of the head, which contributes to our sense of balance.
  If the vestibular system is impaired, perhaps by a bad head cold or by motion sickness, the result is usually an unpleasant period of nausea and dizziness.

• Movement Receptors
  The sensory organs of the vestibular system are found in the inner ear, adjacent to the structures responsible for audition.
  The vestibular structures are divided into two types, the otolith organs and the semicircular canals.
  The otolith organs consist of two separate structures, the saccule and the utricle. The otolith organs provide information about the angle of the head relative to the ground and information about linear acceleration, the rate of movement changes, like when we’re in a plane taking off or in a car pulling away from a stop sign or signal
  Both the saccule and utricle contain hair cells similar to those we encountered earlier in our discussion of audition

• Semicircular Canals

  • Three looping chambers respond to rotational movements of the head. These structures respond to rotational movements of the head and contribute to our ability to walk upright. Rotating the head causes the endolymph within the canals to bend hair cells.

Central Pathways

• Axons originating in the otolith organs and semicircular canals form part of the auditory nerve (cranial nerve VIII). These axons synapse in the vestibular nuclei of the pons and medulla and in the cerebellum

• Connections and Integration

  • Allows us to coordinate information from the vestibular system with other relevant sensory input.

  • Axons from the vestibular nuclei make connections both in the spinal cord and in higher levels of the brain. Input to the spinal cord motor neurons provides a means to adjust our posture to keep our balance.

  • Rotations of the head result in reflexive movements of the eyes in the opposite direction. Because of these reflexes, you maintain a steady view of the world even while riding on the most extreme roller coaster. On the other hand, if your vestibular senses and your visual senses give conflicting information to your brain, you may feel nauseated or dizzy.

Touch

• Our sense of touch begins with our skin, the largest and heaviest organ of the human body. Our skin provides a boundary separating what is inside from what is outside. It prevents dehydration and protects the body from dirt and bacteria.

• Skin Varieties

  • Human skin comes in two basic varieties, hairy skin and glabrous, or hairless, skin. Human glabrous skin is found on the lips, palms of the hands, and soles of the feet.

  • Skin can be divided into the outer layer of epidermis and the inner layer of dermis. Below the dermis, we find subcutaneous tissue, which contains connective tissues and fat.

Touch Receptors

• The majority of the receptor cells for touch are referred to as mechanoreceptors. This term reflects the response of these receptor cells to physical displacement such as bending or stretching.
  In addition to their locations in the skin, mechanoreceptors are also found in blood vessels, joints, and internal organs. Those unpleasant sensations of pressure from a too-full stomach or bladder are provided courtesy of mechanoreceptors in the walls of these organs.

• Mechanoreceptors

  • Categorized according to the structure, size of receptor field, rate of adaptation, and type of information that is processed (refer to Table 7.3). Structurally, mechanoreceptors are either encapsulated or not.
    In encapsulated receptors, the axon fibers are surrounded by a fluid-filled capsule formed of connective tissue.
    Non-encapsulated receptors include the Merkel cells and Ruffini endings. The Merkel cells, like the Meissner corpuscles, are located in the upper areas of the skin, while the Ruffini endings are located at deeper levels.
    We also find free nerve endings distributed within the skin. As their name implies, these receptors do not have any specialized structure but are simply the unmyelinated nerve endings of sensory neurons
    In addition, some receptors wrap themselves around hair follicles and respond to the bending of a hair.

• Receptive Field:

• Area of skin or other tissue that provides information to a particular receptor. Also the density and receptive field size of the mechanoreceptors serving each area
  Meissner corpuscles and Merkel cells both have very small receptive fields, which means that they can identify the borders of very small stimuli.
  In contrast, Pacinian corpuscles and Ruffini endings have very large receptive fields and provide only general information about the borders of stimuli.

• Two-Point Discrimination Test

  • Variation in sensitivity from one part of the body to the next result from the density and receptive field size of the mechanoreceptors serving each area. Fingers and lips are far more sensitive than our backs and the calves of our legs.

• Adaptation Rate:

  • A receptor’s rate of adaptation refers to the length of time it will continue to respond to unchanging stimuli. Meissner corpuscles and Pacinian corpuscles both demonstrate rapid adaptation, and Merkel cells and Ruffini endings are slow adapting.
    Meissner corpuscles, Merkel cells, and Pacinian corpuscles all provide information about pressure.
    Ruffini endings provide input regarding stretch, finger position, and hand grip

• # Touch Pathway

  • Sensory fibers and Classification
    The cell bodies of the mechanoreceptors are located in the dorsal root ganglia and their cranial nerve equivalents. One branch extends to the periphery, ending in a mechanoreceptor. The other branch travels centrally by joining the dorsal roots of the spinal cord.
    The sensory fibers of the peripheral nervous system are classified into four categories based on diameter and speed. The largest fibers, called Aα (alpha-alpha), carry information from the muscles and will be discussed in Chapter 8. The smaller three sets of fibers serve the mechanoreceptors.

  • Dermatomes:
    Area of the skin surface served by the dorsal roots of one spinal nerve
    When the axons from the mechanoreceptors enter the spinal cord, they follow a route called the dorsal column–medial lemniscal pathway. Axons from the dorsal column nuclei form a large band of white matter known as the medial lemniscus, which crosses the midline of the medulla.
    Medial Lemniscus continues to travel rostrally through the medulla, pons, and midbrain before synapsing on the ventral posterior (VP) nucleus of the thalamus
    Touch information from the head reaches the VP nucleus by alternate routes involving a number of cranial nerves.
    These synapses provide opportunities for the cortex to influence the input it receives through descending pathways. In addition, activity in one neuron inhibit its neighbor, leading to a sharpening or enhancement of its signal.

  • Somatosensory Cortex
    The primary somatosensory cortex, also known as S1, is found in the postcentral gyrus of the parietal lobe, just caudal to the central sulcus that divides the parietal and frontal lobes. The secondary somatosensory cortex (S2) is located within the lateral sulcus.
    A map of the body’s representation in the cortex is known as a homunculus.
    the areas of the cortex serving the head and neck are located at the lower, ventral part of the postcentral gyrus, while areas serving the legs and feet extend over the top of the gyrus onto the medial surface of the parietal lobe.

The somatosensory cortex is capable of plasticity. Both reductions and increases in input can initiate plasticity

Somatosensory Disorder

• Clinical studies in humans are consistent with these experiments in monkeys. Amputation of a limb can cause phantom pain, in which pain is perceived as arising in the missing body part, or referred sensations, in which touching a body part such as the cheek is perceived as touch of the missing limb
  Highly trained string musicians, especially those who studied music from a young age, have a larger than normal area of somatosensory cortex representing touch in the fingers
  Damage to the primary somatosensory cortex produces deficits in both sensation and movement of body parts served by the damaged area.

• Neglect Syndrome:Damage to the secondary somatosensory cortex, particularly on the right side of the brain, results in neglect syndrome.

Pain

• Pain is defined as an unpleasant sensory and emotional experience associated with tissue damage. Pain occurring subsequent to an injury is often referred to as acute pain. In contrast, chronic pain refers to pain lasting three or more months. No other sensory modality is as dramatically affected by culture, emotion, context, and experience as our sense of pain.

  • Individual responses to painful stimuli result from genetic, gender, age, and experience.
    Women demonstrate greater sensitivity to pain, while aging is associated with greater sensitivity to some, but not all, types of pain.
    If you’re wondering how pain responses are evaluated experimentally, this is usually done with a cold pressor test

• Receptors for Pain
  Free nerve endings that respond to pain are called nociceptors. Nociceptors respond to a variety of stimuli associated with tissue damage.

• Pain Pathways to the Brain
  Information from the nociceptors is carried toward the CNS by two types of nerve fiber.The faster, myelinated Aδ (alpha-delta) fibers are responsible for that quick, sharp “ouch.” The slower, unmyelinated C fibers are responsible for dull, aching types of pain sensation. Both types of ascending pain fibers use glutamate as their primary neurotransmitter.
  Pain fibers from the skin enter the spinal cord via the dorsal root. Visceral pain fibers follow a similar, but separate route. Once inside the cord, the pain fibers synapse in the substantia gelatinosa, a group of cells in the outer gray matter of the dorsal horn.
  *Melzack and Wall (1965) proposed the gate theory of pain, characterized by a feedback loop in the dorsal horn that determines which signals ultimately reach the brain.
  From the substantia gelatinosa, fibers cross the midline and join the spinothalamic pathway. This leads to the VP and intralaminar nuclei connecting to the ACC. The ACC participates in our emotional responses to pain.

• Managing Pain
  According to the gate theory, activity in touch fibers inhibits the activity of nociceptors in the substantia gelatinosa. This effectively reduces the amount of pain information reaching the brain.
  Being disabled by pain during an emergency is not in the best interests of survival, so it should come as no surprise that extreme stress often reduces the perception of pain.
  A sense of control can reduce the sensation of pain

Summary Table 7.2: Somatosensory Pathways

• Vestibular system

• Touch

• Pain

The Chemical Senses

• Philosopher Immanuel Kant (1798/2006) considered olfaction, or the sense of smell, to be the “most dispensable” sense. Nonetheless, our chemical senses, olfaction and gustation, provide essential warnings of danger, such as smelling smoke from a fire or tasting spoiled food.

Olfaction

• Olfaction begins when air containing olfactory stimuli is taken in through the nostrils and circulated within the nasal cavities connected to the nostrils. The congestion you experience during a bad cold limits this circulation, reducing your ability to smell.

Individuals vary in there scent to smell based on: age, females, smokers, and women using contraceptives.

Olfactory Receptors: The neural receptors for olfaction are contained in a thin sheet of cells within the nasal cavity known as the olfactory epithelium. Unlike most other types of neurons, olfactory receptor cells regularly die and replaced in a cycle lasting approximately four to six weeks.
Unlike most other types of neurons, olfactory receptor cells regularly die and are replaced in a cycle lasting approximately four to six weeks.
The approximately 10,000 olfactory receptor cells in each nostril are bipolar, having two branches extending from the cell body.

Pathway: Axons form olfactory nerve which leads to olfactory bulb -
Olfactory receptors must catalog the over one trillion different smells that we are able to discriminate (Bushdid et al., 2014). Buck and Axel (1991) suggested that mammals can have approximately 1,000 types of receptor cells to accomplish this taskIn vision, two synapses don’t even get you out of the retina.