Lecture 7
Lecture 8
Lecture 9
Lecture 10
Lecture 11
 
 
 
 

Lecture 7
Touch and pain June 1, 2006
Overview of Questions
Are there specialized receptors in the skin for sensing different tactile qualities?
What is the most sensitive part of the body?
Is it possible to reduce pain with your thoughts?
Do all people experience pain in the same way?
Cutaneous System
Skin - heaviest organ in the body
Protects the organism by keeping damaging agents from penetrating the body
Epidermis is the outer layer of the skin, which is made up of dead skin cells
Dermis is below the epidermis and contains mechanoreceptors that respond to stimuli such as pressure, stretching, and vibration

Mechanoreceptors
Merkel receptor - disk-shaped receptor located near the border between the epidermis and dermis
Meissner corpuscle - stack of flattened disks in the dermis just below epidermis
Ruffini cylinder - branched fibers inside a cylindrical capsule
Pacinian corpuscle - onion-like capsule located deep in the skin
Mechanoreceptors - continued
Properties of mechanoreceptor nerve fibers
Temporal properties - adaptation:
Slowly adapting fibers (SA) found in Merkel and Ruffini receptors - fire continuously as long as pressure is applied
Rapidly adapting fibers (RA) found in Meissner receptor and Pacinian corpuscle - fire at onset and offset of stimulation

Properties of Mechanoreceptor Nerve Fibers
Spatial properties - detail resolution
SA1 fibers (Merkel receptor) respond to patterns of grooves
RA2 fibers (Pacinian corpuscle) do not respond to the details of these stimuli
Frequency response
Ranges from .3 Hz  for SA1 fibers 500 Hz for RA2 fibers
 

Pathways from Skin to Cortex
Nerve fibers travel in bundles (peripheral nerves) to the spinal cord
Two major pathways in the spinal cord:
Medial lemniscal pathway consists of large fibers that carry proprioceptive and touch information
Spinothalamic pathway consists of smaller fibers that carry temperature and pain information
These cross over to the opposite side of the body and synapse in the thalamus
 

Maps of the Body on the Cortex
Signals travel from the thalamus to the somatosensory receiving area (S1) and the secondary receiving area (S2) in the parietal lobe
Body map (homunculus) on the cortex shows more cortical space allocated to parts of the body that are responsible for detail
Plasticity in neural functioning leads to multiple homunculi and changes in how cortical cells are allocated to body parts

The homunculus
Somatosensory cortex:  some areas receive more cortical space than others.  Large areas devoted to lips, face, fingers.  Small areas to arm, trunk, legs.  (genitalia:  intermediate).

Perceiving Details
Measuring tactile acuity
Two-point threshold - minimum separation needed between two points to perceive them as two units
Grating acuity - placing a grooved stimulus on the skin and asking the participant to indicate the orientation of the grating

Cortical Mechanisms for Tactile Acuity
Body areas with high acuity have larger areas of cortical tissue devoted to them
This parallels the “magnification factor” seen in the visual cortex for the cones in the fovea
Areas with higher acuity also have smaller receptive fields on the skin
 

Perceiving Vibration
Pacinian corpuscle is primarily responsible for sensing vibration
RA2 fibers associated with them respond best to high rates of vibration
The structure of the corpuscle is responsible for the response to vibration - RA2 fibers without the corpuscle only respond when pressure is applied and removed
 

Perceiving Texture
Katz (1925) proposed that perception of texture depends on two cues:
Spatial cues are determined by the size, shape, and distribution of surface elements
Temporal cues are determined by the rate of vibration as skin is moved across finely textured surfaces
Two receptors may be responsible for this process - called the duplex theory of texture perception
Perceiving Texture - continued
Past research showed support for the role of spatial cues
Recent research by Hollins and Reisner shows support for the role of temporal cues
In order to detect differences between fine textures, participants needed to move their fingers across the surface

Experiment by Hollins et al.
Adaptation experiment - participants’ skin was adapted with either:
10-Hz stimulus for 6 minutes to adapt the RA1/Meissner corpuscle
250-Hz stimulus for 6 minutes to adapt the RA2/Pacinian corpuscle
Results showed that only the adaptation to the 250-Hz stimulus affected the perception of fine textures
 
 

Perceiving Objects
Humans use active rather than passive touch  to interact with the environment
Haptic perception is the active exploration of 3-D objects with the hand
It uses three distinct systems:
Sensory system
Motor system
Cognitive system
Perceiving Objects - continued
Psychophysical research shows that people can identify objects haptically in 1 to 2 sec
Klatzky et al. have shown that people use exploratory procedures (EPs)
Lateral motion
Pressure
Enclosure
Contour following
 

The Physiology of Tactile Object Perception
The firing pattern of groups of mechanoreceptors signals shape, such as the curvature of an object
Neurons further upstream become more specialized
Monkey’s thalamus shows cells that respond to center-surround receptive fields
Somatosensory cortex shows cells that respond maximally to orientations and direction of movement

The Physiology of Tactile Object Perception - continued
Monkey’s somatosensory cortex also shows neurons that respond best to:
Grasping specific objects
Paying attention to the task
Neurons may respond to stimulation of the receptors, but attending to the task increases the response
 

Pain Perception
Pain is a multimodal phenomenon containing a sensory component and an affective or emotional component
Three types of pain:
Nociceptive - signals impending damage to the skin
Types of nociceptors respond to heat, chemicals, severe pressure, and cold
Threshold of eliciting receptor response must be balanced to warn of damage but not be affected by normal activity
 

Types of Pain
Inflammatory pain - caused by damage to tissues and joints that releases chemicals that activate nociceptors
Neuropathic pain - caused by damage to the central nervous system, such as:
Brain damage caused by stroke
Repetitive movements which cause conditions like carpal tunnel syndrome

Pain Perception - continued
Signals from nociceptors travel up the spinothalamic pathway and activate:
Subcortical areas including the hypothalamus, limbic system, and the thalamus
Cortical areas including S1 and S2 in the somatosensory cortex, the insula, and the anterior cingulate cortex
These cortical areas taken together are called the pain matrix

Experiment by Hoffauer et al.
Participants were presented with potentially painful stimuli and asked:
To rate subjective pain intensity
To rate the unpleasantness of the pain
Brain activity was measured while they placed their hands into hot water
Hypnosis was used to increase or decrease the sensory and affective components
Experiment by Hoffauer et al. - continued
Results showed that:
Suggestions to change the subjective intensity led to changes in those ratings and in S1
Suggestions to change the unpleasantness of pain did not affect the subjective ratings but did change:
Ratings of unpleasantness
Activation in the anterior cingulate cortex
 

Cognitive and Experiential Aspects of Pain
Expectation - when surgical patients are told what to expect, they request less pain medication and leave the hospital earlier
Shifting attention - virtual reality technology has been used to keep patients’ attention on other stimuli than the pain-inducing stimulation

Cognitive and Experiential Aspects of Pain - continued
Content of emotional distraction - participants could keep their hands in cold water longer when pictures they were shown were positive
Individual differences - some people report higher levels of pain than others in response to the same stimulus
This could be due to experience or to physiological differences
 

Gate Control Model of Pain Perception
The “gate” consists of substantia gelatinosa cells in the spinal cord (SG- and SG+)
Input into the gate comes from:
Large diameter (L) fibers - information from tactile stimuli
Small diameter (S) fibers - information from nociceptors
Central control - information from cognitive factors from the cortex
 

Gate Control Model of Pain Perception - continued
Pain does not occur when the gate is closed by stimulation into the SG- from central control or L-fibers into the T-cell
Pain does occur from stimulation from the S-fibers into the SG+ into the T-cell
Actual mechanism is more complex than this model suggests

Opioids and Pain
Brain tissue releases neurotransmitters called endorphins
Evidence shows that endorphins reduce pain
Injecting naloxone blocks the receptor sites causing more pain
Naloxone also decreases the effectiveness of placebos
People whose brains release more endorphins can withstand higher pain levels
 

Pain in Social Situations
Experiment by Eisenberger et al.
Participants watched a computer game
Then were asked to play with two other “players” who did not exist but were part of the program
The “players” excluded the participant
fMRI data showed increased activity in the anterior cingulate cortex and participants reported feeling ignored and distressed

Lecture 8
Smell and Taste
June 6, 2006
 

Odors in our world
Multi-billion dollar industry in perfumes
And “under-arm” deodorants.
Ditto for mouth-washes
Real-estate brokers encourage sellers to have coffee brewing when potential buyers come.
Automobile manufacturers now sell and distrubute “new car smell”

Overview of Questions
Why is a dog’s sense of smell so much better than a human’s?
Why does a cold inhibit the ability to taste?
How do neurons in the cortex combine smell and taste?
Functions of Olfaction
Many animals are macrosmatic - having a keen sense of smell that is necessary for survival
Humans are microsmatic - a less keen sense of smell that is not crucial to survive
Experiment by Stern and McClintock
Underarm secretions were collected from 9 donor women
These were wiped on the upper lips of recipient women
Experiment by Stern and McClintock
Results showed that menstrual synchrony occurred since:
Secretions from the donors taken at the beginning of their cycles led to a shortened length of the recipients’ cycles
Secretions from the ovulatory phase lengthened recipients’ cycles
Phermones in the secretions, even though the women did not report smelling them, led to the changes
Detecting Odors
Measuring the detection threshold
Yes/no procedure - participants are given trials with odors along with “blank” trials
They respond by saying yes or no
This can result in bias in terms of when the participant decides to respond
Forced-choice - two trials are given, one with odorant and one without
Participant indicates which smells strongest

Detecting Odors - continued
Rats are 8 to 50 times more sensitive to odors than humans
Dogs are 300 to 10,000 times more sensitive
However, individual receptors for all of these animals are equally sensitive
The difference lies in the number of receptors they each have
Humans have 10 million and dogs have 1 billion olfactory receptors
Detecting Odors - continued
Measuring the difference threshold
Smallest difference in concentration that can be detected between two samples
This research must be done with carefully controlled concentrations using a device called a olfactometer
Research has shown the threshold to be approximately 11%
Identifying Odors
Recognition threshold - concentration needed to determine quality of an odorant
Humans can discriminate among 100,000 odors but they cannot label them accurately
This appears to be caused by an inability to retrieve the name from memory, from a lack of sensitivity
The Puzzle of Olfactory Quality
Researchers have found it difficult to map perceptual experience onto physical attributes of odorants
Henning’s odor prism (1916)
6 corners with the qualities putrid, ethereal, resinous, spicy, fragrant, and burned
Other odors located in reference to their perceptual relation to the corner qualities

The Puzzle of Olfactory Quality - continued
Unfortunately, Henning’s prism has proven of little use in olfactory research
Linking chemical structure to types of smells
Initial attempts showed difficulties since:
Some molecules with similar shapes had very different smells
Some similar smells came from molecules with different shapes

Structure of the Olfactory System
Olfactory mucosa is located at the top of the nasal cavity
Odorants are carried along the mucosa coming in contact with the sensory neurons
Cilia of these neurons contain the receptors
Humans have about 350 types of receptors that each have a protein that crosses the membrane 7 times

Structure of the Olfactory System - continued
Signals are carried to the glomeruli in the olfactory bulb
From there, they are sent to
Primary olfactory (piriform) cortex in the temporal lobe
Secondary olfactory (orbitofrontal) cortex in the frontal lobe
Amygdala deep in the cortex
 
 

Activating Receptor Neurons
Calcium imaging method
Receptors take up calcium ions when they respond
Calcium can be detected by using a chemical that makes the neuron fluoresce
Measuring the decrease in fluorescence indicates the strength of the response
Activating Receptor Neurons - continued
Combinatorial code for odor
Proposed by Malnic et al. from results of calcium imaging experiments
Odorants are coded by combinations of olfactory receptors called recognition profiles
Specific receptors may be part of the code for multiple odorants

Activating the Olfactory Bulb
Olfactory mucosa is divided into 4 zones
Each zone contains a variety of different receptors
Specific types of receptors are found in only one zone
Odorants tend to activate neurons within a particular zone
Specific types of neurons synapse with only one or two glomeruli

Activating the Olfactory Bulb - continued
Optical imaging method
Cortical cells consume oxygen when activated
Red light is used to determine the amount of oxygen in the cells
More oxygen reflects less red light
Measuring the amount of light reflected reveals which areas of cortex are most active

Activating the Olfactory Bulb - continued
2-deoxyglucose (2DG) technique
2DG, which contains glucose, is ingested into an animal
Animal is exposed to different chemicals
Neural activation is measured by amount of radioactivity present
This technique used with behavioral testing shows the pattern of neural activation is related to both chemical structure and to perception
 

Activating the Cortex
Genetic tracing method
DNA cloning is used to create cloned mice
A tracer molecule that stains the specific receptor that was created in the cloning procedure is used
The stain then “traces” the pathway through the olfactory pathways
Activating the Cortex - continued
Zou et al. created two types of mice:
One with new receptor in zone 1 in the mucosa and one with a new receptor in zone 4
Tracers showed that these zones led to different areas in the olfactory bulb
However, in the piriform cortex, the areas of activation overlapped
This might provide information about how complex odors are processed
 

Functions of Taste
Sweetness is usually associated with substances that have nutritive value
Bitter is usually associated with substances that are potentially harmful
Salty taste indicates the presence of sodium
However, there is not a perfect connection between tastes and function of substances
The sense of taste
Any reaction?

Basic Taste Qualities
Five basic taste qualities:
Salty
Sour
Sweet
Bitter
Umami - described as meaty, brothy or savory and associated with MSG
 

Structure of the Taste System
Tongue contains papillae:
Filiform - shaped like cones and located over entire surface
Fungiform - shaped like mushrooms and found on sides and tip
Foliate - series of folds on back and sides
Circumvallate - shaped like flat mounds in a trench located at back
Structure of the Taste System - continued
Taste buds are located in papallae except for filiform
Tongue contains approximately 10,000 taste buds
Each taste bud has taste cells with tips that extend into the taste pore
Transduction occurs when chemicals contact the receptor sites on the tips
 

Structure of the Taste System - continued
Signals from taste cells travel along a set of pathways:
Chorda tympani nerve from front and sides of tongue
Glossopharyngeal nerve from back of tongue
Vagus nerve from mouth and throat
Superficial petronasal nerve from soft palate
Structure of the Taste System - continued
These pathways make connections in the nucleus of solitary tract in the spinal cord
Then they travel to the thalamus
Followed by areas in the frontal lobe:
Insula
Frontal opervulum cortex
Orbital frontal cortex

Neural Coding for Taste
Distributed coding
Experiment by Erickson
Different taste stimuli were presented to rats and recordings were made from the chorda tympani
Across-fiber patterns showed that two substances (ammonium chloride and potassium chloride) are similar to each other
Experiment by Erickson
Rats were then trained by shocking them when they drank potassium chloride
When they were given the choice, they subsequently avoided ammonium chloride
The experiment provides physiological and behavioral evidence for distributed coding
 

Neural Coding for Taste - continued
Specificity coding
Experiment by Mueller et al.
Genetic cloning was used to determine if mice could be created that possessed a human receptor that responds to PTC
Normally, mice don’t have this receptor or respond to this substance
The experiment was successful

Neural Coding for Taste - continued
Experiment by Sato et al.
Recordings were made from 66 fibers in the monkey’s chorda tympani
Results showed that there were fibers that responded best to one of the basic tastes (sweet, salty, sour, and bitter) but poorly to the others
Thus, there are fibers that respond specifically to particular chemicals

Neural Coding for Taste - continued
Evidence exists for both specificity and distributed coding
Some researchers suggest that the neural system for taste may function like the visual system for color
Currently there is no agreed upon explanation for the neural system for taste
The Perception of Flavor
Combination of smell, taste, and other sensations (such as burning of hot peppers)
Odor stimuli from food in the mouth reaches the olfactory mucosa through the retronasal route
The taste of most compounds is influenced by olfaction, but a few, such as MSG are not

The Physiology of Flavor Perception
Responses from taste and smell are first combined in the orbital frontal cortex (OFC)
OFC also receives input from the primary somatosensory cortex and the inferotemporal cortex in the visual what pathway
Bimodal neurons in this area respond to taste and smell as well as taste and vision
Firing of these neurons is also affected by the level of hunger of the animal for a specific food

Individual Differences in Taste
There are different responses to phenylthiocarbamide (PTC) and to 6-n-propylthiouracil (PROP):
Tasters, nontasters, and supertasters
Tasters have more taste buds than nontasters
Tasters have specialized receptors for these compounds
Supertasters appear more sensitive to bitter substances than tasters
 

Lecture 9:
Sound and introduction to hearingJune 13, 2006
 
 

Overview of Questions
If a tree falls in the forest and no one is there to hear it, is there a sound?
What is it that makes sounds high pitched or low pitched?
How do sound vibrations inside the ear lead to the perception of different pitches?
How are sounds represented in the auditory cortex?
Pressure Waves and Perceptual Experience
Two definitions of “sound”
Physical definition - sound is pressure changes in the air or other medium
Perceptual definition - sound is the experience we have when we hear
Sound Waves
Loud speakers produce sound by
The diaphragm of the speaker moves out, pushing air molecules together
The diaphragm also moves in, pulling the air molecules apart
The cycle of this process creates alternating high- and low-pressure regions that travel through the air

Sound Waves - continued
Pure tone - created by a sine wave
Amplitude - difference in pressure between high and low peaks of wave
Perception of amplitude is loudness
Decibel (dB) is used as the measure of loudness
Number of dB = 20 logarithm(p/po)
The decibel scale relates the amplitude of the stimulus with the psychological experience of loudness

Sound Waves - continued
Frequency - number of cycles within a given time period
Measured in Hertz (Hz) - 1 Hz is 1 cycle per second
Perception of pitch is related to frequency
Tone height is the increase in pitch that happens when frequency is increased

Musical Scales and Frequency
Letters in the musical scale repeat
Notes with the same letter name (separated by octaves) sound similar - called tone chroma
Notes separated by octaves have frequencies that are multiples of each other

Range of Hearing
Human hearing range - 20 to 20,000 Hz
Audibility curve - shows the threshold of hearing
Changes on this curve show that humans are most sensitive to 2,000 to 4,000 Hz
Auditory response area - falls between the audibility curve and and the threshold for feeling
It shows the range of response for human audition
 

Range of Hearing - continued
Equal loudness curves - determined by using a standard 1,000 Hz tone
Two dB levels are used - 40 and 80
Participants match the perceived loudness of all other tones to the 1,000 Hz standard
Resulting curves show that tones sound
Almost equal loudness at 80 dB
High and low frequencies sound softer at 40 dB than the rest of the tones in the range
Sound Quality: Timbre
All other properties of sound except for loudness and pitch constitute timbre
Timbre is created partially by the multiple frequencies that make up complex tones
Fundamental frequency is the first harmonic
Musical tones have additional harmonics that are multiples of the fundamental frequency
Sound Quality: Timbre
Additive synthesis - process of adding harmonics to create complex sounds
Frequency spectrum - display of harmonics of a complex sound
Attack of tones - buildup of sound at the beginning of a tone
Decay of tones - decrease in sound at end of tone

The Ear
Outer ear - pinna and auditory canal
Pinna helps with sound location
Auditory canal  - tube-like 3 cm long structure
Protects the tympanic membrane at the end of the canal
Resonant frequency of the canal amplifies frequencies between 2,000 and 5,000 Hz

The Middle Ear
2 cubic centimeter cavity separating inner from outer ear
It contains the three ossicles
Malleus - moves due to the vibration of the tympanic membrane
Incus - transmits vibrations of malleus
Stapes - transmit vibrations of incus to the inner ear via the oval window of the cochlea

Figure 11.12  The middle ear.  The three bones of the middle ear transmit the vibrations of the tympanic membrane to the inner ear.
Function of Ossicles
Outer and inner ear are filled with air
Inner ear filled with fluid that is much denser than air
Pressure changes in air transmit poorly into the denser medium
Ossicles act to amplify the vibration for better transmission to the fluid

The Inner Ear
Main structure is the cochlea
Fluid-filled snail-like structure set into vibration by the stapes
Divided into the scala vestibuli and scala tympani by the cochlear partition
Cochlear partition extends from the base (stapes end) to the apex (far end)
Organ of Corti contained by the cochlear partition

The Organ of Corti
Key structures
Basilar membrane vibrates in response to sound and supports the organ of Corti
Inner and outer hair cells are the receptors for hearing
Tectorial membrane extends over the hair cells
Transduction at the hair cells takes place due to the interaction of these structures
Neural Signals for Frequency
There are two ways nerve fibers signal frequency
Which fibers are responding
Specific groups of hair cells on basilar membrane activate a specific set of nerve fibers
How fibers are firing
Rate or pattern of firing of nerve impulses

Békésys’ Place Theory of Hearing
Frequency of sound is indicated by the place on the organ of Corti that has the highest firing rate
Békésy determined this in two ways
Direct observation of the basilar membrane in a cadaver
Building a model of the cochlea using the physical properties of the basilar membrane
Békésys’ Place Theory of Hearing - continued
Physical properties of the basilar membrane
Base of the membrane (by stapes) is
 3 to 4 times narrower than at the apex
100 times stiffer than at the apex
Both the model and the direct observation showed that the vibrating motion of the membrane is a traveling wave
 

Békésys’ Place Theory of Hearing - continued
Envelope of the traveling wave
Indicates the point of maximum displacement of the basilar membrane
Hair cells at this point are stimulated the most strongly leading to the nerve fibers firing the most strongly at this location
Position of the peak is a function of frequency
 

Evidence for the Place Theory
Tonotopic map
Cochlea shows an orderly map of frequencies along its length
Apex responds best to high frequencies
Base responds best to low frequencies

Evidence for the Place Theory - continued
Neural frequency tuning curves
Pure tones are used to determine the threshold for specific frequencies measured at single neurons
Plotting thresholds for frequencies results in tuning curves
Frequency to which the neuron is most sensitive is the characteristic frequency

Evidence for the Place Theory - continued
Auditory masking experiments
First, thresholds for a number of frequencies are determined
Then, a single intense masking frequency is presented at the same time that the thresholds for the original frequencies are re-determined
The masking effect is seen at the masking tone’s frequency and spreads to higher frequencies more than lower ones
 

Updating Békésy’s Place Theory
Békésy used unhealthy basilar membranes and his results showed no difference in response for close frequencies that people can distinguish
New research with healthy membranes show that the entire outer hair cells respond to sound by slight tilting and a change in length
This is called the motile response and helps to amplify action on the membrane

Response of Basilar Membrane to Complex Tones
Fourier analysis - mathematic process that separates complex waveforms into a number of sine waves
Research on the response of the basilar membrane shows the highest response in  auditory nerve fibers with characteristic frequencies that correspond to the sine-wave components of complex tones
Thus the cochlea is called a frequency analyzer
Pathway from the Cochlea to the Cortex
Auditory nerve fibers synapse in a series of subcortical structures
Cochlear nucleus
Superior olivary nucleus (in the brain stem)
Inferior colliculus (in the midbrain)
Medial geniculate nucleus (in the thalamus)
Auditory receiving area (A1 in the temporal lobe)

Auditory Areas in the Cortex
 Hierarchical processing occurs in the cortex
Neural signals travel through the core, then belt, followed by the parabelt area
Simple sounds cause activation in the core area
Belt and parabelt areas are activated in response to more complex stimuli made up of many frequencies

What and Where Streams for Hearing
What or ventral stream starts in the anterior portion of the core and belt and extends to the prefrontal cortex
It is responsible for identifying sounds
Where or dorsal stream starts in the posterior core and belt and extends to the parietal and prefrontal cortices
It is responsible for locating sounds
Evidence from neural recordings, brain damage, and brain scanning support these findings

Perceiving Pitch and Complex Sounds
Tonotopic maps are found in A1
Neurons that respond better to low frequencies are on the left and those that respond best to high frequencies are on the right
However, early research did not show a direct relationship between pitch perception and the tonotopic map
 

Recent Evidence of Pitch Perception in A1
Effect of training on tonotopic maps
Owl monkeys were trained to discriminate between two frequencies near 2,500 Hz
Trained monkeys showed tonotopic maps with enlarged areas with neurons that responded to 2,500 Hz compared to untrained monkeys
Cases of humans with brain damage to this area show perception difficulties with pitch
 
 
 

Brain scans in humans
Tasks that require pitch recognition activate areas equivalent to the core area in monkeys
Tasks the require recognition of complex stimuli activate areas equivalent to the parabelt area in monkeys
Thus, stimuli that are more complex are processed farther “downstream” in the nervous system
Effect of the Missing Fundamental
The fundamental frequency is the lowest frequency in a complex tone
When the fundamental and other lower harmonics are removed, the perceived pitch is  the same, but the timbre changes
The pitch perceived in such tones is called periodicity pitch

Effect of Experience on the Auditory Cortex
Musicians show enlarged auditory cortices that respond to piano tones and stronger neural responses than non-musicians
Experiment by Fritz et al.
Marmosets were trained to lick a water spout in response to a pure tone embedded within a stream of complex tones
Neurons became quickly tuned to the target frequency and maintained the effect for hours after the testing session
 

Cochlear Implants
Electrodes are inserted into the cochlea to electrically stimulate auditory nerve fibers
The device is made up of
A microphone worn behind the ear
A sound processor
A transmitter mounted on the mastoid bone
A receiver surgically mounted on the mastoid bone

Cochlear Implants - continued
Implants stimulate the cochlea at different places on the tonotopic map according to specific frequencies in the stimulus
These devices help deaf people to hear some sounds and to understand language
They work best for people who receive them early in life or for those who have lost their hearing, although they have caused some controversy in the deaf community
 
 

Lecture 10
 Auditory perceptionJune 15, 2006
Overview of Questions
What makes it possible to tell where a sound is coming from in space?
When we are listening to a number of musical instruments playing at the same time, how can we perceptually separate the sounds coming from the different instruments?
Why does music sound better in some concert halls than in others?
Auditory Localization
Auditory space - surrounds an observer and exists wherever there is sound
Researchers study how sounds are localized in space by using
Azimuth coordinates - position left to right
Elevation coordinates - position up and down
Distance coordinates - position from observer

Auditory Localization - continued
On average, people can localize sounds
Directly in front of them most accurately
To the sides and behind their heads least accurately
Location cues are not contained in the receptor cells like on the retina in vision; thus, location for sounds must be calculated
 
 

Cues for Sound Location
Binaural cues - location cues based on the comparison of the signals received by the left and right ears
Interaural time difference - difference between the times sounds reach the two ears
When distance to each ear is the same, there are no differences in time
When the source is to the side of the observer, the times will differ
 
 
 

Binaural Cues
Interaural intensity difference - difference in sound pressure level reaching the two ears
Reduction in intensity occurs for high frequency sounds for the far ear
The head casts an acoustic shadow
This effect doesn’t occur for low frequency sounds
 
 

Monaural Cue for Sound Location
The pinna and head affect the intensities of frequencies
Measurements have been performed by placing small microphones in ears and comparing the intensities of frequencies with those at the sound source
The difference is called the head-related transfer function (HRTF)
This is a spectral cue since the information for  location comes from the spectrum of frequencies
 

Location Cues: Effects on Behavior
Experimental methods
Free-field presentation - sounds are presented by speakers located around the listener’s head in a dark room
Listener can indicate location by pointing or by giving azimuth and elevation coordinates
 

Location Cues: Effects on Behavior - continued
Headphone presentation of sounds
Advantage - experimenter has precise control over sounds
Disadvantage - cues from the pinna are eliminated, which results in the sound being internalized
Sound can be externalized  by using HTRFs to create a virtual auditory space (VAT)

Judging Azimuth Locations
Experiments by Wight and Kistler
Experiment 1 - used virtual auditory space
HRTFs, ITDs, & ILDs were used to indicate locations that varied from left to right
Listeners were fairly accurate
Experiments 2 & 3 - results showed ITD was the dominant cue for low frequencies and ILD was effective for high frequencies
 

Judging Elevation
ILD and ITD are not effective for judgments on elevation since in many locations they may be zero
Experiment investigating spectral cues
Listeners were measured for performance locating sounds differing in elevation
They were then fitted with a mold that changed the shape of their pinnae
 
 

Experiment on Judging Elevation
Right after the molds were inserted, performance was poor
After 19 days, performance was close to original performance
Once the molds were removed, performance stayed high
This suggests that there might be two different sets of neurons—one for each set of cues
 

The Physiological Representation of Auditory Space
Interaural time-difference detectors - neurons that respond to specific interaural time differences
They are found in the auditory cortex and at the first nucleus (superior olivary) in the system that receives input from both ears
Tonographic maps - neural structure that responds to locations in space
The Auditory Cortex
Even though there are tonographic maps in subcortical areas of mammals, there is no evidence of such maps in the cortex
Instead panoramic neurons have been found that signal location by their pattern of firing
There is also the where stream that shows more specific neural response for location the further upstream the neurons are located in the cortex
 

Identifying Sound Sources
Auditory Scene - the array of all sound sources in the environment
Auditory Scene Analysis - process by which sound sources in the auditory scene are separated into individual perceptions
This does not happen at the cochlea since simultaneous sounds will be together in the pattern of vibration of the basilar membrane

Principles of Auditory Grouping
Heuristics that help to perceptually organize stimuli
Location - a single sound source tends to come from one location and to move continuously
Similarity of timbre and pitch - similar sounds are grouped together
Auditory stream segregation - separation of stimuli into different perceptual streams

Auditory Stream Segregation
Compound melodic line in music is an example of auditory stream segregation
Experiment by Bregman and Campbell
Stimuli were alternating high and low tones
When stimuli played slowly, the perception is hearing high and low tones alternating
When the stimuli are played quickly, the listener hears two streams; one high and one low
 

Auditory Stream Segregation - continued
Experiment by Deutsch - the scale illusion or melodic channeling
Stimuli were two sequences alternating between the right and left ears
Listeners perceive two smooth sequences by grouping the sounds by similarity in pitch
This demonstrates the perceptual heuristic that sounds with the same frequency come from the same source, which is usually true in the environment
 

Principles of Auditory Grouping - continued
Proximity in time - sounds that occur in rapid succession usually come from the same source
This principle was illustrated in auditory streaming
Good continuation - sounds that stay constant or change smoothly are usually from the same source
Good Continuation
Experiment by Warren et al.
Tones were presented interrupted by gaps of silence or by noise
In the silence condition, listeners perceived that the sound stopped during the gaps
In the noise condition, the perception was that the sound continued behind the noise

Principles of Auditory Grouping - continued
Effect of past experience
Experiment by Dowling
Used two interleaved melodies (“Three Blind Mice” and “Mary Had a Little Lamb”)
Listeners reported hearing a meaningless jumble of notes
But listeners who were told to listen for the melodies were able to hear them by using melody schema

Hearing Inside Rooms
Direct sound - sound that reaches the listeners’ ears straight from the source
Indirect sound - sound that is reflected off of environmental surfaces and then to the listener
When a listener is outside, most sound is direct; however inside a building, there is direct and indirect sound
 

Experiment by Litovsky et al.
Listeners sat between two speakers
Right speaker was the lead speaker
Left speaker was the lag speaker
When two sounds were presented simultaneously, listeners heard a centered sound between speakers --
The two sounds became fused

Experiment by Litovsky et al. - continued
When the lead sound was presented
Less than 1 ms before the lag speaker, a single sound nearer the lead speaker was heard
From 1 to 5 ms before the lag speaker, sound appeared to come from lead speaker alone - called the precedence effect
At intervals greater than 5 ms, two separate sounds were heard, one following the other - called the echo threshold

Architectural Acoustics
The study of how sounds are reflected in rooms
Factors that affect perception in concert halls
Reverberation time - the time is takes sound to decrease by 1/1000th of its original pressure
Best time is around 2 sec (1.5 for opera)
 

Factors that Affect Perception in Concert Halls
Intimacy time - time between when sound leaves its source and when the first reflection arrives
Best time is around 20 ms
Bass ratio - ratio of low to middle frequencies reflected from surfaces
High bass ratios are best
Spaciousness factor - fraction of all the sound received by listener that is indirect
High spaciousness factors are best
Interactions Between Vision and Sound
Visual capture or the ventriloquist effect - an observer perceives the sound as coming from the seen location rather than the source for the sound
Experiment by Sekuler et al.
Balls moving without sound appeared to move past each other
Balls with an added “click” appeared to collide
 
 
 

Lecture 11
Speech PerceptionJune 20,2006
Overview of Questions
Can computers perceive speech as well as humans?
Why does an unfamiliar foreign language often sound like a continuous stream of sound, with no breaks between words?
Does each word that we hear have a unique pattern of air pressure changes associated with it?
Are there specific areas in the brain that are responsible for perceiving speech?
The Speech Stimulus
Phoneme - smallest unit of speech that changes meaning in a word
In English there are 47 phonemes:
13 major vowel sounds
24 major consonant sounds
Number of phonemes in other languages varied—11 in Hawaiian and 60 in some African dialects

The Acoustic Signal
Produced by air that is pushed up from the lungs through the vocal cords and into the vocal tract
Vowels are produced by vibration of the vocal cords and changes in the shape of the vocal tract
These changes in shape cause changes in the resonant frequency and produce peaks in pressure at a number of frequencies called formants

The Acoustic Signal - continued
The first formant has the lowest frequency, the second has the next highest, etc.
Sound spectrograms show the changes in frequency and intensity for speech
Consonants are produced by a constriction of the vocal tract
Formant transitions - rapid changes in frequency preceding or following consonants

The Relationship between the Speech Stimulus and Speech Perception
The segmentation problem - there are no physical breaks in the continuous acoustic signal
How do we segment the individual words?
The variability problem - there is no simple correspondence between the acoustic signal and individual phonemes
Variability from a phoneme’s context
Coarticulation - overlap between articulation of neighboring phonemes
 

The Relationship between the Speech Stimulus and Speech Perception - continued
Variability from different speakers
Speakers differ in pitch, accent, speed in speaking, and pronunciation
This acoustic signal must be transformed into familiar words
People perceive speech easily in spite of the segmentation and variability problems
 

Stimulus Dimensions of Speech Perception
Invariant acoustic cues - features of phonemes that remain constant
Short-term spectrograms are used to investigate invariant acoustic cues
Sequence of short-term spectra can be combined to create a running spectral display
From these displays, there have been some invariant cues discovered

Categorical Perception
This occurs when a wide range of acoustic cues results in the perception of a limited number of sound categories
An example of this comes from experiments on voice onset time (VOT) - time delay between when a sound starts and when voicing begins
Stimuli are da (VOT of 17ms) and ta (VOT of 91ms)
Categorical Perception - continued
Computers were used to create stimuli with a range of VOTs from long to short
Listeners do not hear the incremental changes, instead they hear a sudden change from /da/ to /ta/ at the phonetic boundary
Thus, we experience perceptual constancy for the phonemes within a given range of VOT

Speech Perception is Multimodal
Auditory-visual speech perception
The McGurk effect
Visual stimulus shows a speaker saying “ga-ga”
Auditory stimulus has a speaker saying “ba-ba”
Observer watching and listening hears “da-da”, which is the midpoint between “ga” and “ba”
Observer with eyes closed will hear “ba”

Cognitive Dimensions of Speech Perception
Top-down processing, including knowledge a listener has about a language, affects perception of the incoming speech stimulus
Segmentation is affected by context and meaning
I scream you scream we all scream for ice cream

Meaning and Phoneme Perception
Experiment by Turvey and Van Gelder
Short words (sin, bat, and leg) and short nonwords (jum, baf, and teg) were presented to listeners
The task was to press a button as quickly as possible when they heard a target phoneme
On average, listeners were faster with words (580 ms) than non-words (631 ms)
Meaning and Phoneme Perception - continued
Experiment by Warren
Listeners heard a sentence that had a phoneme covered by a cough
The task was to state where in the sentence the cough occurred
Listeners could not correctly identify the position and they also did not notice that a phoneme was missing -- called the phonemic restoration effect
Meaning and Word Perception
Experiment by Miller and Isard
Stimuli were three types of sentences:
Normal grammatical sentences
Anomalous sentences that were grammatical
Ungrammatical strings of words
Listeners were to shadow (repeat aloud) the sentences as they heard them through headphones
Meaning and Word Perception - continued
Results showed that listeners were
89% accurate with normal sentences
79% accurate for anomalous sentences
56% accurate for ungrammatical word strings
Differences were even larger if background noise was present
Speaker Characteristics
Indexical characteristics - characteristics of the speaker’s voice such as age, gender, emotional state, level of seriousness, etc.
Experiment by Palmeri, Goldinger, and Pisoni
Listeners were to indicate when a word was new in a sequence of words
Results showed that they were much faster if the same speaker was used for all the words
Speech Perception and the Brain
Broca’s aphasia - individuals have damage in Broca’s area (in frontal lobe)
Labored and stilted speech and short sentences but they understand others
Wernicke’s aphasia - individuals have damage in Wernicke’s area (in temporal lobe)
Speak fluently but the content is disorganized and not meaningful
They also have difficulty understanding others
 

Speech Perception and the Brain - continued
Measurements from cats’ auditory fibers show that the pattern of firing mirrors the energy distribution in the auditory signal
Brain scans of humans show that there are areas of the human what stream that are selectively activated by the human voice

Experience Dependent Plasticity
Before age 1, human infants can tell difference between sounds that create all  languages
The brain becomes “tuned” to respond best to speech sounds that are in the environment
Other sound differentiation disappears when there is no reinforcement from the environment
Motor Theory of Speech Perception
Liberman et al. proposed that motor mechanisms responsible for producing sounds activate mechanisms for perceiving sound
Evidence from monkeys comes from the existence of mirror neurons
Experiment by Watkins et al.
Participants had their motor cortex for face movements stimulated by transcranial magnetic stimulation (TMS)
Motor Theory of Speech Perception - continued
Results showed small movements for the mouth called motor evoked potentials (MEP)
This response became larger when the person listened to speech or watched someone else’s lip movements
In addition, the where stream may work with the what stream for speech perception