Notes

Outline

Overview of Human-Machine Systems

General Characteristics of Sensory Systems Stimulus Receptor Neural Relay Cortex In Vision Light Rods/Cones LGN of Thalamus Striate In Audition Sound Hair Cells MGN of Thalamus Sup. Temp. G. Other Generalities Always more than one pathway in brain Always more than one brain target Ultimately sensory information is combined

The Physical Stimulus for Audition

The Physical Stimulus for Audition - 2 Amplitude is the difference in air pressure between the compression and rarefaction. The measure of sound amplitude is the relative measure called decibel or dB. Where P = air pressure; P2 = power dB SPL, P2=0.0002 dynes/cm2 which is near the absolute threshold for hearing.

The Physical Stimulus for Audition - 3 Resonance All physical mater will most easily vibrate at certain frequencies. This is true of our ear. Thus some frequencies will more easily enter our ear It helps us determine the frequencies of incoming sounds as we shall see. The physical dimensions are related to but not the same as the psychological dimensions: frequency <> pitch amplitude <> loudness

Anatomy and Physiology of the Ear Three Major Divisions Outer Ear receives sound directs it to the rest of the ear. Pinna - directs sound energy to middle ear and helps  perception of the direction. External Auditory Meatus or Canal - 2.5 to 3 cm long, 7 mm wide Resonates at about 2-4K Hz. Tympanic Membrane

Anatomy and Physiology of the Ear - 2 Middle Ear transmits sound information to inner ear. Ossicles transmit and amplify sound energy. Malleus - Hammer Incus - anvil Stapes - stirrup Eustachian Tube Inner Ear is where transduction of sound information occurs. Cochlea (snail) with the Oval Window Round Window

The Cochlea and Sound Transduction The Cochlea - Latin for seashell which is what it looks like Basilar membrane runs most of the length of the cochlea dividing in the top and bottom. The base is right below the oval window where the sound energy enters The apex is at the other end. Hair Cells are the receptors and run the length of the Basilar Membrane in two sets inner 1 row ~ 3500 outer 3 rows ~20000 Tectorial Membrane - across top of Hair Cells

The Cochlea and Sound Transduction - 2 Auditory Transduction Transduction is the conversion of energy from one form to another, e.g., sound pressure to neural impulses The Traveling wave. Wave set up by action of stapes on oval window Point of Maximal Displacement depends upon the frequency of the tone. High Frequencies near the base. Low frequencies near the apex. The shearing force The traveling wave bends the basilar membrane This bends the hair cells.

Loudness The experience of sound most closely related to amplitude or intensity. Examples of sounds at different dB SPL levels for comparison. Rustling Leaves =~20 dB Average Speaking Voice =~60 dB Heavy Traffic =~80 dB Rock Band =~120 dB Pain/Damage Threshold =~130 to 140 dB  Loudness differs in many ways from intensity. The threshold depends upon intensity and frequency. Intensity doubles every 3 dB; loudness doubles every 8 dB.

Pitch The dimension of sound that most closely relates to frequency. The higher the frequency the higher the pitch. Discrimination between two pitches depends on the frequency of the lower pitch: Weber Fraction:   (f1 - f2)/f2  =  0.004 e.g. (251-250)/250= 0.004 (1004-1000)/1000=0.004 Pitch is not the same as frequency Pitch will change as intensity is increase and frequency is kept constant.

The Interdependence of Loudness and Pitch First studied by Fletcher and Munson (1933). Called Fletcher-Munson Curves or Equal Loudness Contours. Method: Subjects adjusted tone of different frequencies to match loudness of 1000 Hz tone the intensity of 1000 Hz tone was varied over trials. Thus, all tones that match a 1K Hz tone of a given intensity should all be equally loud and connecting those on a graph of intensity by frequency should give an equal loudness contour.

The Interdependence of Loudness and Pitch - 2 As intensity of the 1K Hz tone increase, the contours get flatter. Relates to the Loudness button on your stereo. This relationship again illustrates the difference between physical dimensions and psychological experience.

Application to Human Factors Sound Button on Stereo Most recording are at region where loudness if fairly constant across frequency. We may play at a lot lower level where loudness does depend on frequency Alters what we hear because we lose sensitivity to low and high frequencies faster than middle frequencies. Sound button compensates for this by boosting high and low frequencies.

Fourier Analysis A mathematical procedure to break down complex waveforms in to simple components, usually sinewaves. The ear does something like this.

Fourier Analysis - 2 Let us use this stimulus as our complex wave. It is called a square wave.

How Fourier Analysis Works - Briefly The Frequency Domain Frequency of Sinewave along the x-axis Amplitude of Sinewave along the y-axis

How Fourier Analysis Works - Briefly 2

How Fourier Analysis Works - Briefly 2

Effects of Multiple Tones Beats Perception of intensity changes from two nearby frequencies From constructive and destructive interference Frequency of beating is difference in frequency between the two tones, e.g. 101-100 = 1 Hz beats

Effects of Multiple Tones - 2 Missing fundamental Fundamental is lowest pitch of a tone higher frequencies called harmonics or partials Perceive a same pitch even without  fundamental Allows us to tell female vs. male voices on the telephone.

The Missing Fundamental

Removing the Fundamental

Full vs. Octave

Octave vs. Missing Fundamental

Masking DEFINITION: one tone is rendered less perceptible by another auditory stimulus. Tone Masking low tones will mask higher tones better. due to shape of traveling wave (skewed towards base, higher frequencies). Noise Masking Noise is sound energy that lacks coherence. Beyond a point adding more frequencies to the noise does not increase masking. Critical bands: region of basilar membrane where sound energy is summed together.

Application to Human Factors Consider Noisy Environments How keep all the sounds distinguishable? Consider sirens and other alerting sounds? Is simply loud enough or necessary?

The Perception of Auditory Direction Eyes can see only in one direction at a time.  Ears are not so limited. Interaural Time Difference/Phase Description - sound has to travel farther to ear on farther side of head This difference can be detected if as small as 0.1 msec. Works for clicks and tones with frequencies < 1000 Hz Precedence Effect - Tendency to suppress later arriving parts of a sound

Time of Arrival

Time of Arrival

Time of Arrival

Time of Arrival

The Perception of Auditory Direction - 2 Interaural Intensity Differences Description - Head shadows sound so that farther ear will hear a slightly less intense sound. Just as we suppress later sounds, we suppress less intense sounds. Works best for relatively high frequencies. This ability to  hear sounds from all directions is useful to design alerts.

Interaural Loudness Differences

Signal Detection Theory The Detection Situation The Stimulus is: Subject Judges Stimulus to be:

Sensory Events Underlying STD

Measures of a Sensory Event - d’ and b

Applying Model to Types of Outcomes

Errors in Signal Detection Theory

Effects of d’ on Sensitivity

Effects of b

Receiver Operating Characteristic