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Chapter 6

Chap 6 Summary

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McGill University
PSYC 212
Evan Balaban

Sensory Perception Chapter 6 Notes 2013-10-12 8:51 PM TheAuditory System: PerceptualAspects of Hearing A. Intensity and Loudness •Loudness always refers to the magnitude of the perception created by the sound of a particular physical intensity – sound cannot have a value of loudness only intensity •Characteristics of loudness perception in humans is related to several factors oOne being frequency. 20-20000 Hz being the range of hearing among humans. oPerception of intensity is related to the frequency of sound within this range. oOther factors that have bearing on loudness perception  Use of one ear or both  Duration of sound  Presence of other stimuli such as noise 1. Audibility Threshold •Fundamental issue – absolute sensitivity of the ear and how it relates to sound frequency •Procedural aspects oManner in which sound stimuli are delivered to ear can affect actual threshold values that are obtained  Two procedures used  1. Provide sound through loudspeaker located directly in front of subject in enclosed room •Absolute threshold values obtained by open-ear method called minimum audible field (MAF) •Advantage - they are obtained by free-field measurements and represent a natural way of listening •Disadvantage – a number of other factors can influence results. Like head position or environmental quality  2. Have subject listen to sounds present to both ears through headphones. •Sound pressure is measured or estimated through calibration procedure •Absolute threshold values obtained by closed-ear method known as Minimum Audible Pressure (MAP) •Advantage – can be related to exact pressure values at ear drum and its easier to ensure stimulation is applied equally to both ears •Disadvantage – differences in ear canal can mean pressures will be different among individuals  After gathering data from a number of trials using either methods a psychometric function can be plotted showing relationship between detection performance and intensity. Sound intensity hat produces 50% detection level is chosen for criterion threshold •Absolute Detection Thresholds oHearing sensitivity is dependent on frequency – thus frequency parameter needs to be specified oPure tone of frequency is used to obtain psychometric function – this is then repeated at different frequencies to provide a comprehensive picture of hearing sensitivity. oOnce a set of threshold values are obtained they can be plotted with respect to frequencies at which they were obtained - known as a psychophysical function. oHighly useful psychophysical function is on that related threshold sound detection levels in terms of frequency – called a MinimalAudibility Curve  Examination of MinimalAudibility Curve  Lowest detection threshold (highest hearing sensitivities) – 2000 Hz to 4000 Hz  Sounds below optimal range display progressively higher detection thresholds. •Lowest frequency that can be heard lies between 10 Hz and 20 Hz  Sound frequencies higher than middle range also display progressively higher threshold values. •Anatomical and physiological determinants of minimal audibility curve oFactors responsible for U shaped profile of minimal audibility curve  Conductive element of outer and middle ear  Mechanical response to function of the cochlea – can be eliminated because response is similar across broad range of frequencies  Physiological properties of auditory nervous system – as frequencies increase neurons are better able to summate responses triggered by cycle of the sound wave. Thus higher frequencies result in increased neuronal response and lower detection thresholds This process does not prevail at high frequencies •Factors that affect absolute sensitivity oWay in which data was obtained  Threshold obtained with MAF lower by 6dB than those obtained with MAP oDuration of sound stimulus oWhether sound is applied to both ears or just one  It is total sound pressure applies to auditory system that is important  Auditory system integrates sound energy from both ears – known as Binaural Summation  The auditory system also integrates sound energy over time – known as Temporal Summation •Terminal Thresholds oIntense sounds are felt as well as heard oThe Terminal Threshold is the maximum sound pressure that we can tolerate oRange of intensities for normal hearing function occurs within space bound by minimal audibility curve and terminal threshold 2. Difference Thresholds •Weber’s Law – relationship to sound intensity and frequency oThe Weber constant for sound intensity discrimination varies form 0.1 to 0.4. oWeber constant for sound intensity discrimination Is higher at low intensities and gradually declines as intensity increases. oIntensity discrimination is not affected by sounds frequency when single tones are used. •Determinants of Intensity discrimination ability oAbility to discriminate sound intensity is based on response of auditory neurons oAgreater spread of excitation means that auditory neurons across a broader range of frequencies will be stimulated.  Current view – increased sound intensities produce increased discharge in auditory neurons as well as increased discharge of neurons encoding other nearby frequencies 3. Loudness Perception – Relationship to intensity • The Sone Scale of Loudness oRequired subjects to either rate the perceived intensity of stimulus (Method of Magnitude Estimation) or identify pairs of stimulus intensities that generated a certain ratio of loudness sensations, such as half (Method of Fractionation) oNeeded to define a standard unit. Sone scale  1 sone is equal to the loudness sensation produced by a 1000Hz tone at 40dBspl • The Intensity-Loudness Function oAn increase of 10dBspl was required for every doubling of loudness and a 10dBspl decrease was required for halving the loudness oLoudness grows at a lower rate than intensity because relatively more sound intensity is required to produce a given increase in loudness • The Power Law Relationship oIn Figure 6.6 in the textbook we see a lol-log plot which is means that both the y and x-axis are logarithmic – thus when you plot loudness vs. stimulus you get a log-log plot. This is equivalent to a power function  (L) Loudness given in sones, constant (k) depends on depends on units used for sound pressure (P)  The exponent depicts that this is a negatively accelerating function, although loudness increases with intensity it doesn’t grow rapidly – important implications in everyday life, if two people are talking with same intensity the perceived loudness is not twice the amount but only about 60%. • Binaural Summation oFor any given stimulus intensity – loudness sensation with binaural is always judges to be twice that of monaural. Essentially sounds are perceived to be louder if they are heard through both ears  This has led to the belief that the auditory system is capable of near perfect binaural summation at suprathreshold intensities • Determinants of Loudness Perception oAs sound intensity increases, electrical activity of the cochlear neurons also increases  However neurons are only capable of signaling within restricted range ( 40dB), far less than the 120dB representing dynamic range for humans Solution – have cochlear neurons that only encode discrete intensity regions  Any intensity will stimulate a subset of auditory neurons oSecond means for intensity coding Higher sound intensities produce spread of excitation along basilar membrane That excitation in turn recruits auditory neurons of different cIn regard t haracteristic frequencies The larger the range of active frequency tuned fibres, the larger the sound stimulus 4. Loudness Perception – Relationship to Frequency •Equal Loudness Contours oIn regard to graph 6.7 – frequency vs. sound intensity Each of the equal loudness curves provides comprehensive set of data on the intensity value that produce equal loudness perception across a range of frequencies Progressively higher intensities are needed to achieve equal loudness perception as you move towards extreme ends of frequency spectrum Equal loudness spectrums appear to become flatter at higher intensities  Implies that extremely loud sounds are perceived as being equally loud regardless of frequency •The Phon Scale of Loudness o Unit of measurement used to indicate loudness level across frequencies is the Phon Number of phons at any frequency is equal to intensity in dBspl of 1000Hz when two are judged to be of equal loudness 5. Masking and Noise •We are not generally exposed to a single tone, the normal world contains sounds with a mixture of different frequencies. Thus the auditory signal is much more complex than in the case of a single tone •Masking – General Properties oMost common perceptual consequence of listening to sound mixtures is masking  Our perception of a particular sound is affected by the presence of other sounds through the phenomenon of masking  Low to moderate intensity sounds are often not heard in presence of more intense sounds  When detection threshold is raised because of other sounds it is said to be masked  Amount of masking is the amount of change in the threshold given in dB  Various aspects of auditory masking  Masking effect of single tones  Masking by noise •Both have provided insight into properties of basilar membrane and the way sound frequency information is transmitted to the brain. •Tonal Masking – Frequency Effects oMasking effects of single tone by another is known as tonal masking  Typical experiment would be to choose a fixed frequency and determine how its audibility is affect by another tone.  The most effective masking occurs when the masking and test tone frequencies are the same •Tonal Masking – Psychophysical Tuning curves oIf a particular tone requires a high intensity level for its masking function then it interfered poorly with the principal tone being considered oLess effective masking frequencies are simply processed at lower efficiency by the functional units responsible for coding and transmitting test tone.  Masking profile is usually referred to as a psychophysical tuning curve oAnarrow tuning curve means that the filter has a high degree of frequency selectivity, whereas a broad curve implies a lower selectivity •Tonal Masking – Asymmetric response profile oEach profile has an asymmetric shape in that the high frequency end shows a steep rise and the low frequency end shows a more gentle rise and possibly a plateau oTones whose frequencies are higher than the test tone frequency lose their masking effectiveness much more rapidly than masking tones of lower frequency oThis suggests there is inherent asymmetry in the way sounds are processed by the auditory system.  The source of this can be traced back to the wave produced by the basilar membrane  Wave shows gradual buildup and then sudden decay after reaching max displacement of the basilar membrane  Ahigher masking frequency has reduced effectiveness and therefore a greater intensity setting to produce the same masking effect because the crest of its travelling wave occurs before that of the test one. •Noise Making – Asymmetric Response Profile oBroadband noise – noise that encompasses a large segment of audible frequency range oBandpass noise – noise stimulus that contains a limited range of frequencies  Experiment – masking effect of narrow-band noise upon test frequencies Greatest effect seen for test tones in frequency range of noise band Threshold elevation rapidly diminishes for test tones of lower frequency The test tone is masked to a greater degree when it has a higher frequency than masking stimulus This asymmetry is due to properties of waves travelling
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