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Chapter 10

PSYB51 Ch.10.doc

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Department
Psychology
Course
PSYB51H3
Professor
Matthias Niemeier
Semester
Winter

Description
Ch.10- Hearing in the Environment • Sound Localization (similar to visual depth perception- Ch.6) ◦ like visual depth perception – involves processing and integrating a set of “cues” stimulus aspects that provide indirect evidence about how far away an object is ◦ 2 ears= crucial for determining auditory locations= for most positions in space, the sound source will be closer to one ear than to the other ▪ Cue #1= interaural time difference (ITD)= the difference in time between a sound arriving at one ear versus the other • azimuth= the angle of a sound source on the horizontal plane relative to a point in the center of the head between the ears.Azimuth is measured in degrees, with 0 degrees being straight ahead. The angle increases clockwise toward the right, with 180 degrees being directly behind. • ITD from 0 degrees= 0 secs, ITD from 90 degrees (to the side)= 640 us, 180 degrees (behind)= 0 • shown that listeners can detect ITD of as little as 10 us= detect angle of a sound source to within 1 degree • portion of auditory system responsible for calculating ITD's obviously needs to receive input from both ears= binaural input enters almost every stage of the auditory nervous system after the auditory nerve BUT as info moves upward with every additional synapse the timing between the two ears is likely to be less precise= medial superior olives (MSO's)= first places in the auditory system where inputs from both ears converge= neuron's found in MSO'S whose firing rates increase in response to very brief time differences between inputs from the two ears= ITD detectors form their connections in the first few months of life, and developing ability to use ITD's to localize sounds depends critically on having experience with separate sounds coming from different places= similar developmental sequence to formation of binocular neurons in visual cortex= interpretation of ITD'S is critically dependant on size of head so if babies were already wired with ITD, their sound localization would steadily decline as their ears moved farther apart ▪ Cue # 2= interaural level difference (ILD)= the difference in level/intensity between a sound arriving at one ear versus the other= sound is more intense at the ear closer to the sound source because the head partially blocks the sound pressure wave from reaching the opposite ear • same in reference to azimuth as ITD= +/-90= largest ILD and 0 = smallest • between two extremes, ILD generally correlates with the angle of the sound source but because of the irregular shape of the head, the correlation is not as precise as it is with ITD's **important difference between ITD and ILD: head blocks high-freq. sound much more effectively than low frequency sound because low frequency waves can bend around the head--> so ILD's are greatest for high-frequency tones and the ILD provides the best info about sound source location. ILD's are greatly reduced for low frequencies becoming almost nonexistent below 1000 Hz. This inability to localize low frequencies is the reason it does not matter where in a room you place the subwoofer in your stereo. **another difference= neurons sensitive to ILD's can be found in the lateral superior olives (LSO's) which receive both excitatory and inhibitory inputs. Excitatory connections come from the ipsilateral field and inhibitory inputs from the contralateral ear. The neurons in the LSO are sensitive to ILD because the two ears compete between excitatory and inhibitory inputs from one ear to the next, so when sound is more intense at one ear, connections in that ear are better both at exciting LSO neurons on that side and inhibiting neurons on the other side. • Cone of confusion= a region of positions in space where all sounds produce the same time and level/intensity difference (ITDs and ILDs) • infinite number of cones= widest is the most confusing = real perceptual phenomena= but as soon as you move your head the ITD and ILD of a sound source shift and only one spatial location will be consistent!! • Pinna and head cues= another reason why cones of confusion are not major practical problems 1. complex shape of pinnae with nooks and crannies--> due to shape funnel some frequencies more efficiently than others 2. size and shape of the rest of the body, especially the upper torso, affect which frequencies reach the ear the most easily • due to these two effects= intensity of each frequency varies slightly according to the direction of the sound= variation provides another auditory localization cue • directional transfer function (DTF)= a measure that describes how the pinna, ear canal, head and torso change the intensity of sounds with different frequencies that arrive at each ear from different locations in space (azimuth and elevation) • why listening to music from headphones sounds different than from a concert where you can tell what direction everything is coming from --> binaural recordings make things more lifelike but because everyones pinnae is different, everyone would need their own recordings • change in body size, shape, piercings= suggests that children may update the way they use DTF through development and into adulthood= other sources of info such as vision etc can also provide feedback • Auditory Distance Perception • ITD, ILD, DTF= don't provide information concerning distance btwn sound source and a listner that is more than an arms length away • Cue #1= simplest cue for judging distance of a sound source= relative intensity= easy to percieve relative distances of two IDENTICAL sounds= but like relative size in depth perception, requires assumption that may turn out to be false • effectiveness of relative intensity decreases as distance increases because sound decreases according to the inverse-square law= a principle stating that as distance from a source increases, intensity initially decreases much faster than distance increases, such that the decreases in intensity is equal to the increase in distance squared.Applies to optics and other forms of energy. SO listeners are good at telling differences between two sources that are close to them, but it is harder to perceive the same distance between two sources that are farther away. • Intensity works best as a distance cue when the sound source or the listener is moving ◦ closer the animal moves the higher the intensity of sound ◦ similar to motion parallax in visual depth perception= sounds that are farther away do not seem to change direction in relation to the listener as much as nearer sounds do • Cue #2= another cue for auditory distance= spectral composition= sound-aborbing qualities of air dampen high frequencies more than low frequencies so when sound sources are far away, higher frequencies decreases in energy more than lower frequencies as the sound waves travel from the source to the ear= the farther away the sound source is, the “muddier” it sounds • change in spectral composition is noticeable only for fairly large distances= greater than 1000 metres= analogous to aerial perspective • Cue # 3= relative amounts of direct versus reverberant energy= in most environments, the sound that arrives at the ear is some combination of direct energy (from the source) and reverberant energy (bounced of surfaces of the environment)= informs about distance because when a sound source is close to a listener, most of the energy is direct, whereas reverberant energy provides a greater proportion of the total when the sound source is farther away • COMPLEX SOUNDS • Harmonics • human voice, instruments, most common types of sounds= harmonic sounds • fundamental frequency= the lowest frequency component of a complex periodic sound= with natural vibratory sources there are also energy at frequencies that are integer multiples of the fundamental frequencies (second, third, fourth harmonics) • for harmonic complexes= perceived pitch is determined by the fundamental frequency and the harmonics (“overtones”) add to the perceived richness of sound **listeners hear the missing fundamental= when the first harmonic/fundamental harmonic is absent, and only the others are present, listeners still hear the pitch corresponding to the first because the auditory system is acutely sensitive to the natural relationships between harmonics= can identify the fundamental with just a few other harmonics • explanation for missing fundamental effect= temporal code for pitch= all harmonics of a f
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