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Psychology (9,699)
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Chapter 5

Chapter 5 part 2.doc

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Department
Psychology
Course Code
PSYA01H3
Professor
Steve Joordens

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Psychology- Second Half of Chapter 5 SENSATION Audition - vision involves the perception of objects in three dimensions, at variety of distance, with multitude of colours and textures - complex stimuli may occur at a single point in time or over an extended period - may involve an unchanging scene or rapidly changing one - other sense analyze much simpler stimuli (odour/taste) or depend on time and stimulus change for the development of a complex perception - example: to perceive a solid object in three dimensions by means of touch, must manipulate it-turn it over in our hands or move our hands over its surface - Stimulus must change over time for a full-fledged perception of form to emerge -same for audition: we hear nothing meaningful in an instant SOUND - sound consists of rhythmical pressure changes in the air - as object vibrates, causes air around it to move - when object in phase of vibration in which it moves toward you, it compresses molecules of air - as it moves away it pulls molecules of air further apart - as pressure arrives at ear, it bends eardrum in - following wave of negative pressure(molecules pulled farther apart) causes eardrum to bulge out - sound waves measured in frequency units of cycles per second called Herts(Hz) - human ear perceives vibrations between 30-20 000 Hz - produce corresponding changes in sensations of loudness, pitch - example: consider a loudspeaker, a device that contains a paper cone moved back and forth by a coil of wire located in a magnetic field. Alteration in the electrical current transmitted from an amplifier to this coil cause coil(and paper cone) to move back and forth. If cone begins to vibrate more rapid, pitch of sound increases(that is if the cone moves in and out over a greater distance) - third perception dimension, timbre, corresponds to the complexity of the sound vibration THE EAR AND ITS FUNCTIONS - refer to the ear, usually mean what anatomists call pinna-the flesh-covered cartilage attached to side of head - pinna performs only small role in audition - helps funnel sound through ear through ear canal toward the middle and inner ear, where the business of hearing gets done - eardrum is a thin, flexible membrane that vibrates back and forth in response to sound waves and passes these vibration on to the receptor cells in the inner ear - eardrum attached to the first set of three middle ear bones called the ossicles - three ossicles are hammer(malleus), anvil(incus), stirrup(stapes), because of their shapes M.I.S. - these bones act together, in lever fashion to transmit the vibrations of the eardrum to the fluid-filled structure of the inner ear that contains the receptive organ - bony structure that contains the receptive organ is called the cochlea - the cochlea is filled with liquid - bony chamber attached to the cochlea(the vestibule) contains two openings: oval window and round window - the stirrup presses against a membrane behind an opening in bone surrounding the cochlea called oval window, transmitting sound waves into the liquid inside the cochlea where it can reach the receptive organ for hearing - cochlea is divided into three chambers by two membranes - Basilar membrane: a sheet of tissue that contains auditory receptor cells - As footplate of stirrup presses back and forth against membrane behind oval window, pressure changes in the fluid above the basilar membrane cause basilar membrane to vibrate - Basilar membrane varies in width and flexibility, different frequencies of sound cause different parts of the basilar membrane to vibrate - High-frequency sounds cause end near oval window to vibrate - Middle frequency sounds cause middle to vibrate - Low frequency sounds cause tip to vibrate - Basilar membrane can vibrate freely only if fluid in lower chamber of cochlea has somewhere to go-unlike gases, liquids cannot be compressed - Free space provided by round window - Basilar membrane flexes down, displacement of fluid causes membrane behind round window to bulge out - Basilar membrane flexes up, membrane behind round window bulges in - Sounds detected by neurons-auditory hair cells located on basilar membrane - Auditory hair cells: transduce mechanical energy caused by flexing of basilar membrane into neural activity - Cells possess hair-like protrusions called cilia - Ends of cilia embedded in rigid shelf-tectorial membrane, that hangs over basilar membrane like balcony - When sound vibration cause basilar membrane to flex, cilia are stretched-this pull is translated into neural activity - Mechanical force exerted on cilia, electrical charge across membrane is altered-cause is not fully known, although pressure on cilia is known to increase calcium flow into the hair cells - Calcium currents very fast, alter temporal duration of depolarization - Change in electrical charge cause transmitter substance to be released at a synapses between auditory hair cell and dendrite of neuron of auditory nerve, similar to way that bipolar cells connect to ganglion cell in retina - One hair cell connected to many auditory neurons, unlike case with visual cells, one hair cell has a large effect on subsequent nerve activity - Auditory system deal with sounds that persist for long intervals-intervals long enough to deplete transmitter substance in normal synapses, unlike in visual stimuli that can disappear - Synapses between hair cells and auditory nerve therefore differ in function - Depolarization is strongerresult is more reliable signal passed along auditory nerve DETECTING AND LOCALIZING SOUNDS IN THE ENVIRONMENT - sounds differ in loudness, pitch, timbre - come from particular locations - ears ability to distinguish sounds by their timbre depends on its ability to distinguish loudness and pitch Loudness and Pitch: - originally thought neurons of auditory system represented pitch by firing in synchrony with vibrations of the basilar membrane - learned that axons cannot fire rapidly enough to represent high frequencies that we can hear - young ear hear frequencies of more than 20 000 Hz, but axons cannot fire more than 1000 times per second - therefore: high frequency sounds, must be encoded in some other way - sounds of different frequencies stimulate different groups of auditory hair cells located along the basilar membrane (different frequency sounds cause different parts of basilar membrane to vibrate) - high-frequency/medium-frequency sounds, brain is informed of pitch of a sound by activity of different sets of axons from auditory nerve - medium-frequency sounds waves reach ear, middle basilar membrane vibrates, auditory hair cells located in region are activated - high-frequency sounds activate auditory hair cells located at base of basilar membrane near oval window - Two kinds of evidence indicate pitch is detected in this way: first-direct observation of basilar membrane shows that region of maximum vibration depends on frequency of stimulating tone. Second-experiments found that damage to specific regions of basilar membrane cause loss of ability to perceive specific frequencies - Low frequency sounds are detected by different method: Kiang recorded electrical activity of single axons in auditory nerve and found many that responded to particular frequencies - Presumably these axons were stimulated by hair cells located on different regions of basilar membrane - However, Kiang did not find any that responded uniquely to particular frequencies lower than 200 Hz-yet tones lower than 200 are easily perceived - Lower frequencies (lower than 200 Hz) cause tip of basilar membrane to vibrate in synchrony with the sound waves, neurons that are stimulated by hair cells located there are able to fire in synchrony with these vibrations, firing at the same frequency as sound - Brain counts these vibrations, detects low-frequency sounds(example of temporal coding) - Axons of the cochlear nerve appear to inform the brain of loudness of a stimulus by altering rate of firing - Intense vibrations stimulate hair cells more intensely - Stimulation causes release of transmitter substance, results in higher rate of firing by axons in auditory nerve - This explanation works for axons involved in anatomical coding of pitch: pitch is signaled by which neurons fire, loudness: loudness is signaled by their rate of firing - However; neurons that signal lower frequencies do so with their rate of firing- if they fire more frequently they signal higher pitch-cannot signal both loudness and pitch by same means - Therefore most investigators believe that the loudness of low-frequency sounds is signaled by the number of auditory hair cells that are active at a given time-louder sound excites a larger number of hair cells Timbre -sounds can vary in complexity (you can distinguish between sounds of a clarinet from a violin) - start suddenly or gradually increase in loudness, be short or long, seem thin and reedy or full and vibrant - combining or synthesis of two or more simple tones, each consisting of a single frequency, can produce complex tone
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