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Sensation and perception 2115.docx

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Department
Psychology
Course
Psychology 2800E
Professor
Anthony Skelton
Semester
Fall

Description
Sensation and perception 2115 – Final exam review Physical aspects of sound - Sound can be defined in two different ways o A physical stimulus – sound is pressure changes in the air or other medium o A perceptual response – sound is the experience we have when we hear - Sound is important for o Communication o Object identification o Object location Sound as pressure changes Dark lines = increase in pressure/condensation Light lines = decrease in pressure/ rarefaction - Stimulus occur when the movements or vibrations of an object cause pressure changes in air, water or any other elastic medium that surrounds the object - When sound is coming out of a speaker, the diaphragm of the speaker moves out and pushes the surrounding air molecules together. This process is called condensation - Condensation increases the density of the molecules near the diaphragm. This causes an increase in air pressure above atmospheric pressure. - When the diaphragm moves back in the air molecules spread out in the space surrounding the speaker resulting in rarefaction - Rarefaction causes a decrease in pressure - When repeated over and over again, the pattern of air pressure changes creates a sound wave - Pure Tones Pure tones occur when changes in air pressure occur in a pattern described by a mathematical function called a sine wave. Examples of sound waves that are close to pure tones are whistling of high pitched notes created form a flute. - Amplitude - difference in pressure between high and low peaks of wave o Perception of amplitude is loudness o Decibel (dB) is used as a measurement of loudness o The bigger the difference in high and low peaks of the sound wave the greater the amplitude is, making it louder - Frequency - number of cycles within a given time period (# of sine waves) o Perception of pitch is related to frequency o Measured in Hertz (Hz) = 1 Hz is one cycle per second o The closer together the sine waves are, the higher the frequency which creates a higher pitch Audible range – Frequency - 20Hz – 20,000 Hz - <20Hz = infrasound - >20,000 Hz = ultrasound - Changes with age - Species differences Complex Tones and Frequency Spectra Repetition of waveforms repeating means that the wave is a complex tone called a periodic tone. - Fundamental frequency – first harmonic of a complex tone. Lowest frequency in the frequency spectrum of a complex tone. Higher harmonics, have frequencies that are multiples of the fundamental frequency. Harmonics - Complex tones are made up of a number of pure tone (sine-wave) components added together. This is called the harmonic of the tone - First Harmonic – a pure tone with frequency equal to the fundamental frequency. This is known as the fundamental of the tone. o 200 Hz is the fundamental low - Higher harmonics – pure tones with frequencies that are whole number multiples of the fundamental frequency. Example C from above is double the fundamental frequency, making it a higher harmonic. - Additive Synthesis – creating complex tones by adding together an appropriate number of simple sine waves at harmonically related frequencies. The Audibility Curve Some frequencies have low thresholds – it takes very little sound pressure change to hear them. Other frequencies have high thresholds which take large changes in sound pressure for them to be heard. The audibility curve indicates the threshold for hearing versus frequency, indicates that we can hear sounds between 20-20,000 Hz. We are most sensitive at frequencies between 2,000-4,000 Hz. Timbre With pitch being consistent, another perceptual quality, the tones timbre changes. Timbre is the quality that distinguishes between two tones that have the same loudness, pitch and duration but still sound different. An example – the same note, at same amplitude of a flute and oboe are played. They sound similar but there is still some difference. The flutes sound can be described as clear, while the oboes sound can sound like reedy. - Timbre depends on the time course of an tones attack (buildup of sound at the beginning of the tone) and the tones decay (decrease in sound at the end of the tone). The Auditory System The auditory system consists of: - Pinnae o Structures that stick out from the sites of the head. o Used to determine the location of sounds and it is important for those who wear glasses because it is part of the ear that we could most easily do without - Outer Ear o Consists of the pinna and the auditory canal o The auditory canal is a tube like structure that protects the delicate structures of the middle ear from the hazards of the outside world o Wax in the ear protects the tympanic membrane/ eardrum and keeps the membrane and the structures at a constant temperature o The outer ear also enhances the intensities of some sounds by means of the physical principle of resonance o Resonance occurs when the sound waves that are reflected back from the closed end of the auditory canal interact with sound waves that are entering the canal. This reinforces the sounds frequencies. o The frequency reinforced the most is called the resonant frequency - The Middle Ear o A small cavity that separates the outer and inner ears o Contains:  Ossicles – three smallest bones • Malleus (hammer) – set into vibration by the ear drum and transmits its vibrations to the incus • Incus (anvil) – transmits the vibrations to the stapes • Stapes (stirrup) – the stapes transmits its vibrations to the inner ear by pushing on the membrane covering the oval window o Occicles are important due to the fact that the inner ear consists of water, instead of air. This change in pressure will cause problems transmitting the soundwave. o Middle ear muscles  Attached to the ossicles and at very high sound levels they dampen he vibration which reduces the transmission of low frequency sounds and helps prevent intense low frequency components from interfering with our perception of high frequencies. - The Inner ear o Contains:  Liquid filled cochlea – a snail like structure. The liquid is set into vibration by the movement of the stapes against the oval window. The most obvious feature uncoiled part of the cochlea is in the upper half called the scala vestibuli and the lower half called the scala tympani. These are separated by the cochlear partition.  Cochlear partition – most the whole length of the cochlea. From its base near the stapes to its apex at the far end. The partition transforms the vibrations inside the cochlea into electricity.  Organ of Corti – contains hair cells, receptors of hearing. Contains the basilar membrane and the tectorial membrane • Basilar membrane – controls the vibration of the cochlear partition • Tectorial membrane – located directly over the hair cells. Vibrations of the partition cause the membrane to bend the hair cells by rubbing against them Hair cells and the two membranes - Inner hair cells – responsible for auditory transduction and the perception of pitch - Outer hair cells – amplify the response of inner hair cells by amplifying the vibration of the basilar membrane - The cilia of the outer hair cells are in contact with the tectorial membrane, while the cilia of the inner hair cells are not Movement of the inner hair cell result in the firing of fibres, creating a sound stimulus. Vibrations of the Basilar Membrane - Two ways nerve fibres signal frequency: o Which fibers are responding – specific groups of hair cells on basilar membrane activate a specific set of nerve fibres o How fibres are firing – rate or pattern of firing of nerve impulses How do we perceive pitch? - Place theory – pitch is encoded by the place on the basilar membrane that is stimulated - Frequency (volley) theory – the timing of the sound waves matches the rate of firing of neural impulses to the brain Place Theory - Beskesy - Frequency of sound is indicated by the place on the organ of corti that has the highest firing rate - Bekesy determined this in two ways o Direct observation of the basilar membrane in cadavers o Building a model of the cochlea using the physical properties of the basilar membrane - The lower the maximum electrical response for each frequency the closer it is located to the apex, where the higher maximum electrical responses for each frequency is located closer to the apex - Physical properties of the basilar membrane o Base of the membrane by stapes is  3-4 times narrower than the apex  100 times stiffer than at the apex Evidence for Place Theory - Tonotopic map (electrical response to different frequencies) o Cochlea shows an orderly map of frequencies along its length  Apex responds best to low frequencies  Base responds best to high frequencies Practical Application - Cochlear Implants - Electrodes are inserted into the cochlea to electrically stimulate auditory nerve fibers - The device is made up of o A microphone worn behind the ear – receives sound signals from the environment o A sound processor – divides the sound received by the microphone into a number of frequency bands o A transmitter mounted on the mastoid bone – sends signals o And electrodes implanted along the length of the cochlea – 22 electrodes stimulate the cochlea at different places o This stimulation activates auditory nerve fibers along the cochlea which sends signals to the brain Volley theory - Derived from frequency theory - The timing of the sound waves matches the rate of firing of neural impulses to the brain - Multiple neurons fire in a volley to combine and match the frequency of the original sound stimulus - Place theory works for sounds 4000Hz and less; volley theory for those 4000HZ and greater Central Auditory Pathways The primary auditory receiving area – A1, extends inside the temporal lobe Tonotopic map on the A1. The numbers are characteristic frequencies of neurons in thousands of Hz. Low CFs are on the left and high CFs are on the right. Direction of frequency change and differences in pitches are most influential for producing poorer performances. Duration of sounds and orientation of lines do not affect performance that much. What and where auditory pathways are located How to Damage your Hair Cells - Presbycusis o Greates loss is at high frequencies o Affects males more severely than females o Appears to be caused by exposure to damaging noises or drugs - Noise-induced hearing loss o Loud noise can severely damage the organ of corti o Standards for noise levels at work are set to protect workers o Leisure noise can also cause hearing loss Infant hearing - Fetuses hear in utero o Mothers voice o Dr.Seuss stories - Speech and music preference - Auditory thresholds are higher than adults for low frequency noises - Thresholds are adult like by 12 months - Changes in the outer and idle ear and in the auditory cortex Synaesthesia - Phenomenon of sensory stimulation in one modality creating an experience in another modality - Possible explanation o All senses evolved from a common origin o For synaesthetes, connections between different sensory areas of the brain are not pruned away o Newborns are synaesthetes Chapter 13 Speech Perception What is speech? - An acoustic signal or stimulus – patterns of pressure changes in the air o Created by air that is pushed from the lungs past the vocal cords into the vocal tract - Collection of ordered sounds to which meaning is attached The Acoustic Signal - Vocal Tract – shape is altered by moving articulators which consist of the tongue, lips, teeth, jaw and soft palate - Vowels are produced by vibration of the vocal cords The amplitude of the pressure changes produced for each vowel. The peaks in pressure changes are
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