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which area of the ear is mature at birth and which is not?
the middle ear is mature at birth
it is the cochlea and the processes in the brainstem and beyond that is still maturing after birth
what is the difference in infants and adults in sensitivity to changes in frequency?
At birth, the newborn is less sensitive to sound than an adult
By 6 months threshold is still twice that of the adult. Little change from 6 to 12 months
Hearing gets better slowly over a period of years for children
it is important that there is exposure to high frequency sounds early in infancy for normal development of the neural coding of high frequency sounds, thus leading to better speech and language outcomes for hearing-impaired children if there is intervention prior to 6 months of age
another thing to know
Continuous exposure to loud background noise in the NICU or home will interfere with auditory development and especially frequency discrimination
can infant separate a sound from the background?
7-9-month-old infants have difficulty extracting a tone from an irrelevant sound
they are poorer at using selective attention to separate the sounds
7 year olds are poorer at listening to sounds coming into one ear while a distracting sound is sent to the other (not good at selectively attend to different vocals coming in at different locations)
Discrimination of Rhythm
infants were trained to turn to the right direction when there is a change in the rhythm, thus receiving a reward if they turned in the right direction
it is found that discrimination was poorer in 6 month old infants than older children and adults
why is detection of hearing loss is important?
hearing loss can impair speech, language and cognitive development, thus it is important to intervene at an early age (no later than 6 months is the goal)
types of tests for screening for hearing loss
Auditory brain stem response (ABR) test: this test uses patches, called electrodes, to see how the auditory nerve reacts to sound. a series of clicks are presented
Otoacoustic emissions (OAE) test: microphones placed into the baby's ears detect nearby sounds. The sounds should echo in the ear canal. If there is no echo, it is a sign of hearing loss.
Localization can be described in three-dimensions –the horizontal angle (left and right), the vertical angle (up and down, and depth (is the sound source near and far).
How can we tell sounds that come from the directly in front of us, above or below?
The human outer ear, i.e. the structures of the pinna and the external ear canal, form direction-selective filters (which allows us to discriminate from up and down)
Depending on the sound input direction in the median plane creates-different patterns or bands of frequency are produced. We adapt to the shape of our pinna so if we change the shape of the pinna we adapt and respond to the real position of the source.
Direction-selective reflections at the the pinna, head, shoulders and torso, from the shape of the outer ear we get information about the location on the median plane.
depth cues for sound to tell of a space we are in
The delays or echoes of sounds that reflect off the space tells about the size of the space we are in
We use the difference between the time the direct sound from the source gets to us and the echo to know how far away the sound is
other cues for distance:
high frequencies are more quickly damped by the air than low frequencies. Therefore a distant sound source sounds more muffled (lower in pitch) than a close one, because the high frequencies are attenuated
other cues for distance:
Distant sound sources have a lower loudness than close ones
motion parallax for sound cue for depth
as your head moves the continuous sound would sound like it is lower and louder thus telling that motion can be a cue for us to determine the distant of the source of a sound
Distance Perception of sound
it is found that infants reached more for a sound source that is within their reach more often then when it is out of reach
another study shows that infants reached more oftern to the louder sounding stimulus compared to the lower sound stimulus
The precedence effect is called the law of the first wavefront, or echo suppression
If I hear a sound coming from one direction, and the very same sound comes from some other direction within a matter of a few milliseconds, I hear the sound coming from the direction of the first sound. If the second sound is delayed more than about 7 milliseconds, we hear both sounds, so can’t pick a single direction
DeCasper and Fifer: Before birth mother’s voice is learned study
Operant, Skinnerian, conditioning. The behavior must occur naturally and if it is reinforced the probability of the behavior increases. Evidence of learning. When reinforcement is removed the probability goes down.
5 minutes of sucking to find the average time between bursts of sucking.
Infants got to hear (were rewarded by hearing) their mother’s voice when they increased the delay between bursts of sucking and when they decreased (made the time shorter) they were punished by hearing a mother reading the same story. Other infants were reinforced when delay was decreased
reported that 8 of the 10 infants changed in the predicted direction thus, studies confirm the hypothesis that before birth the fetus is learning something that distinguishes the mothers voice from a strangers