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LESSON 77

Critical periods and neuroplasticity

Year 1
Spring semester
Speech and Language Development
Speech and language
Duration: 11:01

The auditory brain needs sound to develop. In this lesson we look at neuroplasticity, the difference between critical and sensitive periods, the effect of auditory deprivation on the central auditory pathways, P1 latency findings in children with cochlear implants, and the 1-3-6 early identification goals.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson we will learn about neuroplasticity, the brain's ability to change with experience, and the critical period for hearing.
With scientific findings, we will see why early identification and early intervention matter so much.
Neuroplasticity is the ability of the nervous system to change its structure and function in response to experience.
Frequently used connections between nerve cells, called synapses, grow stronger, and new connections form.
Unused connections weaken over time and are pruned; this principle is summed up as use it or lose it.
The auditory cortex follows this rule too: its connections are shaped as it receives regular sound input.
Plasticity lasts a lifetime, but it is much higher in the first years of life.
So infancy and early childhood are the most productive time for shaping the auditory system.
A critical period is a time window with fixed limits, during which input must arrive for a function to develop.
Once this window has closed, the function cannot develop normally, even if input arrives.
The classic example of the concept is the work of Hubel and Wiesel on the visual system of kittens.
This work was among the studies awarded the nineteen eighty one Nobel Prize in Physiology or Medicine.
A sensitive period, by contrast, is a time when learning is easiest and most efficient, but its limits are more flexible.
Learning is still possible after this period, but it progresses more slowly and the outcome is often limited.
Many authors prefer this more flexible concept for hearing and language; in everyday use, the two terms are often used interchangeably.
Not all links of the auditory pathway mature at the same rate.
The cochlea is already largely developed in structure at birth.
The brainstem pathways, however, continue to mature after birth.
The wave latencies of the auditory brainstem response, or ABR, reach adult values at around two years of age.
The auditory cortex is the part that develops for the longest time.
The maturation of cortical layers and connections continues throughout childhood and is not complete until adolescence.
So sound input is needed most in the first years, when cortical connections are forming rapidly.
This shaping happens in a series of stages, and its first condition is regular sound input from the ear.
In the first years of life, the number of synapses in the auditory cortex rises rapidly; more connections form than are needed.
Then the connections used often with sound input grow stronger and become permanent.
Unused connections are gradually pruned throughout childhood.
The result is a mature auditory network that processes speech sounds quickly and efficiently.
If there is no sound input during this process, there is also no experience to decide which connections to keep.
A long lack of sound input during development is called auditory deprivation.
With deprivation, the central auditory pathways cannot mature as they do in children who receive sound input.
Animal studies have shown that, in this case, the connections between the layers of the auditory cortex remain weak.
Auditory areas left unused for a long time may partly turn to the functions of other senses, such as vision and touch.
This change is called cross-modal reorganization.
As reorganization progresses, adapting to sound input provided later may become harder.
Studies also report that if only one ear gets sound for a long time, the brain favors that ear and binaural processing may stay weak.
Some of the strongest evidence for a critical period in humans comes from studies of children with cochlear implants.
A cochlear implant electrically stimulates the auditory nerve and provides sound input to an auditory pathway that has long been silent.
These studies use P1, the first large positive wave of the cortical auditory evoked potential.
P1 latency is long in newborns and shortens throughout childhood as the auditory pathway matures.
So P1 latency is accepted as a measure of the maturity of the central auditory pathways.
In two thousand two, Sharma, Dorman and Spahr compared P1 latency in implanted children by the duration of deafness before implantation.
In children deaf for less than about three and a half years, P1 latency reached age-appropriate values within six months of stimulation.
Based on this finding, the authors proposed a sensitive period of about three and a half years for the central auditory system.
In the same study, responses were quite variable in children implanted between about three and a half and seven years of age.
In most children deaf for more than seven years, P1 latency did not reach age-appropriate values even after long stimulation.
These limits are not a fixed deadline, but the shorter the deafness, the better the chances for the central auditory pathways.
Plasticity is not unique to children; the adult auditory system also adapts to changing input.
For an adult with new hearing aids, sounds can feel more natural and benefit can grow over time; this is called acclimatization.
With bilateral loss and one aid, discrimination in the unaided ear may drop; this is called late-onset auditory deprivation.
Adults who lose their hearing after acquiring language often gain meaningful benefit from an implant, even after a long time.
So plasticity exists throughout life, but it is not as fast or as broad as during development.
Findings on plasticity also guide the time goals of early identification programs.
The Joint Committee on Infant Hearing, or JCIH, recommends the 1-3-6 goal: screening is completed in the first month.
For a baby who does not pass screening, diagnosis is made by the end of the third month.
Appropriate devices and early intervention services begin by the end of the sixth month.
The JCIH twenty nineteen statement recommends a more ambitious 1-2-3 goal for programs that have met this goal.
The aim of these goals is for intervention to begin in the first months, when the auditory brain is most plastic.
The pioneering study on early identification and language is by Yoshinaga-Itano and colleagues, published in nineteen ninety eight.
In this study, children whose hearing loss was identified before six months had clearly higher receptive and expressive language scores.
In children whose loss was identified after six months, language scores stayed lower on average.
This difference was seen regardless of the degree of loss and helped universal newborn screening become widespread.
Later studies also stressed the importance of family involvement and daily device use, alongside early intervention.
When identification and intervention are delayed, the effects can spread to many areas of language.
The child's vocabulary grows more slowly than that of peers.
Grammatical structures and word endings, the fine details of language, are learned later and with more difficulty.
Speech intelligibility may be affected, especially for high-frequency consonants that are not heard well.
When the language foundation stays weak, literacy and school achievement may also be affected.
Communication difficulties may also show in the child's friendships and self-confidence.
A tip from the clinic: in early intervention, fitting the device is not enough; it has to be worn.
The brain receives sound input only while the device is on the ear.
At follow-ups, review the device's datalogging and discuss daily wearing time with the family.
If wearing time is short, ask why: the earmold, feedback, the child pulling the device off or the family's worries may be the cause.
Solving these barriers early ensures the most productive months of the sensitive period are not missed.
Neuroplasticity is the ability of the nervous system to change with experience, and it is highest in the first years.
Maturation of the auditory cortex depends on regular sound input; without input, auditory deprivation develops.
P1 findings in children with cochlear implants point to a sensitive period of about three and a half years.
For this reason, the targets are screening by one month, diagnosis by three months and intervention by six months.
Test yourself: P1 latency is used as a measure of what?
The correct answer is B: the maturity of the central auditory pathways.
P1 latency shortens through childhood; in implanted children, its change after stimulation traces the sensitive period.
In the next lesson we will discuss how hearing loss affects a child's language development, and the role of family involvement.
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