Critical Periods in First Language Acquisition: Key Evidence

[00:00:01]well okay we're back so in the first part of this ledger we gave a kind of overview of what the critical period hypothesis is all about and how it relates to language in this one we're going to investigate in a little bit more detail evidence for and against the critical period for first language acquisition so this is CP for l1 a critical period for first language acquisition if a child does not acquire a first language by the end of some critical period then they will never acquire a first language normally so what kinds of evidence have been brought to bear on this question excuse me well one kind of evidence that is frequently cited and included in introductory linguistics classes and is sort of very famous and well-known has to do with so-called feral children what does that mean well it's children who through some series of unfortunate events end up deprived of linguistic input at least that's usually presumed right and so here are two famous examples on the right here we have Victor the wolf boy of a Vale home and he was a child who was found wandering in the woods outside a town in I believe the Alps in France a Navajo and was found there I was brought to the town physician and they decided that this boy had been like raised by wolves in the woods or something like that he didn't speak any French or any other language he was filthy and had been surviving on who knows what in the woods that he appeared to have sort of no contact with other human beings and so he was taken in by the physician in this town and was you know civilized by him and was treated kindly by all accounts but he never learned to speak a language normally I mean they estimated or the physician estimated his age when he was found to be 12 years old so the idea here is Victor the wolf boy never had any linguistic input he was like by wild animals in the woods or something like that and so he never acquired a language normally therefore language has a critical period that's the idea from Victor the picture on the left is from Jeanne and there was a movie made about Jeanne in the 1990s called Nell in a very famous case of unfortunately child abuse where Jeanne had a psychotic father who kept her locked in the basement essentially until she was I want to say in her teen years when she was found eventually and was rescued from this horrible situation they kept her locked in the basement they didn't talk to her she was never taught a language or was never exposed to a language was just kept in the state of awful deprivation for many many many years as a child and then once she was rescued and was brought to live somewhere else she did eventually acquire some ability to communicate but never learned English at anything like the level of a native speaker and so again this was taken to be more evidence for critical periods and first language acquisition if you deprive a child of first language input then they never show normal or typical native language maturation that's the basic idea here so open-and-shut case right what's proved well no not so fast right so there's many many problems with this kind of evidence for one thing it's very rare there's only a few cases that are famous in the history of like human knowledge and so you know even if we were sure about the details of all of these it's still only a couple people you wouldn't want to conclude anything too universal from looking at a couple specific people that's one thing the main problem here to my mind is that all of these cases are going to be confounded with abuse hardship distress of some sort right so we know that jeanne was hardly abused as a child and was mistreated and neglected and that could impact your ability to do any kind of cognitive task regardless of what kinds of genetic predispositions you have for language learning we have no idea what happened to Victor but presumably he went through some hardship in his early life that could have independently interfered with his ability to learn any kind of a language or to do anything else because these children by definition have had exceptional early lives they're not typical early lives and you don't know whether the difference between them and a typical native speaker is the language deprivation deprivation or all the other extraordinary and awful things that have happened to these children it's impossible to attribute their language difficulties to exposure to language rather than any of the other awful stuff that happened to them basically so that's a big big problem with this kind of evidence I would finally hear well say not finally there's two more things here one of it it's possible that these children were atypical before language deprivation we have no idea but for sure in you know 19th century France if you had a child who was developmentally disabled it's entirely possible that a French peasant would have just sent the kid out into the woods to die basically maybe there was already some sort of cognitive issue before they were ever neglected abused or abandoned in the first place we can't say one way or the other because we don't have enough information about what happened to these children and finally at least in the case of Victor and to some extent in the case of Jeanne as well we don't actually have any evidence for the lack of linguistic input or for how much linguistic input they got we have no idea how much language Victor was exposed to maybe he grew up until age 10 in a normal French village listening to normal French people speak and then he wandered off in the woods one day we have no idea that he was he was deprived of linguistic input so in general this type of evidence from feral children well it's you know sort of interesting and spectacular and and you know very salient right it's not actually very good quality evidence for concluding anything about the human language faculty and so this shows up in textbooks and and so on and so forth but I'm actually language scientists don't take this kind of evidence very seriously and yeah obviously there's this idea out there it's referred to as the forbidden experiment there's this idea that well if you could systematically deprive a child of a first language exposure and then see what happens that would tell you about you know whether there's a critical period or not and obviously that experiment can't be done because it would be unethical so that sometimes referred to as the forbidden experiment and it's worth noting that feral children like Victor and Jeanne are not actually like the forbidden experiment because their situations were totally uncontrolled for non linguistic factors so here's some other ways of thinking about how we might get evidence how are you gonna get evidence about first first language critical periods if we can't do the forbidden experiment then what are we gonna do well here's the logic no exposure to a language and tails no normal acquisition maybe even very limited exposure would entail yes acquisition that's another possibility if it's really a critical period is the sort of magical biological period where you absorb everything about language maybe even ridiculously small amounts of exposure would entail acquisition so there's gonna be two sides of the coin here we'll look at cases of exceptional deprivation on the one hand and that would include the feral children although again as I've said I don't think we weigh that evidence very strongly excuse me congenitally deaf children are actually going to be a hugely important source of evidence here children who are born deaf tend to be deprived of linguistic input until some age when their deafness is discovered and depending on when and where they're born there are various ways that they might be treated for their deafness and at various ages so we'll see some evidence from there and yes there anything else can anybody can think of in terms of exceptional deprivation were thinking about a little bit and then there's also cases of exceptional retention this is sort of the other side of the coin if you're just exposed to a tiny little bit of language during your critical period is there evidence that you retain things exceptionally that would also be some kind of evidence for at least a very sensitive period during a very young age and one of the easiest populations to study here would be adoptees all right so that would certainly be something to look at maybe heritage speakers although there's been less done about that so let's take a look at some of the these other kinds of evidence here so yeah there's really sort of two degrees of deprivation that we might be talking about when we talk about deprivation so here's the evidence for critical periods from bird imprinting and pictured here this fun-looking gentleman is Konrad Lorenz with a whole bunch of baby ducks following him around why are they following him around well it turns out that ducks have a biological drive to imprint on a parent in the first 18 hours after they're born and this is true even in cases where the environment deprives that organism of its normal imprinting target in those cases this imprinting Drive can focus itself on a non canonical target and the famous one in this case was Conrad Lawrence's wading boots they're roughly the size and shape of a duck the chicks that saw him before they saw their mothers in the environment learned to imprint on his boots and they would follow him and around in his wading boots and he'd take cute pictures that get put in biology textbooks so this is the famous example of him excuse me imprinting for baby ducks and so abstracting away from the ducks and the specific imprinting behavior we can sort of state this in more general terms you have some biological drive to imprint on something and even in cases where you get deprived of your normal something maybe that critical period that drive can focus itself on a non-canonical or unusual target so with respect to languages what kind of targets would I be talking about here well generally it's going to be fur ducks at something like roughly duck sized and colored objects in my immediate environment in language it's gonna be well the native language of my surrounding community that's what the target for ug is gonna be it's gonna be whatever language is being spoken around me as I'm an infant so what would happen if the environment deprived a child of native language of surrounding community that construct what if it just wasn't there what would happen in that case well we could find cases maybe of partial deprivation this would be a case where you have for instance lots of people but no common native language amongst them total deprivation maybe feral children right so no people around them no language around them although it's very hard to know there could be another kind of deprivation here though which doesn't involve feral children it would involve children were there again lots of people around but no language at all so take a moment and see if you can think about what I'm talking about in terms of that first possibility with lots of people around no common native language and that last possibility with lots of people around with no language whatsoever so partial deprivation here lots of people with no common native language this unfortunately has occurred pretty frequently in human history usually in cases of slavery where slaves are taken from one part of the world and brought to another part of the world in a particular African slavery in the new world in the Caribbean in southern United States and in certain other areas in the world often brought together groups of African people from totally different culture with genetically unrelated languages where there was absolutely no way for these communities to communicate amongst themselves because they all spoke totally different languages from different language families and they were being thrown together on these horrific conditions and ships and on plantations and this is frequently a case where you end up with you know midsize communities of people who have no common native language and maybe no common language at all right at least at the beginning there could be a couple other situations where this kind of situation arises but slavery unfortunately is the most common in human history and we have some information about what happens in cases like these what I'm about to tell you is sometimes referred to as the pidgin to Creole cycle and it is a very old idea in contact linguistics that is sort of again it appears in textbooks and my teachers in this class it has recently this whole idea of the pidgin and Creole cycle has recently been questioned very very seriously by Creole scholars who believe that this is just and some people think this is an inaccurate way of talking about the development of these creole languages and I will say that I am very receptive to those concerns but actually they're not totally relevant to the critical period aspect of it so the parts of pidgin and Creole hood that I'll be talking about in this slide I think are largely independent from the historical problems with the idea of a pidgin to Creole cycle in a way that won't materially make a huge difference to the point being made here so I'm gonna keep this material in the class but anytime you see pidgin the Creole cycle know that this is nowadays very very controversial that this might not be exactly the right story about the way these languages develop but the story about generations of speakers here as far as we know is still roughly correct the main relevant fact about these cases is that in these circumstances adults linguistically appear to behave differently than children what does this mean well the language that arises in the first generation this is the adults who are thrown into this situation who don't have a common language the language that they develop is usually referred to as a pidgin and then the language that later generations develop is generally referred to as a Creole what does that mean oh sorry I thought I have a slide where I explain those terms I guess I don't what is a pidgin the idea of a pidgin is that it is a simplified language that basically takes lexical items from all the substrate languages around all the languages spoken by the people in the community and it basically pins lexical items together right so there's no real functional categories there's no inflection there's not a lot of morphology you basically just are taking mostly content words nouns verbs and adjectives from the all the languages being spoken in the environment and you're sort of putting them after one another in ways that roughly reflect natural language syntax and you can get enough out of this to convey a meaning so you know I you know you know some words of English but you don't really know how to speak the language and we can agree on terms for you know sky and blue and I say sky blue it's pretty clear what that means right that's the basic idea behind a pidgin it's basically a bunch of words without any grammatical much grammatical machinery no functional categories no determiner is no inflection nothing like that interestingly later generations in these communities so the children who were born into these communities who grew up in these communities with the pidgin being spoken develop reintroduce spontaneously generate all of the grammatical machinery of a normal language so there's again a raging debate with extreme disagreements about whether Creoles have any properties that distinguish them from so-called normal languages but they certainly don't have very many they have rich inflection rich functional categories just like any other language in the world and so the only way for them to get that stuff is for children who are acquiring the languages natively to spontaneously restore grammatical information into these so called pidgin languages that are have less grammatical detail inflection functional elements the conclusion here is that well children's biological drive to learn languages is still intact even in these fractured slavery communities and that drive to learn whatever kinds of learning biases children have their universal grammar imposes structure even when their input is unstructured where the idea here is that pigeons in some regards in terms of inflection morphology different kinds of functional architecture and categories are impoverished input yet then children learning these languages natively have such a strong drive for all of these grammatical components that it imposes structure on that input can you think of any other type of conclusion that you might get from these whom from looking at such phenomena I'll leave that one actually for the end of the for the next set of slides because there's gonna be an interesting idea there here's a maybe even more spectacular case of the emergence of different kinds of languages from conditions of social contact so this is going to correspond to the example that I mentioned there that isn't feral children where you have total deprivation with lots of people around but no language at all and so in many parts of the world including until recently Nicaragua which is a country in Central America there were no schools or Institute's for the Deaf so this was true until the late 1970s in Nicaragua you so deaf children who were born deaf were just isolated at home and there was no standardized language for them no other deaf children for them to communicate with and so they could communicate with their families you know using some sort of home sign system which is kind of like a pidgin version of a sign language where you have gestures that mean something and you're able to sort of make a passable communication system out of them but they don't have any of the structural complexity of a real language and this was true up until the late 1970s when a progressive government created two such schools so overnight suddenly well not overnight but over a short period of time there was suddenly these large communities of deaf children who were put together into residential schools and they had about 400 students and these were all deaf children who had never been exposed to normal linguistic input and they just got thrown together into these large groups what do you think happened next pause and answer that question here's what happens older students so maybe roughly in their teens and preteens they developed a system of communication that's referred to as lenguaje they're seniors or signals Nicaraguan se so there's this difference in Spanish between lenguaje and idioma and I'm not entirely clear to me what this difference is but somehow this one is taken to be more structured than this one in any case this first lenguaje version of it is Cajun or Creole like in its lack of functional structure right so there's you know normal sign languages are basically just like spoken languages in terms of their grammatical complexity there's lots of complicated morphology inflection different kinds of functional and grammatical categories in a sign language it works just like a spoken language but this first generation the lenguaje version lacked most of that functional structure that you find in real sign languages younger students however who were acquiring this Nicaraguan sign language as a native language they spontaneously developed verb agreement functional morphology things like this and all of this was documented in real time by a linguist from MIT by Judy Shepard Kegel there's an interesting kind of fun backstory here so she was a sign language linguist who was teaching I don't know I think she was up she's certainly at the it was at the University of Southern Maine for many years I don't know if that's where she was teaching at the time but in any case she was an expert on sign language linguistics the Nicaraguan government had built these schools for the deaf with the idea that they were gonna teach these deaf children to write in Spanish and read in Spanish and maybe to read lips and to become educated citizens that will help build a better world and they were really gonna improve the condition of these children and they got the children there and found that they were all making these gestures back and forth to one another and they were able to communicate with one another in ways that the staff of the school couldn't understand and so they called up Judy Shepard Kegel who's an expert on sign language linguistics and they said hey you've got to come down here our children won't behave they keep talking to each other in this gesture system can you stop them right that was why she was called down to Nicaragua in the first place and she got there was obviously very excited and documented several of the stages in the creation of Nicaraguan sign language spontaneously amongst a community of deaf Nicaraguan youths right so this is again pretty compelling evidence that there is a strong drive for particular forms of linguistic knowledge is the spontaneous emergence of a sign language amongst children who have basically been deprived of language for their entire lives up to that point now again you might look at that and say well that's extremely rare it's only ever happened once maybe we shouldn't take it too seriously there is a second case that's been documented that is much like this I'm the açaí Bedouin sign language so Bedouin czar a community of traditionally nomadic people who sort of live in these very arid areas and I gather that in this particular group the else I eat there is some degree of inbreeding in the group and the rates of congenital deafness for whatever reason within this community are ten to a hundred times as high as in the normal population with the result that there are many many many deaf people in this community of veterans and so here just as in the Nicaraguan case there's enough deaf people in the community you reach this critical mass and they've spontaneously developed a sign language that has all of the complicated grammatical structure of any other sign language this one has also been documented by linguists and so it looks like this is not an exceptional thing it looks like this is you know as best as we can tell a fairly normal occurrence when deaf children are thrown together in large enough numbers so the logic of these examples is that young children have a drive to learn language languages that is so intense it can be imposed on systems for which it was not biologically intended and I've put that in scare quotes assuming that you know when humans were evolving the ability to use languages it was probably with spoken languages certainly not with pidgin systems as far as we know and probably not with gestural systems although that said every spoken language also has a gestural component to it there are conventionalized ways that physical gestures fit in into spoken languages and those are you know in some sense as much of the part of the grammar as a lot of other phonological aspects of systems the other interesting point here is that children seem to differ to some degree from adults in this regard so you don't find the adult first generation slaves thrown into these communities or the older Nicaraguan deaf children thrown into these communities you don't find them spontaneously imposing grammatical structure in the same way as young children do this is taken to be again more evidence for a critical period for sort of emergence of grammatical complexity in children here's some more interesting evidence again with congenitally deaf children or started not these are not all congenitally deaf children some of them went deaf at a later later age and this is from Johnston's paper showing tracking eventual performance on sort of grammatical functional morphology test in American Sign Language from native signers so those who are congenitally deaf and that's the leftmost group here you can see that they have a very you know high scores relatively high scores on this test of producing ASL morphology I think this is so this is not the ability to perceive the morphology it's the ability to sign like a native and there's some range amongst these native signers but they're you know uniformly pretty good at this go to children who acquired sign language as young children rather than having who been exposed to it since birth these are people who acquired it a little bit later you can see that you know maybe there's some decline in performance but they're basically up near the same level as the native signers late acquires these are people who acquired American Sign Language after puberty on the other hand have qualitatively different levels of performance on certain grammatical properties of ASL of American Sign Language then early learning and native signers right so this is again evidence I believe all these people were born deaf some of them were deprived of first language input by having not learned sign language until a later age and those learners never quite reach the same level of proficiency as signers who were exposed to the sign language much much earlier so again pretty direct evidence that once you reach a certain age and fail to have gotten linguistic input in a first language things very quickly drop off a cliff afterwards so that's taken to be evidence for critical periods in first language acquisition this is a more recent literature that's come out with regard to congenitally deaf children and it involves the development of cochlear implants so these are digital electronic devices that directly interface with the auditory nerve in your inner ear that basically sends signals that nerve and they're not anywhere near as sensitive or capable of encoding as many differences as a real human auditory nerve so they're much more coarser grained but they can encode enough information and transmit it into your brain that with training it can allow people to hear not maybe quite as much as somebody who isn't congenitally deaf but much better than was ever possible with hearing aids or other kinds of assistive devices in the past so this has been a huge topic of research and speech science and more applied speech science and one of the questions here is well what's the difference between people who get this linguistic exposure from their cochlear implants at a younger age versus an older age because these are fairly recent developments and some much older people have chosen to get cochlear implants some very young infants have had them who are born deaf and we can ask well what's the difference in their eventual attainment after they get these devices so what this particular chart is showing us here is how on the horizontal axis how long has it been since you got your cochlear implant on the vertical axis how well did you do on a consonant production task in English again and so this is not actually measuring their ability to hear anything even though that's what the cochlear implant does this is really measuring their implicit linguistic knowledge because it's trying to see how well they've learned how to produce consonants right which requires more than just being able to hear and the different lines here represent different groups so this solid line is group a1 this is people who had a implantation implantation I believe in this one it is at under one year old so during early infancy these two lines are children getting implants from age 1 to about 7 and yeah there's not much difference between them one of them's like 1 2 3 years old one of them is 3 to 7 years old something like that there's not much difference and then you get to these 7 to 10 year olds down here and they are again not acquiring knowledge as quickly despite so everybody's learning more the longer they have the cochlear implant in that's why all of these slopes are positive this gray line is a hearing aid so this is not a cochlear implant at all it's a control saying well this is how well you learn but just our sort of primitive hearing aid devices and the people who get these implants after 7 years old are not necessarily doing much better than the people who just had hearing aids the traditional technology but you're finding a faster rate of learning and higher eventual attainment with children who got these implanted younger and with implantation in early infancy you can make the case this is really a qualitatively different pattern that they're acquiring they're looking much more accurate and learning much faster and on a different kind of curve than children who got these implants later on so again showing that there seems to be some kind of a sensitive age for I guess phonetics in this case and early infancy matters a lot in this case so we briefly talk about these two other papers that were up on the website it's been about 32 minutes yes I won't spend too long on these but here's what these are showing so Polly and colleagues in their 2003 paper wanted to know whether Korean adoptees living in France showed some sign of exceptional retention of Korean and so they basically had their Korean subjects go through a battery of tests trying to for instance discriminate between these three very hard Korean phonemes that are three different kinds of voiceless stops that are really hard for French speakers and of course they found that most of the adoptees couldn't do this at all and then they yeah they tried a whole bunch of other kinds of tasks asking hey do these people still know something know anything about Korean and they basically found no they found not no evidence that those add up T's from Korea retained any knowledge about Korean so the this is thought to bear on the critical period hypothesis the first language acquisition because it's again that flip side of the coin well maybe if you do get some exposure even if it's very little exposure at a very early age maybe you end up retaining that because the critical period is so strong and they had the children whose age of adoption went up to I think about ten or twelve years in this study yet they found no effect that there was no retention of Korean knowledge for these children and that was an interesting result it suggested that the exceptional retention thing just doesn't work out but that's not how language works until there's a paper in 2017 from Troy and colleagues and what they showed is a very similar experiment I think it was done in the Netherlands rather than France but again with Korean adoptees and again trying to train them on this difficult phonemic distinction between tense lakhs and aspirated stops in standard Korean or in Seoul Korean and here they again started by you know seeing whether people could do this and the add up T's could not do it at any better rate than the native Dutch speakers who were the control group here but then they gave them training they trained them using a bunch of tasks with feedback to figure out how to either produce or perceiver both some of these these stop sounds and what they found here was that you know everybody showed some amount of learning the Dutch people showed some learning but the Korean adoptees the faster even though they didn't start out recognizing these phonemes better they learn to discriminate them faster than the dutch-speaking controls so that is super fascinating right it basically means that no they didn't exactly retain the knowledge from having been infants in Korea but they regained that knowledge more readily than people who hadn't had that experience of being in Korea as an infant so if that result holds up and again this is just one experiment and I'd want to probably see a lot more data before I accept this uncritically but it does appear to be the case that even very limited exposure in early infancy or early childhood can leave some kind of a lasting trace on people's linguistic knowledge as adults that was the point of the toy at all paper so basic summary here we it's very hard to get very direct evidence for critical period effects in first language learning especially because the relevant experiment which would be just depriving people of language is impossible for ethical reasons but all the little partial bits of evidence that we've been able to group together from different kinds of aberrant and unusual social phenomena from you know feral children to slavery to congenital deafness all seem to point in the same direction which is that it looks very very likely that there are critical period effects in first language acquisition and that people who are deprived of input or have degraded input during early childhood and during infancy end up having lasting differences in the way that they acquire languages compared to people with so-called normal linguistic input so I think it's hard to be a hundred percent certain but the general consensus for is there a critical period for first language acquisition the general consensus seems to be yes next time we'll look at second language acquisition and we'll see that there is very very little consensus there