BACKGROUND
statistical learning can be described as the process of extracting the statistical properties of the data input.
in speech, the statistical properties include the transitional probabilities between the different linguistic items, such as phonemes and syllables.
statistical learning of speech and sensitivity to social cues help infants in the rapid acquisition of their first language  <cit> .
from  <dig> months of age, infants use computational strategies to learn the distributional patterns of sounds  <cit> , the non-adjacent dependencies required to learn grammar  <cit> , and the sequential probabilities  <cit> , as well as the stress patterns  <cit> , necessary to perform word segmentation.
in the visual domain, infants as young as  <dig> months have been shown to learn statistical regularities in sequences of visual stimuli  <cit> .
although these skills are all based on computational mechanisms, they may, however, be based on very different underlying processes.
recently, the mapping of different basic skills possibly used in language learning during the first year of life has undergone tremendous progress.
for instance, it has been found that the skills necessary to discriminate different stress patterns in speech develop only after  <dig> months of age  <cit> , even though newborns are able to process some rhythmic information in speech  <cit> .
still at the age of  <dig> months, infants use statistical rather than prosodic cues for recovering words from strings of syllables  <cit> .
furthermore, eight-month-old infants adopt these units as possible words in their native language  <cit> .
recent results also suggest that even newborn infants learn patterns of repetition in adjacent syllables located in tri-syllabic pseudowords  <cit> .
however, the possibly very early capabilities of statistical learning that could exist as early as at birth have remained undiscovered.
these skills could have an important contribution to the rapid language development that infants exhibit during the first months of life.
kooijman, hagoort, and cutler  <cit> , aiming at determining whether 10-month-old infants detect previously learned words from continuous speech, found that event-related potentials  of 10-month-olds showed a greater negative deflection from  <dig> to  <dig> ms from the stimulus onset for familiar than unfamiliar words.
the words in the experiment were learned in isolation, however.
furthermore, sanders, newport, and neville  <cit> , studying adult erps to three-syllable pseudowords hidden within a syllable stream, found an enhanced n <dig> amplitude and, for the high learners, an enhanced n <dig> amplitude for the pseudoword onsets after training.
even before training, the n <dig> amplitude was greater for the initial syllable of the pseudoword compared to that for the medial and final syllables, suggesting implicit learning of transitional probabilities in adult population.
additional evidence for the n <dig> and n <dig> erp components as indices of adult statistical segmentation have recently been discovered .
in order to determine the availability of computational mechanisms for word segmentation at birth, we recorded erps in two successive experiments from  <dig> healthy sleeping neonates presented with a stream of syllables containing statistical cues to word boundaries.
in the experiment  <dig>  we employed a paradigm previously used to determine statistical word segmentation in adults with magnetoencephalography .
in experiment  <dig>  we replicated the results of the first experiment with minor modifications to the research design.
ten different three-syllable pseudo-words were constructed so that each syllable was  <dig> ms in duration, with 200-ms breaks between the consecutive syllables and after the last syllable of a word.
each syllable belonged to one triplet only.
the pseudo-words were played in a random order, but with equal transitional probabilities from a word to each of the other words .
thus, the sequence contained no morphological or prosodic cues to word boundaries .
the resulting, seemingly random, stream of syllables was played for  <dig> minutes.
the sequences of syllables were acoustically controlled by placing syllables of a similar type, such as diphthongs, in all three positions of the words .
thus, the acoustical properties of the syllables could not contribute to the learning of the word boundaries.
consequently, before learning the word boundaries, the responses to the initial syllable should not differ from those to the second and third syllables because of the similar acoustical properties of the syllables.
in order to assess the nature of the prospective processes responsible for detecting the word onsets, we also included unexpected syllables after  <dig> minutes of exposure.
these unexpected syllables, occurring after every 2– <dig> words , consisted of the medial and final syllables of the previously-learned pseudowords and were presented alone between the intact pseudowords .
one fourth of the deviant syllables were novel syllables not present earlier in the experiment.
in experiment  <dig>  the inventory of the syllables was same as in experiment  <dig>  but rearranged to create pseudowords with a different placement in the three syllable positions .
this was to ensure that no acoustic characteristics of the pseudowords could affect the effects measured.
additionally, the novel unexpected syllables were not used in experiment  <dig>  still keeping the frequency of unexpected syllables identical with experiment  <dig> by increasing the number of unexpected medial and final syllables.
the syllables used in experiment  <dig>  medial  and final  syllables were also used as unexpected syllables added between standard pseudowords.
the syllables used in experiment  <dig>  the final syllables  of experiment  <dig> were shifted to become initial syllables .
medial  and final  syllables were also used as unexpected syllables added between standard pseudowords.
there were no novel unexpected syllables in experiment  <dig>
RESULTS
experiment 1
a three-way anova  was calculated on the average erp amplitudes on consecutive  <dig> ms bins that shifted in steps of  <dig> ms .
all the standard syllables  except those in the triplets immediately following unexpected syllables were included in the averages.
a significant main effect of syllable on four consecutive bins was taken as evidence for differences between the responses for different syllables.
this criterion was reached in the latency range of 260– <dig> ms relative to stimulus onset.
the most significant bin, i.e., the one with a time range of 340– <dig> ms , was selected as the latency of the effect.
the response for s <dig> had a greater negative deflection than those for s <dig> and s <dig> .
post-hoc lsd tests confirmed that the response to s <dig> significantly differed from s <dig>  and s <dig> , but s <dig> did not differ from s <dig> .
we found a significant main effect of location  caused by the absolute amplitude differences between the different scalp locations.
however, the direction of the effect was similar in all four anterior positions .
finally, we found a significant main effect of hemisphere , with the amplitudes being overall more negative over the left hemisphere than over the right one.
no significant interactions were found.
to further investigate the observed negativity at the word boundary, we included the initial syllables immediately following an unexpected syllable in the comparison, excluding the first  <dig> minutes of the data .
however, no significant main effects or interactions were found.
this was likely due to the relatively low signal-to-noise ratio for the unexpected syllables due to a smaller number of repetitions compared to the within-word syllables, as well as the smaller number of repetitions for the within-word syllables, as the trials recorded during the first  <dig> minutes of the experiment were excluded.
consequently, the unexpected syllables were not analysed in experiment  <dig>
experiment 2
a significant main effect of syllable was reached in the latency range of 290– <dig> ms relative to stimulus onset.
the most significant bin, i.e., 310– <dig> ms , was selected as the latency of the effect.
similarly to experiment  <dig>  the negative deflection for s <dig> was larger in amplitude than those for s <dig> and s <dig>  post hoc lsd tests confirmed that the response to s <dig> significantly differed from that to s <dig>  but not from that to s <dig> , and the response to s <dig> did not significantly differ from that to s <dig> .
we found a significant main effect of location  caused by the absolute amplitude the differences between different scalp locations.
however, also in experiment  <dig>  the direction of the effect was similar in all four anterior positions .
we also found a significant interaction between location and hemisphere , but no significant main effect of hemisphere .
finally, we combined the data of the most significant bins in the two experiments and applied a four-way anova  to verify the replication of the results obtained in experiment  <dig>  we found no main effect of experiment  nor any interaction including experiment .
as expected, we found significant main effects of syllable  and location .
there was also a significant main effect of hemisphere  and an interaction of hemiphere and location , with the responses over the left hemisphere being larger especially at the central electrode site than those over the right hemisphere.
no other main effects or interactions were found.
lsd tests of the combined data confirmed that the response for s <dig> was larger in amplitude than those to s <dig>  and s <dig> , but the response to s <dig> did not differ from that to s <dig> .
discussion
we recorded erps of  <dig> sleeping newborn infants, presented with repeating pseudowords containing occasional unexpected syllables.
we also recorded a further set of data of  <dig> additional infants, because using a novel research design, it was important to replicate the findings.
furthermore, one could argue that the novel unexpected syllables used in experiment  <dig> could provide additional information with respect to the location of the word boundaries.
thus, the unexpected novel syllables were not used in experiment  <dig>  as the unexpected syllables may considerably contribute to the perceptive processes of the infants, however, it was important to preserve the unexpected medial and final syllables in experiment  <dig> to enable the direct comparison between the results of these two experiments.
in experiment  <dig>  we found that the neonate brain began to respond specifically to the pseudowords embedded in a seemingly random stream of syllables.
a larger negative deflection was detected in the response to the initial syllable s <dig> compared with those to the other two syllables, s <dig> and s <dig>  in experiment  <dig>  we found differences at a similar latency between the responses to the different syllables.
a larger negative deflection was detected in the response to s <dig> compared to that in the response to s <dig>
because the corpus used in the experiment consisted of  <dig> different words, it was highly unlikely to remain in auditory sensory memory during the experiment due to its limited capacity and interference caused by continuous stimulation  <cit> .
furthermore, due to the large number of different syllables and the acoustically balanced positioning of the syllables, learning could not be specific to any single items.
therefore, the results demonstrate efficient segmentation of continuous speech into distinct units solely on the basis of the statistical properties.
the properties used could either be the transitional probabilities between consecutive syllables, a strategy that 8-month-old are able to use  <cit> , or they may be based on learning the frequencies of co-occurrence of consecutive syllables, higher within words than between them.
one could also argue that the difference between the frequencies of individual s <dig> syllables and s <dig> or s <dig> syllables  could have provided an additional cue for distinguishing these syllables from each other.
however, as even the largest differences in frequency  were very small, it is unlikely that this subtle variation could have been detected or in any way utilised by the infants within the limited time of the experiment.
the frequencies of different acoustic syllables  shown in percentages for the last  <dig> minutes  and the entire experiments.
during the first  <dig> minutes , the probabilities of s <dig>  s <dig>  and s <dig> syllables were equal .
the results obtained in experiment  <dig> did not replicate the significant difference between s <dig> and s <dig>  found in experiment  <dig>  it is reasonable, however, to assume that the difference between s <dig> and s <dig> is more prominent than that between s <dig> and s <dig>  as the grand-averaged responses to the whole pseudowords show an overall positive trend .
as the neonate erps typically have a very large inter-individual variation, it could result in the less prominent difference between s <dig> and s <dig> being evident in the erps of some individuals only.
additionally, the syllable stimuli used for s <dig> and s <dig> in experiment  <dig> were used for s <dig> and s <dig>  respectively, in experiment  <dig>  thus, the difference between the responses to s <dig> and s <dig> in experiment  <dig> and the lack of a difference between the responses to s <dig> and s <dig> in experiment  <dig> indicate that the observed effects were not caused by any acoustical characteristics of the syllables.
it should also be noted that when the data from experiments  <dig> and  <dig> were combined, the response to s <dig> showed a larger negative deflection than those to both s <dig> and s <dig>
the observed differences between the responses to s <dig> and s <dig> may be due to various reasons.
firstly, the lack of predictability caused by the smaller transitional probability at the word boundary could be processed differently from the way of how completely predictable within-word transitions were processed.
this could be seen as anticipatory processes emerging during the processing of the final syllable or even before it.
on the other hand, the onset response could be interpreted as a specific reaction to the beginning of a new word.
such a word onset response may be explained by pseudowords being distinguished and memorised in the brain or by cerebral memory traces of the relationships between syllable pairs within the words.
the underlying processes need to be studied further before any final conclusions on the nature of the learning mechanisms can be made.
the differences seen between the responses to the initial syllables and those to the final syllables of the words suggest that these syllables may be the key elements of detecting the word boundaries.
it is also interesting to note that in a similar task, an enhanced negativity for the onset syllable is also observed in the adult n <dig> and n <dig> responses .
CONCLUSIONS
we found that newborn infants are sensitive to the statistical properties of a stream of speech.
more specifically, our results show that even the neonatal brain effectively segments word-like units from a stream of syllables using only the transitional probabilities or frequencies of co-occurrence between the syllables.
these results hence suggest a role for statistical learning in the early acquisition of words.

 statistical learning can be described as the process of extracting the statistical properties of the data input. in speech, the statistical properties include the transitional probabilities between the different linguistic items, such as phonemes and syllables. statistical learning of speech and sensitivity to social cues help infants in the rapid acquisition of their first language. from months of age, infants use computational strategies to learn the distributional patterns of sounds, the non-adjacent dependencies required to learn grammar, and the sequential probabilities, as well as the stress patterns, necessary to perform word segmentation. in the visual domain, infants as young as months have been shown to learn statistical regularities in sequences of visual stimuli. although these skills are all based on computational mechanisms, they may, however, be based on very different underlying processes. recently, the mapping of different basic skills possibly used in language learning during the first year of life has undergone tremendous progress. for instance, it has been found that the skills necessary to discriminate different stress patterns in speech develop only after months of age, even though newborns are able to process some rhythmic information in speech. still at the age of months, infants use statistical rather than prosodic cues for recovering words from strings of syllables. furthermore, eight-month-old infants adopt these units as possible words in their native language. recent results also suggest that even newborn infants learn patterns of repetition in adjacent syllables located in tri-syllabic pseudowords. however, the possibly very early capabilities of statistical learning that could exist as early as at birth have remained undiscovered. these skills could have an important contribution to the rapid language development that infants exhibit during the first months of life. kooijman, hagoort, and cutler, aiming at determining whether 10-month-old infants detect previously learned words from continuous speech, found that event-related potentials of 10-month-olds showed a greater negative deflection from to ms from the stimulus onset for familiar than unfamiliar words. the words in the experiment were learned in isolation, however. furthermore, sanders, newport, and neville, studying adult erps to three-syllable pseudowords hidden within a syllable stream, found an enhanced n amplitude and, for the high learners, an enhanced n amplitude for the pseudoword onsets after training. even before training, the n amplitude was greater for the initial syllable of the pseudoword compared to that for the medial and final syllables, suggesting implicit learning of transitional probabilities in adult population. additional evidence for the n and n erp components as indices of adult statistical segmentation have recently been discovered. in order to determine the availability of computational mechanisms for word segmentation at birth, we recorded erps in two successive experiments from healthy sleeping neonates presented with a stream of syllables containing statistical cues to word boundaries. in the experiment we employed a paradigm previously used to determine statistical word segmentation in adults with magnetoencephalography. in experiment we replicated the results of the first experiment with minor modifications to the research design. ten different three-syllable pseudo-words were constructed so that each syllable was ms in duration, with 200-ms breaks between the consecutive syllables and after the last syllable of a word. each syllable belonged to one triplet only. the pseudo-words were played in a random order, but with equal transitional probabilities from a word to each of the other words. thus, the sequence contained no morphological or prosodic cues to word boundaries. the resulting, seemingly random, stream of syllables was played for minutes. the sequences of syllables were acoustically controlled by placing syllables of a similar type, such as diphthongs, in all three positions of the words. thus, the acoustical properties of the syllables could not contribute to the learning of the word boundaries. consequently, before learning the word boundaries, the responses to the initial syllable should not differ from those to the second and third syllables because of the similar acoustical properties of the syllables. in order to assess the nature of the prospective processes responsible for detecting the word onsets, we also included unexpected syllables after minutes of exposure. these unexpected syllables, occurring after every 2– words, consisted of the medial and final syllables of the previously-learned pseudowords and were presented alone between the intact pseudowords. one fourth of the deviant syllables were novel syllables not present earlier in the experiment. in experiment the inventory of the syllables was same as in experiment but rearranged to create pseudowords with a different placement in the three syllable positions. this was to ensure that no acoustic characteristics of the pseudowords could affect the effects measured. additionally, the novel unexpected syllables were not used in experiment still keeping the frequency of unexpected syllables identical with experiment by increasing the number of unexpected medial and final syllables. the syllables used in experiment medial and final syllables were also used as unexpected syllables added between standard pseudowords. the syllables used in experiment the final syllables of experiment were shifted to become initial syllables. medial and final syllables were also used as unexpected syllables added between standard pseudowords. there were no novel unexpected syllables in experiment  experiment 1 a three-way anova was calculated on the average erp amplitudes on consecutive ms bins that shifted in steps of ms. all the standard syllables except those in the triplets immediately following unexpected syllables were included in the averages. a significant main effect of syllable on four consecutive bins was taken as evidence for differences between the responses for different syllables. this criterion was reached in the latency range of 260– ms relative to stimulus onset. the most significant bin, i.e., the one with a time range of 340– ms, was selected as the latency of the effect. the response for s had a greater negative deflection than those for s and s. post-hoc lsd tests confirmed that the response to s significantly differed from s and s, but s did not differ from s. we found a significant main effect of location caused by the absolute amplitude differences between the different scalp locations. however, the direction of the effect was similar in all four anterior positions. finally, we found a significant main effect of hemisphere, with the amplitudes being overall more negative over the left hemisphere than over the right one. no significant interactions were found. to further investigate the observed negativity at the word boundary, we included the initial syllables immediately following an unexpected syllable in the comparison, excluding the first minutes of the data. however, no significant main effects or interactions were found. this was likely due to the relatively low signal-to-noise ratio for the unexpected syllables due to a smaller number of repetitions compared to the within-word syllables, as well as the smaller number of repetitions for the within-word syllables, as the trials recorded during the first minutes of the experiment were excluded. consequently, the unexpected syllables were not analysed in experiment experiment 2 a significant main effect of syllable was reached in the latency range of 290– ms relative to stimulus onset. the most significant bin, i.e., 310– ms, was selected as the latency of the effect. similarly to experiment the negative deflection for s was larger in amplitude than those for s and s post hoc lsd tests confirmed that the response to s significantly differed from that to s but not from that to s, and the response to s did not significantly differ from that to s. we found a significant main effect of location caused by the absolute amplitude the differences between different scalp locations. however, also in experiment the direction of the effect was similar in all four anterior positions. we also found a significant interaction between location and hemisphere, but no significant main effect of hemisphere. finally, we combined the data of the most significant bins in the two experiments and applied a four-way anova to verify the replication of the results obtained in experiment we found no main effect of experiment nor any interaction including experiment. as expected, we found significant main effects of syllable and location. there was also a significant main effect of hemisphere and an interaction of hemiphere and location, with the responses over the left hemisphere being larger especially at the central electrode site than those over the right hemisphere. no other main effects or interactions were found. lsd tests of the combined data confirmed that the response for s was larger in amplitude than those to s and s, but the response to s did not differ from that to s. discussion we recorded erps of sleeping newborn infants, presented with repeating pseudowords containing occasional unexpected syllables. we also recorded a further set of data of additional infants, because using a novel research design, it was important to replicate the findings. furthermore, one could argue that the novel unexpected syllables used in experiment could provide additional information with respect to the location of the word boundaries. thus, the unexpected novel syllables were not used in experiment as the unexpected syllables may considerably contribute to the perceptive processes of the infants, however, it was important to preserve the unexpected medial and final syllables in experiment to enable the direct comparison between the results of these two experiments. in experiment we found that the neonate brain began to respond specifically to the pseudowords embedded in a seemingly random stream of syllables. a larger negative deflection was detected in the response to the initial syllable s compared with those to the other two syllables, s and s in experiment we found differences at a similar latency between the responses to the different syllables. a larger negative deflection was detected in the response to s compared to that in the response to s because the corpus used in the experiment consisted of different words, it was highly unlikely to remain in auditory sensory memory during the experiment due to its limited capacity and interference caused by continuous stimulation. furthermore, due to the large number of different syllables and the acoustically balanced positioning of the syllables, learning could not be specific to any single items. therefore, the results demonstrate efficient segmentation of continuous speech into distinct units solely on the basis of the statistical properties. the properties used could either be the transitional probabilities between consecutive syllables, a strategy that 8-month-old are able to use, or they may be based on learning the frequencies of co-occurrence of consecutive syllables, higher within words than between them. one could also argue that the difference between the frequencies of individual s syllables and s or s syllables could have provided an additional cue for distinguishing these syllables from each other. however, as even the largest differences in frequency were very small, it is unlikely that this subtle variation could have been detected or in any way utilised by the infants within the limited time of the experiment. the frequencies of different acoustic syllables shown in percentages for the last minutes and the entire experiments. during the first minutes, the probabilities of s s and s syllables were equal. the results obtained in experiment did not replicate the significant difference between s and s found in experiment it is reasonable, however, to assume that the difference between s and s is more prominent than that between s and s as the grand-averaged responses to the whole pseudowords show an overall positive trend. as the neonate erps typically have a very large inter-individual variation, it could result in the less prominent difference between s and s being evident in the erps of some individuals only. additionally, the syllable stimuli used for s and s in experiment were used for s and s respectively, in experiment thus, the difference between the responses to s and s in experiment and the lack of a difference between the responses to s and s in experiment indicate that the observed effects were not caused by any acoustical characteristics of the syllables. it should also be noted that when the data from experiments and were combined, the response to s showed a larger negative deflection than those to both s and s the observed differences between the responses to s and s may be due to various reasons. firstly, the lack of predictability caused by the smaller transitional probability at the word boundary could be processed differently from the way of how completely predictable within-word transitions were processed. this could be seen as anticipatory processes emerging during the processing of the final syllable or even before it. on the other hand, the onset response could be interpreted as a specific reaction to the beginning of a new word. such a word onset response may be explained by pseudowords being distinguished and memorised in the brain or by cerebral memory traces of the relationships between syllable pairs within the words. the underlying processes need to be studied further before any final conclusions on the nature of the learning mechanisms can be made. the differences seen between the responses to the initial syllables and those to the final syllables of the words suggest that these syllables may be the key elements of detecting the word boundaries. it is also interesting to note that in a similar task, an enhanced negativity for the onset syllable is also observed in the adult n and n responses.  we found that newborn infants are sensitive to the statistical properties of a stream of speech. more specifically, our results show that even the neonatal brain effectively segments word-like units from a stream of syllables using only the transitional probabilities or frequencies of co-occurrence between the syllables. these results hence suggest a role for statistical learning in the early acquisition of words. 
these results demonstrate that neonates can efficiently learn transitional probabilities or frequencies of co-occurrence between different syllables, enabling them to detect word boundaries and in this way isolate single words out of fluent natural speech. we found evidence that sleeping neonates are able to automatically extract statistical properties of the speech input and thus detect the word boundaries in a continuous stream of syllables containing no morphological cues. syllable-specific event-related brain responses found in two separate studies demonstrated that the neonatal brain treated the syllables differently according to their position within pseudowords. to test the statistical learning skills at birth, we recorded event-related brain responses of sleeping neonates while they were listening to a stream of syllables containing statistical cues to word boundaries. statistical learning is a candidate for one of the basic prerequisites underlying the expeditious acquisition of spoken language.
 statistical learning is a candidate for one of the basic prerequisites underlying the expeditious acquisition of spoken language. infants from months of age exhibit this form of learning to segment fluent speech into distinct words. to test the statistical learning skills at birth, we recorded event-related brain responses of sleeping neonates while they were listening to a stream of syllables containing statistical cues to word boundaries.  we found evidence that sleeping neonates are able to automatically extract statistical properties of the speech input and thus detect the word boundaries in a continuous stream of syllables containing no morphological cues. syllable-specific event-related brain responses found in two separate studies demonstrated that the neonatal brain treated the syllables differently according to their position within pseudowords.  these results demonstrate that neonates can efficiently learn transitional probabilities or frequencies of co-occurrence between different syllables, enabling them to detect word boundaries and in this way isolate single words out of fluent natural speech. the ability to adopt statistical structures from speech may play a fundamental role as one of the earliest prerequisites of language acquisition. 