Not All Words Are Equally Acquired: Transitional Probabilities and Instructions Affect the Electrophysiological Correlates of Statistical Learning

Abstract

Statistical learning (SL), the process of extracting regularities from the environment, is a fundamental skill of our cognitive system to structure the world regularly and predictably. SL has been studied using mainly behavioral tasks under implicit conditions and with triplets presenting the same level of difficulty, i.e., a mean transitional probability (TP) of 1.00. Yet, the neural mechanisms underlying SL under other learning conditions remain largely unknown. Here, we investigated the neurofunctional correlates of SL using triplets (i.e., three-syllable nonsense words) with a mean TP of 1.00 ( easy “words” ) and 0.50 ( hard “words” ) in an SL task performed under incidental (implicit) and intentional (explicit) conditions, to determine whether the same core mechanisms were recruited to assist learning. Event-related potentials (ERPs) were recorded while participants listened firstly to a continuous auditory stream made of the concatenation of four easy and four hard “words” under implicit instructions, and subsequently to another auditory stream made of the concatenation of four easy and four hard “words” drawn from another artificial language under explicit instructions. The stream in each of the SL tasks was presented in two consecutive blocks of ~3.5-min each (~7-min in total) to further examine how ERP components might change over time. Behavioral measures of SL were collected after the familiarization phase of each SL task by asking participants to perform a two-alternative forced-choice (2-AFC) task. Results from the 2-AFC tasks revealed a moderate but reliable level of SL, with no differences between conditions. ERPs were, nevertheless, sensitive to the effect of TPs, showing larger amplitudes of N400 for easy “words,” as well as to the effect of instructions, with a reduced N250 for “words” presented under explicit conditions. Also, significant differences in the N100 were found as a result of the interaction between TPs, instructions, and the amount of exposure to the auditory stream. Taken together, our findings suggest that triplets’ predictability impacts the emergence of “words” representations in the brain both for statistical regularities extracted under incidental and intentional instructions, although the prior knowledge of the “words” seems to favor the recruitment of different SL mechanisms.

Age-Related Differences in Auditory Statistical Learning of Complex Speech Streams: Evidence From ERPs

“…Note that a triplet with a TP of 1.00 means that a given stimulus (e.g., a syllable) is always followed by another stimulus (e.g., another syllable) in the stream, thus reducing the variance along with the aSL ability can be measured. Besides, as Soares et al 9 recently pointed out, removing all signs of uncertainty from the input does not mimic what occurs in natural languages, which also reduces the possibility of aSL results to be related with other linguistic outcomes. Indeed, in natural languages, syllables, as well as other linguistic units (e.g., phonemes, morphemes) do not follow each other with 100% of certainty, as the same syllable can occur in many different words at different syllable positions such as the syllable /cur/ in words as /cur.va.ture/, /in.cur.sion/ or /re.oc.cur/.…”

“…Indeed, in natural languages, syllables, as well as other linguistic units (e.g., phonemes, morphemes) do not follow each other with 100% of certainty, as the same syllable can occur in many different words at different syllable positions such as the syllable /cur/ in words as /cur.va.ture/, /in.cur.sion/ or /re.oc.cur/. Additionally, the 2-AFC task is an o ine post-learning task, measuring the result of the learning that has occurred in the previous familiarization phase, and not the processes underlying that result [see 9,28,34 for an extended discussion]. Thus, it is possible that even though children and adults might not differ in terms of aSL outcomes with linguistic materials, they might nevertheless differ in the mechanisms they recruit to reach those outcomes.…”

“…Although a large number of studies have shown that the ability to extract regularities from the sensory environment, an ability known as statistical learning (SL 1 ), is observed in young children [e.g., [1][2][3][4][5][6] ] and adults [e.g., [7][8][9][10] ], little is known about how this ability, assumed to play a central role in human cognition [see 11 ], changes throughout development.…”

“…Thus, further research using other tasks and/or methodologies is required to get a deeper understanding of how aSL with linguistic materials might change throughout development. Moreover, it is also important to note that most studies tested aSL using three-syllable nonsense words presenting the same level of predictability, i.e., transitional probabilities (TP) of 1.00, which might also contribute to limit not only our understanding of the conditions under which aSL works, but, importantly, the chances of age-related differences in aSL to be observed [see, however, 9,[30][31][32][33] for works using triplets with different TPs]. Note that a triplet with a TP of 1.00 means that a given stimulus (e.g., a syllable) is always followed by another stimulus (e.g., another syllable) in the stream, thus reducing the variance along with the aSL ability can be measured.…”

“…This work aimed to directly analyze these issues by examining the electrophysiological correlates (ERP) elicited during the familiarization phase of an aSL task with linguistic materials (syllables) while 5 yearsold children and young adults (university students) were exposed to the repetition of eight three-syllable nonsense words -four with TPs of 1.00 (high-TP 'words') and four with TPs of .50 (low-TP 'words')presented in two different blocks to further examine how ERPs might change as a function of words' predictability and length of exposure. Furthermore, to examine the role that previous knowledge of the to-be-learned-regularities might play on aSL -an issue that is gaining increased interest given the proposal that previous linguistic knowledge might interfere with SL outcomes [see 35,36 ] -participants performed the aSL task, rstly, under implicit and, subsequently, under explicit conditions to minimize the role of individual differences in performance [see 32 and also 9 for a similar procedure]. Behavioral data were also collected from the standard 2-AFC task presented after the familiarization phase of each of the aSL tasks.…”