Transition From Implicit To Explicit Sequence Knowledge And The Underlying Brain Mechanism

When contacting with the surrounding environment,people can acquire the knowledge of structural relationship among objects or events in an unintentional and automatic way without conscious awareness,which is called "implicit learning".Implicit learning plays an important role in supporting individual adaptability and thus attracts much attention from researchers.In view of the fact that rules among interrelated events in real life often tends to be certain sequence regularities defined by chronological order,researchers put forward the serial reaction time（SRT）task to study implicit learning.In the SRT task,generally,a series of visual stimuli are presented successively on different locations,and participants are required to make corresponding key-press response to the position of each stimulus;according to whether the sequence of stimuli positions conforms to a certain regularity,these stimulus materials can be classified into regular sequence stimulus and random sequence stimulus.Typical results observed in previous studies are that,after a period of learning,participants’ reaction time（RT）to regular sequence stimuli is significantly shorter than that to random sequence stimuli,indicating that participants have acquired the sequence knowledge;however,the participants do not have conscious awareness of the sequence regularity,which suggested that implicit sequence learning has occurred to participants.Studies based on SRT task found that,after the occurrence of implicit sequence learning,participants may eventually become consciously aware of the sequence regularity if they continue to experience more learning sessions of the SRT task.At this point,participants’ previously-acquired implicit sequence knowledge has transitioned into explicit sequence knowledge,that is,the "transition from implicit to explicit sequence knowledge" has happened.This transition is thought to be highly related to conscious access of unconscious information,of which the two somehow have shared mechanism,and hence makes researchers put effort into exploring it.However,previous studies only conducted awareness measure once after all the SRT task sessions had completed so as to judge whether participants were consciously aware of the sequence regularity.Therefore,it can only explain whether the transition happened during all SRT task sessions,but cannot determine participants’ specific transition session,nor can it explore what kind of behavioral and brain activity changes happened in the transition session.At present,the behavioral characteristics of the transition from implicit to explicit sequence knowledge still need to be further explored,and what changes happened in brain activities lead to this transition remains unclear.To address these questions,with adopting a nine-session SRT task and utilizing session-by-session awareness measure,the present study detected the occurrence of transition from implicit to explicit sequence knowledge and located the specific transition session.Combining with electroencephalograph（EEG）,magnetoencephalogram（MEG）,and rhythmic transcranial magnetic stimulation（r TMS）techniques,the present study systematically explored individuals’ behavioral characteristics and brain mechanism of transition from implicit to explicit sequence knowledge.Experiment 1 adopted cross-sectional comparison between groups.Four groups of participants were asked to respectively complete one-session,two-session,eightsession,and nine-session SRT tasks.In terms of sequence learning effect,the RT difference between regular sequence stimuli and random sequence stimuli（i.e.,learning performance）was not significant in the first session,but this difference became significant from the second session and further widened in the following sessions.In experiment 1,the session-by-session awareness measure was applied during the task to detect participants’ implicit-to-explicit transition and locate the specific transition session,besides,a traditional post-task awareness measure was also conducted after each group of participants completed all assigned learning sessions.The receiver operating characteristic（ROC）curve test based on signal detection theory showed that these two methods,i.e.,the session-by-session awareness measure and post-task awareness measure,have good consistency in classifying whether the participants experienced the transition and eventually become aware of the sequence regularity.Based on the specific transition session detected by session-by-session awareness measure,the results of model-fitting analysis in experiment 1 revealed that the learning performance increased sharply at the session of transition,emerging a switchpoint of growth,rather than at the other sessions without transition,which suggested that this switchpoint occurred specifically at the transition session.On the basis of experiment 1,experiment 2 used EEG to explore the changes of neural oscillatory activity in relation to the transition from implicit to explicit sequence knowledge.In experiment 2,all participants were asked to complete the nine-session SRT task,during which the session-by-session awareness measure was also implemented to pinpoint the transition session.The behavioral results were the same as those of experiment 1.It showed that there was an abrupt increase in the RT difference between regular and random sequence stimuli at the transition session,that is,the switchpoint of the learning performance increment was emerged,and also this switchpoint was specific to the transition session.In the neural oscillation results of experiment 2,there was a link between the transition from implicit to explicit sequence knowledge and the change of theta oscillation.Specifically,the transition from implicit to explicit sequence knowledge involves both the pre-transition session（i.e.,the session before the occurrence of transition）and the transition session（i.e.,the session when the transition occurred）.Time-frequency results of EEG showed that parietal theta oscillation was significantly enhanced in the pre-transition session,and its strength significantly predicted participants’ learning performance increment in the transition session;while in the transition session,the significant enhancement of theta oscillation was absent in the parietal region but took place in frontal.Experiment 3 required all participants to perform the nine-session SRT task and applied the session-by-session awareness measure to pinpoint the transition session.Experiment 3 utilized MEG to investigate changes of theta oscillation at cerebral cortex level,and further analyzed signal transfer of theta oscillation between different cortical regions to reveal the changes of brain activities underlying the transition from implicit to explicit sequence knowledge.The cortical source localization of theta oscillation was performed based on the MEG signal co-registered with structural MRI,and then the signal transfer of theta oscillation was measured with phase transfer entropy（PTE）technique.The behavioral results were consistent with experiment 1 and experiment 2.The results of theta oscillation at cortex level showed that,in the pre-transition session,there was not only significant precuneus theta oscillation in parietal cortex,but also a significant signal transfer of theta oscillation from the precuneus to the superior frontal gyrus（SFG）.And also,both the strength of the precuneus theta oscillation and signal transfer significantly predicted participants’ learning performance increment in the transition session.In the transition session,those significant precuneus theta oscillation and theta oscillation signal transfer from the precuneus to the SFG disappeared while significant SFG theta oscillation appeared.These results indicated that,the precuneus theta oscillation and the precuneus-to-SFG theta oscillation signal transfer in the pretransition session,and the prefrontal theta oscillation in the transition session,were related to the transition from implicit to explicit sequence knowledge.In experiment 4,consistently,all participants were required to perform the ninesession SRT task.Before each SRT task session,r TMS and neural navigator were applied to modulate the theta oscillation in the cortical region（precuneus）where was the origination of theta oscillation signal transfer observed in experiment 3.The precuneus theta oscillation was intensified in one group of participants while suppressed in the other one,and the EEG data were meanwhile recorded.The results showed that,comparing to the r TMS-suppressive group,the theta oscillation in the precuneus was significantly strengthened in the r TMS-intensive group,as well as a stronger neural signal transfer from the precuneus to the SFG.Also,the transition from implicit to explicit sequence knowledge had happened to more participants in the r TMS-intensive group,in which RT difference between regular and random sequence stimuli increased abruptly at the transition session with a switchpoint of learning performance increment appeared.In addition,the increment of learning performance at transition session in the r TMS-intensive group was significantly larger than that in the r TMS-suppressive group.The present study can be drawn as follows: 1)in multi-session SRT task,after implicit sequence learning occurred,the transition from implicit to explicit sequence knowledge would be happened with learning more sessions.At the transition session pinpointed by session-by-session awareness measure,the RT difference of participants to regular sequence stimuli and random sequence stimuli increased abruptly,and learning performance increment exhibited a switchpoint which specifically emerged at the transition session.2)the present study revealed the process of brain activity in the transition of participants.When the transition emerged,there was an active prefrontal SFG theta oscillation which played a role in being aware of sequence regularity.Furthermore,the present study found that such activation of prefrontal theta oscillation was ignited by signal transfer of theta oscillation from precuneus toward SFG in the session previous to the emergence of transition.Specifically,the significant precuneus theta oscillation appeared in the pre-transition session along with a significant theta oscillation signal originated from precuneus transferring toward SFG,and both of the strength could predict the learning performance increment at transition session.Moreover,in the transition session,those significant theta oscillation and signal transfer both disappeared while active theta oscillation was observed in SFG.3)with utilizing r TMS to either intensify or suppress the strength of theta oscillation in cortical origination（precuneus）of theta oscillation signal transfer for two groups of participants,the strength of theta oscillation signal transfer from precuneus toward SFG was correspondingly changed,and both the incidence of implicit-to-explicit sequence knowledge transition and the increment of learning performance at the transition session showed corresponding changes as well.Therefore,the above theta oscillation and signal transfer had a causal link to the transition from implicit to explicit sequence knowledge.In conclusion,the present study clarified the behavioral characteristics of the transition from implicit to explicit sequence knowledge,that is,a switchpoint of sharply-increased learning performance,and revealed that the signal transfer of theta oscillation from precuneus to prefrontal cortex serves as a mechanism underpinning the transition of implicit to explicit sequence knowledge from the perspective of neural oscillation and oscillatory signal transfer.