Involuntary Consciousness During Implicit Memory Retrieval:An Event-Related Potential Study

According to the classical definition of cognitive science,memory is divided into explicit memory (declarative memory) and implicit memory (non-declarative memory).A memory is implicit if has nothing to do with consciousness during the information retrieval from storage,or is otherwise explicit.Under this notion,the implicit memory refers to unconscious memory abilities such as skills (e.g.,riding a bicycle adeptly or playing a musical instrument adeptly).While explicit memory refers to the conscious recall of specific facts and episodes.Yet,prior evidence from behavioral or neuroimaging studies of explicit memory based on patients with brain injuries suggest that consciousness seems to be a weak criterion for distinguishing between implicit and explicit memory.Thus,a new model proposes that memory types should be distinguished by the processing characteristics during the memory retrieval rather than by consciousness.Specifically,it is worth noting that previous evidence for differentiating memory types largely comes from studies of explicit memory conducted on patients with brain injuries,such as subjective behavioral studies or neuroimaing studies at high spatial resolution.However,to date,research on healthy subjects at high temporal dimension during implicit memory retrieval is limited.Thus,the purpose of this thesis is to examine whether implicit memory retrieval is accompanied by consciousness based on the above research background.The present study contained two parts,which were electroencephalography(EEG) experiment and theoretical modeling analysis.(1) EEG experiment:we used two types of Shepard tone pairs with different pitch intervals as auditory materials and recruited healthy human subjects who had undergone long-term musical learning as experimental group (Amateur musicians) and subjects who had not received long-term musical learning as control group (Non-musicians).We designed a passive oddball paradigm for the retrieval of implicit memory in which an auditory stream of Shepard tones with musical pitch interval contrasts were delivered to the subjects,and whole-head recordings of brain activities in response to different Shepard tone pairs were collected from two groups.Then,we let subjects know the rules of the auditory stream and measured behavioral responses to identification of pitch interval in amateur musicians and non-musicians after EEG recording.(2) Theoretical analysis:the dipole source analysis of ERP components was made by software of brain electrical source analysis (BESA) based on the principle of equivalent current dipole(ECD),volume conduction and linear superposition.In addition,effective functional connectivity was calculated utilizing dynamic casual modeling (DCM) based on the free-energy principle of Bayesian brain prediction,and then the optimal model was selected by the Bayesian model selection (BMS) for calculating the effective functional connectivity between the brain regions associated with auditory cognition.Shepard tones with musical pitch interval contrasts evoked a mismatch negativity response (MMN),a neural marker of implicit memory,which reflects the automatic deviance detection at early auditory stage,in the subjects with long-term musical training,but not in the subjects without.Therefore,we can confirm that amateur musicians induced implicit memory successfully.Notably,such MMN response was followed by a salient P3 component,and P3 is generally regarded as a key feature of conscious perception,which suggests that the implicit memory retrieval of amateur musicians is accompanied by consciousness.Then,the dipole source analysis of MMN and P3 revealed that bilateral temporal generators accounting for MMN response,and dipoles of P3 component moved from the frontal lobe to the insular area.Moreover,according to the model selection of DCM and the calculation of the effective connectivity within brain regions,significant effective connectivity of the backward connection from the frontal lobe to the temporal lobe of the right hemisphere was found in amateur musicians.Additionally,correct ratio of the amateur musician’s responses to the pitch interval recognition was significantly higher than that of the non-musicians in the behavioral test,and amateur musicians’behavioral test accuracy was significant and positive correlated with the amplitudes of MMN and P3 evoked by amateur musicians.To sum up,this study demonstrates ERP evidence for involuntary consciousness during implicit memory retrieval and suggests that implicit and explicit memories may not necessarily have to be clearly differentiated by whether consciousness is involved.On the one hand,ERP results (MMN) propose that long-term musical learning induces an adult’s ability of automatic pitch interval processing,and such capacity can be integrated into the automatic detection of implicit memory.Source analysis of the P3 revealed moving dipoles from the frontal lobe to the insular,a brain region closely related to conscious attention.Thus,such results propose involvement of involuntary consciousness during the implicit memory retrieval and present a case of involvement of involuntary consciousness in the implicit memory retrieval,as well as suggest a potential challenge to the classical definition of implicit memory.On the other hand,measures of effective connectivity showed distinguishable backward connectivity from the right frontal lobe to the ipsilateral temporal lobe in amateur musicians,which demonstrates that top-down processing is involved in the implicit memory retrieval of musical pitch intervals.These findings are consistent with those indicating the involvement of top-down processing in the retrieval of explicit memory,which indicates that implicit memory and explicit memory may share an analogous underlying mechanism.In conclusion,results suggest that implicit and explicit memories may not necessarily have to be clearly differentiated by whether consciousness is involved and that aspect of memory processing such as top-down process,might be considered as an effective factor in defining types of memory.Additionally,the current study proposes that long-term musical learning can induce the changes of the plasticity in the central nervous systems including the central auditory system,and top-down signal may be associated with memory retrieval and memory consolidation during auditory cognitive processing.Moreover,this study may provide a potential mechanism for our understanding of learning and memory in humans.Results of the present study further indicate that for amateur musicians,due to the accumulated memory reserves from long-term musical learning experience,their brains can distinguish the tone pairs with different pitch intervals related to musical elements even without the involvement of consciousness.Thus,the prediction of the auditory stimulus will be more precise and detailed,which will result in automatic change detection when the auditory stimulus is not consistent with that of prediction.However,when it comes to non-musicians,non-musicians did not have experiences of long-term musical learning,and their brains did not contain music-related memory reserves.Therefore,their prediction of the auditory stimulus is not as precise as that of amateur musicians.Taken together,all these results support the theory of Bayesian brain prediction based on the free energy principle.