The Neural Mechanism Of Statistical Summary Representation Of Facial Expressions

Owing to the limited attentional resources and capacity of working memory,the visual system must refine input visual stimuli through efficient encoding.Hence,the visual system tends to process features in the set as a whole,extracting statistical information of the set.This representation of the statistical properties of the set is defined as "statistical summary representation".An increasing number of behavioral studies investigated processing dimensions,processing strategies and processing capacity of statistical summary representation of facial expressions.But the study in exploring the neural mechanism of statistical summary representation is still in its infancy.Recently,a small quantity of researchs comparing differences in neural activities between ensemble and individual conditions to explore the neural mechanism of statistical summary representation.But this cannot rule out quantitative differences in stimuli that contribute to neural activity differences.Specifically,differences in neural activity between individual and ensemble condition may included neural activity elicited by the mental process of "statistical summary representation",may also included differences in the number of stimuli that interfere with neural activity.Hence,the present study employ adaptation paradigm to address this issue.The term “Neural adaptation” refers to the phenomenon in which the neural response to the stimuli is reduced when the same stimuli is repeated or prolonged.After adapting to the same stimuli,the more consistent the test stimuli is with the emotional intensity of the adaptive stimuli,the greater neural adaptation effect.Many previous studies used adaptation as a tool,focused on the change in responses caused by adaptation stimuli to reveal the processes of encoding stimuli properties.The present study consists of two experiments,Experiment Ⅰ is a behavior experiment,which demonstrate that participants could represent statistical summary information of facial expressions that are presented for a long time.Experiment Ⅱemploy adaptation paradigm combine with electroencephalogram(EEG)to examine the neural mechanism of statistical summary representation of facial expressions,If neural adaptation could be observed,suggesting neurons represent facial expressions of adaptation phase as an average emotional face.Then the brain activity related to adaptation(repetition suppression)is the neural mechanism of statistical summary representation of facial expressions.ExperimentⅡ is an one-factor within-subjects design in which four faces of varying emotional intensity are presented in adaptation phase and a single face is presented in test phase.There are three types of the test stimuli,namely mean condition(mean emotional face of the previously adapted set),member condition(a member of the previously adapted set),other condition(a member from other adapted set).First,time domain results showed there is a negative deflection at the N250 time window,and there are significant differences among conditions.Specifically,the mean condition elicit lower neural responses than other/member conditions in the time window of 250～360 ms over occipital–temporal brain areas,it revealed that neural adaptation occured when the mean emotional face was presented.This indicated that participants represent statistical summary information of facial expressions,extracting the mean emotional face when presented adaptation stimuli.Thus,neural response decreased when the mean emotional face was presented again.In addition,neural adaptation of the mean condition is greater than the member condition.According to reversed hierarchical theory and predictive coding models,the conscious perception begins at the hierarch’s top,following a reverse hierarchical course.higher areas that represent global features "explains" activity in lower areas which represent local features via feedback projections.In the case of our study,we propose that when the adaptation facial expressions were presented as an mean emotional face in higher areas,this “expectation” or ‘‘understanding’’ of the mean emotional face is sent back to lower visual areas that represent members of the set,resulting in suppression of the represent of members.The mean condition significantly contributed to theta band enhancement.Since theta band is associated with psychological integration,theta oscillations under the integration condition are greater than the non-integration condition.We think maybe the mental process of statistical summary representation involves psychological integration.Specifically,related neurons that representing statistical summary information integrates various neural signals that representing emotional set of faces to form an average representation.Second,time-frequency analysis revealed that alpha and beta ERD of 10～20Hz in the time window of 200～400ms for the mean condition in the test phase were smaller than the other condition.Functionally,alpha/beta band ERD reflecting the degree of cortical activation,lower ERD signal intensity indicates lower cortical activation.It suggest that the brain is less active than the other condition when presenting the mean condition,that is,neural adaptation occurs.Finally,the mean condition significantly contributed to theta band enhancement.Since theta band is associated with psychological integration,theta oscillations under the integration condition are greater than the non-integration condition.We think maybe the mental process of statistical summary representation involves psychological integration.Specifically,related neurons that representing statistical summary information integrates various neural signals that representing emotional set of faces to form an average representation.Together,the present study suggests that N250 may be the neural indicator of statistical summary representation of facial expressions.In addition,when the adaptation facial expressions were presented as a mean face in higher areas,this“expectation” or ‘‘understanding’’ of the mean emotional face is sent back to lower visual areas that represent members of the set,resulting in suppression of the represent of members.