Cross-Frequency Coupling Mechanism Of Lexical Cohort Activation In Speech Production

Oral language is the most important means of communication in people’s daily lives.The cognitive process of expressing thoughts through oral language is called speech production,which involves a crucial step of selecting appropriate vocabulary to convey meaning,known as the lexical selection stage.Mainstream models of speech production suggest that,during the process of lexical selection,not only the target word but also semantically related words are activated in parallel through semantic network,competing with each other,forming a "semantic cohort".However,previous studies did not examine how neural activity supports the activation of semantic cohorts.Recent research suggests that multi-item parallel processing is achieved through cross-frequency coupling(CFC)between theta and gamma neural oscillations,where the processing of each item is represented by gamma oscillations occurring at different phases of theta cycles;each time the theta oscillation repeats,the items represented by gamma oscillations are re-activated in turn.This mechanism of gamma nested within theta(i.e.,CFC)allows for the parallel activation of multiple items.Based on this,the present study hypothesizes that theta-gamma coupling is also the neural mechanism underlying the activation of semantic cohorts,and two experiments were conducted to test this hypothesis.Experiment 1 employed the classical category naming paradigm and EEG technology to investigate the relationship between semantic cohort activation and cross-frequency coupling(CFC)between theta and gamma.The experiment used a single-factor two-level(category type: homogeneous vs.heterogeneous)design,where participants were required to repeatedly name a set of pictures in a block.In the homogeneous block,the pictures were semantically related(belonging to the same category or theme,e.g.,apple,banana,peach,watermelon,cherry,etc.),while in the heterogeneous block,the pictures had no semantic connection.The pictures in the homogeneous and heterogeneous blocks were exactly the same,but the grouping method was different.Behavioral results showed that compared to the heterogeneous condition,participants had significantly longer naming latencies in the homogeneous condition,indicating the classic semantic interference effect.This classic effect is explained by semantic competition among naming items within a semantically related cohort,which mutually activate each other through the semantic conceptual network.The EEG results indicated that both ERP show that category type modulated the ERP.More importantly,we found that,in the left frontal scalp(near Broca’s area),the thetagamma coupling strength was significantly higher in the homogeneous condition than in the heterogeneous condition.This result suggests that theta-gamma coupling may support the parallel activation of semantic cohorts during speech production.Based on the results of Experiment 1,Experiment 2 further investigated the causal relationship between theta-gamma coupling and parallel activation of semantic cohorts.According to the mechanism of theta-gamma coupling,the lower the frequency(i.e.,longer oscillation cycle)of the theta involved in the coupling,the more gamma oscillations are nested,resulting in more items maintained in parallel activation.If the parallel activation of semantic cohorts is indeed achieved through theta-gamma coupling,then lower-frequency theta oscillations can maintain more semantic competition items and lead to a larger semantic competition effect.We used transcranial alternating current stimulation(t ACS)technology to manipulate the frequency of theta involved in theta-gamma coupling by applying electrical stimulation at different frequencies within the theta band to the Broca’s area.Experiment 2 also used the category naming paradigm,with a 2(category type:homogeneous vs.heterogeneous)× 3(stimulation type: 4 Hz,7 Hz,sham)experimental design.As expected,we observed that theta frequency modulated the semantic interference effect: the classic semantic interference effect was found in the sham condition,significantly enhanced in the 4 Hz condition,and disappeared in the 7Hz condition.This result causally confirms that theta-gamma coupling is the neural mechanism underlying the parallel activation of semantic cohorts.In summary,this study has revealed the cross-frequency coupling mechanism of semantic cohort activation in speech production from both correlation and causal perspectives.The results indicate that the parallel activation process of semantic cohorts,as multiple items,is achieved through theta-gamma coupling during speech production.These findings have potential implications for clinical interventions and treatments of naming disorders.