The Study Of Long-term Memory Traces For Words In Children With Cochlear Implants

Objective Language experiences can develop long-term memory traces for speech units in the brain,which ensures efficient processing of speech sounds.Prelingually deafened children lack sufficient auditory input before cochlear implantation and probably miss the critical period of development of long-term memory traces.However,whether the experience-dependent long-term memory traces exist in the brain of prelingually deafened children with cochlear implants(CIs)remains unclear.Methods We recruited 20 prelingually deafened CIs and 20 normal hearing(NH)children,and selected 6 disyllables([zi4 ji4]/handwriting,[zi4 ji3]/myself,[nian2 ji4]/age,[nian2 ji2]/grade,[zi4 ji2] and [nian2 ji3])which were comprised of 5 monosyllabic stimuli([zi4],[nian2],[ji2],[ji3] and [ji4]).Two Mandarin Chinese disyllables([zi4 ji4] and [nian2 ji4])were standard stimuli and four disyllables([zi4 ji2],[zi4 ji3],[nian2 ji3] and [nian2 ji2])were deviant stimuli in two blocks.We presented the participants with Mandarin Chinese spoken disyllables in an oddball paradigm and recorded event-related potentials.Each disyllable was defined as a meaningful word or a meaningless pseudoword by tonal information of the second syllable.We recorded the event-related potentials(ERPs)in response to auditory stimuli and examined possible differences of MMN responses between words and pseudowords in each group.Auditory and speech abilities of CI children were evaluated by Categories of Auditory Performance(CAP),Speech Intelligibility Rate(SIR)and Meaningful Auditory Integration Scale(MAIS).After the EEG experiment,we performed two behavioral tests to assess children's abilities of word recognition and speech comprehension.Results The amplitude of the mismatch negativity(MMN)elicited by words was larger than that elicited by pseudowords in NH children.However,such enhancement was not found in CI children.Behavioral results showed longer recognition points for words and lower accuracies for speech comprehension in CI children than those in NH children.Furthermore,increased theta power for words compared with pseudowords was only found in NH children.Conclusion In the present study,words evoked higher MMN amplitude than pseudowords did in NH children.However,this enhancement of brain responses was not found in CI children,indicating deficits of long-term memory traces for words.CI children also shows lower efficient speech processing revealed by longer MMN peak latencies and later recognition points compared with NH children.Increased theta power for words than that for pseudowords only in NH children,but not in CI children.Furthermore,theta oscillations are involved in activation of long-term memory traces for speech sounds.Our results suggest that early speech input may be crucial to formation of long-term memory traces for speech units.