OBJECTIVE:
This study examined the impact of spectral resolution on the processing of lexical tones and the number of frequency channels required for a cochlear implant (CI) to transmit Chinese tonal information to the brain.
METHODS:
ERPs were recorded in an auditory oddball task. Normal-hearing participants listened to speech sounds of two tones and their CI simulations in 1, 4, 8, or 32 channels. The mismatch response elicited by speech sounds and CI simulations in different numbers of channels were compared.
RESULTS:
The mismatch negativity (MMN) was observed for speech sounds. For the 1-channel CI simulations, deviants elicited a more positive waveform than standard stimuli. No MMN response was observed with the 4-channel simulations. A reliable MMN response was observed for the 8- and 32-channel simulations. The MMN responses elicited by the 8- and 32-channel simulations were equivalent in magnitudes and smaller than that elicited by speech sounds.
CONCLUSIONS:
More than eight frequency channels are required for a CI to transmit Chinese tonal information. The presence of both positive and negative mismatch responses suggests multiple mechanisms underlying auditory mismatch responses.
SIGNIFICANCE:
The current findings of spectral resolution constraints on the transmission of tonal information should be taken into account in the design of the CI devices. 目標：本研究探討頻譜解析度（spectral resolution）對於字詞聲調處理的影響，以及人工電子耳需要多少頻道數方能將中文聲調訊息傳遞至大腦加以處理。
方法：本研究於聽覺新異刺激作業（auditory oddball task）中記錄時間相關腦電位（event-relatd potentials，ERPs）。具正常聽力的實驗參與者被要求注意觀看無聲影片，同一時間實驗者以中文語音“yi”的一聲及四聲分別作為新異刺激作業的標準（standard）刺激以及離異（deviant）刺激，播放這兩個刺激的語音及其具有1，4，8，及32頻道的人工電子耳模擬聲音（cochlear implant simulations），藉以比較語音以及不同頻道數人工電子耳模擬聲所引發的不匹配反應（mismatch response）。
結果：當以語音為實驗刺激時，作為離異刺激的第四聲相對於作為標準刺激的第一聲如原先預期，引發顯著的不匹配負波（mismatch negativity，MMN）。當以一個頻道的人工電子耳模擬音為刺激時，離異刺激相對於標準刺激引發較為正向的不匹配反應；以四個頻道的模擬音為實驗刺激時沒有觀察到任何不匹配反應；以八個以及卅二個頻道的模擬音為刺激則引發相同大小的不匹配負波，但其振幅皆比以語音為刺激時所引發的不匹配負波小。
結論：人工電子耳的頻譜解析度至少需要達到八個頻道才能有效傳遞中文聲調訊息。本實驗中觀察不匹配反應可以是正的亦可是負的，顯現聽覺不匹配反應有多重機制。
重要性：對於人工電子耳的設計應考慮將本研究所發現關於頻譜解析度對於聲調訊息傳遞的影響。
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