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J Neural Eng. 2020 Mar 5;17(2):022001. doi: 10.1088/1741-2552/ab702e.
The N400 is an event related potential that is evoked in response to conceptually meaningful stimuli. It is for instance more negative in response to incongruent than congruent words in a sentence, and more negative for unrelated than related words following a prime word. This sensitivity to semantic content of a stimulus in relation to the mental context of an individual makes it a signal of interest for Brain Computer Interfaces. A complicating aspect is the number of factors that can affect the N400 amplitude. In this paper, we provide an accessible overview of this range of N400 effects, and survey the three main BCI application areas that currently exploit the N400: (1) exploiting the semantic processing of faces to enhance matrix speller performance, (2) detecting language processing in patients with Disorders of Consciousness, and (3) using semantic stimuli to probe what is on a user's mind. Drawing on studies from these application areas, we illustrate that the N400 can successfully be exploited for BCI purposes, but that the signal-to-noise ratio is a limiting factor, with signal strength also varying strongly across subjects. Furthermore, we put findings in context of the general N400 literature, noting open questions and identifying opportunities for further research.
N400 是一种事件相关电位，它是对概念上有意义的刺激做出的反应。例如，在句子中，与一致的词相比，不一致的词会产生更负的 N400；与相关的词相比，在一个启动词之后，不相关的词会产生更负的 N400。这种对刺激的语义内容与个体心理背景之间关系的敏感性，使其成为脑机接口感兴趣的信号。一个复杂的方面是，有许多因素可以影响 N400 幅度。在本文中，我们提供了对这一系列 N400 效应的概述，并调查了目前利用 N400 的三个主要 BCI 应用领域：（1）利用面部的语义处理来增强矩阵拼写器的性能，（2）检测意识障碍患者的语言处理，（3）使用语义刺激来探测用户的想法。通过对这些应用领域的研究，我们表明 N400 可以成功地用于 BCI 目的，但信噪比是一个限制因素，信号强度也在很大程度上因受试者而异。此外，我们将研究结果置于一般 N400 文献的背景下，注意到存在的问题，并确定了进一步研究的机会。
