Behavioral and neural measures of confidence using a novel auditory pitch identification task

Observers can discriminate between correct versus incorrect perceptual decisions with feelings of confidence. The centro-parietal positivity build-up rate (CPP slope) has been suggested as a likely neural signature of accumulated evidence, which may guide both perceptual performance and confidence. However, CPP slope also covaries with reaction time, which also covaries with confidence in previous studies, and performance and confidence typically covary; thus, CPP slope may index signatures of perceptual performance rather than confidence per se. Moreover, perceptual metacognition – including neural correlates – has largely been studied in vision, with few exceptions. Thus, we lack understanding of domain-general neural signatures of perceptual metacognition outside vision. Here we designed a novel auditory pitch identification task and collected behavior with simultaneous 32-channel EEG in healthy adults. Participants saw two tone labels which varied in tonal distance on each trial (e.g., C vs D, C vs F), then heard a single auditory tone; they identified which label was correct and rated confidence. We found that pitch identification confidence varied with tonal distance, but performance, metacognitive sensitivity (trial-by-trial covariation of confidence with accuracy), and reaction time did not. Interestingly, however, while CPP slope covaried with performance and reaction time, it did not significantly covary with confidence. We interpret these results to mean that CPP slope is likely a signature of first-order perceptual processing and not confidence-specific signals or computations in auditory tasks. Our novel pitch identification task offers a valuable method to examine the neural correlates of auditory and domain-general perceptual confidence.

Methods
32-channel EEG data (Ag/AgCl) were recorded using a BrainVision ActiCap system (Brain Products GmbH, Munich, Germany) via the BrainVision Recorder software (Version 1.21.0303, Brain Products GmbH, Munich, Germany) at 500 Hz sampling frequency. All electrodes were mounted in an elastic cap according to the international 10/20 system (Oostenveld & Praamstra, 2001), and were grounded with a forehead ground and online referenced to Cz. Impedances were kept below 20 kΩ on average, and an antialiasing low-pass fi2lter of 245 Hz was applied via an ActiCHamp amplifier (Brain Products GmbH, Munich, Germany). Conductive gel was applied to maintain the contact between the electrodes and the scalp. Event and response triggers were synchronized with the recorded EEG signal via TTL signals sent through the VPixx DATAPixx system.

观察者可通过自信感受区分正确与错误的知觉决策。顶叶中央正电位累积速率（centro-parietal positivity build-up rate，CPP斜率）被认为是累积证据的潜在神经标志物，其可同时调控知觉表现与自信水平。然而，CPP斜率同样与反应时存在共变关系——既往研究中反应时亦与自信相关，且表现与自信通常亦存在共变；因此CPP斜率可能反映的是知觉表现的相关特征，而非自信本身。此外，知觉元认知（包括其神经关联）的研究大多集中于视觉领域，仅有少数例外。因此，我们对视觉以外领域中知觉元认知的领域通用神经标志物仍缺乏认知。
本研究设计了一种新颖的听觉音调识别任务，对健康成年人开展行为实验并同步采集32通道脑电图（electroencephalogram，EEG）数据。被试在每一试次中先看到两个音调标签，二者的音程距离各不相同（例如C调与D调、C调与F调），随后聆听一段单一声调；被试需判断哪个标签为正确匹配，并评定自身的自信程度。
我们发现，音调识别的自信程度随音程距离变化，但识别表现、元认知敏感性（即每一试次中自信与准确率的共变程度）以及反应时均未表现出类似变化。有趣的是，尽管CPP斜率与表现及反应时存在共变关系，但它并未与自信呈现显著共变。据此我们认为，在听觉任务中，CPP斜率大概率是一阶知觉加工的标志物，而非与自信相关的特异性信号或计算过程。本研究开发的新型音调识别任务，为探究听觉领域及领域通用的知觉自信的神经关联提供了极具价值的研究方法。

方法
本研究采用BrainVision ActiCap系统（德国慕尼黑Brain Products GmbH公司）配合BrainVision Recorder软件（版本1.21.0303，德国慕尼黑Brain Products GmbH公司）采集32通道银/氯化银（Ag/AgCl）脑电图数据，采样率为500 Hz。所有电极按照国际10/20系统（Oostenveld & Praamstra, 2001）安装于弹性帽中，以额区电极为接地参考，并在线参考至Cz电极。电极阻抗平均控制在20 kΩ以下，通过ActiCHamp放大器（德国慕尼黑Brain Products GmbH公司）施加245 Hz的抗混叠低通滤波。使用导电膏维持电极与头皮之间的接触良好。事件与反应触发信号通过VPixx DATAPixx系统发送的TTL信号与录制的脑电图信号实现同步。