在每个约 1.36 公斤、含约 860 亿个神经元与数兆突触的人脑中，非侵入式工具如 fMRI 与 EEG 只能呈现低解析度的群体活动。极少数情况下，癫痫手术患者接受脑内电极植入，使研究者能记录单一神经元的放电。此类研究揭示单细胞层级的反应可对外界刺激（如气味、计算、记忆任务）呈现高度特异性，提供无法以群体平均讯号推导的细部机制。

「概念神经元」研究显示，一个细胞可专门对特定概念放电，并可跨模态回应影像、文字或语音，同时一个细胞能编码数十个相关概念。2024 年研究进一步显示，嗅觉皮质神经元同时对实际气味与其相关影像或文字作出反应，支持嗅觉也使用概念式表征，而非仅依感受器驱动。这些结果皆依赖单细胞电极技术，而一般族群研究无法取得此解析度。

以相同方法，研究者发现特定神经元对小数量（1–4）呈现更精确的放电，并展现偏好数字的特征函数，而较大数字的编码精准度下降。续作显示大脑将「0」置于心理数线上，但以与其他数字不同的特殊方式表征，使其成为统计结构中的离群点。

In each roughly 3-pound human brain with about 86 billion neurons and trillions of synapses, noninvasive tools such as fMRI and EEG provide only coarse population-level activity. Rare opportunities arise when epilepsy patients receive implanted electrodes, allowing single-neuron recordings. These studies reveal highly specific responses to stimuli such as odors, arithmetic, and memory tasks, exposing mechanisms unobtainable from averaged signals.

Research on “concept neurons” shows that a single cell can respond selectively to a specific concept and across modalities, firing to an image, written name, or spoken cue, while each cell can encode dozens of related concepts. A 2024 study showed piriform cortex neurons respond both to actual odors and to pictures or words of those odors, supporting a concept-based representation of smell rather than purely receptor-driven processing. Such findings depend entirely on single-cell electrode access.

Using similar methods, researchers found neurons that prefer specific small numbers, with firing precision highest for 1–4 and decreasing for larger quantities. Follow-up work showed the brain places zero on the mental number line but represents it distinctly, making zero an outlier within the numerical code.