Sex categorization is essential for mate choice and social interactions in many animal species. In humans, sex categorization is readily performed from the face. However, clear neural markers of face-sex categorization, i.e., common responses to widely variable individuals from one sex, have not been identified so far in humans. To isolate a direct signature of rapid and automatic face-sex categorization generalized across a wide range of variable exemplars, we recorded scalp electroencephalogram (EEG) from 32 participants (16 females) while they were exposed to variable natural face images from one sex alternating at a rapid rate of 6 Hz (i.e., 6 images per second). Images from the other sex were inserted every 6th stimulus (i.e., at a 1-Hz rate). A robust categorization response to both sex contrasts emerged at 1 Hz and harmonics in the EEG frequency spectrum over the occipito-temporal cortex of most participants. The response was larger for female faces presented among male faces than the reverse, suggesting that the two sex categories are not equally homogenous. This asymmetrical response pattern disappeared for upside-down faces, ruling out the contribution of low-level physical variability across images. Overall, these observations demonstrate that sex categorization occurs automatically after a single glance at natural face images and can be objectively isolated and quantified in the human brain within a few minutes.

中文翻译：

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快速而自动的面部性别分类的一种生态措施。

在许多动物物种中，性别分类对于伴侣选择和社交互动至关重要。在人类中，很容易从面部进行性别分类。然而，迄今为止，在人类中还没有明确的面部性别分类的神经标志物，即对来自一种性别的广泛变异的个体的共同反应。为了隔离在广泛的可变样本中快速，自动进行的面部性别分类的直接特征，我们记录了32名参与者（16名女性）的头皮脑电图（EEG），而他们却暴露于来自不同性别的可变自然面部图像6 Hz的快速速率（即每秒6张图像）。每第六次刺激（即以1 Hz的频率）插入来自另一性别的图像。在大多数参与者的枕颞皮质上，EEG频谱在1 Hz和谐波处均出现了针对两种性别对比的强大分类响应。在男性面孔中出现的女性面孔的反应大于相反的现象，表明这两个性别类别并非同质。这种不对称的响应模式在上下颠倒的脸部消失了，排除了图像之间低级物理变化的影响。总体而言，这些观察结果表明，一眼看到自然的面部图像，性别分类就会自动发生，并且可以在几分钟之内在人脑中进行客观地分离和量化。这表明这两个性别类别不是同质的。这种不对称的响应模式在上下颠倒的脸部消失了，排除了图像之间低级物理变化的影响。总体而言，这些观察结果表明，一眼看到自然的面部图像，性别分类就会自动发生，并且可以在几分钟之内在人脑中进行客观地分离和量化。这表明这两个性别类别不是同质的。这种不对称的响应模式在上下颠倒的脸部消失了，排除了图像之间低级物理变化的影响。总体而言，这些观察结果表明，一眼看到自然的面部图像，性别分类就会自动发生，并且可以在几分钟之内在人脑中进行客观地分离和量化。