To mitigate dictionary attacks or similar undesirable automated attacks to information systems, developers mostly prefer using CAPTCHA challenges as Human Interactive Proofs (HIPs) to distinguish between human users and scripts. Appropriate use of CAPTCHA requires a setup that balances between robustness and usability during the design of a challenge. The previous research reveals that most usability studies have used accuracy and response time as measurement criteria for quantitative analysis. The present study aims at applying optical neuroimaging techniques for the analysis of CAPTCHA design. The functional Near-Infrared Spectroscopy technique was used to explore the hemodynamic responses in the prefrontal cortex elicited by CAPTCHA stimulus of varying types. The findings suggest that regions in the left and right dorsolateral and right dorsomedial prefrontal cortex respond to the degrees of line occlusion, rotation, and wave distortions present in a CAPTCHA. The systematic addition of the visual effects introduced nonlinear effects on the behavioral and prefrontal oxygenation measures, indicative of the emergence of Gestalt effects that might have influenced the perception of the overall CAPTCHA figure.

中文翻译：

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视觉功能在基于文本的验证码中的作用：针对可用安全性的fNIRS研究

为了减轻字典攻击或类似的对信息系统的不希望的自动攻击，开发人员通常更喜欢使用CAPTCHA挑战作为人类交互证明（HIP）来区分人类用户和脚本。正确使用CAPTCHA要求在挑战设计期间在健壮性和可用性之间取得平衡的设置。先前的研究表明，大多数可用性研究都使用准确性和响应时间作为定量分析的测量标准。本研究旨在应用光学神经成像技术对CAPTCHA设计进行分析。使用功能性近红外光谱技术探讨了由各种类型的CAPTCHA刺激引起的前额叶皮层的血流动力学响应。这些发现表明，左，右背外侧和右背核前额叶皮层中的区域对验证码中存在的线遮挡，旋转和波浪变形的程度做出响应。视觉效果的系统添加在行为和前额氧合措施上引入了非线性效应，表明格式塔效应的出现可能影响了对CAPTCHA数字的感知。