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. )e 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.

翻译：为抵御字典攻击或类似针对信息系统的恶意自动化攻击，开发者多采用验证码挑战作为人机交互证明来区分人类用户与脚本程序。合理使用验证码需要在设计挑战时平衡其鲁棒性与可用性。既有研究表明，多数可用性研究将准确率与响应时间作为定量分析的衡量标准。本研究旨在运用光学神经成像技术分析验证码设计，采用功能性近红外光谱技术探索不同类型验证码刺激所诱发的前额叶皮层血氧动力学反应。研究发现，左右侧背外侧前额叶皮层与右侧背内侧前额叶皮层对验证码中线条遮挡程度、旋转角度和波浪形变产生响应。视觉效应的系统性添加对行为数据和前额叶血氧指标产生了非线性影响，表明格式塔效应的出现可能影响了整体验证码图形的感知过程。