  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.
