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The Role of Visual Features in Text-Based CAPTCHAs: An fNIRS Study for Usable Security
Emre Mülazimoğlu1, Murat P Çakır2, Cengiz Acartürk1,2
1Cyber Security Department, Middle East Technical University, Ankara 06800, Turkey.
Computational Intelligence and Neuroscience
|May 31, 2021
Summary
This study used neuroimaging to analyze CAPTCHA designs, finding that visual distortions like occlusion and rotation impact brain activity in the prefrontal cortex. These findings offer insights into optimizing CAPTCHA usability and security.
Area of Science:
- Cognitive Neuroscience
- Human-Computer Interaction
- Information Security
Background:
- CAPTCHAs (Completely Automated Public Turing tests to tell Computers and Humans Apart) are crucial for distinguishing users from automated attacks.
- Existing usability studies primarily rely on accuracy and response time, potentially overlooking cognitive factors.
- Optical neuroimaging offers a novel approach to assess CAPTCHA design's cognitive load and user experience.
Purpose of the Study:
- To investigate the neural correlates of CAPTCHA perception using functional Near-Infrared Spectroscopy (fNIRS).
- To analyze how varying visual distortions in CAPTCHAs affect hemodynamic responses in the prefrontal cortex.
- To explore the relationship between visual complexity, Gestalt principles, and CAPTCHA usability.
Main Methods:
- Employed functional Near-Infrared Spectroscopy (fNIRS) to measure brain activity.
- Presented participants with CAPTCHA stimuli featuring different levels of line occlusion, rotation, and wave distortions.
- Collected behavioral data (accuracy, response time) alongside neuroimaging data.
Main Results:
- Significant hemodynamic responses were observed in the left and right dorsolateral and right dorsomedial prefrontal cortex.
- Increasing visual distortions (occlusion, rotation, wave effects) led to non-linear changes in brain oxygenation and behavior.
- Evidence suggests the emergence of Gestalt effects influencing the perception of CAPTCHA legibility.
Conclusions:
- fNIRS can effectively measure the cognitive load associated with different CAPTCHA designs.
- Prefrontal cortex activity is sensitive to specific visual manipulations in CAPTCHAs.
- Findings indicate that Gestalt principles play a role in how users perceive and process CAPTCHA security challenges, informing future design for better usability and robustness.
