Captcha stands for Completely Automated Public Turing test to Tell Computers and Humans Apart, a standard security mechanism in every web based applications used to differentiate humans and bots. Various Captcha security methods such as text, image, video and audio based methods have emerged over the years. All the traditional Captcha methods are frequently suffered by OCR attacks, laundry, brute force, pixel count and other automated bot attacks. Humans smash computers and robots in cognitive activities. In this way cognition based Captcha security methods becomes popular and replaces the traditional Captcha methods in web applications. This paper analyzes the various cognition based Captcha security methods employed in different web applications and their merits and demerits.