The Inaccessibility of CAPTCHA: Alternatives to Visual Turing Tests on the Web
http://www.w3.org/TR/turingtest/

W3C
2005

Abstract:

A common method of limiting access to services made available over the Web is visual verification of a bitmapped image. This presents a major problem to users who are blind, have low vision, or have a learning disability such as dyslexia. This document examines a number of potential solutions that allow systems to test for human users while preserving access by users with disabilities.