Deciphering CAPTCHAs: What a Turing Test Reveals about Human Cognition


  • Hannagan Thomas
  • Ktori Maria
  • Chanceaux Myriam
  • Grainger Jonathan

document type



Turning Turing's logic on its head, we used widespread letter-based Turing Tests found on the internet CAPTCHAs) to shed light on human cognition. We examined the basis of the human ability to solve CAPTCHAs, where machines fail. We asked whether this is due to our use of slow-acting inferential processes that would not be available to machines, or whether fastacting automatic orthographic processing in humans has superior robustness to shape variations. A masked priming lexical decision experiment revealed efficient processing of CAPTCHA words in conditions that rule out the use of slow inferential processing. This shows that the human superiority in solving CAPTCHAs builds on a high degree of invariance to location and continuous transforms, which is achieved during the very early stages of visual word recognition in skilled readers.

more information