A method of user logon to a computing device or computer system that, distinct from requiring entry of a set of known logon credentials such as a username and password, introduces an additional thought-directed user interface whereby the user must respond to one or more prompts that measure the user's cognitive function at the time of logon or during an active logon session. The user's responses to these prompts are evaluated for several purposes, including determining whether the user demonstrates the required level of cognitive function to gain access to the computer system or continue an active logon session. The user's responses and associated data may also be stored and retrieved at a later time for various purposes, including determining whether and to what extent the user's level of cognitive function is improving, diminishing, or remaining static over time.