Determining human stimuli at computing devices

Techniques for granting access to an application or service based on a detected human stimulus at a computing device include detecting a stimulus that is generated by an external entity and independent of a CAPTCHA or CAPTCHA-like challenge. If the stimulus is included in a set of human stimuli, access to the application or service may be granted. Otherwise, access may be denied. The detected stimulus may be ad-hoc, or may be a response to an explicit challenge other than a CAPTCHA or CAPTCHA-like challenge. A background application may continuously test over time for ad-hoc stimuli that are consistent with a human presence, and may maintain or deny access accordingly. The detected stimulus may include changes in states of components of the computing device that are related to spatial orientation and/or location. Access to the application or service may be granted/denied based on the detected stimulus and an additional criteria.