Methods for authenticating a genuine presence of a human involve directing one or more modulated probes towards a body part of the human, receiving a response to the probes from the body part, and analyzing the response to determine whether it contains spectral characteristics that match a class of responses to such probes for the human body part in a human population. Replay attacks are countered by varying the modulation of the probe temporally, spatially, and spectrally each time authentication is performed. The probes may include electromagnetic radiation, acoustic beams, or particle beams that generate a detected reflection, absorption pattern, scintillation, or fluorescence response of the body part. The analysis of the response may be directed to one or more of temporal, spatial, and spectral variations in accordance with the nature of the probes and the modulation.