1. A noninvasive method to determine the cerebral blood flow velocity response to face recognition tasks of a human subject, including steps of: (a) obtaining a subject's baseline cerebral blood flow velocity in cerebral arteries on both sides of the brain using a transcranial Doppler ultrasound instrument with two probes placed on the temples and sample volumes focused on cerebral vessels on both sides; (b) simultaneously with (a) obtaining the mean blood flow velocity in both pairs of cerebral arteries at baseline; (c) testing the subject with face recognition tasks on the screen of a digital computer and using a computer input peripheral device while simultaneously monitoring the mean blood flow velocity during each stage of the task in real-time; (d) determining the response of the brain using mean cerebral blood flow velocity to a particular imaginary or real face; (e) determining the response of the brain using side-to-side differences in mean cerebral blood flow velocity to a given target face; (f) determining the response of the brain using side-to-side differences in mean cerebral blood flow velocity to a given composite face; (g) determining the response of the brain using side-to-side differences in mean cerebral blood flow velocity to a given facial expression; (h) simultaneously with (g) determining if there is a match of the response of the brain using side-to-side differences in mean cerebral blood flow velocity to a given face compared to the response evoked by target face; and (i) simultaneously with (h) assessing if by varying the emotional expression of the given face the match in response to target face could be enhanced.