1. A method for determining a subject's response and/or interest toward an object, surface, or visual material, comprising: using a hardware sensor coupled to the object, surface, or visual material, the hardware sensor sensing at least one index of the subject's attention specifically toward the hardware sensor together with the object, surface, or visual material; and using a processor to output a result indicating the subject's response and/or interest in the object, surface, or visual material based on the sensed at least one index of the subject's attention; wherein sensing does not elicit a change in the at least one index of the subject's attention; wherein the at least one index is eye contact or head and/or face orientation; wherein sensing includes: (i) obtaining alternating odd and even video frames of at least one of the subjects' pupils including determining whether a frame was illuminated by on-axis or off-axis illumination and whether a pixel is classified as non-zero or zero; (ii) subtracting the alternating odd and even video frames to produce a subtracted frame; (iii) detecting at least one pupil and a glint in the subtracted frame; and (iv) determining alignment of the at least one pupil and the glint; wherein alignment of the at least one pupil and the glint confirms eye contact of the subject and is related to the subject's response and/or interest toward the object, surface, or visual material.