The output activity from the brain ranges from overt motor responses, like the grasping of an item, to very subtle autonomic responses like sweating or heart rate increases. When it comes to consumer behavior, the most relevant are eye-movements, facial muscle responses, GSR, heart rate and breathing.
Whenever we consider something of importance, we move our eyes to put the item in the center of our visual field, where vision is sharpest and most sophisticated. We do this about three times per second, often without consciously knowing it. Eye tracking tells us directly what draws our attention, and whether things get noticed.
When emotions are strong, they may become visible on the face; we smile, frown, or show an expression of fear. With a camera, these expressions can be captured and analyzed. The technique offers the possibility to grasp consciously expressed emotions that may change too quickly to all verbalize at the same time. It is questionable to what extent unconscious emotions can be captured. We don’t use it (image from MobiLab at IVP)
When we are emotionally aroused (e.g. frightened or thrilled), the body prepares by several autonomic responses, such as an increase in breathing and heart rate, and an increase in skin conductance (GSR). These can be picked up using standard medical recording equipment. While of great value in measuring emotional arousal, the valence of the emotions (whether they are positive or negative) cannot be established.