It's probable that the psychological effect of having the light on is distorting how they perceive the insects hitting. I think the only scientific way to deal with this would be to make a bug spraying machine that can spray bugs at someone's face in different patterns. It may take a little time to find the right pattern, but when the subject thinks it matches what they experience with the headlamp they could say "Yes! Yes! That's the one!" (or they could just put their thumbs up and smile if they prefer not to open their mouth)