Technological devices are occasionally useful in marketing research. Galvanometers can measure the interest or emotions aroused by exposure to a specific ad or picture. The tachistoscope flashes an ad to a subject with an exposure interval that may range from less than one hundredth of a second to several seconds. After each exposure, the respondent describes everything he recalls. Eye cameras study respondents' eye movements to see where their eyes land first, how long they linger on a given item, and so on