In the study of perceptual abilities of infants, a number of techniques are used to determine infants'
responses to various stimuli. Because they cannot verbalize or fill out
questionnaires, indirect techniques of naturalistic observation are used as the
primary means of determining what infants can see, hear, feel, and so forth. Each of these methods compares
an infant's state prior to the introduction of a stimulus with its state during
or immediately following the stimulus. The difference between the two
measures provides the researcher with an indication of the level and duration
of the response to the stimulus.
问题是the difference between the two measures 的two measures 是哪俩个measures