What ability is measured in “infant psychophysics”?

The magazine American Scientist recently celebrated Davida Teller’s “forced-choice preferential looking,” a psychophysical method that yields a detection/discrimination function for human infant vision. The method was soon adopted and modified for auditory detection/discrimination. Eventually, sufficient data accumulated for a retrospective (“Human auditory development,” L. Werner et al.). In the auditory trials, an assistant holds an infant exposed to auditory stimuli, and a hidden observer records the infant’s behavior, from which an analyst constructs a psychometric function or other quantifier of detection/discrimination. But whose detection/discrimination is quantified, exactly? Consider the roles of the laboratory personnel. The analyst (a receptor) interprets the data (stimulus) supplied by the hidden observer, who is an interpreter (receptor) of the behavior (stimulus) of the infant/infant-holder duo, who constitute a probe (receptor) of the acoustical environment. Altogether, the detection/discri...