Past studies have found equivocal support for the ability of young infants to discriminate infant-directed (ID) speech information in the presence of auditory-only versus auditory + visual displays (faces + voices). Generally, younger infants appear to have more difficulty discriminating a change in the vocal properties of ID speech when they are accompanied by faces. Forty 4-month-old infants were tested using either an infant-controlled habituation procedure (Experiment 1) or a fixed-trial habituation procedure (Experiment 2). The prediction was that the infant-controlled habituation procedure would be a more sensitive measure of infant attention to complex displays. The results indicated that 4-month-old infants discriminated voice changes in dynamic face + voice displays depending on the order in which they were viewed during the infant-controlled habituation procedure. In contrast, no evidence of discrimination was found in the fixed-trial procedure. The findings suggest that the selection of experimental methodology plays a significant role in the empirical observations of infant perceptual abilities. (Contains 2 footnotes and 2 figures.)