Abstract
Metacontrast masking is a type of backward masking, in which target perception is impaired when the target is followed by a mask which does not overlap the target. Although simpler types of backward masking in which the mask overlaps the target are shown to occur in infants, it is as yet unknown whether metacontrast masking occurs in infants. In this study, we investigated whether metacontrast masking occurs in 3–8-month-old infants. We measured shape discrimination using a habituation method, and tested if the discrimination was disturbed by masking. In the habituation phase, one of the two shape stimuli (horizontal and vertical ellipses) was presented for 250 ms repeatedly until infants habituated to it. In the test phase, the familiar trial (the ellipse stimulus used in the habituation was presented) and novel trial (the other ellipse stimulus was presented) were presented alternately, and infants’ looking time was measured. If infants could discriminate the shapes, looking time would be longer in the novel trial (novelty preference). The two SOA conditions were tested. In the long SOA condition, a mask was presented for 100 ms after the shape stimulus with 450-ms ISI (700-ms SOA). In the short SOA condition, the mask was presented after the shape stimulus with 0-ms ISI (250-ms SOA). We found that, in 7–8-month-old infants, novelty preference was observed in the long SOA condition but not in the short SOA condition. This suggests that metacontrast masking occurs in 7–8-month-old infants. However, in 3–6-month-old infants, novelty preference was observed not only in the long SOA condition but also in the short SOA condition. The results suggest that metacontrast masking does not occur in 3–6-month-old infants, and furthermore, that they are able to perceive a stimulus that older infants cannot because of masking.