But that's possible! Infants, pupils, and impossible events.

Infants' expectations of the world around them have been extensively assessed through the violation of expectation paradigm and related habituation tasks. Typically, in these tasks, longer looking to impossible events following familiarisation with possible equivalents is taken to reflect surprise at their occurrence, thus revealing infants' knowledge. In this study, the role of learning during the task itself is explored by switching the archetypal approach on its head and familiarising infants to impossible events. In a partial replication of Jackson and Sirois (2009), nine-month-old infants were presented with short video clips of toy trains moving around a circular track. A tunnel over a short section of the track meant trains were briefly occluded as they completed a circuit. In impossible versions of events, the train switched colours while occluded by the tunnel. Both looking times and pupil dilation were used as dependent measures. Using a factorial design in which perceptual (novelty-familiarity) and conceptual (possible-impossible) variables were independently and jointly analysed, we show that infants showed greater responding to possible events than to impossible events following familiarisation. Pupil dilation data successfully allowed for more precise interpretation of infants' perception of events than could have been achieved through looking times alone. These findings suggest a central role for learning in violation of expectation tasks, and also further support the use of pupil dilation as a dependent measure in infancy work.