Categorical perception along the happy–angry and happy–sad continua in the first year of life

The current study investigated 6-, 9- and 12-month old infants' ability to categorically perceive facial emotional expressions depicting faces from two continua: happy-sad and happy-angry. In a between-subject design, infants were tested on their ability to discriminate faces that were between-category (across the category boundary) or within-category (within emotion category). Results suggest that 9- and 12 month-olds can discriminate between but not within categories, for the happy-angry continuum. Infants could not discriminate between cross-boundary facial expressions in the happy-sad continuum at any age. We suggest a functional account; categorical perception may develop in conjunction with the emotion's relevance to the infant.