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.

Afterimage induced neural activity during emotional face perception.

The N170 response differs when positive versus negative facial expressions are viewed. This neural response could be associated with the perception of emotions, or some feature of the stimulus. We used an aftereffect paradigm to clarify. Consistent with previous reports of emotional aftereffects, a neutral face was more likely to be described as happy following a sad face adaptation, and more likely to be described as sad following a happy face adaptation. In addition, similar to previous observations with actual emotional faces, we found differences in the latency of the N170 elicited by the neutral face following sad versus happy face adaptation, demonstrating that the emotion-specific effect on the N170 emerges even when emotion expressions are perceptually different but physically identical. The re-entry of emotional information from other brain regions may be driving the emotional aftereffects and the N170 latency differences.

Context-dependent categorical perception of surprise.

Evidence regarding the categorical perception of surprise facial expressions has been equivocal. Surprise is inherently ambiguous with respect to valence: it could be positive or negative. If this ambiguity interferes with categorical perception, disambiguating the valence might facilitate categorical perception. Participants identified and discriminated images that were selected from expression continua: happy-fear, surprise-fear, happy-surprise. Half were presented with a context for the surprise expressions indicating positive or negative valence. Both groups had a typical identification curve, but discrimination performance was better predicted by identification in the context condition for happy-fear and surprise-fear continua, suggesting that categorical perception was facilitated by the disambiguating context.

Categorical perception of emotional facial expressions in preschoolers.

Adults perceive emotional facial expressions categorically. In this study, we explored categorical perception in 3.5-year-olds by creating a morphed continuum of emotional faces and tested preschoolers' discrimination and identification of them. In the discrimination task, participants indicated whether two examples from the continuum "felt the same" or "felt different." In the identification task, images were presented individually and participants were asked to label the emotion displayed on the face (e.g., "Does she look happy or sad?"). Results suggest that 3.5-year-olds have the same category boundary as adults. They were more likely to report that the image pairs felt "different" at the image pair that crossed the category boundary. These results suggest that 3.5-year-olds perceive happy and sad emotional facial expressions categorically as adults do. Categorizing emotional expressions is advantageous for children if it allows them to use social information faster and more efficiently.

Mapping emotion category boundaries using a visual expectation paradigm.

Past research showing categorical perception of emotional facial expressions has relied on identification and discrimination tasks that require an explicit response via keypress. Here we report a new paradigm for investigating the category boundary of emotional facial expressions that, instead, relies on an implicit response--eye direction. Participants were trained to expect a target stimulus on a particular side of the monitor, predicted by an emotional expression on a face image. An eye-tracker then recorded eye movements of participants as they viewed novel intermediate facial-expression stimuli. Anticipatory eye movement was taken as evidence of categorisation. Results from two experiments suggest that this implicit method can be used to determine category boundaries, and that the boundaries found with this method are similar to those found with the keypress response.