Facial expressions are vital for communication and are thought to be recognsied universally, across many cultures. There are two leading, opposing models that attempt to explain how the perception of facial expressions is achieved. The dimensional model assumes that expressions are perceived along general dimensions of emotion, according to characteristics that are common to all emotions, such as valence. In contrast, the categorical model suggests that facial expressions are perceived according to predefined expression classifiers, which are specific to discrete emotion categories. Therefore, it is unclear whether the perception of facial expressions is a dimensional or categorical process. The current study aimed to address the categorical vs dimensional model debate, by using a novel combination of measurements and stimuli. In total, 30 participants’ expression categorisation, reaction time, and face-viewing gaze distribution to different facial expressions were measured. Happy expressions were morphed with angry, sad, fearful, and disgust expressions at different intensities to create ambiguous facial expressions. If significant differences between the different expression morph combinations were found for the expression categorisation, reaction times, and gaze behaviour, it may indicate a categorical process. However, if significant differences between the different expression combinations were not found, it may uncover a valence-general mechanism, as proposed by dimensional model. Analyses of variance revealed that there were significant differences between the different expression morphs at the same ambiguity proportions on the expression categorisation performance, reaction times, and gaze behaviour. Therefore, the novel findings indicate that an emotion-specific process, as opposed to a valence-general mechanism may be responsible for the perception of ambiguous facial expressions, supporting the categorical perspective. The implications for the categorical vs dimensional model debate, as well as suggestions for future directions are discussed.
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