Does the categorization difficulty elicit the uncanny-valley-like phenomenon without animacy?

We feel strong eeriness for non-human objects that are highly similar to humans (a phenomenon known as uncanny valley). Previous research has accounted for this phenomenon based on animacy perception (e.g., the avoidance to threat or the lack of mind) or cognitive processes irrespective of animacy (categorization difficulty hypothesis). However, whether animacy perception is responsible for uncanny valley to be elicited is unclear at present. This is because all the previous studies used objects related to animacy (creature-like features) as stimuli (e.g., faces). Therefore, the present study examined whether uncanny valley occurs for non-animacy objects, using geometric figures (square, circle, and triangle). In Experiment 1, we made stimulus figures by systematically morphing an original geometrical figure (e.g., circle) into another figure (e.g., triangle). Participants first categorized which original figure (e.g., circle or triangle) the morphed figure was perceived as and then evaluated its likability with a 7-point Likert scale. As the result, we found the uncanny-valley-like phenomenon with non-animacy objects; that is, the figures that took longer times to be categorized (difficult-to-categorize objects) were rated as being less likeable as compared to the figures that were quickly categorized (easy-to-categorize objects). The subsequent experiments hypothesized that the categorization difficulty deteriorates perceptual and cognitive fluency of the morphed figures, resulting in the lower evaluation for them. To assess this hypothesis, we independently manipulated the perceptual (Experiment 2) and cognitive (Experiment 3) fluency of to-be-evaluated figures and investigated how the uncanny-valley-like phenomenon is modulated in response to the processing fluency. The results demonstrated the uncanny-valley-like phenomenon, but no effects of the processing fluency on it. These results suggest that the categorization difficulty is one factor underlying the uncanny-valley-like phenomenon, but the deteriorated processing fluency associated with it does not elicit the phenomenon by itself. We conclude that the uncanny-valley-like phenomenon can be robustly elicited by the stimuli without animacy (or creature-like features), which are in favor with the categorization difficulty hypothesis; that is, animacy perception is not a necessary condition for the occurrence of uncanny valley.