Artificial emotion display is a key feature of social robots to communicate internal states and behaviors in familiar human terms. While humanoid robots can draw on signals such as facial expressions or voice, emotions in appearance-constrained robots can only be conveyed through less-anthropomorphic output channels. While previous work focused on identifying specific expressional designs to convey a particular emotion, little work has been done to quantify the information content of different modalities and how they become effective in combination. Based on emotion metaphors that capture mental models of emotions, we systematically designed and validated a set of 28 different uni- and multimodal expressions for the basic emotions joy, sadness, fear and anger using the most common output modalities color, motion and sound. Classification accuracy and users' confidence of emotion assignment were evaluated in an empirical study with 33 participants and a robot probe. The findings are distilled into a set of recommendations about which modalities are most effective in communicating basic artificial emotion. Combining color with planar motion offered the overall best cost/benefit ratio by making use of redundant multimodal coding. Furthermore, modalities differed in their degree of effectiveness to communicate single emotions. Joy was best conveyed via color and motion, sadness via sound, fear via motion and anger via color.

This article presents an experiment exploring the possible impact of avatar's body part visibility on players' experience and performance using current Virtual Reality (VR) gaming platform capacities. In an action-based VR game, a player sees an avatar in first person perspective which is replicating his/her hand, head and body motion. In contrast to the expected outcome from non-game VR contexts, our results did not reveal significant differences with an avatar presenting an increasing number of visible body parts. The body ownership, immersion, emotional and cognitive involvements as well as the perceived control and difficulty were not improved with a more coherent virtual body. This tends to confirm the strong performance aspect of action-based games, whereby control efficiency and enemy awareness is paramount, and could overcome the perceptual, behavioural or emotional effects of avatar embodiment. Digital games are indeed prone to create an intense flow state typically reducing self-awareness, and focusing on the game completion and high performance achievements. However, further experiments with full-body tracking and different game types are necessary to confirm this trend. This research outcome motivates further analysis of the mutual influence of bottom-up and top-down factors of avatar embodiment causing psychophysical effects. In addition, it provides useful indications for VR game developers and researchers on possible effects and evaluation methods.