Mutual Masking and Perceptual Simultaneity in Electrical Muscle Stimulation and Vibration Haptics

Multimodal haptic feedback that combines electrical muscle stimulation (EMS) and vibrotactile signals can create richer, more immersive experiences than those using a single modality. EMS delivers kinesthetic feedback by inducing muscle contractions, simulating force sensations that complement tactile stimuli from mechanical vibrations. However, presenting these stimuli concurrently can lead to perceptual interference, where one modality masks or alters the perception of the other. Temporal alignment between stimuli is also critical, as asynchrony can affect the perceived quality of haptic sensations. To investigate these phenomena, we conducted three user studies with a total of 40 participants (12, 12, and 16, respectively), focusing on mutual masking effects and temporal order perception between EMS and vibration. Our findings suggest that vibration can alleviate the tingling and discomfort commonly associated with EMS, effectively mitigating these unwanted sensations. Conversely, the presence of EMS increases the Just Noticeable Difference (JND) in vibration frequency discrimination, indicating a decrease in sensitivity to vibratory changes. Additionally, participants generally perceived the stimuli as simultaneous when EMS preceded vibration by 100 to 200 milliseconds. We discuss these findings and present four design guidelines for multimodal haptic rendering with EMS and vibrations in user applications.