When space rules time: the Kappa effect with concurrent visual and tactile stimulation

The spatial and temporal dimensions of a subjective experience are intertwined. In the Kappa effect, the spatial distance between consecutive visual stimuli proportionally influences the perceived duration of the inter-stimulus interval. Despite its relevance, to our knowledge no study has investigated such an illusory effect in concurrent multimodal perception through visual and tactile elicitations. To overcome this limitation, we characterise the Kappa effect on vision and somatosensation also through the assessment of duration perception in the sub-second range when sensory inputs are delivered on the palm or forearm using wearable devices. The effect of unimodal visual/tactile elicitations was also compared to several bimodal stimulations. Results showed that the illusory effect, which arose in both unimodal conditions, produces significantly higher distortions in vision. In the multimodal condition, when visual stimuli were at not-equidistant spatial locations, the integration with the tactile channel did not diminish the Kappa effect. Conversely, the Kappa effect vanished when the visual stimuli were at equidistant spatial locations, independently from the tactile stimuli location. These findings shed light on the human perceptual mechanisms, suggesting that the perceptual channel with lower variance in the spatial discrimination has a preeminent role in determining illusory perceptions in the time domain.