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
characterize 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 perceptual channel with lower variance in
the spatial discrimination has a preeminent role in determining illusory
perceptions in the time domain.