Growing evidence shows that early sensory areas are involved in cross-modal processing, challenging the traditional view that confined multisensory analysis to high-level temporal and parietal areas. Haptic signals have been shown to influence the dynamics of binocular rivalry, a form of perceptual bistability that engages competition between monocular signals (Lunghi et al, 2010, Current Biology, 20(4), R143-144). We investigated the role of spatial proximity, type of tactile stimulation (active or passive) and cross-modal attention in promoting fusion between vision and touch during binocular rivalry of orthogonally oriented gratings (±45°, size 2.5°, SF 2c/cm). By varying the haptic orientation (±7.5°, ±15°, ±30°), we further investigated the orientation selectivity of the interaction. We found that both active exploration and passive tactile stimulation boosted the visual stimulus congruent with the tactile one (engraved grating), both prolonging conscious perception and restoring it from binocular rivalry suppression. The interaction was strictly tuned (less than one octave) to visuo-haptic spatial frequency and orientation. We also found that voluntary attention played a minor role in mediating the interaction, while temporal alignment and spatial proximity between visual and tactile stimuli were necessary for fusion of the cross-sensory signals, this latter result suggesting that the visual and the somatosensory spatial maps are aligned. Taken together, our results points to a very early multisensory interaction site, possibly V1.

Cross-modal processing; binocular rivalry; psychophysics