Signals arising from different sensory modalities are integrated into a coherent percept of the external world. Here we tested whether the integration of haptic and visual signals can occur at an early level of analysis by investigating the effect of touch on binocular rivalry. Visual stimuli were orthogonal (vertical or horizontal) Gabor Patches (spatial frequency: 3.5 cpd or 5 cycles/cm, patch size: 1.5 deg, contrast: 45%), presented foveally against a grey background (7.8 cdm-2) and displayed alternatively to the two eyes through Ferro-Magnetic Shutter goggles (driven at the monitor frame rate, 120 Hz). At random intervals subjects explored briefly (a??3 s) a haptic stimulus (sinusoidal milled Plexiglas, 5 cycles/cm) oriented vertically or horizontally. The task was to report the perceived orientation of the visual stimulus. We measured the probability of switching perception during haptic stimulation and during periods of visual-only stimulation of comparable duration. When the orientation of the haptic stimulus was orthogonal to the dominant visual percept, perception switched towards the haptic orientation; the probability of a switch was significantly higher than during visual-only stimulation. Similarly, when the haptic orientation was parallel to the dominant visual percept, maintenance of that percept was significantly more probable. We repeated the experiment varying the spatial frequency of the visual (1.3 and 3 cycles/cm) and the haptic stimuli (1.3, 2, 3 and 4 cycles/cm) and we showed that the effect is spatial-frequency tuned, occurring only when visuo-haptic spatial frequencies coincided. Our results indicate that a visual stimulus, rendered invisible by binocular rivalry suppression, can nonetheless revert to consciousness when boosted by a concomitant haptic signal of congruent orientation and spatial frequency. Given that suppression is thought to occur early in visual analysis, our results suggest that haptic signals modulate early visual processing.
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