Abstract
Many recent studies show that the human brain integrates information across the different senses and that stimuli of one sensory modality can enhance the perception of other modalities. Here we study the processes that mediate cross-modal facilitation and summation between visual and tactile motion. We find that while summation produced a generic, non-specific improvement of thresholds, probably reflecting higher-order interaction of decision signals, facilitation reveals a strong, direction-specific interaction, which we believe reflects sensory interactions. We measured visual and tactile velocity discrimination thresholds over a wide range of base velocities and conditions. Thresholds for both visual and tactile stimuli showed the characteristic “dipper function,” with the minimum thresholds occurring at a given “pedestal speed.” When visual and tactile coherent stimuli were combined (summation condition) the thresholds for these multisensory stimuli also showed a “dipper function” with the minimum thresholds occurring in a similar range to that for unisensory signals. However, the improvement of multisensory thresholds was weak and not directionally specific, well predicted by the maximum-likelihood estimation model (agreeing with previous research). A different technique (facilitation) did, however, reveal direction-specific enhancement. Adding a non-informative “pedestal” motion stimulus in one sensory modality (vision or touch) selectively lowered thresholds in the other, by the same amount as pedestals in the same modality. Facilitation did not occur for neutral stimuli like sounds (that would also have reduced temporal uncertainty), nor for motion in opposite direction, even in blocked trials where the subjects knew that the motion was in the opposite direction showing that the facilitation was not under subject control. Cross-sensory facilitation is strong evidence for functionally relevant cross-sensory integration at early levels of sensory processing.
Highlights
Integration and interactions between touch and vision The different sensory modalities provide redundant information about the environment
Both visual and tactile motion produced the characteristic “dipper function,” where the thresholds initially decreased with base speed to a minimum at base speeds around 0.1 cm/s, rose, roughly in proportion to base speed
The results show that the two forms of motion are processed by similar mechanisms, which interact with each other, possibly at an early level of analysis
Summary
Integration and interactions between touch and vision The different sensory modalities provide redundant information about the environment. Much evidence over the last decade has shown that our nervous system integrates signals from different modalities to maximize the information available for perception and action (e.g., Ernst and Bulthoff, 2004) It is not entirely clear whether these processes occur at low sensorial levels or higher decisional levels. Much psychophysical evidence demonstrates integration of visual and auditory motion stimuli, but the integration would not be functionally useful in discrimination direction of motion, of a common object, as it occurs for both the same and opposite directions of motion (Meyer and Wuerger, 2001; Wuerger et al, 2003; Alais and Burr, 2004a). Area SI and SII, primary somatosensory areas respond well to visual stimuli (Keysers et al, 2004; Blakemore et al, 2005; Schaefer et al, 2005) and neurophysiological evidence support the presence of multimodal interactions at the level of the single neuron (e.g., Stein et al, 2001; Rowland and Stein, 2007)
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