Tactile masking within and between hands:Insights into spatial coding of touch at the fingers

Recent work has shown that touches on the body can be represented according to multiple spatial codes (Haggard, 2006; Schicke & Röder, 2006). For instance, a tactile stimulus at the index finger of the right hand activates a spatial code finger-specific (index), hand-specific (right) and space-specific (the finger’s position occupies in external space). Here we examine in two experiments the relative importance of these multiple spatial codes by adapting a tactile masking paradigm for stimuli presented at the index or middle fingers of either hands. Twelve participants performed a go-no-go task where a vibro-tactile pattern (target) presented to one finger during a presentation of a second pattern (non-target) to either the same finger (i.e., left index, right index) or a second finger (i.e., left index, left middle finger). Participants had to respond only if the target finger was stimulated. In this experiment we found a clear tactile masking interference effect. In particular, this interference was referring to a large activation of a spatial code finger-specific. In a second experiment we modified the position of one hands. More in deep, we rotated the hands alternatively of 180° on the horizontal plane. This transformation has modified the degree of interference between the fingers. In fact, the tactile masking on the opposite hand, respect to the target, was changed with relation to the fingers position in the external space. This suggests that with a mirror image position of the hands the critical spatial codes is finger-specific, while modifying the hand position a critical role was played also by space-specific spatial code.