Effective masking is revealed by an increase in the detection threshold i.e., a reduction in sensitivity. We used a tactile masking technique in which tactile sensitivity (the reciprocal of threshold) on one forearm is measured in the presence of a vibrating stimulus (the mask) on another part of the body. We define this as the intensity at which the stimulus is detected 75% of the time. Threshold refers to the minimum stimulus that is needed to detect a stimulus. Here, the phenomenon of long-range tactile masking is explored by investigating how vibration applied to one forearm might affect tactile sensitivity thresholds on the opposite forearm. A goal of such long-range masking experiments is to indicate how components of the body's representation in the brain may be functionally linked. Long-range tactile masking effects have been shown between mirror-symmetric points on the hand and arm 5, 7 - 9but these studies have been largely restricted to looking at the hands and fingers 7, 10, with more extensive parts of the whole body being largely ignored. In contrast, few studies have looked at contralateral tactile masking in which the masking and probe sites may be far removed. While tactile masking has been studied extensively over the years, research has mainly investigated ipsilateral tactile masking using electrical stimulation 2, 3, pressure 4, and vibrotactile stimulation 5, 6. This is a technique pioneered by von Bekesy 1to reveal location interactions, especially lateral inhibition, between areas of skin that are adjacent on the body surface. Tactile masking is where a tactile stimulus at one location on the body alters the perception of a touch at another location. This manuscript describes a behavioural protocol that can be used for studying contralateral tactile masking. The use of this technique is beneficial as a behavioural measure for exploring which parts of the body are functionally connected and whether the two sides of the body interact in a somatotopic representation. Careful controls are required to rule out direct effects of the remote stimulus, for example by mechanical transmission, and also attention effects in which thresholds may be altered by the participant's attention being drawn away from the stimulus of interest. To explore the matching of corresponding points across the body, we can measure the spatial tuning and effect of posture on contralateral masking. Occasional reports have indicated that touches on one hand or forearm can affect tactile sensitivity at corresponding contralateral locations. Such effects can provide insight into how components of the body's representations in the brain may be linked. Masking effects have also been demonstrated in which a tactile stimulus alters the perception of a touch at a distant location. Masking, in which one stimulus affects the detection of another, is a classic technique that has been used in visual, auditory, and tactile research, usually using stimuli that are close together to reveal local interactions.
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