Here we explore contralateral tactile masking between the forearms in which tactile detection thresholds are modulated by vibration applied to a remote site. The details of which remote sites have an effect can tell us about how the body is represented in the brain.
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. Masking effects have also been demonstrated in which a tactile stimulus alters the perception of a touch at a distant location. Such effects can provide insight into how components of the body’s representations in the brain may be linked. Occasional reports have indicated that touches on one hand or forearm can affect tactile sensitivity at corresponding contralateral locations. To explore the matching of corresponding points across the body, we can measure the spatial tuning and effect of posture on contralateral masking. 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. 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. This manuscript describes a behavioural protocol that can be used for studying contralateral tactile masking.
Tactile masking is where a tactile stimulus at one location on the body alters the perception of a touch at another location. This is a technique pioneered by von Bekesy1to reveal location interactions, especially lateral inhibition, between areas of skin that are adjacent on the body surface. While tactile masking has been studied extensively over the years, research has mainly investigated ipsilateral tactile masking using electrical stimulation2,3, pressure4, and vibrotactile stimulation5,6. In contrast, few studies have looked at contralateral tactile masking in which the masking and probe sites may be far removed. Long-range tactile masking effects have been shown between mirror-symmetric points on the hand and arm5,7–9but these studies have been largely restricted to looking at the hands and fingers7,10, with more extensive parts of the whole body being largely ignored. 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. 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. Threshold refers to the minimum stimulus that is needed to detect a stimulus. We define this as the intensity at which the stimulus is detected 75% of the time. 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. Effective masking is revealed by an increase in the detection threshold i.e., a reduction in sensitivity. The technique can be used in conjunction with other manipulations such as varying limb position or movement to explore their effects on the effectiveness of masking.
Here we used vibrotactile stimuli as the masking stimulus. The advantage of this is that the frequency, and hence the receptor type that it stimulates can be controlled. The technique could be extended to look at pain using electrical stimuli as the probe or mask or both. Also, any site can be used as the masking site allowing the investigation of acupuncture sites for example.
All of the experiments were approved by the York Ethics board and all participants signed informed consent forms. The experiments were performed in accordance with the Treaty of Helsinki.
1. Stimuli
2. Experimental Setup and Design
Figure 1. Experimental Design. This figure shows the set up of the experiment and the materials used. See text for details.
3. Experimental Procedure
4. Data Analysis
Analyses of the data was reported in13. Tactile sensitivity (expressed relative to the thresholds measured in the control condition) on the forearm was significantly reduced (thresholds were significantly increased) when vibrotactile masking stimulation was applied to the opposite arm (Figure 2A), demonstrating a contralateral masking effect between forearms. The effect depended on the position of the masking stimulus on the masking arm, with the largest effect occurring when the mask and test sites corresponded. Figure 2B shows that posture also plays a role on the effectiveness of masking. The masking effect was considerably stronger when the arms were touching compared to when they were parallel (3.3 dB compared with 0.52 dB).
Figure 2. Typical data obtained using this technique. (A) The elevation in threshold is plotted as a function of the position of the mask (shown as blue arrows in the cartoon above the graph). (B) The elevation in threshold at the probe site on the left arm caused by a masking stimulus on the corresponding site on the right arm depends on whether the arms are touching or parallel. All data are expressed in dB relative the thresholds obtain with vibration applied to a control site on the shoulder (green arrow). Standard errors are shown. N = 15. Data redrawn from D'Amour and Harris, 2014.
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Here, a detailed protocol for contralateral tactile masking is described and previously published results using the technique to test tactile detection thresholds are shown. The advantage of this method is that thresholds are measured using a psychophysically rigorous technique. The two-alternative forced choice (2AFC) procedure is relatively insensitive to response bias and therefore from attentional effects. The adaptive staircase procedure for honing in on the actual threshold value is very efficient as most of the data are collected with stimulus intensities close to the threshold level. Blindfolding the participant and having them look straight ahead throughput the data collection period further reduced attentional effects.
It is technically very demanding to measure the actual pressure applied by a tactor. It is not sufficient to calibrate the device beforehand because the pressure exerted will also depend on how tightly the tactor is bound to the skin surface. Thus we are only able to make statements about changes in thresholds rather than the absolute values. Since in this experiment we are only looking for changes brought about by the masking stimulus, this is not a concern in this design.
By interleaving relatively short blocks (about 10 min) for each condition (i.e., each position of the masking stimulus) and presenting them in a sequence that was counterbalanced between participants the alertness of the participant is maintained.
Contralateral masking can be useful for exploring the representation of the body in the brain by revealing details of which parts are functionally connected to others. This technique provides behavioural5,7,8,13–15 evidence to support neurophysiological16–18 and neuroimaging data19–23 that suggest that the integration of somatosensory inputs from the two sides of the body occurs in a somatotopic representation. In these experiments, the effect of arm location was briefly examined by comparing masking when the hands were touching or parallel. Though a difference was found, it cannot be concluded whether it is caused from actual skin contact or arm position. In a set of new experiments, we have taken these methods and tested a variety of different arm positions of both the test and masking arms. These findings will help address whether long-range masking effects occur before or after postural information has been added.
The technique is extremely flexible and can be used to investigate any manner of interactions between different parts of the somatosensory system. For example, the frequency content of masking or testing stimulation can be varied to optimally stimulate rapidly adapting or slowly adapting sub-systems. A potential limitation of these methods is the tactile stimuli used. Using different detection and masking stimuli (such as size, frequency, duration, etc.) might reveal different results especially when measuring the spatial tuning of the masking effect. A smaller masking stimulus would allow for better precision and allow for more accurate measurements of specific areas. For future applications, this protocol could be modified by testing the masking effect using a wide range of tactile stimuli.
Research has typically concentrated on studying masking and tactile perception on the hands and fingers with relatively few studies examining the whole body14,24–27. Future directions could include testing contralateral masking on more extensive areas of the body, which might reveal unexpected connections between other body parts or within a limb that could shed light on how the three-dimensional body is represented within the brain.
The authors have nothing to disclose.
LRH was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada. SD was partly supported from the NSERC CREATE program.
C-2 tactor | ATAC Technology; Engineering Acoustics, Inc. | http://www.atactech.com/PR_tactors.html | |
Magic Wand | Hitachi | http://magicwandoriginal.com/magic-wand-original/ | |
FC5 Foot Pedals | Yamaha Corporation | http://ca.yamaha.com/en/products/music-production/accessories/footpedals/fc5/?mode=model | |
MATLAB | The Mathworks, Inc. | http://www.mathworks.com/products/matlab/ | |
Velcro | Velcro Industries B.V. | http://www.velcro.com/ |