Testing Tactile Masking between the Forearms

Summary

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.

Abstract

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.

Introduction

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 stimulation^2,3, pressure⁴, and vibrotactile stimulation^5,6. In contrast, few studies have looked at contralateral tactile masking in which the maski....

Protocol

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

1. Tactile Detection Stimulus
   1. Use a tactor (1.17" diameter and 0.30" thick) to deliver tactile stimuli of 250 Hz vibration for 100 msec. Using a purpose-built tactor provides a linear relationship between the amount of travel and the voltage applied.
   2. Control tactile stimulus delivery with a 64 bit sound card.
      1. To drive the tactor, treat it as a loudspeaker. Connect a stereo audio amplifier to a 64-bit....

Results

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.......

Discussion

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 da.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
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/