Testing Tactile Masking between the Forearms

The overall goal of this psycho-physical experiment is to show how touching one place can affect the perception of a touch at another location. This technique of long-range tactile masking can make important contributions to the field of perception. For example, it can tell us how the brain represents the body's surface.

An advantage of this technique is that it is non-invasive, and yet, even though it's a perceptual measure, it can tell us information about how the brain is wired up. Demonstrating this technique will be Sarah. She's a graduate student in my lab, and she developed the techniques that we're demonstrating today as part of her master's thesis.

Begin by preparing the tactile detection stimulus system that will be used during this experiment. Use a tactor with a 1.7 inch diameter and 0.30 inch thickness to deliver tactile stimuli with 250 hertz vibration for 100 milliseconds. Also use a 64 bit stereo sound card to control the tactile stimulus delivery.

To do this, first connect a stereo audio amplifier to the sound card that will drive the tactor as a loud speaker. Next, connect the tactor to one of the outputs of the amplifier, such as the left speaker out. Then, connect the second channel to a loudspeaker to provide auditory signals.

In the computer code that is controlling the experiment, generate the waveform that will be used to vibrate the tactor, and another waveform for the auditory signal. Put each of these two waveforms into a two-dimensional array. Attach the tactor to a strap with a mechanical base fastener so that it can be placed on the participant.

For the masking stimulus, use an electric handheld vibrator with a four centimeter diameter, with 83 hertz vibration when set on low. Select the sites where the stimulus is to be applied based on the areas where masking is to be tested. After the tactile detection stimulus system is ready, arrange the computer and other equipment for the experiment on a desk with a chair in front of it for the experimenter.

Then, set up a table on which participants will rest their left arm, where tactile stimuli will be applied, and place a chair to the right side of the table. Also place an armrest stand next to the chair, on which the participants rest their right elbow. Add cushioning to the table, and an armrest for comfort.

Next, place the loudspeaker on a mechanically isolated surface. Arrange foot pedals that are connected to the left and right buttons of a computer mouse so that the participant's feet can rest comfortably on them. Be sure to make a list of the order in which the conditions will be run, and use this list during the experiment to determine where to hold the masking stimulus for each block.

Enter the number of trials into the computer program code and ensure that each block is no longer than 10 minutes to maintain the participant's attention. Program a two alternative forced choice tactile detection task in which stimulus intensity is controlled with Bayesian adaptive psycho-physical staircase psychometric procedure. For each trial, present two one second intervals, marked by three beeps, with a tactile stimulus presented in the middle of one of the intervals.

Have the participant indicate in which interval the stimulus was presented by means of the foot pedals. The computer scores the response as correct or wrong, and the adaptive staircase chooses the next value to be presented accordingly. This determines a threshold value for detecting vibration at the test site.

Prior to the experiment, obtain written informed consent from the participant. Then, seat them in the chair with their feet resting comfortably on the response foot pedals and explain the experimental procedure. Measure and record the length of the particpant's dorsal surface of the left forearm.

Then, apply the strap so the tactor is positioned in the middle of the arm, halfway between the inner angle of the elbow and the wrist crease. Wrap a tensor bandage loosely around the arm several times to hold the tactor in place. Now, instruct the participant to place their right elbow on the arm rest, and rest their left arm on the table.

Then, blindfold the participant to prevent them from looking at the stimulus or the masking location, and instruct them to look straight ahead throughout the experiment. Start with a block of 20 practice trials without the masking stimulus, in order to familiarize the participant with the tactile detection task, and to allow them to become familiar with the stimuli. Following the practice run, first ensure that the arms and the masking stimulus are in the correct positions prior to each experimental block.

Run the blocks of conditions in a random, counter balanced order. For each condition, refer to the list of conditions made earlier, and cross off each condition as it is run. Be sure to hold the masking stimulus throughout each block of data collection trials, maintaining approximately constant pressure.

Run the experimental program with the set number of trials as previously selected, and introduce a two minute rest period after each short trial block. After a given trial block is completed, select the next site for the masking stimulus, and run the program again, and repeat this until all trials are completed. In this figure, the elevation and threshold is plotted as a function of the position of the mask.

Tactile sensitivity on the forearm was significantly reduced when vibro-tactile masking stimulation was applied to the opposite arm, demonstrating a contralateral masking effect between forearms. Here, these data show 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.

Once mastered, this technique can be done in about half an hour if it's performed properly. Although it's necessary to repeat the experiment on at least ten participants to obtain statistical validity. While attempting this procedure, it's important to remember to maintain the pressure at the masking site and to keep the participant's attention at a location that does not vary, usually straight ahead.

Following the procedure, variations can be performed in order to answer additional questions, such as the dependency of these effects on the characteristics of the masking stimulus. After its development, this technique has paved the way for researchers in the field of tactile research to explore whether this phenomenon might generalize to more than just touch sensations, but also pain. After watching this video, you should have a good understanding of how to test the influence of remote tactile stimulus on touch perception at a specific site.