The overall goal of this psychophysical procedure is to determine the threshold for vibration detection in tactile spatial acuity. This method can help us ask some quite interesting questions about sense medicine through physiology, in particular, about the sense of touch. The main advantage of this method is that we can obtain quantitative information about mechani-sensory profiles in individuals and this information can be used for both for clinical diagnosis as well as for basic research.
We first applied this method in a twin study to test whether touch traits in people are heritable. Visual demonstration of this method is critical as depiction of the procedure as well as the adaptive method we use are complex. Begin to assemble the components of the device by placing a 40 cm by 80 cm smooth surface board on a table.
Then, place a brass bar on this board. Afterwards, ready the controller unit. Next, connect the response box and the monitor device to the data acquisition system.
Then, screw the custom-made stimulating probe to the moving part of the piezoelectric actuator. Finally, mount the piezoelectric actuator with the probe on the balance brass bar. Begin by bringing the subject into a quiet room and seat them near the device.
Inform the subject about the testing procedure and have them sign a written consent form. Then, place their arm on the board and pad the little finger with medical dough to minimize movement. Next, place the brass bar on the board and use a water level to position the probe horizontally on the little finger of the tested hand just below the nail bed.
Avoid skin contact with the edges of the circular flat probe. Prior to starting the test, ensure that the procedure is understood by presenting both an easy to perceive vibration stimulus, and a hard one by varying the amplitude until the subject perceives a vibration. Then, start the test by running a script that uses a two alternative forced-choice procedure with the up-down adaptive method.
For each trial, randomly administer a vibration stimulus during one of the two intervals which are visually indicated to the test subject as one or two on the screen in front of them. Ask the subject to indicate if the first or second interval contained the vibration stimulus by pressing the number one or two on the response box. Have the subject make a guess response if they are not sure when the stimulus was presented.
Complete a total of six to nine trials, and repeat the same vibration stimulus at the one amplitude level at least six times consecutively. If all responses are correct, reduce the stimulus intensity level for the subsequent trial series. Based on the decision rule of the adaptive method, grant the subject more trials at the same stimulus intensity if the subject makes errors in a trial series.
Reduce the stimulus intensity if the stimulus is correctly identified in at least five trials, and incorrectly in less than two trials. Increase the stimulus level if the subject makes two incorrect responses in a trial series or more than one incorrect response and fewer than five correct responses. Document the change in the direction of stimulus intensity as the reversal point.
Change the stimulus intensity according to reversal point number. For example, prior to the third reversal point, Change by four intensity levels. Or at the third reversal point, change by two intensity levels.
Otherwise, by one level. End testing when the subject completes a total of eight reversals. Finally calculate the vibration detection threshold or VDT by taking the median of the stimulus amplitude value of the last six reversals.
Begin by providing instructions about the task in a quiet room set at a temperature between 20 and 30 degrees Celsius. Next, blindfold the subject using shielded eye glasses. Place the dominant hand on a table with the palm surface facing up.
During each trial, apply the tactile acuity cube or TAC to the finger pad at one of two grating orientations. Vertically or horizontally aligned to the long axis of the finger. Apply the gratings of the TAC to the finger pad of the finger for two seconds so that the cube exerts its whole weight on the finger.
Then, ask the subject to determine the orientation of the alignment before the cube is removed from their finger. Start with the largest grating at 6.0 millimeters and decrease the grating width after two correct identifications of the orientation. Then, test the next smaller width and continue with the stepping rule until the subject makes and incorrect response.
Then, document this grating width as a reversal point. Increase the grating width step-wise again until the two orientations of the width are determined correctly again. Finally, end the test after the completion of 13 reversals and calculate the tactile grating orientation threshold by taking the median of the grating widths of the last 10 reversals.
Here, a typical testing session to determine the vibration detection threshold at 125 Hz is shown. A decrease in stimulus intensity requires at least six sequential correct responses. Furthermore, a change in the direction of stimulus intensity marks a reversal point in the trial series track and requires at least two incorrect responses.
The stimulus intensity is changed in steps of four levels when the number of reversals is less than three, followed by two levels at the third reversal, and finally one level steps to finally determine the threshold. For the tactile cube experiment, where each trial consists of two stimulations, vertical and horizontal, the threshold is 1.6 millimeters, which is the median value of the grating widths of the last 10 reversal points. Once mastered, this technique can be done in about 30 minutes, if it's done properly.
After watching this video, you should get a good idea about how to measure vibration detection threshold as well as tactile acuity in humans.
