Measurement of Vibration Detection Threshold and Tactile Spatial Acuity in Human Subjects

Summary

Here, we present protocols to determine vibration detection thresholds and tactile acuity using psychophysical methods in man.

Abstract

Tests that allow the precise determination of psychophysical thresholds for vibration and grating orientation provide valuable information about mechanosensory function that are relevant for clinical diagnosis as well as for basic research. Here, we describe two psychophysical tests designed to determine the vibration detection threshold (automated system) and tactile spatial acuity (handheld device). Both procedures implement a two-interval forced-choice and a transformed-rule up and down experimental paradigm. These tests have been used to obtain mechanosensory profiles for individuals from distinct human cohorts such as twins or people with sensorineural deafness.

Introduction

Specialized mechanosensory receptors in the skin mediate the perception of vibration and grating orientation. Each mechanosensory receptor type is tuned to detect distinct features of tactile stimuli^1,2. This property provides the psychophysical basis for a differentiated assessment of mechanosensory function by using tests that deliver simple sinusoidal oscillations (vibration) or fine gratings. It is known that psychophysical thresholds for vibration perception are lower for high than low frequency vibration^3,4. Two types of rapidly adapting mechanoreceptors associated with the Meissner and Pacinian corpuscles primarily detect low (10-40 Hz) an....

Protocol

The testing protocol was approved by the Charité-Universitätsmedizin Ethics Committee.

1. Vibration Detection Threshold (VDT)

1. Device and Testing Protocol Assembly – Pretesting
   1. Assemble the components of the device according to Figure 1A. Place a smooth-surfaced board (40 cm x 80 cm) on a table. Place the brass bar on the board.
   2. Connect the piezoelectric actuator (vibration stimulator) to the controller unit.
   3. Connect the response box and the monitor device to the data acquisition system (see supplemental code file).
   4. Connect the data acquisition system to a computer (or....

Results

The piezoelectric actuator provides the vibration stimulus to the subject. The vibration stimulus has a total duration of 1.8 sec and is presented only once during a trial during the first or second interval (Figure 2A). The rise and fall time at the onset and offset of the stimulus is determined by the functions (1-e^-bt)∙Amplitude∙sine(frequency), and (e^-bt)∙Amplitude∙sine(frequency), respectively, where b is set at 9.1. The rise and fall time at on.......

Discussion

The techniques used to evaluate VDT vary as to device specifications, hardware, and testing protocols. The International Organization for Standardization specifies the methods and procedures to analyze and interpret vibrotactile thresholds including recommendations for the various components of a vibrometer (ISO 13091-1 and 2^26,27) The described testing system abides to the relevant ISO recommendations for testing frequency range (4-125 Hz), method (variant of up-down staircase and forced choice), probe size (.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Piezo actuator	Physik Instrument, Germany	P-602.1L	The linear piezoelectric actuator, with integrated position sensor and motion amplifier, contains a piezoceramic material that elongates and contracts when voltage is applied. The piezoelectric actuator travels up to100 µm. The actuator is equipped with a flexure guide that ensures straight motion without tilting or lateral offset. The displacement is linear and calibration is done and checked by the manufacturer. It is recommended that on-axis movement of the probe be checked under the microscope. According to the manufacturer, the stimulus amplitude dampens by less than 20% at oscillating frequencies of 1000 Hz. This can be checked by using a force or displacement measuring device (e.g. force transducer from Kleindiek).
Piezo Amplifier/Servo Controller	Physik Instrument, Germany	E-665	E-665 amplifier/controller drives and controls the displacement of a low-voltage piezoelectric actuator in a system with sensor position feedback (SGS sensors). The servo-controller provides the option for closed loop operation. When applying sinusoidal and oscillating stimuli the amplitude signal deviates from the set amplitude starting from 500 Hz and reaches a maximum decrease of 20% at 1000Hz.
LabChart Software	ADInstruments, USA	LabChart 7, MLU60/8	Can create, store and run macro of the psychophysical testing algorithm.
PowerLab	ADInstruments, USA	PowerLab 4/35 PL3504	Data Acquisition Hardware. Used with LabChart software.
Brass bar	Custom-made		Bar made of pure brass, weighs 15.5 kg. When the peizoelectric actuator is mounted on the brass bar it should exert a force of 30 g weight on skin surface.
Monitor	Custom-made		To mark the 1st and the 2nd interval. The monitor indicates to the subject the time intervals during which the stimulus may be presented.
Response box	Custom-made		The subject indicates the interval at which stimulus occurred.
Board	Custom-made		Upper surface should be smooth (Plastic), lower surface made of foam to prevent stray vibration ot be transmitted to the stimulating pobe.
Probe	Custom-made		A flat circular probe with smoothed edges (thermoplastic material) attached to a screw head. The screw should be of appropriate size to be tightened directly to the moving part of piezoelectric actuator. Size of the probe can be according to preference; in our case, diameter 8.21 mm and surface area 52.9 mm2.
Labchart Script			Can be sent on request. See supplementary code file.
Tactile Acuity Cube	MedCore		The cube is comprised of 6 sides each containing a grating (bar and groove) whose widths are 0.75, 1.25, 1.75, 3.0, 4.5, and 6.0 mm.