Physitemp Instruments VIBRATRON II - Manual (Page 5)

4.1 The recommended method of testing is a “two-alternative” forced choice procedure.” For each
trial the subject is required to determine which of the two rods is actually vibrating. The position of
vibration is under experimental control, determined by a randomized sequence. The intensity of the
sequence is similarly under the control of the experimenter and is determined by a testing algorithm (see
below.) Prior to testing, all subjects should be allowed an adaptation period of between 10 and 15 min-
utes, during which they can become accommodated to room temperature.
During this period each subject should be given an opportunity to become familiar with the testing appa-
ratus and with the expected vibratory sensations. The experimenter can use this time to instruct the sub-
ject as to the appropriate length and force with which to contact the vibrating rod. An ideal duration for
contact is approximately one second, while the force should be less than that required to blanch the nail.
This adaptation period allows the experimenter to determine the appropriate voltage level at which to
begin testing. A number of vibration intensities should be set and sampled by the subject. For the initial
trial, the experimenter should set the voltage at the level detectable by the subject 100% of the time.
At the beginning of each testing session the subject should be issued the following instructions:
“Please press your finger against each rod in sequence for approximately one second. During
each trial you will be allowed to touch the rods only once. Only one of the rods will be vibrating and
you must decide whether it’s on the right or the left. The task will become increasingly more difficult
and I understand that you will be guessing on many of the trials.”
To determine accurate vibration thresholds, the experimenter must be concerned with the following
details:
I) The subject should be consistent in the location of touch and in the approximate force
applied to the vibrating surface. Instructions such as “please press more firmly” can be issued during
testing to ensure trial to trial consistency.
II) Throughout testing, the sounds and motions associated with changing the stimulus position
should be presented between each trial. For conditions where the stimulus postion remains unaltered, the
“dummy” switch must be utilized.
III) The subject must take care not to contact the rods or any part of the vibration unit’s surface
between trials.
IV) The subject should be carefully screened from viewing the instrument settings or the data
sheet.
TESTING ALGORITHMS
If the subject is correct on the initial trial, the vibration unit setting should be reduced by approximately
5 - 10% for the next trial, and this process should be continued until the first error. This percentage is not
an exact requirement but rather a guideline. When the subject makes his/her first error, the identical volt-
age setting should be repeated twice for a total of three trials at that level. If the position of the stimulus
is correctly identified on two of the three trials, the voltage setting should be raised 5 - 10%. If errors are
made at two successive settings at a given level, presentation of the third stimulus is not necessary.
All levels below 0.7 vibration units should be repeated twice - even if the subject selects the correct
stimulus position.
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