Ethical considerations

Warning

It is the experimenter’s responsibility to ensure all relevant ethical standards are met in the course of using the FAB device. In addition to the participant exclusions noted in our Instructions for participants, experimenters must take precautions to exclude any other participants who could be expected to suffer harm in the course of the experiments.

Weight, force, pressure, and perceived stimulus intensity

Studies using the original FB device delivered painful pressure-stimuli to the fingers. This was achieved by lowering a 2kg [1] weight (force = approx 19.6N) attached to a lucite knife 0.5mm wide, which made contact with an area in the region of 5mm2 of the tested finger.

Because the area in contact with the knife varied with width of the finger (and this varies by partipant) the peak pressure delivered could vary substantially between participants. We estimate that studies using the original FB device applied pressures in the range of 3.14MPa to 5.23MPa (megapascals). However, many Forgione Barber devices subsequently used in research have not conformed to the original specification, and in our experience most used a knife probe approximately 2mm in width, equating to a peak pressure of approximately 1.31MPa.

The nature of the relationship between peak pressure, total force applied, perceived stimulus intensity, and pain is not fully understood. However we have found that, in use, participants with narrow fingers typically find FB stimuli substantially more painful than those with wider fingers. Furthermore, the large individual-differences typically observed in response to FB stimuli may be in part due to the shape of participants’ finger bones; those with wider, flatter finger bones may experience less pain, purely as an artifact of the device design.

Our new device replaces the linear knife-probe with a circular probe, 5mm in diameter. Holding the force applied constant, we anticipate a peak pressure of 1.25MPa. Thus, the stimulus is well below the pressures delivered by the FB device as originally specified, and also slightly lower than the peak force delivered by later machines which did not fully conform to the specification. However our new device ensures that all participants recieve the same peak force and pressure in all stimuli.

Note

Calculations of pressure from knife size and assumed contact areas are available in this spreadsheet pressures.xslx.

[1]Some non-standard devices delivered as much as 3kg (29.4N) of force, e.g. [1]

Stimulus intensity, duration and safety

Pressure stimuli in the range delivered by the FB and FAB devices are typically not immediately painful; the sensation of pain typically builds over a duration of 10-20 seconds. Most previous studies capped stimuli at 2 minutes in duration, although many participants opt to end the trial early, typically between 40 and 80 seconds.

However, because the FAB device can be used dynamically, we make several observations:

  1. Pressure stimulus can be introduced gradually, rather than all at once.
  2. Pressure intensity can be varied through the trial. The current model is fast enough to pulse from rest to full-pressue at approximately 0.5hz, potentially facilitating wind-up type effects.
  3. Peaks and troughs in pressure applied may provide opportunities to make several measurements
  4. Participants are blind to the actual pressure applied, and so measurements could be taken where participants hold pain constant, but vary weight instead.

However, we don’t recommend that trials on an individual finger last longer than 2 minutes at full stimulus intensity. Where less-painful stimuli are applied, somewhat longer trials may be appropriate, but 5 minutes with no more than 1 minutes spent at > 50% intensity, should probably be considered an upper bound until more data are available on safety and participant-acceptability.