Should I switch to MouseMet EvF?

Von Frey filaments

They may not be perfect, but they are tried and tested with a huge body of published data. You know what thresholds to expect and you know the method will detect a change in the treatment group. So why change? Why move to a new method, especially one that is not directly comparable in terms of the threshold forces recorded?

On this page I’m going to do three things:

  • List the pros and cons of filaments
  • List the pros and cons of MouseMet EvF
  • Highlight the possible shortcomings of some other EvF systems
Good things about filaments
  • They’ve been around a long time and lots of people use them
  • There’s a huge body of published data so it’s easy to compare data (although see below)
  • They’re cheap. A set costs a few hundred pounds
  • They’re simple.
  • They all have different diameters. This is sometimes cited as a limitation but in fact it’s a good thing: it “stretches” the force scale, amplifying the measurement of treatment effect compared with any method that uses a constant diameter probe. Here’s a pdf that explains further.
  • Not everyone uses them in quite the same way, so comparing data is not as easy as it should be.
    • People apply them at different rates
    • Some people pause after touching on to see if the mouse reacts whereas others go straight into the ramp
    • Some view from the side, some underneath, some with cameras
  • It’s possible to use them when they are damaged. If they are already bent, they will still buckle, but not at the same force. It’s not generally possible to replace individual filaments in a set, so people tend to keep going even when the thinner ones should be thrown away.
  • The size and profile of the contact changes drastically as it buckles: it’s then presenting something between a semicircle and an edge to the plantar surface. So if the mouse reacts just as it buckles, is that a response to that filament or not?
  • There is confusion over the up-down method, commonly used to analyse von Frey filament data; some people don’t use it properly.
  • It’s very repetitive work and therefore prone to errors in sequencing the filaments or in data entry (due to operator fatigue or boredom) . Here’s a recent Topcat poster where we quantify some of this.
Good things about MouseMet EvF
  • It gives a ramped stimulus with a guide line on the graph so the operator can see that they are applying the stimulus at the same rate each time (1g/sec). Other workers in other labs will hopefully be doing the same.
  • Every application of the probe gives a threshold, whereas with the up-down method a minimum of 5 readings is needed. So it’s much faster, with less stress to the mice.
  • It’s easier to use: the transducer has a rotary action, making it insensitive to hand tremor, and the runs are adjustable in height so that the operator can sit with their elbows on the bench. This creates a much more stable platform than hovering your hand, trying to push a filament up without shaking. And as all workers are using the same MouseMet runs, viewing angles and therefore consistency are improved.
  • The probe area is constant so you lose the amplification effect of filaments.
  • It’s more expensive.
Other EvF systems
  • Some systems consist of a commercial force transducer of an inappropriately high force range mounted in a hand held “wand”, with a probe on the measurement side to contact the rodent’s foot. The high force range means that the transducer is only ever operating at the very bottom of its range, where noise, linearity and repeatability are generally most significant.
  • This probe assembly may also have a significant mass compared to the forces being measured (0.1-7g). This means that the transducer will indicate a force if the probe is subjected to an acceleration along its axis (Force=Mass*Acceleration) so you can generate numbers just by waving the probe around in air, without ever going near the mouse. (Click here for a paper written by Dr MJ Dixon while studying the effect at the National Physical Laboratory in the 1980s).