![]() ![]() This is likely attributable to visual afterimage processing, although a more tangible explanation remains elusive. The perceived intersection between the triangles seems to be therefore influenced by the contrast and the color of the triangles. It is of interest that, upon introducing color to the white triangle (purple or red), the readings obtained from the black triangle showed lower values in comparison to the colored triangle (video). Remarkably, the observed discrepancy between the 2 scales can be eliminated by simply darkening the white triangle on the fork with a marker ( figure, E and F). It is unclear as to why or when a modification was made and one of the black triangles was substituted with a white triangle. 2, 3 Of note, the original publication from Rydel and Seiffer depicts the tuning fork with 2 black triangles on the dampers ( figure, B). There is currently no report distinguishing between the black and white scales and both are used interchangeably. This is best appreciated during the motion of the prongs (video on the Neurology® Web site at ). This discrepancy can be as much as 25% or 2 units in the scale of 0 to 8. Values obtained from the virtual intersect of the white triangle are consistently lower in comparison to that from the black triangle ( figure, C and D). We have observed consistent discrepancy between the values obtained from the black triangle in comparison to the white triangle on the Rydel-Seiffer fork. (F) Rydel-Seiffer tuning fork with 2 black triangles in motion showing equivalent reading with intersect at 4. ![]() (E) Same Rydel-Seiffer tuning fork with the white triangle blacked out at rest. ![]() (D) Rydel-Seiffer tuning fork in motion with the black triangle showing intersect between 4 and 5 (on the 0 to 8 scale) and white triangle showing intersect between 3 and 4 (discrepancy is better appreciated in the video). (B) Original design of the tuning fork as published by Rydel and Seiffer in 1903 (reprinted with permission of Springer). (A) An example of a modern-day Rydel-Seiffer tuning fork. The position of the intersect is recorded on an arbitrary scale from 0 to 8 once the subject is no longer perceiving vibration. The intersect between the 2 virtual triangles moves up the scale with gradual attenuation of the amplitude of the oscillation. After the fork is snapped into motion, the prongs start to oscillate and the illusion of 2 triangles becomes visible on each damper. The modern Rydel-Seiffer tuning fork has a black triangle on one prong and a white triangle on the second prong ( figure, A). 3 Vibration threshold measurement with the Rydel-Seiffer tuning fork is therefore increasingly used in outcome measures assessing neuropathy from diverse etiologies: chemotherapy-induced peripheral neuropathy, 4 Charcot-Marie-Tooth disease, 5 multifocal motor neuropathy, 6 and immune-mediated neuropathies. 2 It has also been validated with the use of more modern sensory nerve action potentials. The Rydel-Seiffer tuning fork has been used extensively over the past century and normative values have been published. ![]() 1 In their paper, they described an adaptation of the standard tuning fork to make it semiquantitative on an 8-point scale in measuring vibration sensing. Seiffer published the use of what is now commonly known as the Rydel-Seiffer tuning fork. ![]()
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