Research Project

Using Digital Image Correlation to Quantify Skin Deformation by von Frey Monofilaments

Using digital image correlation to quantify the skin deformation associated with von Frey monofilaments

March 15, 2022

Anika Kao, Chang Xu, Gregory Gerling

Damage to the nervous system can diminish tactile acuity. To assess the extent of sensory impairments, clinicians commonly examine regions of a patient’s skin by touch using thin monofilaments, such as von Frey and Semmes-Weinstein. One’s ability to perceive low force monofilaments indicates absolute threshold and thereby the extent of impairment. However, while monofilaments are prescribed to bend at defined forces, there are no empirical measurements of skin surface’s response.

The skin’s surface is the point of origin for encoding touch information. Our perceptions of tactile acuity are shaped by some combination of factors involving the skin, afferents, and various elements of the central nervous system. In effort to better understand how our sense of touch is encoded, we might first ask – how does the skin move?

In this work, we study how the mechanical states of deformation at the skin surface, in response to indentation by von Frey monofilaments, drive just noticeable percepts at absolute detection and discrimination thresholds.

Experimental setup and data collection using DIC

Using a non-contact optical tracking method called digital image correlation, we quantify the differences in skin deformation at perceptual thresholds. Using stereo camera calibration and unique patterns applied to the skin we can obtain specific 3D displacement from 2D images. The speckling method shown in panel B includes a base layer of washable black paint followed by a spray of white speckles, creating unique patterns of high contrast and density to be tracked from frame to frame.

Experimental apparatus. (A) Indenter with load cell and von Frey monofilament attached. Three cameras around the indenter synchronously capture images for surface reconstruction via digital image correlation (DIC). (B) A von Frey monofilament makes contact with a speckled index finger. Randomly applied speckles(white) applied over base layer (black) create a unique pixel pattern to enable surface tracking. (C) – (D) Resultant 3D DIC tracking of skin deformation while in contact with monofilament, where point cloud shows displacement magnitude early and later into the indentation. (E) First principal Lagrangian strain.

Biomechanical metrics

3D skin surface deformation quantified to generate four derived metrics. (A) 3D point cloud showing displacement magnitude at maximum indentation. A cross-sectional plane is overlaid on the 3D point cloud. (B) The undeformed cross-section before contact and at maximum indentation. (C) Boundaries of radial deformation at maximum indentation for monofilaments between 0.07 - 4.0 g. (D) – (G) Four derived metrics, where plots show raw data(points) from one measurement trial with one participant fit by an exponential function.

Following the DIC analysis, to further quantify the deformation at the skin surface, we defined four derived skin deformation metrics, including penetration depth into the surface, strain across the skin surface, radial deformation emanating from the contact point, and area between 2D cross-sections. These metrics consider the extent to which the skin is both stretched laterally and indented normally, calculated from the initial contact frame through the buckle frame, to visualize changes in skin deformation up to that monofilament’s calibrated force.

Distinct states of skin deformation measured

Skin deformation metrics in response to von Frey monofilaments (force range: 0.07 – 4.0 g). (A, C) Experimental data from all participants and trials. Solid curves indicate an exponential fit to the raw data. Shaded regions represent the upper and lower 95% confidence intervals of the fit. (B,D) Plots showing metrics for only the 0.07 g monofilament with adjusted y-axis scale to illustrate DIC tracking of deformation at the micrometer level.

Using monofilaments ranging from 0.07 – 4.0 g, we measured clear separation across all four skin deformation metrics and between all six monofilaments, outside of their 95% confidence intervals even amidst variance between individuals and trials. Across metrics and monofilaments, the data ramp upward upon initial skin contact within 0.5 s, and subsequently reach a plateau at the calibrated force at 0.5 to 1.5 s. The 3D DIC method used here produces range and resolution such that we can record penetration depth as low as 6.1 microns and strain values of 0.34%.

Comparison of skin deformation and psychophysics

Results of psychophysical evaluation of absolute detection and discrimination threshold. (A) Absolute detection results showing average percent correct of trials in ascending and descending order inbox whisker plots, where experiments conducted with and without paint on the finger pad skin yielded a negligible difference. (B) Discrimination results for pairs of monofilaments.

With respect to absolute detection (A), the force threshold – set at 80% correct due to the experimental paradigm – was encountered at 0.4 g across both no paint and paint conditions, with the 0.07 g monofilament just beneath this threshold. Psychophysical discrimination trials (B) show that participants could discriminate only the smallest monofilament pair at levels of 75% correct. A decline in discriminability was observed with higher monofilament force. While we measured no systematic impact of paint on perceptual response, we would like to note, that these findings are restricted to the range of monofilaments used in this study (0.02 – 4.0 g) and a relatively modest cohort of participants.

The empirical quantification of skin states at perceptual thresholds may ultimately aid clinicians in better understanding the neurological origins of particular sensory impairments. The results indicate that this approach indeed achieves sufficient resolution and range to capture distinct states of skin deformation at just noticeable thresholds of absolute detection and discrimination.

References

  • A. R. Kao, C. Xu and G. J. Gerling, Using Digital ImageCorrelation to Quantify Skin Deformation with Von Frey Monofilaments [DOI]. IEEE Transactions on Haptics, 2021.

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