# Measurement System for Finger Skin Displacement on a Textured Surface Using Index Matching
## 2 Related Work  
Tactile perception relies on multiple processes. These can be broadly divided into the conversion of object shape into skin vibration, the conversion of skin vibration into receptor-firing activity, and the processing of neural firing patterns in the brain. In this vein, neurophysiologists have attempted to measure the relationship between texture shape and receptor firing patterns. [PERSON] et al. [10] and [PERSON] et al. [11] performed experiments to measure the neural activity of receptors in the fingertips of monkeys. They used simple textures, such as artificially created planes and steps, to record SAI and RAI temporal firing-pattern changes. [PERSON] et al. [12; 13] measured how fingertip receptors undergo spatiotemporal changes when presented with a matrix of dots. From this, they found a greater relationship between roughness and tactile receptor activity via spatial changes than via temporal changes at 1-2-mm dot intervals.  
Skin vibrations have been recorded to reveal how texture shapes are decoded by the skin. [PERSON] et al. [2] used an accelerometer and a laser Doppler module to measure the similarity between vibrations and groove texture shapes when traced with a fingertip. [PERSON] et al. [14] measured contact vibration data for a large number of natural textures using accelerometers, position sensors, and force sensors attached to pens and attempted to reproduce these results on a tactile display. [PERSON] et al. [15] proposed a method of indirectly measuring the shear displacement of the finger surface using an accelerometer to measure changes on the finger side.  
The behavior of spatial skin displacement has also been measured to elucidate patterns, such as stick-slip and shape perception. Numerous measurements have been made using transparent measurement samples and cameras. [PERSON] et al. [6] measured the behavior of the finger surface on flat and very large uneven surfaces after defining the feature points of the fingertips, including sweat pores and fingerprints. [PERSON] et al. [16] used a glass prism to measure the true and apparent contact areas based on the contact force. In another study, a similar optical measurement setup was used to measure changes in the moisture content and the contact area of the skin during the grasping of an object [17]. [PERSON] et al. [18] proposed a method to discriminate the force exerted on a fingertip and its orientation using the initial shear displacement of the fingertip contact area. [PERSON] et al. [19] proposed a method using GelSight tactile sensors to detect normal, shear, and torsional loads on contact surfaces on planar and curved surfaces. Such optical measurements of fingertip surfaces are not only used to reveal tactile perception but also for human-computer interfaces. [PERSON] et al. [20] used a fingerprint image to identify which fingers were in contact with the display.  
In summary, it can be said that, although the temporal changes on textures have been measured using accelerometers, fingertip behavior on very fine textures has not been directly and spatiotemporally measured. Although there have been clear examples of spatial measurements, most of the measured samples were of planar or large uneven shape that did not interfere with optical observations. The direct quantitative observation of spatiotemporal behavior is essential for presenting higher quality textures.