Science Literature

For most people, the extraordinary sophistication of each of our senses is only dimly appreciated. Touch is a good example. Most of us struggle when feeling Braille characters, but they are a lifeline to those who can sense and understand them. The problems of gaining effective tactile feedback are well-known to robotics engineers, who know that solutions do not come easily.

Musicians know the importance of precise control of finger position, pressure and micro-movements (Source here)

Human touch sets the standard in robotics research, as this provides an example of "exquisite tactile sensitivity". We have four types of mechanoreceptor that convey information about touch. Perception of fine features is associated with Pacinian corpuscles located in our skin. New research has sought a better understanding of how textural information is gained by finger/substrate contact.

"Because there is currently no way to measure experimentally the subcutaneous stress using a human subject, our approach is based on the use of a biomimetic tactile sensor whose functioning principle and main geometrical characteristics are matched to those of the human fingertip. This allows us to test, in particular, the role of epidermal ridges (fingerprints) in this transduction process."

Fingerprints can be inferred to have a function. The ridges are not random in size and distribution, but show a distinctive pattern. Although a design inference seems obvious, it is not at all clear what functionality fingerprints provide. Two hypotheses are to be found in the literature. The first is that fingerprints improve grip (analogous to the tread on car tyres). The second is to enhance tactile perception by "increasing the subsurface strain with respect to the surface deformation." The new research validates the second hypothesis.

"Here, we show that fingerprints may have a strong impact on the spectral filtering properties of the skin in dynamic tactile exploration."

After describing the experimental work and the findings, the authors present three conclusions. First, and most significant, is that the regular ridged topography of fingerskin is an effective amplifier of the minute vibrations generated by surface sliding. Amplification factors of up to 100 are reported. Furthermore, the inter-ridge distances are optimised to amplify the relevant frequencies. The mechanical system works best at a frequency that is of -

"the order of the best frequency of the Pacinian fibers that mediate the coding of fine textures. Fingerprints thus allow for a conditioning of the texture-induced mechanical signal that facilitates its processing by specific mechanoreceptors."

Secondly, observations are reported which are suggestive of why our fingerprints are not straight (we can compare ourselves with "macaque monkeys [that] have ridges parallel to the long axis of their fingers"). Experiments show that the amplification effect is "strongly dependent on the orientation of the ridges with respect to the scanning direction". So, at the level of hypothesis, it is suggested that our elliptical ridges respond well, whatever the direction of sliding.

"In humans, fingerprints are organized in elliptical twirls so that each region of the fingertip (. . .) can be ascribed with an optimal scanning orientation."

Thirdly, the observed response behaviour of the sensory system is suggestive of several other valuable performance characteristics, described as "interesting functional consequences of fingerprints".

"Remarkably, the response function of the fingerprinted system displayed in Fig. 3 is analogous to a Gabor filter because it provides both spatial and spectral resolution. Such filters are classically used in image analysis and have been identified in visual systems at the neural level. They are known to provide orientation discrimination, contrast enhancement, and motion detection."

In a commentary article by Miller, the thoughts of a biomedical engineer are documented:

"It's a really interesting finding because it demonstrates the extent to which the physical and mechanical properties of a sensor can perform a computation". Selecting and amplifying the signals important for texture perception could in principle be accomplished within the nervous system. [. . .] But in this case, it seems to be the design of the hardware rather than the programming of the neural software that does the trick.

So, next time you are contemplating your fingerprints, it is worth reflecting, not only on the fact that you are unique, but also on the difference fingerprints make to your quality of life. By coupling hardware design with programming of neural software, you have been equipped with a tactile sensory system that beats any robot capability. All this talk of hardware design features and software programming is fully consistent with the conceptual framework of Intelligent Design, but it must be an aggravation to those who want to ban the D-word from biology.

The Role of Fingerprints in the Coding of Tactile Information Probed with a Biomimetic Sensor
J. Scheibert, S. Leurent, A. Prevost, and G. Debregeas
Science, 323, 13 March 2009: 1503-1506 | DOI: 10.1126/science.1166467

Abstract: In humans, the tactile perception of fine textures (spatial scale <200 micrometers) is mediated by skin vibrations generated as the finger scans the surface. To establish the relationship between texture characteristics and subcutaneous vibrations, a biomimetic tactile sensor has been designed whose dimensions match those of the fingertip. When the sensor surface is patterned with parallel ridges mimicking the fingerprints, the spectrum of vibrations elicited by randomly textured substrates is dominated by one frequency set by the ratio of the scanning speed to the interridge distance. For human touch, this frequency falls within the optimal range of sensitivity of Pacinian afferents, which mediate the coding of fine textures. Thus, fingerprints may perform spectral selection and amplification of tactile information that facilitate its processing by specific mechanoreceptors.

See also:

Miller, G. Fingerprints Enhance the Sense of Touch, Science, 323 (30 January 2009), 572b | DOI: 10.1126/science.323.5914.572b