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Biomechanics of Human Fingerpad-Object Contact

The goals of this research are (1) to determine the growth and motion of contact regions and the associated force variations over time between the human fingerpad and carefully chosen transparent test objects whose microtexture, shape or softness is varied in a controlled manner and (2) Experimental measurement of the surface deformations of human fingertips under shaped indentors. The results obtained are being used to gain a deeper understanding of the neurophysiological and psychophysical data we have already obtained for the same test objects.

To measure the in vivo surface deformations of the fingerpad under various tactile stimuli, we have designed a videomicroscopy system together with a high precision tactile stimulator. The videomicroscopy system consists of a set of video zoom lenses attached to a high-resolution CCD camera, whose output can either be digitized into the computer system memory in real time at about 20 frames/s, or stored on a laserdisk at 30 frames/s for off-line digitization. The zoom lenses enable continuous variation of magnification, with the field of view covering the entire fingerpad, or just a few fingerprint ridges. The tactile stimulator is composed of a linear stepper motor with a microstepping drive. The motor is controlled by a 80386 PC, with a specified indentation velocity commanded by a 80486 PC via a digital link. To record the contact force, a strain gage based single degree of freedom force sensor is mounted on the motor to which a transparent test object can be attached for both biomechanical and psychophysical experiments. This method allows the force and video data to be synchornized and stored in the 80486 PC. With this setup, we are able to investigate how the skin-object contact region changes with indentation velocity and force. In active touch experiments the subject contacts a stationary specimen, whereas in passive touch experiments the stimulator moves the specimen to indent a stationary finger at a given velocity. High contrast images of the contact interface are achieved with coaxial and other fiberoptic lighting.

Videomicroscopy of the fingerpad-object contact regions

Using the test facility described above, we have performed a set of experiments with human subjects to investigate the relationship between the contact force, contact area and compliance of the object. The experiments involved active indentation of transparent compliant rubber specimens and a glass plate with the subjects' fingerpads. Static video images of the contact regions were captured at various force levels and magnifications. In order to minimize the effects of non-uniform illumination, we implemented homomorphic image processing algorithms with or without image decimation. The processed images showed that contact regions consisted of discontinuous `islands' along each finger ridge, with clear distinction between contact and non-contact regions over the entire field of view.

Results show that for objects whose compliances are discriminable, even when the overall contact areas under a given contact force are the same, the actual contact areas can differ by a factor of two or more. The actual pressure distribution, which acts only within the discontinuous contact islands on the skin, will therefore be radically different for the objects. Consequently, a spatio-temporal neural code for object compliance emerges with far higher resolution than an intensive code such as the average pressure over the overall contact area. These results are in agreement with our hypothesis that the neural coding of objects with deformable surfaces (such as rubber) is based on the spatio-temporal pressure distribution on the skin. This was one of the conclusions from our previous psychophysical, biomechanical and neurophysiological experiments (Srinivasan and LaMotte, 1995;1996).

Measurement of Surface Deformation of Human Fingerpads

The finite element models described previously need to be verified by comparing the experimentally observed skin surface deformations with those predicted by the finite element models under the same mechanical stimuli. The experimental data was obtained by indenting human fingerpads with several cylindrical and rectangular indentors and acquiring images of the undeformed and deformed fingerpad using the videomicroscopy setup (Roby, Dandekar, and Srinivasan, 1994; Roby and Srinivasan, 1995). Fine markers were placed on the fingerpad and the skin surface deformation was measured by tracking the displacements of the markers in the high resolution video images. The same experiment was simulated using the finite element models of the human fingertip and the displacements of corresponding points were compared with the experimental data. The displacements predicted by the multilayered 3D model matched the experimental data quite well (Dandekar, 1995).

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Last Updated: May 8, 2002 1:45 PM Comments: David Schloerb