How Human can Discriminate between Convex and Concave
Shape from the Tactile Stimulus
Masashi Nakatani, Naoki Kawakami and Susumu Tachi({nakatani, kawakami, Tachi}@star.t.u-tokyo.ac.jp)
Department of Information Physics and Computing, 7-3-1 Hongo, Bunkyo-Ku
Bunkyo-Ku Tokyo 113-8656 JAPAN
Keywords: tactile signal processing; pin matrix;

Introduction
Computational model of how vision system reconstructs theworld information from visual stimulus has been studied sofar (Marr 1982). Many of the famous achievements done byDavid Marr have been applied to the computer visionstudies in the engineering field. On the other hand, little isknown about how people can reconstruct 3D informationfrom 2D stimulus distribution in the sense of touch. Invision, shape recognition has been revealed to depend on
information such as stimulus intensity difference, texture,contour, or shading. Analogous to vision, a candidate forunderstanding the shape in touch can be the contour or edge
location of an object.
In tactile sensation, contour or edge locations arerepresented by the information on pressure (nominal tohuman body) or that on friction (tangential to human body).
Take Braille (embossed dots on paper or metal plate) as an
example of a tactile shape with 3D information. When areader's finger strokes over Braille, the nominal force(pressure) and the tangential force (friction) are presented
according to the nominal force which the reader applies.
However, it has been demonstrated that both of these twokinds of forces are not necessarily required to recognize
Braille. For example, Optacon (Bliss, 1970) is the firstdevice that enables the user to read the characters by using
vibrating pins which move only up and down at highfrequency. Hayward et al (2005) found that horizontal force
applied to the finger can give bumpy sensation, and they
developed a tactile display with this principle. Although thisis a good compromise to develop tactile display, there
remains a possibility that presenting only nominal ortangential force can be ambiguous, resulting in incorrectinterpretation. One evidence for such possibility was foundby Nakatani et al.(2005); they found that convex or concave
judgment with a pin type tactile stimulator can be lessaccurate if the displacement of the pin is 0.1 mm. This
perceptual discrepancy can be a possible way to understandthe mechanism of tactile signal processing in human.
Therefore, we conducted the experiment to examine howfrequently such convex and concave inversion occurs.

in diameter) with various center-to-center spacing (0.5, 1.0,1.5, 2.0 mm) (Figure 1). The pins' movement is constrainedby close-tolerance holes in fiberglass boards to allow freemovement in the vertical direction and essentially prohibit
motion in the vertical direction. Users place the index finger
on top of the pin array grasping the boards with thumb andmiddle finger, then stroke the array over the test surface.
Each pin samples the height information from the surface
pattern and relays it to the finger tip. Subjects (N=9, eightmales and two females, averaged age was 24.6) evaluatedwhether the contact surface contained raised or indented
surface in two-alternative forced choice procedure. Subjects
stroked their bare finger or their finger with pin matrix overthe stimulus(0.1 mm raised or indented circle (diameter = 25mm) embossed on an aluminum plate).

Results and Discussion
The result is shown in the right side of Figure 1. All subjects
could correctly answer whether the surface was convex orconcave with over 90 % accuracy with their bare finger. On
the other hand, when touching the convex surface with thepin matrix of 0.5 mm or 1.0 mm spacing, two of sevensubjects could not correctly answer (less than 75 %) thegeometry. This implies that high spatial frequency
component disturbed the interpretation of a shape in touch.

Figure 1: Schematics of pin matrix and the result.

References

Materials and Methods

Marr, D. (1982). Vision. New York: W. H. Freeman andCompany.
Bliss, J. C. (1970). Dynamic tactile displays in manmachine systems. IEEE Transactions on Man-Machine
Systems, 11, 58-65.
Nakatani, M., Kajimoto, H., Kawakami, N. & Tachi, S(2005). High Density Pin Matrix, Proceedings of theSecond Annual Conference of the Applied Perception andGraphic Visualization (p. 169). La Coruna, ACM.
Hayward, V., & Cruz-Hernandez, J. M. (2000) Tactile
Display Device Using Distributed Lateral Skin Stretch,
Proceedings of the Haptic Interfaces for Virtual
Environment and Teleoperator Systems Symposium (p.
1309-14. Florida, IEEE.

We used a passive high density pin matrix (Nakatani et al.,2005) as a stimulator of subjects’ finger. This passive device
is composed of steel pins (0.3, 0.8, 1.3, 1.5 mm, respectively1820