Inspired by the work of Bach-y-Rita (1972) on tactile-visual sensory substitution, over the past 10 years we have been developing a computerized platform called Tactos (Lenay, Gapenne, Hanneton, Marque, & Genouelle, 2003). One of the main aims of this platform is to enable persons who are visually impaired (that is, those who are blind or have low vision) to read computerized two-dimensional (2-D) shapes (Gapenne, Rovira, Ali Ammar, & Lenay, 2003). The platform has been used in the context of fundamental research on perception (Sribunruangrit, Marque, Lenay, Gapenne, & Vanhoutte, 2004), and for the design of new interfaces for perception and communication (Ziat, Gapenne, Stewart, & Lenay, 2007). The fact that this platform makes it possible to read simple computerized shapes has also provided the conditions for a longitudinal study of the teaching of mathematics, particularly geometry, to a small group of high school students who are congenitally or adventitiously blind (Rovira & Gapenne, 2008). In the setting of this longitudinal study, we have been particularly interested in the activities of categorization of shapes and the attribution of meaning to shapes. Beyond the simple perception of shapes, we wanted to evaluate the capacity of these students to categorize geometric line drawings defined by several dimensions. In addition, we wanted to study and compare the categorization strategies used by these students when they performed identical tasks either with traditional material (on thermoformed paper) or with the Tactos system. The Tactos platform (see Figure 1) consists of a graphics tablet and its pen, a computer, and a tactile stimulator (2 standard 8-pin electronic braille cells). The participant can explore a line drawing by moving the pen on the graphic tablet. The Tactos software provides the link between the position of the pen and the stimulators: The 16 (2 x 8) pins of the braille cells are activated in an all-or-none fashion whenever the participant encounters the shape (which is represented by black pixels) being explored by means of the pen. The tactile stimulation is delivered on the free hand. It is to be noted that perception with this instrument does not involve any symbolic coding--the activation of one or several pins on the braille cells corresponds solely to contact with the virtual object--and that at any one moment, access to the object (a 2-D shape in this case) is partial--the capture zone of 64 pixels is not large enough to cover the entire shape, so that global perception requires active displacements by the participant. After acquiring effective exploratory strategies (which takes about 20 hours of learning), the students were able to perceive and recognize simple shapes. [FIGURE 1 OMITTED] We conducted a systematic comparison of the categorization strategies that the three students used when exploring 2-D line drawings, which were presented either as raised forms on the traditional thermoformed paper or as computerized forms on the graphic tablet of the Tactos device. More precisely, the participants first performed a recognition exercise in each of these two contexts to verify that they could properly recognize the line drawings used for the categorization and that they were sensitive to the three dimensions that were used (size, orientation, and shape). Then, two categorization tasks were tested: a free-classification task and a sorting task into predefined categories. The main object of our study was to investigate the students' understanding of geometric line drawings and thus to compare the categorization in two contexts of perception: direct perception with bare fingers using the traditional material and instrumental perception using the Tactos device. METHODS Participants The participants were three high school students who were blind from birth and were aged 14 at the time of the observation. Two students (one boy, Michael, and a girl, Belinda) attended a community school, and the other girl Janice attended a specialized school for students who are visually impaired. …