Proprioceptive contributions to tactile identification of figures: dependence on figure size

Studies on the recognition of objects or shapes explored by a hand or finger have concentrated on the relative merits of different methods of exploration--in particular, whether the exploring hand or finger is moved by the subject himself (active), is guided by an assistant (passive), or remains stationary with the shape being moved by an assistant (tactile). These factors, together with another variable--the size of the explored shape--were investigated in twelve normal adults whose task, without the aid of vision, was to explore and identify various shapes with the tip of the extended right index finger. Two series of shapes were used, a large (15 cm) and small (3 cm) series, both of which consisted of twelve shapes each being a variant of the letter 'S'. The shapes were outlined as rows of Braille-like dots on a flat surface. Exploration was also tested in the proprioceptive mode, in which the exploring hand was guided by an assistant around a shape which was simply drawn on the surface so as to give a minimum of tactile information. For neither large nor small shapes could we demonstrate differences between active, passive and proprioceptive exploration. For the small shapes, the active, passive and proprioceptive modes gave no advantage over purely tactile assessment. However, for the larger shapes, active, passive and proprioceptive modes allowed better identification than movement of the shape. Thus, purely cutaneous signals appear sufficient for optimal discrimination of small figures but proprioceptive signals, generated by movement, are needed for optimal discrimination of larger figures.