Haptic shape perception from force and position signals varies with exploratory movement direction and the exploring finger

We investigated how exploratory movement influences signal integration in active touch. Participants judged the amplitude of a bump specified by redundant signals: When a finger slides across a bump, the finger's position follows the bump's geometry (position signal); simultaneously, it is exposed to patterns of forces depending on the gradient of the bump (force signal). We varied amplitudes specified by force signals independently of amplitudes specified by position signals. Amplitude judgment was a weighted linear function of the amplitudes specified by both signals, under different exploratory conditions. The force signal's contribution to the judgment was higher when the participants explored with the index finger, as opposed to the thumb, and when they explored along a tangential axis, as opposed to a radial one (pivot congruent to shoulder joint). Furthermore, for tangential, as compared with radial, axis exploration, amplitude judgments were larger (and more accurate), and amplitude discrimination was better. We attribute these exploration-induced differences to biases in estimating bump amplitude from force signals. Given the choice, the participants preferred tangential explorations with the index finger-a behavior that resulted in good discrimination performance. A role for an active explorer, as well as biases that depend on exploration, should be taken into account when signal integration models are extended to active touch.