Exploratory pressure influences haptic shape perception via force signals

In this study of the haptic perception of small bumps, we investigated the influence of exploratory movement variation on signal integration and the percept's reliability. When sliding across a bump on a surface, the finger follows the geometry of the bump (i.e., the position signal). At the same time, patterns of forces depending on the gradient of the bump act on the finger (i.e., the force signal; Robles-de-la-Torre & Hayward, 2001). Consistent with the maximum likelihood estimation (MLE) model, haptically perceived shape can be described by a weighted average of the shapes signaled by the position and force signals (Drewing & Ernst, 2006; Ernst & Banks, 2002). Here, we found that the weights of the position and force signals and the reliability of the shape percept depend on the pressure (and velocity) of the exploratory movement (Experiment 1). These effects could not be traced back to pressure effects on the reliability of the single signals, as would be predicted from the MLE model (Experiment 2). However, we found that the signal-specific shape estimate derived from the force signal increases with increasing pressure (Experiment 3), and this bias can explain the perceptual effects of exploratory pressure.