Do Humans Sense Finger Deformation or Distributed Pressure to Detect Lumps in Soft Tissue?

Detecting and localizing hard lumps in soft tissues is an important medical procedure. A psychophysics experiment measured minimum indentation forces required to detect a hard ball embedded in a soft rubber model. Ball size and finger indentation speed were varied. Threshold detection forces increased with ball size and decreased with indentation speed. The shape of the finger and contact pressure distribution on the finger were measured at the threshold force. The relative deformation of the finger induced by the ball was determined by comparing the shape of the finger indenting models with and without a ball at the same indentation force. The maximum relative deformation was nearly constant for the different ball sizes and increased significantly as indentation speed decreased. The measured pressure distributions show a considerable change across different ball sizes and indentation speeds. This suggests subjects sensed deformation of the fingerpad induced by the ball and not changes in the pressure distribution.