A comparison of force and depth of skin indentation upon psychophysical functions of tactile intensity

Tactile sensory intensities related to force applied to the skin, and depth of skin indentation were measured with a magnitude estimation procedure at various sites on the left hand of four human subjects. These same skin sites were measured for 'compressibility' - that is, the indentation depths that resulted from controlled forces. Graphic examination of the magnitude estimation data indicated that, in most cases, growth of sensory intensity was relatively shallow at the lower stimulus intensities, and steeper at higher stimulus intensities. The 'breakpoint' between the shallow and the steep legs of the psychophysical functions was routinely found between 0.30 and 0.40 mm of indentation, and between 12.0 and 20.0 mN of force. Two subjects consistently produced positively accelerating psychophysical functions, whereas the other two produced negatively accelerating or nearly linear functions above the breakpoint. Differences in skin compressibility did not systematically alter the exponent of the psychophysical functions, regardless of the stimulus dimension (i.e., force or depth of skin indentation). Psychophysical functions based on controlled depth of skin indentation, at a constant rate of indentation, consistently produced higher r² values than psychophysical functions based on controlled force. When the exponents of psychophysical functions based on controlled skin indentation were compared across different regions of the hand, the values were ordered such that dorsum of hand>finger>thenar. It was concluded that tactile sensory intensity is more closely related to depth of skin indentation than to force, but only when the rate of skin indentation is controlled.