Invariant temporal characteristics of manipulative hand movements.

The measurement of eight manipulative serial hand movements showed a clear distribution of their temporal characteristics into two distinct groups. When the hand was used as a sense organ during active touch the finger movements across objects were restricted to a slow performance range below 2 Hz. Recordings from single mechanoreceptive afferents and calculations of their receptor densities indicated that these movements have to be slow to match the temporal requirements of the sequential sampling process from the mechanoreceptor populations. In contrast, manual skills not associated with the collection of sensory information like handwriting, typing or pencil shading, were performed rapidly. Their frequencies were close to those of fastest possible tapping. Evidence is provided that the different frequency groups are associated with distinct sensory control processes. The low frequency group represents movements involving focal sensory control (Julesz 1984). The high frequency group is not performed open-loop but monitored by preattentive sensory processes. The results indicate a dual sensory control mode operating in separate frequency domains of movement.