Hand shaping in the rat: conserved release and collection vs. flexible manipulation in overground walking, ladder rung walking, cylinder exploration, and skilled reaching.

Hand shaping in terrestrial mammals is adapted to many functions including walking, climbing, exploration, and skilled manipulation. Nevertheless, hand shaping is not well described in the rat (Rattus norvegicus) although the species is used to study the evolution of movement, the neural control of movement, and to model impairments that can result from brain injury. In the present study, rat hand movements were examined in standardized tests of overground walking, horizontal or inclined ladder rung walking, exploring a vertical wall of a cylinder, and skilled reaching for food. Behavior was filmed with high-speed (250-1000 f/s) video camera from which frame-by-frame behavioral and kinematic analyses (Peak Motus) were made. There were three hand actions common to all tasks. In release, the hand pushed off or was lifted from a substrate; in collection, the digits were closed and flexed though the midpoint of limb transport; and, in manipulation, the hand and digits were shaped to contact or grasp a target. The movements of release and collection, although variable in character, speed and duration, were very similar in the different tests. The movement of manipulation featured greater specialized digit use and varied sensory control (olfaction, vibrissae, and tactile senses) in different tasks. Conserved release and collection vs. the variability of manipulation is discussed in relation to the evolution, neural control, and neural commitment underlying hand movements.