Skilled forelimb reaching for pasta guided by tactile input in the rat as measured by accuracy, spatial adjustments, and force

Rats are capable of reaching for food with a single forelimb, but since they locate the target of their reach using olfaction, it is unclear how they adjust their limb movement to compensate for errors. Although it is thought that their reaching movement is ballistic and can only be adjusted by trial and error, whether they cap use haptic cues to aid in locating and identifying a target has not been examined. The present study addressed this question by allowing rats to reach through a slot for rigidly held pieces of uncooked pasta of varying thickness, which could be oriented vertically or horizontally from different points around the slot and which were attached to a force transducer. The tasks required that animals not only adjust their reach and grasp to the target's location but also identify the target based on its texture. Acquisition curves were made of head orientation, limb transport trajectories, number of attempts per success, paw orientation, breaking direction and force of the grasp. A haptic discrimination test used pasta and similar sized metal rods with different tactile properties as discriminanda. The results indicated that whereas postural orientation and limb transport trajectory were not modified as a function of target orientation, paw orientation and grasp force did vary as a function of the sensory qualities of the target object, and the rats could make a haptic discriminative choice of a target object. The results show that the rat is capable of adjusting paw movements using haptic information, suggesting that somatosensory features of sensorimotor control of limb and paw movements in carnivores and primates are shared by rodents. This commonality points to a conservation of motor control in mammals: explains some of the idiosyncratic features of rat reaching behavior, and confirms that rodents provide a good model for investigating sensorimotor functions. (C) 2000 Elsevier Science B.V. All rights reserved.