Functional slip reflexes aid bimanual grasp

When an object is held between the outstretched hands, feedback mechanisms help to maintain the grasp. This paper reports a stereotyped bimanual response in which the hands grasp an object tighter after it begins to slip. The response has both uncrossed and crossed components, each hand responding to slip across its own fingertips, as well as to slip from the opposite hand. Electromyographic (EMG) responses to slip occur at latencies between those of a monosynaptic stretch reflex and a voluntary response. The latency to EMG on the perturbed side is about 50 ms, and 80 ms on the unperturbed side. The size of the crossed response depends on the recent experience and expectation of the subject. Smaller crossed responses occur after a series of unimanual stimuli, or when stimulus to only one hand is expected. By analogy to postural reflexes of fixed latency but adaptive amplitude, this response should be classified as a functional slip reflex.