Cutaneomuscular reflexes (CMRs) can be recorded in the hand muscle of human subjects after stimulation of a digital nerve. We hypothesized that repeated synchronous stimulation of nerves from two digits may lead to long-term plastic changes in CMR, by the mechanisms of spike-timing-dependent plasticity (STDP). To test this idea, we conducted experiments in 27 healthy human volunteers. After baseline measurement of CMR, one of four 30-min-long stimulation conditions were tested
the CMR was then remeasured. The four conditions were simultaneous index finger and thumb stimulation
asynchronous index finger and thumb stimulation
thumb 5 ms before index finger stimulation
and thumb-only stimulation. Neither the early (E1) nor late excitatory (E2) components of the CMR showed consistent changes after any stimulation condition. The inhibitory (I1) component was slightly reduced in all cases. To understand why paired stimulation did not produce long-term changes, we conducted a further experiment. In this, we measured the CMR in response to simultaneous stimulation of index finger and thumb, compared with a prediction expected if the responses summed linearly. This revealed sublinear summation, possibly indicating partial response saturation after stimulation of only one digit. We argue such a pattern prevents paired stimuli from generating especially reliable and well-timed outputs relative to synaptic inputs in downstream neurons, which is required to produce plasticity by STDP.