Optimal parameters for microstimulation derived forelimb movement thresholds and motor maps in rats and mice

Intracortical microstimulation (ICMS) is a technique that was developed to derive movement representations (motor maps) of the motor cortex, and was originally used in cats and the capuchin monkey. In more modern experiments, ICMS has been used in rats and mice to assess and interpret plasticity of motor maps in response to experimental manipulation; however, a systematic determination of the optimal ICMS parameters necessary to derive baseline motor maps in rats and mice has not been published. In the present manuscript, we describe two experiments. We first determined the optimal stimulation frequency, pulse number, neocortical depth, and current polarity to achieve the minimum current intensity (movement threshold) to elicit forelimb movements in rats and mice. We show that experimentally naïve rats and mice differ on several of these ICMS parameters. In the second experiment, we measured movement thresholds and map size in states of enhanced neocortical inhibition by the administration of diazepam, as well as neocortical sensitization as the result of repeated seizures. We conclude that movement thresholds are inversely related to motor map size, and that treatments result in a widespread shift the balance between excitation and inhibition in motor neocortical layer 5 influences both movement thresholds and map size.