English: Original representation of the hypothesis that the directional accuracy and speed of upper limb movements are achieved via circuits tuned variably by local inhibitory mechanisms, as described and illustrated in Georgopoulos and Carpenter (2015) and Mahan and Georgopoulos (2013). Afferent inputs (excitatory) are shown in cyan. The red cells are inhibitory interneurons. The pyramidal cell labelled P is subject to focused excitatory drive. The other pyramidal cells make up the “excitatory fringe”. It is hypothesised that “strong input” to inhibitory interneurons (A) sharpens the locus of motoneuron excitation by attenuating the contribution of the excitatory fringe. This results in a reduction of the directional tuning width and the generation of an accurate and short population vector. In contrast, “weak input” to inhibitory interneurons (B) results in an increase in the directional tuning width and the generation of a less accurate and longer population vector – corresponding to a faster but less accurate movement.