Fig. 8.

Schematic illustration of hypothetical neuronal pools assessed by conventional (A-SICI) and threshold-tracking (T-SICI) techniques. This diagram is based on an assumption that the size of the neuron pool under investigation is defined by the stimulation intensity. Triangles represent upper (cortical) motoneuron pool, circles – inhibitory interneurons projecting onto motoneurons. In A-SICI, the size of the motoneuron pool (represented in a by 3 of 4 neurons; grey triangles) that will be tested and will generate an unconditioned motor response is pre-determined by the test stimulus (TS) intensity. As the intensity of the conditioning stimulus increases (b to d), inhibitory (GABA-ergic) interneurons are progressively recruited (black circles), exerting increasingly stronger inhibitory effect on the upper motoneuron pool (black triangles). As a result the conditioned MEP amplitude decreases (b, c) and is eventually abolished (d). Although even more inhibitory interneurons might be recruited by stronger conditioning stimuli (e), this cannot be further quantified (as the inhibited neurons are not activated by the test stimulus) thus producing a ‘floor effect’. By contrast, in T-SICI, test stimulus intensity is adjusted to counteract the effect of the inhibitory interneurons so that a small response (represented by a single grey neuron) is always obtained (g to j). Although potentially different subsets of motoneurons are assessed at different conditioning stimulus levels, this allows the inhibitory potential of GABA-ergic interneuron pool to be fully evaluated. Arrows indicate change in MEP amplitude (pink) and test stimulus intensity (blue). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)