Fig. 2.
Schematic illustration of linear and nonlinear interactions between two successive impulsive-like stimuli. When applied separately, the two stimuli x1(t) and x2(t) elicit the output waveforms y1(t) and y2(t), respectively (top). When the two stimuli are applied in succession and there is no nonlinear interaction between them, the total output yL(t) is simply the addition between y1(t) and y2(t) (bottom, solid line). On the other hand, when nonlinear interactions between the two stimuli/responses occur, the total output yNL(t) is not equal to y1(t) + y2(t) (bottom, dashed line). In the hypothetical case illustrated at bottom, the response to the second stimulus is suppressed by 50% by the presence of the first stimulus. Hereby we examine if the principle of superposition holds and also if the event-related potentials (ERPs) elicited are identical, i.e., whether y1(t) = y2(t) in the case of somatosensory stimuli, for the range of ISIs considered.