Fig. 3.

Sketch of technique used to measure double-referent phase of single-unit interneuron (bottom trace) activity during normal rostral scratching with respect to the hip-flexor ENG burst (middle trace) and the integrated full-wave rectified hip-flexor ENG (top trace). The start of the burst of the integrated full-wave rectified hip-flexor ENG was defined as 0.0 (and 1.0) phase; the end of the burst was defined as 0.5 phase. In this example, the start of the unit burst occurred when hip-flexor activity was 74% complete. This ON-phase occurred at 0.37 (= 0.74/2). The end of the unit burst occurred when hip-flexor quiescence was 54% complete. This occurred at an OFF-phase of 0.77 [= 0.5 + (0.54/2)]). See Materials and Methods for details of double-referent phase calculation (Berkowitz and Stein, 1994b). The unit was active during 26% of the hip-flexor burst; in this example of a single cycle shown for illustration, we term this 26% overlap. In the remainder of the paper, overlap percentage was calculated on the basis of mean ON- and OFF-phases for each unit.