Fig. 3. A cancellation signal appeared in an AEN after five 1 min periods of coupling parallel fiber stimulation to an electrosensory stimulus. (A) Raster plot shows activity of an AEN before, during and after the five 1 min couplings of a 2 µV dipole stimulus to direct parallel fiber stimulation. No measurable change is found after a single 1 min coupling test (P>0.05) during the precoupling period. However, the cancellation signal (or negative image) following five 1 min coupling cycles appears as soon as parallel fiber stimulation is resumed in the post-coupling period (P<0.01), and then fades within minutes as parallel fiber stimulation is given alone. Note that since the parallel fiber inputs are the source of the cancellation signal in the AEN there is no cancellation signal present during the rest periods when the parallel fiber stimulation is stopped along with the electrosensory stimulus. (B) Histograms (30 trials each) from the same data as in the raster plots in A, representing the AEN activity before, during and after a single 1 min coupling test. (C) Histograms before, during and after the five 1 min episodes of coupling. Note the presence of the cancellation signal (seen as a negative image of the initial response of the AEN) when parallel fiber stimulation is resumed after the five 1 min couplings (P<0.01). Also note that during the five 1 min couplings, the response of the AEN to the dipole stimulus gradually declines with each additional coupling cycle. Rasters, histograms and labels as in Fig. 2.