\subsection*{Electrophysiology}

We recorded electrophysiological data from X cells from Y different fish.

\textit{Surgery}. Twenty-two E. virescens (10 to 21 cm) were used for
single-unit recordings. Recordings of electroreceptors were made
from the anterior part of the lateral line nerve.
Fish were initially anesthetized with 150 mg/l MS-222 (PharmaQ,
Fordingbridge, UK) until gill movements ceased and were then
respirated with a constant flow of water through a mouth tube,
containing 120 mg/l MS-222 during the surgery to sustain anesthesia.
The lateral line nerve was exposed dorsal to the operculum. Fish were
fixed in the setup with a plastic rod glued to the exposed skull bone.
The wounds were locally anesthetized with Lidocainehydrochloride
2\% (bela-pharm, Vechta, Germany) before the nerve was exposed.
Local anesthesia was renewed every 2 h by careful application of
Lidocaine to the skin surrounding the wound.
After surgery, fish were immobilized with 0.05 ml 5 mg/ml tubocurarine (Sigma-Aldrich, Steinheim, Germany) injected into the trunk
muscles. 
\sout{Since tubocurarine suppresses all muscular activity, it also
suppresses the activity of the electrocytes of the electric organ and thus
strongly reduces the EOD of the fish. We therefore mimicked the EOD
by a sinusoidal signal provided by a sine-wave generator (Hameg HMF
2525; Hameg Instruments, Mainhausen, Germany) via silver electrodes
in the mouth tube and at the tail. The amplitude and frequency of the
artificial field were adjusted to the fish’s own field as measured before
surgery.} After surgery, fish were transferred into the recording tank of the
setup filled with water from the fish’s housing tank not containing
MS-222. Respiration was continued without anesthesia. The animals
were submerged into the water so that the exposed nerve was just above
the water surface. Electroreceptors located on the parts above water
surface did not respond to the stimulus and were excluded from analysis.
Water temperature was kept at 26°C.\footnote{From St\"ockl et al. 2014}

\textit{Recording. }Action potentials from electroreceptor afferents were
recorded intracellularly with sharp borosilicate microelectrodes
(GB150F-8P; Science Products, Hofheim, Germany), pulled to a resistance between 20 and 100 M and filled with a 1 M KCl solution.
Electrodes were positioned by microdrives (Luigs-Neumann, Ratingen,
Germany). As a reference, glass microelectrodes were used. They were
placed in the tissue surrounding the nerve, adjusted to the isopotential line
of the recording electrode. The potential between the micropipette and the
reference electrode was amplified (SEC-05X; npi electronic) and lowpass filtered at 10 kHz. Signals were digitized by a data acquisition board
(PCI-6229; National Instruments) at a sampling rate of 20 kHz. Spikes
were detected and identified online based on the peak-detection algorithm
proposed by Todd and Andrews (1999).
The EOD of the fish was measured between the head and tail via
two carbon rod electrodes (11 cm long, 8-mm diameter). The potential
at the skin of the fish was recorded by a pair of silver wires, spaced
1 cm apart, which were placed orthogonal to the side of the fish at
two-thirds body length. The residual EOD potentials were recorded
and monitored with a pair of silver wire electrodes placed in a piece
of tube that was put over the tip of the tail. These EOD voltages were
amplified by a factor of 1,000 and band-pass filtered between 3 Hz and
1.5 kHz (DPA-2FXM; npi electronics).
Stimuli were attenuated (ATN-01M; npi electronics), isolated from
ground (ISO-02V; npi electronics), and delivered by two carbon rod
electrodes (30-cm length, 8-mm diameter) placed on either side of the
fish parallel to its longitudinal axis. Stimuli were calibrated to evoke
defined AM measured close to the fish. Spike and EOD detection,
stimulus generation and attenuation, as well as preanalysis of the
data were performed online during the experiment within the
RELACS software version 0.9.7 using the efish plugin-set (by J.
Benda: http://www.relacs.net).\footnote{From St\"ockl et al. 2014}

\textit{Stimulation.} White noise stimuli with a cutoff frequency of 300{\hertz} defined an AM of the fish's signal. The stimulus was combined with the fish's own EOD in a way that the desired AM could be measured near the fish. Amplitude of the AM was 10\% (?) of the amplitude of the EOD. Stimulus duration was between 2s and 10s, with a time resolution of X.