J. exp. Biol. 171, 107-125 (1992)
Printed in Great Britain © The Company of Biologists Limited 1992
107
SUPPRESSION OF COMMON MODE SIGNALS WITHIN THE
ELECTROSENSORY SYSTEM OF THE LITTLE SKATE
RAJA ERINACEA
BY DAVID BODZNICK*, JOHN C. MONTGOMERY!
AND DAVID J. BRADLEY
Marine Biological Laboratory, Woods Hole, MA 02543, USA
Accepted 23 June 1992
Summary
The electroreceptors of elasmobranchs are strongly modulated by thefish'sown
ventilation but this source of potential interference is suppressed within the
medulla. The mechanism for the suppression is thought to be based on the
common mode nature of the ventilatory noise, i.e. it is of the same amplitude and
phase for all of the electroreceptors, compared with environmental electric fields
which affect the receptors differentially. Evidence for the common mode
suppression hypothesis is provided here in skates by the observation that the
response to an artificial common mode stimulus that is independent of ventilation
and delivered through an electrode inserted into the animal's gut is also
suppressed by the medullary neurons; the extent to which a particular neuron
suppresses the responses to the gut stimulus and to ventilation is similar. In
addition, a potential modulation of 5-150/iV is measured between the skate's
interior and the sea water during ventilation and this appears to be responsible for
the self-stimulation. By passing d.c. or sinusoidal currents through the gut
electrode it is demonstrated that this ventilatory potential is due to the variable
shunting of a standing d.c. potential across the fish's skin by the opening and
closing of the mouth and gill slits during ventilation. Osmoregulatory ion-pumping
appears to contribute to the production of the d.c. potential.
Introduction
Elasmobranch fishes possess an acutely sensitive electrosensory system with
electoreceptor organs known as ampullae of Lorenzini. These ampullary organs
are arranged in a series of 4-6 clusters on each side of the body. At each cluster,
the receptor cells are located in subdermal alveoli at the blind endings of canals
* Present address: Department of Biology, Wesleyan University, Middletown, CT 06457,
USA.
t Present address: Department of Zoology, University of Auckland, Auckland, New
Zealand.
Key words: elasmobranch, electroreception, noise suppression, reafference, medulla, Raja
erinacea.
108
D. BODZNICK, J. C. MONTGOMERY AND D. J. BRADLEY
that radiate outwards and open to the exterior via skin pores. The receptor
epithelium and the canal walls have a very high resistance in comparison with the
conductivity along the canals themselves. With this arrangement, afferent firing
rate effectively measures changes in potential difference between the sea water at
the skin opening and the basal surface of the receptor cells in the interior of the
animal. The best-documented behavioral role of the elasmobranch electroreceptors is in prey detection, where the system is used to detect the d.c. and lowfrequency bioelectric fields of other animals in the aquatic environment. However,
it is also possible that uniform electric fields such as those induced by the motion of
the animal itself or of ocean currents within the earth's magnetic field provide
useful information for orientation and navigation. The anatomy, physiology and
behavioral uses of the elasmobranch electrosense are reviewed in Bodznick and
Boord (1986), Kalmijn (1988) and Montgomery (1988).
The d.c. bioelectric fields of prey animals appear to result from differences in
the electrical properties among the various body surfaces the animals have in
contact with the surrounding water (Kalmijn, 1988) and in many cases probably
include a major contribution from the potentials produced across ion-exchange
surfaces. In fishes these d.c. potentials are thought to be modulated at low
frequencies by changes in resistance along the current pathway caused by the
opening and closing of the mouth and gill slits during ventilation. One problem
inherent in the electrosensory system is that the animal must detect extremely
weak bioelectric fields produced by other animals on top of its own very similar
bioelectric fields. The electric fields produced by elasmobranchs are typically of
lower amplitude than those produced by bony fishes (Kalmijn, 1974) but, even so,
recordings from primary afferent fibers of the electrosensory system in the
thornback ray, Platyrhinoldis triseriata, show afferent input to be strongly
modulated by the animal's own ventilation (Montgomery, 1984). An interesting
observation is that, in the secondary neurons of the electrosensory system in
Platyrhinoidis, this ventilatory modulation is virtually absent. The animals are able
to distinguish between afferent activity due to their own ventilation (reafference)
and that due to extrinsic fields and they can effectively suppress the ventilatory
reafference in the brain.
What is the mechanism for this noise suppression? Montgomery (1984) showed
that ventilatory modulation is of similar amplitude and phase in all afferent fibers
from one ampullary cluster; in other words, ventilatory modulation is common
mode. Afferents from canals of opposite orientation respond differentially to
extrinsic fields, but show a very similar response to ventilation. This indicates that
the ventilatory reafference is primarily driven by a potential modulation between
the animal's interior and the surrounding water rather than by the small potential
differences measured in the water around the fish during ventilation. These
observations support an earlier suggestion by Kalmijn (1974) that the grouping of
ampullae into clusters sharing a common internal reference may permit the
elimination of unwanted noise by a mechanism of common mode suppression.
New and Bodznick (1990) have recently shown that suppression of ventilatory
Electrosensory common mode signal suppression
109
reafference is also present in the little skate (Raja erinacea), though apparently to
a lesser degree than that shown in Platyrhinoidis. They found that in Raja afferent
modulation during ventilation is effectively common mode within all the electrosensory afferents, even those from opposite sides of the body, and direct evidence
was obtained for a contribution of contralateral input to the noise suppression
mechanism.
The demonstration that ventilatory modulation is common mode shows that
noise suppression could operate via a common mode suppression mechanism, and
this is further supported by the importance of the contralateral input for the
suppression (New and Bodznick, 1990). However, other possibilities exist. For
example, in the weakly electric fish Gnathonemuspetersii, ampullary electroreceptors respond to the animal's own electric organ discharge. This unwanted
reafference is removed by an elegant mechanism of modifiable efference copy (for
a review, see Bell, 1986). Although no evidence was found for this mechanism in
Raja (New and Bodznick, 1990), one aim of the present study was to provide a
more direct test of the common mode suppression hypothesis versus other
mechanisms by determining whether an experimentally imposed common mode
electrosensory input, independent of ventilation, would also be suppressed within
the central nervous system. In addition, we provide new information on the
characteristics and origins of the bioelectric fields associated with ventilation in
elasmobranchs.
Materials and methods
Experiments were performed on 20 specimens of the little skate Raja erinacea
Mitchill at the Marine Biological Laboratory in Woods Hole, MA. Animals were
caught in short trawl tows in Vineyard Sound and kept in cooled sea water until
their return to the laboratory, where they were maintained in holding tanks at a
temperature of 14°C. For experimentation, the animals were anesthetized by
immersion in tricaine methanesulfonate (approximately 0.02%), the cranium was
opened to expose the brain, which was then decerebrated by a transection at the
optic chiasm. Some animals were then paralyzed by intravenous injection of
tubocurarine chloride (3 mg kg" 1 ). In others, the spinal cord was transected about
1 cm behind the brain to permit normal ventilatory movements while immobilizing
the animal's trunk and tail. A salt-bridge electrode of PE90 tubing filled with
1.5% agar in sea water was inserted into the gut via the anus. An additional
Ag/AgCl electrode, made from 0.2mm diameter silver wire coated with Teflon
except near the tip, was implanted through a small skin incision into the interior of
the animal in the region between the hyoid and buccal ampullary clusters on the
head. The skin incision was then sealed with tissue adhesive (Histoacryl,
Trihawk). The animals were positioned on a Plexiglas head holder to stabilize the
brain for microelectrode recording. A Plexiglas plate inserted through the mouth
provided support for the cranium, but also meant that the mouth was held partly
open. In the case of paralyzed animals, a stream of oxygenated sea water directed
into the mouth provided for respiration. The rays were immersed in sea water up
110
D . B O D Z N I C K , J. C. MONTGOMERY AND D . J. BRADLEY
to the level of the cranial opening, and the water temperature in the experimental
bath was regulated at 8-10°C. These procedures followed NIH guidelines for care
and use of experimental animals and were approved by the Institutional Animal
Care and Use Committees of the Marine Biological Laboratory and Wesleyan
University.
Electrosensory afferent activity was recorded with glass micropipettes (4 mol 1~'
NaCl; 25 MQ) in the anterior lateral line nerve within the cranium. Platinumblack-tipped indium electrodes (2-5 [xm tip diameter, 2-7 MQ) were used to make
recordings from neurons within the dorsal octavolateralis nucleus, where ascending efferent neurons (AENs), the principal output neurons, were identified by
antidromic stimulation from the lateral mesencephalic nucleus. Single unit activity
was recorded during ventilation, during stimulation from the gut electrode and
during stimulation by uniform extrinsic electric fields. Spike data were analyzed
from poststimulus time histograms of 25-50 ventilatory cycles or stimulus
presentations. The response was measured as the peak-to-peak change in firing
rate produced by the stimulus or, in the case of a zero-firing rate nonlinearity, the
increase above spontaneous firing rate was measured and this valued was doubled
to obtain the effective peak-to-peak modulation.
Ventilation was monitored with the piezoelectric crystal of a phonograph
cartridge coupled to movements of the branchial chamber. This signal was used to
trigger the histogram program at the beginning of expiration. The stimulus
delivered through the gut electrode was a continuous 1 or 2 Hz sine wave centered
about zero and applied between the gut electrode and salt bridges (1.5 % agar in
sea water) located along all four sides of the experimental aquarium. The
amplitude was adjusted to give a 20 juV peak-to-peak signal measured between the
interior of the animal and an indifferent electrode placed in the seawater bath.
These amplitude and frequency values were chosen to approximate normal
ventilatory potential modulations recorded from skates. Longitudinal and transverse uniform fields were applied through salt bridges as a continuous 2 Hz sine
wave, typically at a peak-to-peak amplitude of 2juVcm~'. This intensity is within
the linear range of the intensity response functions of afferents and dorsal nucleus
neurons. The response of an electrosensory unit to a uniform field depends on the
orientation of its canal with respect to the field. For this reason, the overall
response of units to uniform fields was taken as the vector addition of their
responses to the two fields presented at right angles. The receptive field of units
was located by a small (5 mm) roving dipole electrode.
Potential modulations measured between the Ag/AgCl electrodes inside the
body and those in the water during ventilation were amplified by a PAR model 113
d.c. differential preamplifier with half-amplitude low-frequency cut-off at 0.03 Hz.
In four animals, this so-called ventilatory potential was monitored after the
intravenous injection of l-4mmol of NaCl (1 or 2 ml injections into the caudal
vein) to salt-load the animal. This was done in order to stimulate increased
osmoregulatory ion transport and thereby to assess its contribution to the
potential. In two other cases, the internal electrode was implanted in an otherwise
Electrosensory
common
mode signal suppression
111
intact animal to record the size of the ventilatory potential and to monitor changes
induced by placing the animal in diluted sea water.
Results
Ventilatory potential
With a Ag/AgCI electrode inserted beneath the skin of a skate, an electrical
potential modulation is measured between the fish's interior and the surrounding
sea water coincident with ventilatory movements. This ventilatory potential lasts
1-3 s but can vary greatly in both waveform and amplitude among fish (Fig. 1A)
and in a single fish at different times. Most often it takes the form of a biphasic
wave, which is inside-negative during expiration and then inside-positive during
inspiration, but in other cases the biphasic potential is reversed in polarity. More
rarely the ventilatory potential is monophasic, triphasic or even more complex.
In two unrestrained and unoperated animals the ventilatory potential amplitude
was 5-27 fxV. The ventilatory potential in operated animals set up for physiological
recordings ranged in peak-to-peak amplitude from less than 5/iV to more than
150 fiV, but was usually 15-25 fiV. Gradual changes in amplitude were often seen
in the hour immediately after surgery. Cycle by cycle changes in the intensity of
the normal breathing movements affected the ventilatory potential amplitude (see
bottom record, Fig. 1A), and occasional 'coughs', when water was vigorously
ejected through the spiracle, generally caused brief, large inside-positive ventilatory potentials.
The pattern of firing of afferent electrosensory fibers during ventilation was as
one would predict given their known excitatory response to a cathodal stimulus
located at the skin pore. That is, when the internal potential became positive
relative to the sea water, the firing rate in the afferents was accelerated; negative
inside potential suppressed afferent firing (Fig. 1A). As noted previously (Montgomery, 1984; New and Bodznick, 1990), this is true regardless of the location of
the skin pore of a particular receptor on the body surface. The degree of afferent
modulation was also directly proportional to the amplitude of the co-occurring
ventilatory potential (Fig. IB).
The ventilatory potential was investigated experimentally. Placing a skate in
70 % sea water resulted in a threefold increase in the ventilatory potential over
30min; it returned to normal soon after the fish was returned to full sea water.
Salt-loading animals by intravenous injection of l-4mmol of NaCl in order to
stimulate enhanced osmoregulatory ion transport caused a relatively small or no
immediate change in the potential, but after a lag of 5-20 h the potential increased
dramatically to a peak of 230-350 fiV and then returned to more normal levels
after 72-96 h (Fig. 2).
The origin of the ventilatory potential was studied further with the use of a
seawater-agar bridge electrode inserted through the anus into the fish's gut.
Current from a constant-current isolator, passed between the gut electrode and
salt-bridge electrodes on the four sides of the tank, created an electrical potential
112
D. BODZNICK, J. C. MONTGOMERY AND D. J. BRADLEY
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Fig. 1. Modulation of electrosensory afferent firing closely follows the waveform and
amplitude of a potential modulation recorded between the skate's interior and the
surrounding sea water during ventilation. (A) Three examples from different fish show
simultaneous recordings of primary afferent firing (top trace) and the ventilatory
potential (bottom trace) recorded with a subdermal Ag/AgCI electrode relative to a
second electrode in the water. Upward deflections in the ventilatory potential records
represent increased positivity inside the fish relative to the sea water. Periods of
expiration (E) and inspiration (I) are indicated under recordings. Calibration:
horizontal, 0.5 s for all traces; vertical, traces 1, 2 and 5, 50 j.iV cm~\ traces 3, 4 and 6,
100 ^V cm"1. (B) Peak-to-peak afferent modulation is shown as a function of the
ventilatory potential amplitude recorded simultaneously. Filled triangle indicates the
mean response of afferents to a 20^V gut stimulus.
between the inside of the fish near the ampullary clusters in the head and the
surrounding sea water that, as detailed below, appeared to be nearly the same over
all areas of the skin surface that were under water. A 3-15 Hz sinusoidal potential
produced by a current stimulus (200-300 ^iA) through the gut electrode and
monitored between the internal Ag/AgCI electrode and the indifferent electrode
80
Electrosensory common mode signal suppression
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Fig. 2. The effect of intravenous injection of either 1 or 2 mmol of NaCl on the
amplitude of the ventilatory potential modulation in two skates.
in the sea water showed an amplitude modulation during ventilation due to
apparent changes in the resistive pathway between the animal's interior and the
sea water outside. The peak-to-peak amplitude of the potential created across the
body surface decreased during expiration and increased during inspiration
(Fig. 3A), coinciding with the separate phases of the usual bipolar ventilatory
potential. In cases where the waveform of the ventilatory potential was nearly
monophasic, the resistance changes measured in this way were similarly unidirectional and coincident with the ventilatory potential (Fig. 3C).
In animals in which the normal ventilatory potential was otherwise quite small, a
d.c. potential created by passing d.c. current of 100-250fiA through the gut
electrode resulted in a large normal-looking ventilatory potential modulation
measured with the subdermal Ag/AgCl electrode (Fig. 3B). Reversing the
polarity of the gut current reversed the polarity of each phase of the ventilatory
potential. In animals with larger ventilatory potentials, a d.c. gut stimulus of the
correct polarity and amplitude canceled and even reversed the polarity of the
normal ventilatory potential (Fig. 3C). These results support the hypothesis that
the ventilatory potential is a modulation of a standing d.c. potential that exists
across the fish's body surface. The d.c. gut stimulus required to cancel the normal
ventilatory potential provides an estimate of this standing d.c. potential. The d.c.
potential change created by a gut stimulus just sufficient to suppress the normal
ventilatory potential ranged from 20-450 yiV, and in almost all cases the required
polarity of the gut stimulus was negative. In two animals exhibiting a ventilatory
potential of reversed phase, i.e. inside-positive on expiration, negative on
inspiration, a positive d.c. gut stimulus suppressed the ventilatory potential and a
negative potential enhanced it.
114
D. BODZNICK, J. C. MONTGOMERY AND D. J. BRADLEY
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Fig. 3. (A) Resistance changes between the skate's interior and the surrounding sea
water during ventilation are indicated (lower trace) by the varying amplitude of the
potential created when a sinusoidal current stimulus (12 Hz) is delivered between a
salt-bridge electrode in the fish's gut and similar electrodes on the sides of the
aquarium. Potential was measured in this and all records of the figure with a subdermal
Ag/AgCl electrode relative to a second electrode in the sea water; positive is upwards
in all traces of the figure. Periods of expiration (E) and inspiration (/) are indicated
beneath the traces. The upper trace is the normal ventilatory potential (VP)
modulation recorded from the animal at approximately the same time. Note the first
periods of the ventilatory cycle in the two traces are accurately aligned for comparison.
Calibration marks at lower right indicate: horizontal, 0.8 s; vertical, upper trace 25 ftV,
lower trace lOOjtiV. (B) Effects of passing d.c. current between the gut electrode and
sea water on the ventilatory potential. The top trace shows that the normal ventilatory
potential is nearly undetectable in this skate. The lower traces show normal-looking
biphasic ventilatory potentials recorded while delivering the indicated d.c. currents
throughout the trace. At onset, 125 fiA current caused a d.c. potential shift of about
200fiV between the skate's interior and sea water. Calibrations: horizontal, 0.8s;
vertical, 50,uV. (C) As in B, but in this skate the ventilatory potential (top), which is a
simple monophasic positive wave, is canceled (middle) and then reversed in polarity
(bottom) by the d.c. currents indicated. The onset of — 150/uA current created a d.c.
potential shift of 200 ;uV, indicating that the normal standing d.c. potential in thisfishis
200/xV inside-positive. In the bottom record the increased amplitude of the transcutaneous potential created by sinusoidal current (3.3 Hz) from the gut electrode (as in A)
indicates a simple monophasic resistance increase between the skate's interior and the
sea water, which is coincident with the ventilatory potential. Calibrations: horizontal,
0.4s; vertical, top three traces 50juV, bottom trace 125^V.
Electrosensory common mode signal suppression
115
Central suppression of common mode signals
The gut electrode was used to create a common mode stimulus, that is one of
nearly the same amplitude and phase, for all of the electroreceptors as a means of
testing the hypothesis of a central common mode rejection mechanism for
suppressing ventilatory reafference. The potentials created by the gut electrode
stimulus were measured with the subdermal Ag/AgCl electrode in the region of
the ampullary clusters and an indifferent electrode, which was moved to a series of
locations around the skate, in the bath. The records in Fig. 4 show that the gut
electrode produced a good common mode stimulus with a nearly identical
potential modulation for virtually all of the electrosensory canals with skin pores
beneath the water surface, though the field in the region of the most caudal hyoid
pores in some animals was slightly phase-shifted and smaller in amplitude than at
other locations (Fig. 4, position E). The significant exception to this was found in
the case of the dorsal and medial hyoid pore group. The skin pores of these
receptors were located above the water surface in the experimental situation, and
50 ixV
0.1s
Fig. 4. The gut stimulus is nearly identical for all electroreceptors in the water. On the
left are shown the potentials recorded between the subdermal Ag/AgCl electrode
(asterisk on skate drawing) and a similar electrode positioned in the sea water at the
locations shown on the right. An exception is the reversed-phase potential recorded on
the skin above the water at F. Dashed line around F indicates water level.
116
D. BODZNICK, J. C. MONTGOMERY AND D. J. BRADLEY
the potential on the surface of the skin in this region was phase-reversed in
comparison with the potentials recorded in the sea water.
The common mode nature of the gut stimulus was confirmed in recordings from
more than 200 primary electroreceptive afferents. Afferents from canals of
different orientations and locations on the body responded differentially to
uniform electric field stimuli, but gave nearly identical responses to the gut
stimulus (Fig. 5). The ampullary organs of skates in a uniform electric field are
sensitive to the potential drop across the skin plus the drop within the animal's
body along the length of the ampullary canal. Thus, as illustrated in Fig. 5,
afferents from electroreceptor organs with longer canals appear to be more
sensitive to uniform fields than those from organs with shorter canals. However,
the amplitude of the receptors' responses to the gut stimulus was independent of
canal length, indicating that the skate's interior, at least in the region of the
ampullary clusters in the head, is nearly isopotential with respect to the gut
stimulus and that the effective voltage drop is across the skin. The amplitude of the
response to the gut stimulus was also similar to the afferent modulation produced
by a ventilatory potential of the same magnitude (Fig. IB). The common mode
nature of the response to the gut stimulus was evident in all afferents with a
receptive field located in the water. In several instances afferents were found
whose receptive fields could not be located. These were presumed to be in the
dorsomedial hyoid group and their responses were phase-reversed, consistent with
the polarity of the field produced by the gut stimulus in that location.
The responses of 62 AENs and 85 primary afferents to the gut stimulus and to
uniform electric fields in the water were compared. A signal to noise ratio (S/N)
for each electroreceptive unit studied was defined as the overall response of the
unit to uniform fields of 2 /iV cm"' (signal) divided by its response to the 20 //V gut
stimulus (noise). The S/N in primary electroreceptor afferents was relatively
homogeneous and averaged 0.96 (S.D. 0.7, N=85) (Fig. 6). In contrast, the S/N
for AENs was quite variable, even within a single animal. Approximately onethird had S/N values similar to that of primary afferents and the remaining twothirds were larger, indicating suppression of the response of these AENs to the gut
stimulus compared with that to extrinsic fields (Figs 6, 7). The mean S/N for all
AENs recorded was 4.6 (S.D. 9.1, N-62), a value 4.8 times higher than the
average for primary afferents.
A subset of cells in Fig. 6 was recorded from five animals that were not
paralyzed by curare injection but instead had spinal transections and were
ventilating naturally. For both primary afferents and AENs in these animals the
ventilatory modulation was also measured and an S/N value was determined
where the signal was again the response to uniform fields (2 Hz sinusoidal stimuli;
2jitVcm~l) but the noise was the peak-to-peak ventilatory modulation. The
uniform field stimuli were presented without a fixed phase relationship to the
ventilatory cycle, which had a variable 1.5-3 s period. For primary afferents the
mean S/N was 1.2 (S.D. 0.7, N=47) and, as with the gut stimulus, the S/N for
AENs was quite variable but averaged 4.8 (S.D. 6.2, N=28), four times higher than
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D. BODZNICK, J. C. MONTGOMERY AND D. J. BRADLEY
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2ftVcm~' uniform electric field and noise is the response to a 20,uV gut stimulus, for
primary electroreceptive afferents and ascending efferent neurons of the dorsal
nucleus in the medulla.
for the afferents (Fig. 8). For these AENs the extent to which their responses to
the gut stimulus and to the ventilatory potential were suppressed compared to
uniform fields was positively correlated (Fig. 9, r=0.66).
Discussion
Ventilatory potential
The feeble, low-frequency electrical potentials recorded in the water near fishes
during their ventilatory movements were among the first bioelectric fields
identified as natural stimuli for the electric sense of elasmobranchs (Kalmijn,
1974). Such electric fields guide the well-aimed approach of elasmobranch
predators to their concealed prey (Kalmijn, 1971). Through physiological recordings from the electroreceptors of freely ventilating elasmobranchs it later became
evident that potentials associated with the animal's own ventilation were also a
potent source of electrosensory self-stimulation (Montgomery, 1984). From the
common mode nature of the ventilatory reafference, it was inferred that the
effective ventilatory potential responsible for self-stimulation was that existing
between the animal's interior and the surrounding water rather than the weak
potential distribution measured in the sea water. In the current study we have
directly measured a ventilatory potential modulation between the animal's interior
and the sea water with a Ag/AgCl electrode placed beneath the skin of the head,
and it is evident from the correlation of electroreceptor firing with the waveform
and amplitude of the potential that it is responsible for the ventilatory reafference.
Although in our physiological set-up the animal was restrained by a head clamp
Electrosensory common mode signal suppression
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Fig. 7. Poststimulus time histograms (30 trials each) from two ascending efferent
neurons (AENs) and a primary afferent recorded at almost the same time as AEN97
illustrate suppression by the AENs of responses to the gut stimulus and ventilation
compared with extrinsic uniform fields. The signal to noise ratio of these units for the
gut stimulus and ventilation, respectively, were: P170, 1.1, 1.3; AEN97, 27.0, 5.9;
AEN99, 4.1, 3.2.
and a mouthpiece that holds the mouth partly open, the ventilatory potential is not
simply an artifact of this arrangement as it was recorded with a similar waveform
and amplitude in two unoperated, unrestrained animals breathing naturally.
Control measures made while probing the body surface over the recording
electrode demonstrated that the ventilatory potential is also not a movement
artifact.
The ventilatory potential is probably due to modulation of a standing d.c.
potential between the animal's interior and the water by the variable shunting of
the potential across the gills during ventilation. This model is supported by
experiments in which a current was passed between the gut electrode and the sea
water. Altering the standing d.c. potential by passing a d.c. gut current in this
2s
120
D. BODZNICK, J. C. MONTGOMERY AND D. J. BRADLEY
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ventilation.
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(75 "c
W)
15
r=0.66
x!
fiel
5
c
3
00
20-
10
50
5
10 15 20
25
Signal to noise ratio for uniform
field/ventilation
30
Fig. 9. Suppression of the gut stimulus by each AEN shown in Fig. 8 (as measured by
the signal to noise ratio) is plotted versus the cell's suppression of ventilatory
reafference.
manner predictably altered the ventilatory potential modulation. Thus, a sizable
ventilatory potential of the usual waveform can be created in an animal in which
none is otherwise apparent solely by changing the d.c. potential, and in other cases
a normal ventilatory potential can be canceled and reversed with the appropriate
polarity of d.c. gut stimulus. It is difficult to measure the actual size of the small
d.c. potential between the fish's interior and the water because of the unknown
contribution to the measurements of the galvanic half-cell potential of the
implanted electrode. Measurements of the abrupt change in potential at the onset
Electrosensory common mode signal suppression
121
of the d.c. gut current required to cancel exactly the normal ventilatory potential
modulation do permit a measure of this d.c. potential, which in most cases turned
out to be 200-300 ^V but ranged from near 0 to 450 fiV. This method may
underestimate the actual potential by a small amount because of the capacitance
across the animal's body wall. In most cases the measured d.c. potential was
inside-positive relative to the water, but in a few cases it was inside-negative,
accounting for the reversed polarity of the potential fluctuations seen during
ventilation in these animals.
Evidence of the impedance changes between the interior of the animal and the
surrounding water is also provided by the gut stimulus. In this case, a sinusoidal
current stimulus created a potential modulation across the skin that was reduced in
amplitude during expiration when the gill and spiracles were open and enhanced
during inspiration when the gill clefts were tightly shut. Thus, the partial shunting
of the inside-positive d.c. potential found in most fish can account for the shift to a
more negative potential during expiration and, similarly, a positive potential shift
accompanies the resistance increase during inspiration. The reverse is true when
the d.c. potential is inside-negative.
The natural origin of the d.c. potential modulated during ventilation probably
includes diffusional potentials across permeable surfaces, but a major contribution
must also come from active ion pumping. The change in ventilatory potential
observed within minutes of transferring a skate to dilute sea water and also the
relatively small but short-latency changes seen in some animals after NaCl
injection are probably due to diffusional potentials from permeable epithelia such
as that of the gills. However, the much larger increases in ventilatory potential
with salt-loading were only observed after a delay of several hours, which is more
consistent with active osmoregulatory ion transport having been turned on. Burger
(1962) found that after intravenous NaCl injection in the dogfish, Squalus
acanthias, the rectal gland exhibited a rise in NaCl secretion only after a lag of
30-75 min. The rectal gland, which is the chief osmoregulatory organ in elasmobranchs (for a review, see Shuttleworth, 1988), creates substantial transepithelial
potentials in secreting sodium and chloride. Studies on an isolated rectal gland
preparation from the dogfish indicate that during active ion secretion the lumen of
the gland, which opens into the gut, may be several millivolts negative compared
with the plasma (Siegel et al. 1976). The gill is also known to be involved in active
ion pumping in elasmobranchs and may make an additional but unknown
contribution to the d.c. potential. Osmoregulatory physiologists who have
measured d.c. potentials between the interior of fishes and the surrounding water
using intraperitoneal salt-bridge electrodes have generally attributed these to
branchial sources because of their similarity to transbranchial potentials measured
in vitro (Bentley et al. 1976). It is notable that both gill ion permeability and the
d.c. potentials measured in vivo are an order of magnitude smaller in elasmobranchs than in marine teleosts (Shuttleworth, 1988; Evans, 1980). As stated
earlier, the bioelectric fields measured in the water during ventilation are also
much smaller in elasmobranchs than in marine teleosts (Kalmijn, 1974).
122
D . B O D Z N I C K , J. C. MONTGOMERY AND D . J. BRADLEY
It follows from the above considerations that the ventilatory potential and thus
electrosensory reafference could be influenced by any factor that affects either the
standing d.c. potential or the relative resistive changes during ventilation and,
therefore, might be expected to be quite changeable under natural circumstances.
The standing d.c. potential could be readily influenced by an intermittent dietary
salt load, by swimming into the hyposaline water of an estuary, or by pH changes
in the water or blood (Bentley et al. 1976). Resistive changes during ventilation are
affected by variations in ventilatory movements as indicated above, but could also
be affected by other factors such as the changes in gill circulation and permeability
that accompany stress or strenuous activity. Circulating catecholamines may affect
branchial circulation (Taylor, 1985) and also suppress rectal gland activity by
restricting its blood supply (Shuttleworth, 1988).
The ampullary electroreceptors of elasmobranchs are responsive to extremely
low frequencies, faithfully following potential modulations below 0.05 Hz (D.
Bodznick, unpublished observations). However, at least over a reasonable range
they are not sensitive to the absolute d.c. level and, by a process of accommodation, can apparently maintain high sensitivity to small potential modulations
against a range of d.c. reference levels (Murray, 1965). The accommodation
mechanism is not completely understood (Bennett and Clusin, 1976), but its
importance for ampullary electroreceptor function is clear given the d.c. potentials
measured here between the fish's interior and the sea water.
Suppression of common mode signals
A central mechanism for rejecting common mode signals is particularly well
suited for suppressing electrosensory reafference caused by the ventilatory
potential because, although the potential can vary greatly in intensity and
waveform at different times and in different animals, it is, nevertheless, always a
common mode stimulus for the electroreceptors. Direct evidence for a common
mode suppression mechanism in the medulla is provided here by experiments
using the common mode stimulus created with the gut electrode. In our
physiological set-up the gut electrode provides an effective means of creating a
stimulus of common amplitude and phase for all the electroreceptors, with the
exception of the dorsomedial hyoid group in which the skin pores are out of the
water. This was verified by measuring the potential distribution in the water
around the skate and, perhaps more convincingly, by monitoring the responses of
a large population of primary afferent fibers. The major potential drop caused by
this stimulus appears to be across the skin of the skate since receptor canal length
had no apparent effect on the sensitivity of the response of the afferent fibers to
the gut stimulus. Although there was considerable variability among AENs, most
units exhibited significant suppression of the response to the gut stimulus
compared with responses to extrinsic uniform fields. The uniform fields were
similar in frequency and intensity to the gut stimulus but, unlike the gut stimulus,
they affected the electroreceptors differentially. That the response to a common
mode stimulus with no temporal relationship to ventilation is suppressed within
Electrosensory common mode signal suppression
123
the brain is clear evidence for the existence of a common mode rejection
mechanism. This result does not rule out the possibility that other mechanisms,
such as efference copy, also contribute to suppressing ventilatory reafference.
However, in unparalyzed animals most individual AENs appeared to suppress the
gut response and ventilatory reafference to about the same degree, despite
considerable variation in the suppression among the AENs. Suppression of
ventilatory modulation and of the response to the gut stimulus had a positive
correlation coefficient (r) of 0.66 in the 29 cells measured and omitting one large
outlying value, results in an r value of 0.76. This indicates that other mechanisms
may not play a major role in suppressing the reafference due to ventilation at this
level of the brain. The results are also important because they establish the gut
electrode as a means of creating a well-controlled common mode stimulus that
may be useful for further characterizing the medullary circuits responsible for
suppressing ventilatory noise.
In this study, as in a previous one on Raja erinacea (New and Bodznick, 1990),
most AENs exhibited at least some ventilatory modulation. Some cells were
strongly affected. This is in marked contrast to the case in Platyrhinoidis triseriata
(Montgomery, 1984), in which virtually none of the dorsal nucleus neurons were
affected by ventilation. The reason for this difference is not known. In the studies
on Raja the device that stabilized the head also restricted movements of the mouth
during ventilation; no such restraint was used with Platyrhinoidis. It is possible
that there is a proprioceptive component to the normal suppression of ventilatory
reafference that has been altered in Raja. In other ways the methodologies of the
experiments with Platyrhinoidis and Raja were very similar and it may be that the
results simply reflect a species difference. This would suggest that electrosensory
sensitivity might be more adversely affected by ventilatory reafference in Raja
than in Platyrhinoidis, but this can only be determined through behavioral studies.
The reasons for the variation observed among AENs in this study on Raja with
respect to their cancellation of common mode stimuli are also not known. In the
case of ventilatory reafference it is possible that some of the apparent variation in
the suppression is actually due to variation in the amplitude of the ventilatory
potential itself among animals or during a single experiment. We attempted to
minimize this by periodically sampling primary afferents throughout the experiment to ensure that the ventilatory reafference was consistently strong, but some
variation was unavoidable. However, the suppression of responses to the gut
stimulus was as variable among AENs as suppression of responses to ventilation
and the positive correlation between the suppression of the gut stimulus and
ventilatory reafference indicates that other factors are more important. Differences in the general health and condition of the fish might account for some
variation. However, units in even a single electrode track sometimes showed quite
different degrees of common mode suppression.
In the recording set-up used in these studies the top of the head is above the
water level and the skin openings of the dorsomedial hyoid ampullae are out of the
water. As a result, this small group of 10-12 electroreceptors on each side
124
D . BODZNICK, J. C. MONTGOMERY AND D . J. BRADLEY
appeared to respond 180° out of phase with the other receptors during stimulation
by the gut electrode and presumably also during ventilation. This should
significantly degrade the common mode signal suppression in any units for which
these ampullae provide inhibitory inputs. Similarly, AENs with excitatory
receptive fields amongst these dorsomedial receptors should show poor common
mode suppression, though none was apparently included in our sample of units.
This artifact of the set-up seems to be a probable source of some of the variation
observed among AENs. Lastly, since the extent of common mode signal
suppression was measured in each case in comparison to the cell's response to
uniform electric fields, some of the apparent variation among AENs in their
suppression of common mode stimuli may actually be due to variations in their
uniform field sensitivity. Recent studies on the carpet shark (Bodznick and
Montgomery, 1992) indicate that differences in AEN receptive field configurations
might result in just such differences in uniform field sensitivity among the AENs.
This variation would be beyond that expected as a result of varying ampullary
canal lengths, which affect uniform field sensitivity in AENs and primary afferents
alike.
Despite the variation, the effective suppression of an imposed artificial common
mode stimulus by many AENs is the best evidence to date for a central common
mode suppression mechanism. Direct evidence of the mechanism should also be
found in the receptive fields of AENs, which must have inhibitory areas in addition
to their small, ipsilateral excitatory fields. Several factors have made a convincing
demonstration of these inhibitory areas difficult. However, this has recently been
accomplished in experiments on the carpet shark, the results of which are reported
in the following paper.
The authors thank Greg Hjelmstad for helpful discussion and technical
assistance during a part of the study. The work was funded by an NSF grant to
D.B. and Fullbright Fellowship to J.C.M.
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