Sensitivity to novel feedback at different phases of a gymnotid electric organ discharge
Institut für Biologie I, Hauptstrasse 1, Albert-Ludwigs-Universität Freiburg, D-79104 Freiburg, Germany
* Author for correspondence (e-mail: schustef{at}uni-freiburg.de)
Accepted 15 July 2002
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Summary |
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Key words: electric organ, gymnotid, electrolocation, communication, novelty response, feedback, electric fish, Gymnotus carapo
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Introduction |
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To our knowledge this notion of a `dual EOD system' was first introduced by
Trujillo-Cenóz et al.
(1984) for the best-studied
pulse-type gymnotid, Gymnotus carapo. A clear picture has emerged of
how the sequential activation of electrocytes that differ in their morphology,
innervation pattern and the number of spike-generating faces, generates the
pattern of current flow during its complex electric organ discharge (EOD)
(Trujillo-Cenóz et al.,
1984
, 1989; Lorenzo et al.,
1988
,
1990
,
1993
; Caputi et al.,
1989
,
1993
;
Macadar et al., 1989
).
Recently, the notion has received support in this species by studies of
Castelló et al. (2000
)
and Aguilera et al. (2001
).
These studies demonstrated a foveal region of high sensitivity at the head in
which the self-produced currents produced during the final phase of the EOD
(V4) were negligible, while currents of earlier phases
(V1 and V3) were predominant. Thus,
only these latter currents appear to play a role during electrolocation at the
fovea. In contrast, of the EODs produced by a distant conspecific, the
currents of the final phase V4 were predominant at the
fovea. These findings suggest a dual system in which current produced during
the V1 and V3 phases of the EOD would
be used for electrolocation and current produced during the later
V4 phase would be used for communication.
Here, we explore a novel approach to analyze directly the importance of the
different phases of a gymnotid EOD for electrolocation. In this approach the
EOD feedback is changed selectively during particular phases within an EOD in
which known parts of the electric organ are active. Moreover, the duration and
magnitude of the feedback-changes within these selected phases are directly
measured ensuring that feedback could be changed when selected parts of the
electric organ were active. The `novelty response', an increase in the
discharge rate in response to novel EOD feedback, is used to assess the
animal's ability to detect a feedback change triggered at various phases of
the EOD waveform. We apply this approach to test the notion of a dual EOD
system in G. carapo. If this fish uses a dual EOD system as recently
suggested (Aguilera et al.,
2001), then it should generally be sensitive to feedback changes
that are triggered during V3 (or V1)
but not to changes triggered during V4. In particular,
this should hold in the trunk/tail region of the fish in which all major
currents (V2 to V4) are present so
that a lack of sensitivity to changes in a particular current cannot simply
result from the absence of that particular current.
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Materials and methods |
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Basic experimental arrangement
Fig. 1A shows the basic
arrangement used to trigger a brief change in feedback within the desired
phase of one single EOD. The experimental animal rested in a cage (described
below) placed in the centre of the tank at half the height of the water column
(7.5 cm above ground). Two silver wires at the front and back end of the tank
monitored the headtail EOD. This signal served (i) as a phase reference
to assay which part of the electric organ was active (e.g. see
Caputi, 1999) and (ii) to
monitor the inter-EOD intervals of the fish. Feedback changes were elicited by
briefly closing a fast electronic switch (MAX 323, Maxim) that connected two
Ag/AgCl pellets, placed 8 cm apart, in preassigned positions at the trunk/tail
position on the side of the fish. A virtual-ground input, current-sensitive
preamplifier was used (EG&G 5182) to monitor the shunted EOD current that
flowed in the external circuit between the pellets when the switch was closed.
Surprisingly, a simple circuitry sufficed to reliably place the switch closure
at the desired phases of the EOD. During an experiment, a reference pulse
marked the onset of one selected EOD and triggered a generator that issued a
second rectangular pulse of suitable duration and delay with respect to the
reference pulse. This second pulse closed the electronic switch at the desired
phase within the selected EOD. The reference pulse, switch-closure command,
current in the shunting circuit and headtail EOD were monitored on a
4-channel oscilloscope (Yokogawa DL 1200A) and could be fed into a computer
(programmes written in Turbo Pascal and DAPL language/C++; Processor card DAP
3000a/212, Microstar Labs).
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Monitoring the response
When an experiment began, a rectangular pulse coactivated (i) a counter
module and (ii) an electronic switch (MAX 324, Maxim) that fed the recorded
EODs into the computer. After 200 inter-EOD intervals had been recorded, the
counter module produced a reference pulse to mark the onset of the 201st EOD
and to command switch closure at the chosen phase within that EOD. The
computer continued to record the 300 inter-EOD intervals that followed after
switch closure. The switch was closed either during a particular phase of the
201st EOD or, as a control, in the silent inter-EOD interval 4 ms after the
reference pulse. As recognized in earlier studies
(Bennett and Grundfest, 1959;
Szabo and Fessard, 1965
;
Larimer and Macdonald, 1968
;
Heiligenberg, 1980
;
Meyer, 1982
), such controls
are important to assess whether the fish responded to electrical artefacts
associated with the switch closure rather than to the redistribution of EOD
current. Examples of responses to switch closure during the EOD are shown in
Fig. 1B. Animals responded to
novel feedback with a transient decrease in the inter-EOD interval, a response
termed novelty response (NR). Because this response varied considerably in its
magnitude as well as its time course, a comparison of the efficiency of
different stimulus regimes in eliciting responses required that these regimes
were presented in random alternation. Meeting this requirement ensured that
differences in the responses could not be caused by stimulus-unrelated changes
in responsiveness.
Time allowed between successive stimuli
To avoid habituation to the feedback changes the animal was given a rest of
3 min between successive experiments. This limited the amount of tests that
could be made within the maximally possible interval of 6 h during which a
fish would rest quietly in its cage. However, a time of 3 min was chosen as
prior experiments had indicated significantly lower response probabilities
when a rest of only 2 min or 1 min was allowed.
Determining differences in response probability between the different
phases
The probability with which a change in EOD feedback at a given phase
elicited a novelty response was determined off-line as follows. A computer
program randomly selected and displayed a trace recorded during the
experimental day. The trace showed the 200 pre-stimulus intervals, an
indicator of stimulus timing and the 300 post-stimulus intervals (see
Fig. 1B). The traces displayed
were recorded either when the switch was closed within a particular phase of
the EOD, in the silent interval between EODs or in the absence of switch
closure. No information hinted at the stimulus condition and no stimulus
artefacts were present. An observer had to decide whether the currently
displayed trace showed a response or not. After this decision the next trace
was selected, displayed, and so on. Only after all traces of an experimental
day had been judged in this manner were the numbers of responses and failures
displayed for the different stimulus conditions and the controls. This
procedure ensured that changes in the response criterion used by the observer
could not cause differences in response probability among the phases within
the EOD, or between them and the controls. Moreover, two types of control were
also evaluated. (i) Controls in which a sequence of 500 inter-EOD intervals
were displayed that were recorded without stimulation. This type of control
was important to determine the `baseline' number of false responses that are
expected because random fluctuations in the inter-EOD intervals (e.g. see the
pre-stimulus intervals in Fig.
1B) may be scored as a response. (ii) Controls with the switch
briefly closed after the EOD. If the responsiveness obtained during these
controls was significantly higher than the `baseline' probability it would
indicate that the fish responded to an electrical signal generated during
switch closure and not necessarily only to changes in EOD feedback. However,
these controls never yielded higher than baseline probabilities. Statistical
comparisons of response probabilities were done using
2-tests.
Construction of the cage
A rigid cage with all its edges carefully rounded (so that the sensitive
skin of the fish could not be hurt in an attempted escape) was built on a CNC
machine (Hermle U630T). It was made from transparent Plexiglas, was 30 cm long
and had dimensions of 3x3 cm. The left and right sides of the cage had
nine windows of inner dimensions 2.8x2.8 cm that were covered with rigid
plastic mesh (square openings of approximately 1.6 mm, separated by
approximately 0.1 mm), whereas the top and bottom sides of the cage consisted
of a regular array of 48 equally spaced bars with 3 mm spacings between bars
that were oriented orthogonally to the cage's longer side. The spacings
between the bars allowed two `doors' (Plexiglas frames with the plastic mesh)
to be inserted so that the fish could be confined (longitudinally) in the
center of the cage. Plastic mesh was glued to the bottom side of the cage to
prevent the fish from sticking their elongated tails out of the cage. The cage
could be firmly suspended from above by means of two rigid Plexiglas rods
(diameter 8 mm) fixed at the top of the cage.
General experimental procedure
At the beginning of an experimental day, a rectangular positioning plate
(20x30 cm; aluminium) was positioned on a marked region on the tank's
floor. A central vertical rod mounted at the plate marked the point at which
the centre of the cage's lower side was to be positioned. Two higher vertical
rods could be inserted in preset positions on the plate so as to mark the
position of the two shunting electrodes at the side of the cage in half of its
height. Thus one of three set lateral distances 0, 1, or 2 cm from the cage
could be chosen. The experimental animal was very carefully placed into the
cage, often by means of a suitable funnel. Great care was exercised that the
fish did not damage its sensitive skin and the tip of its tail because such
damage might yield significant deviation from the response patterns observed
in the present study. Using the positioning device, the cage was brought into
its fixed position and the two shunting electrodes (with a fixed distance of 8
cm between them) were brought into position using a manipulator. Then, the
positioning device was removed and the fish was allowed some rest. Next, the
delays with respect to the phase-reference pulse needed to place the shorting
at the desired phases within the EOD were determined. After a further pause,
experiments started with one test every 3 min for 4-6 h. In these tests
stimulus conditions were randomly varied. At the end the fish was carefully
removed from the cage and transferred to its home tank.
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Results |
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Measuring the shunted EOD current
In the present approach it was necessary to control the waveform and the
magnitude of the EOD current that could be shunted at the various phases of
the EOD. This was attempted by inserting a virtual-ground current amplifier in
the external circuit between the two shunting electrodes (see
Fig. 1A). As a first step in
this analysis, we measured the current when the electronic switch that
connected the two electrodes was closed for long intervals, in the order of
seconds. As illustrated in Fig.
3, this analysis revealed two components of the current flow. (i)
An `offset' that decays slowly when the switch is closed for extended periods.
This component is recorded also in the absence of a fish or a sending-dipole
electrode. It is much larger for Ag (or Cu) wires than for the Ag/AgCl pellets
used here and is also obtained when a mechanical switch is used instead of the
fast electronic switch. These features indicate that it probably results from
a difference in the polarization of the two Ag/AgCl electrodes. In the present
arrangement (with the two electrodes placed longitudinally on the side of the
fish) this component never elicited a behavioral response. (ii) The second
component is the shunted EOD current. This is seen as a modulation on top of
the offset. As shown below the presence of this component elicited responses
in the present arrangement.
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By keeping the switch closed until the offset has decayed it is possible to
record only the second component, the shunted EOD current
(Fig. 3B). However, in the
desired experiments with brief switch closure the offset will not have decayed
and it was thus important to know whether the shunted EOD current could be
determined in the presence of an offset due to slight differences in electrode
polarization or whether it would be necessary to select electrodes in which
this difference was below a certain tolerable level. Our analysis shows that
an offset such as that obtained with standard Ag/AgCl electrodes is tolerable
and does not confound the determination of the shunted EOD current. With such
electrodes and under the conditions of the present experiments the offset
simply adds to the shunted EOD current. As illustrated in
Fig. 3B, the EOD modulations
recorded in the absence of the offset (i.e. when the offset had decayed) were
approximately equal to those that occurred on top of the offset. At least,
within the required precision of about ±0.1 µA, it is justified to
say that the shunted EOD current simply added to the offset. Additional
evidence for this was obtained in experiments in which the distance was
changed between the shunting electrodes and either a fish (e.g. see
Fig. 7 below) or an artificial
dipole sender. In all these experiments the modulations on top of the offset
decayed with distance but were independent of the amount of the offset. The
findings are compatible with an equivalent circuit in which a capacitance in
the current path (a good model for Ag/AgCl electrodes; e.g.
Meyer, 1982) accounts for the
slow decay of the DC potential due to different electrode polarization while
the rapid EOD-induced modulations are readily fed through.
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So far we have dealt only with an interpretation of the current measured over extended periods of switch closure. But there are still two problems: (i) how can the shunted EOD current be determined when the switch is closed for only 100µs, and (ii) does it flow only during the interval of switch closure or do capacitive effects cause the shunted EOD current to flow over longer periods? Recordings, such as those shown in Fig. 4, provide a surprisingly simple answer to both questions. In the present arrangement there was never prolonged current flow for switch closure times down to 20µs. Moreover, the magnitude of the shunted EOD current that flows over the external circuit during the brief switch closure could directly be predicted from measurements in which the switch was closed for an extended period. This is illustrated in Fig. 4, which shows how the magnitude of current pulses obtained as the switch was briefly closed at various EOD phases (of successive EODs) retrace the modulation obtained during a prolonged switch closure.
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These general features were first demonstrated in experiments with
artificial dipole senders [T-shaped electrodes with carbon electrodes inserted
at the openings of the horizontal shaft; similar to electrodes used by Westby
(1975)] that spread currents
similar to the animal's. Using these senders, and determining the current that
was shunted during switch closure, we tried to select an experimental
arrangement in which the magnitude of the shunted EOD current would be as
robustly stable as possible with respect to slight variations in the position
and orientation of the sender. In the tests carried out the artificial dipoles
were placed at either (i) the bottom or (ii) the centre of the tank. Two
dipole lengths were used: 4 cm and 12 cm. Shunt electrodes were placed at the
height of the dipole at lateral distances as in the later experiments. We then
analyzed how the amount of current shunted during switch closure varied in
each of the four possible sender configurations because the sender's average
position was changed slightly in each of four possible ways: (a) ±10 mm
horizontal displacement orthogonal to the dipole axis, (b) ±10 mm
vertical displacement orthogonal to the dipole axis, (c) ±5 mm
displacement along the dipole axis and (d) ±5° rotation in the
horizontal plane along the centre of the dipole. Briefly, the shunted current
showed most stability with respect to the various modes of position changes
when the sender was at the tank's centre and when the distance between the
shunting electrodes was smaller than the dipole's length. The present
arrangement of a narrow cage placed in the centre of the tank was chosen as a
result of these tests.
Detection of novelties in feedback during a single EOD
All fish showed a significant probability of response when EOD feedback was
changed during only 100 µs within only one of its EODs. In the experiments
of Figs 5 and
6 the shunting electrodes were
placed directly at the boundary of the fish's cage at the indicated positions
close to the animal's trunk and tail (as indicated in the respective insets of
Fig. 5). The shunting
electrodes were briefly connected for 100 µs either at one of the major
phases of the EOD or in the silent interval between EODs (control; C). For
shunting of EOD feedback, the period during which the switch was closed was
adjusted such that it centered at one of the extrema V2,
V3 or V4. Interval recordings without
switch closure served to determine the `baseline' percentage of responses that
was assigned because of spontaneous rate fluctuations (dotted horizontal lines
in the diagrams of Fig. 5; see
Materials and methods). Fig. 5
shows the response probabilities determined in a series of experiments
involving all animals (N=8; with about 25 tests for each stimulus
condition). In addition, Fig. 6
reports the magnitude of EOD current (mean ± S.D.) that could be
shunted in these experiments.
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Several features of the behavioral data shown in
Fig. 5 are noteworthy. (i) All
fish showed a highly significant percentage, at least in one of the phases
tested, of responses to the feedback changes during one single EOD. (ii) This
was not because the fish detected an electrical artefact signal spread during
switch closure. As in previous studies
(Bennett and Grundfest, 1959;
Szabo and Fessard, 1965
;
Larimer and Macdonald, 1968
;
Heiligenberg, 1980
;
Meyer, 1982
) placing switch
closure in the silent interval between successive EODs allowed us to
demonstrate that the animals responded to the feedback changes. The absence of
any significant difference between the baseline, caused by spontaneous rate
fluctuations, and the controls (C) with switch closure in the silent inter-EOD
interval shows that the fish responded to changes in EOD feedback and not to
an artefact of switching. (iii) In the presently studied trunk/tail region of
G. carapo there appears to be significant sensitivity to changes in
EOD feedback during all the major phases,
V2-V4, of the discharge. Only for fish
3 (phase V2) and fish 7 (phases V2 and
V3) were the response levels not significantly above
baseline. Thus, feedback changes within all phases appear to be evaluated to
signal novelties at the trunk/tail region. (iv) Although significant response
probabilities were observed at all major phases of the EOD, there were notable
differences among them: responsiveness in V4 was
significantly higher than in V2 for fish 1 and for fish
3-6. With one exception, the highest response levels were obtained in
V4, although the differences between response probability
in V3 and V4 were only significant for
fish 3 and 4 in the present series of tests. However, in later experiments
that involved more tests, the apparently different responsiveness in
V3 and V4 was also proven significant
for fish 6 (e.g. see Figs 7,
8) and for fish 8. The final
head-negative phase V4 appears thus to be the phase of
highest sensitivity for the detection of novel feedback in the trunk/tail
region. Fish 2 showed a remarkably high response probability during all three
phases. This pattern deviated considerably from that found in all other fish.
However equal sensitivity was also found in later tests at larger distances of
the shunting-electrodes and smaller magnitudes of the shunted EOD current. (v)
The findings indicate that the exact location of the shunt electrodes within
the trunk/tail region is not critical for the sensitivity pattern because it
was observed even though the relative position of the shunting electrodes
varied among fish (as detailed in the insets of
Fig. 5). This is in accord with
expectations, based on the longitudinally homogeneous distribution of
electroreceptors within this region
(Westby, 1975
;
Watson and Bastian, 1979
;
Castelló et al., 2000
),
that different subareas within this region should contribute equally to the
detection of novel EOD feedback.
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Did the differences in sensitivity at the different phases arise because different amounts of currents could be shunted in the different phases? This question is addressed in Fig. 6, which shows the magnitude of EOD current shunted in phases V2, V3 and V4 under the experimental conditions of Fig. 5. These measurements showed three main results: (i) EOD current could demonstrably be shunted at all three phases (V2V4), (ii) in all fish the magnitude of the feedback changes elicited in the experiments was higher in V3 compared with V4, and (iii) in V2 the shunted EOD current was not much below that shunted in V4. However, in contrast to the current measurements, the behaviorally detected sensitivity was less in V2 than in V4 and appeared to be higher in V4 than in V3.
To test whether the distribution of sensitivity at the three phases V2V4 was mainly determined by the magnitude of the feedback changes, we varied the distance of the shunting electrodes from the side of the fish and, concomitantly, the absolute amount of EOD current that could be shunted (N=4 fish; fish 2-4,6). In the example shown in Fig. 7 (fish 6), the upper traces characterize the stimulus. Each trace shows, for a given distance, the amount of EOD current that could be shunted during switch closure at any phase of the EOD. Below, the response probabilities are reported for feedback changes at V2, V3 and V4 at each of the three tested lateral distances. These experiments comprise several days of testing in which both phases and distances were varied randomly from trial to trial (quick changes in the lateral distance were aided by a micromanipulator). In each of the fish tested this way the distribution of sensitivity across phases, although not the absolute sensitivity, was constant irrespective of the changes in the amount of the EOD current that could be shunted at the different distances.
Sensitivity to novel feedback in other phases of the EOD
Switch closures of only 100 µs duration during a single EOD were also
applied at other phases within the EOD of G. carapo (N=3;
fish 3,4,6). (i) Centered at the zero crossings of the headtail
recording of the EOD between V2 and
V3, and between V3 and
V4, as well as centered 80 µs after the
V4 peak. (ii) During phase V1 in which
the current results from postsynaptic potentials (PSPs) of the rostral sides
of doubly innervated abdominal electrocytes.
The example in Fig. 8 illustrates the findings obtained for the additional phases after V1. In the respective experiments, a full set of tests was conducted in which all phases (including V2V4 and controls) were tested in a random sequence so that the respective response probabilities could be safely compared. The main results were as follows. (i) Sensitivity is high even for novelties in feedback that occur at the decaying phase of V4. Here, the response probability was generally significantly above baseline and above the responsiveness of all other phases except V4, in which responsiveness was not significantly different. (ii) Placing the centre of the 100 µs shunting period at a reversal of current direction did not lead to significant changes in responsiveness. Response probability was not significantly different whether the feedback change occurred during V3 or during the zero-crossings between V2 and V3 or between V3 and V4.
Only a few behavioral experiments were done to address the question of how phase V1 contributes to sensitivity at the trunk/tail region. Because this phase is produced by abdominal electrocytes the resulting current flow that could be shunted in the trunk/tail region during this phase was small. Thus a low response probability was expected when switch closure was placed within this phase. To place switch closure within this phase, the EOD was picked up with two differential electrodes close to the abdominal site were V1 is generated. The identity of the V1 signal was confirmed by the simultaneous recording of the headtail EOD in the standard manner. A 100 µs shunt was triggered immediately as the locally recorded V1 signal rose above a fixed threshold. The shunt could thus be placed in V1, about 150 µs before the first head-negative deflection V2. The results of tests made with 3 fish (3,4,6), with the shunting electrodes placed in the trunk/tail regions indicated in Fig. 5, showed a response probability that was not significantly different from the baseline. Hence, the current produced during V1 seems to play a negligible role for electrolocating objects in the trunk/tail region of the animal. This could have been the case if foveal receptors were involved in the detection of feedback changes that are triggered at the trunk and tail region. The findings, therefore, make their involvement in the presently observed responses unlikely.
Responses to briefer switch closure within a single EOD
To explore the smallest possible period of switch closure within a single
EOD that elicited a significant response level, experiments were performed in
which the period was centered at the most sensitive phase
V4 (N=3; fish 3,4,6). Electrodes were placed at
the closest possible distance to the trunk/tail region as before. The
responsiveness to three periods of switch closure was examined: 100 µs, 50
µs and 20 µs. An analysis of the current flow in the shunting circuit
confirmed that EOD current was shunted only for these periods and indicated
that the analysis of the magnitude of the shunted EOD current extends to
periods of switch closure of 20 µs. None of the fish showed significant
responsiveness when the feedback changes occurred only for 20 µs during
V4. However, for all three fish responsiveness
significantly above baseline was found at 50 µs.
Fig. 9A illustrates this with
the data for one fish.
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The possibility of recording behavioral responses to feedback changes that occur for 50 µs in a single EOD permits, in principle, a refined analysis in which sensitivity within the EOD can be tested with better resolution. Fig. 9B shows corresponding results obtained with one fish in which the prolonged series of corresponding experiments could be completed (fish 6 of Fig. 5). 11 phases within the EOD (plus the control outside the EOD and the determination of the baseline) were analysed. For each test, phases were randomly selected. Thus only a few tests could be conducted each day for a given phase and the experimental procedure needed to be repeated for an extended period. Two series could not be continued until significant data were gathered: fish 3 lost the tip of its tail after 5 experimental days and fish 6 became so active in its cage that the 50 µs switch closures could no longer reliably be placed at the desired phases of its EOD. However, the results of the only successfully completed series (Fig. 9B) do not indicate a finer pattern of how sensitivity is distributed within an EOD. They confirm the finding that feedback changes in V4 are most readily responded to despite the fact that the largest magnitude of EOD shunting occured at phase V3.
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Discussion |
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Feedback changes as quantifiable stimuli
Shunting the self-produced EOD current has been instrumental in the past in
demonstrating the capability of weakly electric fish to detect changes in
feedback from their own EODs (Bennett,
1965; Bennett and Grundfest,
1959
; Enger and Szabo,
1965
; Hagiwara et al.,
1965
; Szabo and Fessard,
1965
; Harder et al.,
1967
; Larimer and Macdonald,
1968
; Macdonald and Larimer,
1970
; Moller,
1970
; Heiligenberg,
1980
; Meyer,
1982
). However, in these experiments no attempt was made to
directly monitor the magnitude and waveform of the shunted EOD current. In the
present study, controlling the shunted current was crucial (i) to confirm the
extremely short duration of the feedback changes, (ii) to demonstrate that the
feedback changes were robust despite slight changes in the position of the
animal during prolonged testing and, finally, (iii) to assure that sizable
current changes could be induced in each of the major phases and that a lack
of responsiveness to feedback changes in a particular phase of the EOD was not
small simply because little current could be shunted during that phase. From
our findings feedback changes elicited by switch closure qualify as
quantifiable stimuli.
Changes of EOD feedback have previously been studied with two main goals:
(i) to simply distort the normal distribution of self-produced EOD current on
the skin and (ii) to simulate a small object at a defined location in the
surroundings of the fish. With the first goal it is reasonable to place the
shunting electrodes to maximize the current that can be shunted when the two
electrodes are connected outside the water. Probably for this reason, all
previous work that aimed at the first goal used electrodes placed a
considerable distance apart laterally to the fish's body or in front of the
head and behind the tail. To mimick a point-like object, however, Heiligenberg
(1980) used more closely
spaced shunting electrodes. In the present study of sensitivity to novel
feedback in the trunk/tail region an arrangement appropriate for the first
goal could be chosen. This is because Westby
(1975
), Castelló et al.
(2000
) and Aguilera et al.
(2001
) have shown a homogeneous
distribution of electroreceptors with no fovea in the region of our study, so
that responses obtained to feedback changes at two distant electrodes are
expected to be mainly driven from similar receptor populations.
Responses to feedback changes within fractions of a single EOD
To our surprise feedback changes that affect only a fraction of a single
EOD were sufficient to elicit a significant novelty response. This is
surprising as we analysed responses to novel feedback in the least sensitive
region of the fish (Westby,
1975; Castelló et al.,
2000
; Aguilera et al.,
2001
). Several earlier experiments have already demonstrated that
changed feedback during only one EOD could be sufficient to elicit a response
(Szabo and Fessard, 1965
;
Harder et al., 1967
;
Meyer, 1982
). Interestingly,
Heiligenberg, studying the `electromotor' following response of the gymnotid
fish Hypopygus (Heiligenberg,
1974
) and the novelty response of the mormyrid
Brienomyrus (Heiligenberg,
1976
), found that the ability of these animals to electrolocate
deteriorated only when foreign pulses consistently coincided with (or briefly
preceded) its EOD. Additionally, experiments in Hypopomus suggested
that the detection of novelties depended on several EODs being affected in
succession (Heiligenberg,
1980
); thus, the integration of feedback from successive EODs
appeared essential. It is likely that the feedback changes associated with
switch closure in the present study were much larger than in Heiligenberg's
study and an integration mechanism would not be excluded in which smaller
feedback changes would need to be integrated over several EODs before a
response is elicited. However, the present findings exclude an `all or none'
mechanism in which it would be essential that feedback from a considerable
number of EODs is affected in order that novelty is detected and a
rate-acceleration response elicited.
The differences in sensitivity are not caused by differences in the
shunted EOD current
The observed differences in sensitivity to feedback changes triggered at
various phases of the EOD could have simply reflected the different amount of
current that could be shunted during the various phases. However, this simple
view can be excluded from our direct measurements of the shunted current.
Surprisingly, the general pattern of sensitivity at the different phases of
the EOD did not reflect the magnitude of EOD current that we were able to
shunt within these phases. The shunted EOD current was clearly largest during
phase V3 (e.g. Fig.
6). Yet the response probability was highest to shunting triggered
at phase V4. Similarly, the EOD currents shunted during
phase V2 were not much less than those shunted during
V4, yet the response probability was lowest to shunts
triggered at phase V2. Thus, a simple channeling of
current appears not to explain the observed sensitivity pattern in the
trunk/tail region of G. carapo. Studies in curarized fish, with the
EOD substituted by an appropriate mimic, would be needed to understand whether
the sensitivity pattern originates from the properties of individual receptor
units or derives from a more central comparison of the responses of receptors
at different locations.
Could the observed sensitivity pattern be partly due to an involvement of
the foveal receptors? This would be expected when the shunt triggered between
the trunk and tail electrodes produced sufficiently large changes in the local
EOD at the fovea. Two lines of evidence make this unlikely. (i) The study of
Aguilera et al. (2001)
indicated a negligible role of the V4 current at the
fovea. Thus, a contribution of foveal receptors to the sensitivity at this
particular phase seems negligible. (ii) The experiments in which the shunt
between the electrodes placed at the tail and trunk was triggered during
V1 indicated a lack of responses. This would not be
expected if foveal receptors contributed significantly to the response to
shunts at the trunk/tail region.
Macdonald and Larimer
(1970) have applied continuous
trains of external low-amplitude electrical pulses between two electrodes
placed at the head and tail of a G. carapo. They report that
switching this train from consistently coinciding with one extreme of the EOD
to the next resulted in novelty responses. The magnitude of the novelty
response observed in their `phase-switching' experiment with external pulses
depended on which particular phases the phase switching was made between, and
was maximal when switching occurred between V2 and
V3. It is tempting to compare the result of this
phase-switching experiment with our present findings. Provided that details of
the experimental situation were comparable, then the response profile in a
phase-switching experiment should, to a first approximation, be the derivative
(with respect to phase) of a response profile recorded in our study.
Accordingly, the sensitivity patterns shown in
Fig. 5 would be compatible with
Macdonald and Larimer's findings.
Relation of the present findings to the dual EOD hypothesis
The complicated discharge pattern of gymnotid electric organs has prompted
the hypothesis of a dual EOD system in which different parts of the electric
organ may produce currents that serve different roles in communication and
electrolocation. Perhaps the first clear statement of such a dual EOD system
was made by Trujillo-Cenoz et al.
(1984) who stated,
"...the innervation pattern described here suggests the possibility that
a dual EO system has evolved in G. carapo. The doubly innervated
electrocytes, perhaps with a less fixed output, may play a communication role.
The population of singly innervated electrocytes may subserve the well-known
electrolocative function." Recently, the general notion of a dual system
has received support by the study of Aguilera et al.
(2001
) who demonstrated in the
foveal region of the jaw of Gymnotus that current produced during the
final phase V4 of the EOD was negligible. In contrast,
when the EOD of a distant conspecific is monitored at the fovea, the phase
V4 of the conspecific's EOD is predominant. These findings
led to the suggestion that the currents generated during the final phase
V4 might be predominantly used for communication, whereas
the currents from the phases V1 and V3
were suggested to be relevant for electrolocation.
A critical test of this hypothesis is to examine the trunk/tail area of the
fish in which all current components contribute so that their general role as
carriers of electrolocation signals can be assessed using the novel method
introduced here. The hypothesis would predict that the responsiveness should
be largest when feedback changes are triggered in the presumably
electrolocation-important phases while responsiveness should be small or
absent in the presumably communication-relevant phase V4.
Our findings do not support this prediction. Not only can G. carapo
in principle detect changes in EOD feedback that are triggerd in any phase of
its EOD but also its sensitivity is highest in the phase
V4 for which a communication role was suggested. The
present findings therefore suggest that currents produced during
V4 are not specialized for a role in communication.
However, they do not seem to be specialized for electrolocation either. In the
region of highest sensitivity the measurements of Aguilera et al.
(2001) show a small
contribution of V4 current, which would be difficult to
understand if it played a major role for electrolocation also at the fovea.
Rather, the evaluation of feedback changes during the phases
V1 and V3 that are most important at
the fovea would seem to be required for electrolocation at the fovea.
The findings do not disprove the general notion of a dual EOD system. In
fact, it could still be that the original suggestion, cited above, by
Trujillo-Cenoz et al. (1984)
is correct and that the reduction of currents during phase
V4 at the fovea, together with the present finding that
the remaining phases can also be used for electrolocation, might explain the
presumably higher importance of V1 and
V3 for electrolocation at the fovea as inferred from the
results of Aguilera et al.
(2001
). What is clear, however,
is that a strict specialization of currents and functions seems not to hold,
at least not in the electric organ of G. carapo.
It might, therefore, be more useful to pursue other ways of understanding
the puzzling complexity of EOD generation in pulse-type gymnotid fish.
Electrolocating `gymnotid-style' by means of an electric organ that produces
enormous local variations in the waveform of the transcutaneous currents could
have more intrinsic advantages. For instance, early measurements by Bastian
(1977) clearly raise the
possibility that local differences in the EOD may give rise to an interesting
cue in the detection of purely resistive objects. Rather than simply changing
the amplitude of the local EOD (as is certainly the case for a `point object')
(e.g. von der Emde, 1999
),
purely resistive objects, whose sizes are in the range of the local
inhomogeneity of the transcutaneous flow of EOD current, could well lead to
phase changes as well as changes in the amplitude spectrum of the local EODs.
Unfortunately, the possibility that such cues are actually used has, to our
knowledge, not yet been explored. It is not unlikely that the possibility to
exploit such cues, besides other important constraints such as sexual
selection, predation, energetic costs (e.g. see
Stoddard, 1999
;
Hopkins, 1999
), could also
have played a role in shaping the complex electric organs of pulsetype
gymnotid fish.
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Acknowledgments |
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References |
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