First published online March 8, 2005
Journal of Experimental Biology 208, 961-972 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01481
Contextual effects of small environments on the electric images of objects and their brain evoked responses in weakly electric fish
Ana Carolina Pereira,
Viviana Centurión and
Angel Ariel Caputi*
Department of Integrative and Computational Neurosciences, Instituto
de Investigaciones Biológicas Clemente Estable, Av. Italia 3318.
Montevideo Uruguay
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Fig. 1. Methods. (A) Diagram of the electrode arrangement for recording the local
self-generated field (sLEOD) and the head to tail field (htEOD). The black
rectangle represents the position of the tube in the experiments shown in Figs
2,
3,
4,
5,
6,
7. (B) Diagram of the stimulus
object and the electrode arrangement for recording the local self-generated
field (sLEOD) when shunting the poles of the object with a switch-selected
resistor (arrow) in order to control its longitudinal resistance. We recorded
the voltage drop (V) between object contacts and calculated the current flow
through them. The electromotive force and internal resistance of the
equivalent source that `illuminates' the object were estimated from the
characteristic voltage vs current plot (see
Fig. 5). (C) Diagram showing
the position of the electrodes used for recording field potentials at the
electrosensory lobe. (D) Diagram of the pen and the U-shaped structure
moved along the fish axis in order to change the reafferent stimuli while
recording evoked field potentials at the electrosensory lobe. Left, top view
of the set up; right, cross section of the pen.
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Fig. 2. Effect of tube conductivity on the sLEOD. (A) Local self-generated field
(sLEOD) at the electrosensory fovea under three conditions: inside a plastic
tube (top), control (middle) or inside a metal tube (bottom). Each trace
represents 64 average sLEODs from one fish. Each color corresponds to a same
individual. (B) Normalized sLEOD waveforms under the three conditions in one
fish.
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Fig. 3. Effect of relative position of the fish-tube on the sLEOD. (A) Amplitude
(sV3) and (B) waveform (quotient sV4/sV3) as
a function of the tube position relative to the fish. Broken line corresponds
to open field. Insets show the sLEOD waveform at the electrosensory fovea when
this region is in the middle of the tube (left inset), at the opening of the
tube (middle inset) and when the fish is outside the tube (right inset). Inset
locations correspond to the abscissa.
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Fig. 4. The effects of tube conductivity on object images. (A,B) r.m.s. value of
the sLEOD as a function of object longitudinal resistance when the fish is
inside the tube (red, metal; blue, plastic) compared to the respective
controls. Values are means ± S.D. of the measurements made
in four different fish. When the fish is inside a tube the r.m.s. values of
the sLEOD are proportional to the r.m.s. values obtained in control condition
(C; red, metal; blue, plastic). Insets: sLEOD waveforms comparing effects of
the metal (red) and plastic (blue) tubes to the control condition (black);
right, short circuit; left, open circuit. The color key for control, metal and
plastic is common to all panels.
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Fig. 5. Effect of a plastic tube on the equivalent source `seen' by the object.
Voltage measured between the object tips is plotted as a function of the
current through the object. (A) Global measurement correlating voltage r.m.s.
value vs current r.m.s. value. (B–D) Plots corresponding to
each of the EOD peaks, V2, V3 and V4
respectively. Plot sizes were adjusted to match in height the equivalent
electromotive forces inside the tube. Note that the relative change in
V4 is larger than V2, and this in turn is larger than
V3.
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Fig. 6. The increase in carrier amplitude has a simple physical explanation. While
the rostrally generated sLEOD is attenuated by the tube (A), the caudally
generated sLEOD is increased (B). These results can be explained by a simple
electrical model (C,D). The abdominal EO acts mainly as a `voltage' source but
the caudal acts mainly as a `current' source. The voltage generated by the
abdominal source is therefore relatively independent of the external load and
the current generated by the tail source is relatively independent of the
external load. In the tube, the external load opposing the generation of
current by the abdominal source increases. Therefore the current and the sLEOD
at the fovea diminish. In contrast, the part of the current generated by the
caudal source that is shunted through the water in the open field, is forced
rostrally because of the presence of the non conductive tube. This causes an
increase of the sLEOD at the fovea.
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Fig. 8. The field potential responses at the electrosensory lobe. (A) Effects of
placing the fish inside the tube on the field potential responses. Top, fovea
at the middle of tube length; middle, fovea at the tube opening; bottom, fish
outside the tube. (B) Field potential variability induced by changes in
electrosensory stimuli (obtained from a different fish than in A). Bottom,
post EOD averaged trace recorded in open field. Top, post EOD standard
deviation of the signal when a plastic tube was moved in a step-like manner
(three epochs using different speed, black traces) or in an oscillatory manner
along main axis of the fish (three epochs using different frequencies, black
traces) compared with the standard deviation in open field when EOD novelty
responses were mechanically provoked by tapping the aquarium (red trace). (C)
Peak-to-peak amplitude of the fast electrosensory response as a function of
the r.m.s. value of the sLEOD. FEP, fast electrosensory pathway; SEP, slow
electrosensory pathway.
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Fig. 9. Different components of the slow electrosensory pathway responses. (A)
Color map indicating the field potential response (color coded) when the tube,
initially placed caudally to the fish (fish outside the tube, control
condition), is moved up to the point in which the jaw and the rostral opening
of the tube coincide (where the sLEOD is the largest). In this color map the
horizontal dimension corresponds to the time after the EOD and the vertical
dimension to the sequence of EODs. In order to quantify the change imposed by
the presence of the tube the control averaged profile was subtracted from each
raw trace. Control averaged profile was calculated from the first 50
consecutive evoked responses (fish was outside the tube). These data are
represented in (B). The changes in the response (expressed as a percentage of
the control) at the two different times representative of the early and late
responses (marked by the vertical lines in B), are shown in (C) and (D).
Traces in (E) show the presence of a rapidly adapting potential evoked by the
change in sLEOD. Red traces correspond to the first three evoked responses
just after the placement of the tube opening at the level of the jaw. Green
and blue traces are the averaged responses in the steady state control and
maximum sLEOD conditions, respectively.
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Fig. 10. Different components of the late response. (A) Relative position of the
fish and the tube during a sequence of 150 EODs. The limits of the shaded area
correspond to the positions of the snout and the tip of the tail,
respectively. (B) The corresponding amplitude of the sLEODs during the same
sequence. (C) Color map of the electrosensory lobe field potentials evoked by
each sLEOD. In this color map the voltage is color-coded, the horizontal
dimension corresponds to the time after the EOD and the vertical dimension
corresponds to the sequence of EODs. Each color line represents the response
elicited by the sLEOD at the position of the tube represented at the left.
Note that in this animal the late response shows a transient component (arrow)
and a slowly decaying component (double arrow) occurring at different times
after the EOD.
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© The Company of Biologists Ltd 2005
),
and in (E) as a function of the change in r.m.s. value (