Electroreception in G. carapo: detection of changes in waveform of the electrosensory signals
Departamento de Neurofisiología Comparada, Instituto de Investigaciones Biológicas Clemente Estable, Unidad Asociada a Facultad de Ciencias, Universidad de la República, Av. Italia 3318, Montevideo, Uruguay
* Author for correspondence (e-mail: angel{at}iibce.edu.uy)
Accepted 10 December 2002
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Summary |
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Key words: electric image, electrosensory, electric fish, Gymnotus carapo, feature discrimination
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Introduction |
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At every point on the fish's skin, the transcutaneous current results from
the sum of the basal field in the absence of the object plus the effect of the
object, considered as a virtual electric source
(Lissmann and Machin, 1958;
Sicardi et al., 2000
;
Budelli and Caputi, 2000
). The
electromotive force of such an `object equivalent source' decreases as a
function of object distance and as a function of the absolute value of the
object impedance. Since impedance is a complex magnitude, frequency of the
local field at the site of the object is an additional important parameter
determining the electromotive force of the `object equivalent source'. Thus,
the local characteristics of the electrosensory carrier at the site of the
object are critical in measuring object impedance, and differences in the
organisation of electrogeneration between species must imply differences in
their impedance discrimination strategy.
Mormyriform pulse fish from Africa have a short electric organ located in
the tail that generates a basal field with the same waveform everywhere. The
waveform is biphasic, with an initial head-positive phase followed by a
head-negative phase. In these fish, the waveform of the virtual electromotive
force generated by an object is independent of the object's position relative
to the fish body. The stimuli resulting from different objects located at the
same place in the fish's environment are distributed in a two-dimensional
domain in which the axes are the peak-to-peak amplitude of EOD-induced current
and the ratio between positive and negative phases of this current. Two
receptor types, one sensing the peak-to-peak amplitude of the self-generated
local electric organ discharge (sLEOD) and the other sensing a
waveform-related parameter, allow the fish to locate the object in the
environment and to measure `perceptual distances' between points in the
two-dimensional domain defined by the amplitude and waveform axes
(Bell, 1990; von der Emde,
1990
,
1993
;
von der Emde and Ronacher,
1994
; von der Emde and Bell,
1994
; von der Emde and
Bleckmann, 1997
).
Although capacitance discrimination is well-demonstrated in gymnotids, a
detailed description of the stimulus domain is lacking and the mechanism of
capacitance detection in pulse gymnotiforms is not yet understood
(von der Emde, 1999). The
electric organ of pulse gymnotid fish is quite different from that of pulse
mormyriform fish. Pulse gymnotids have a long electric organ that extends
along 90% of the fish body. The organ is not homogenous along its length and
it generates complex spatiotemporal fields and waveforms that are highly
dependent on its position in the field
(Bastian, 1977
;
Watson and Bastian, 1979
;
Caputi, 1999
;
Assad et al., 1999
). This
suggests that the strategy for impedance discrimination must be quite
different in pulse gymnotids from that in pulse mormyriforms.
Wave gymnotids emit a quasi-sinusoidal carrier that is modulated in phase
and amplitude by nearby objects. Electroreceptors are well tuned to the main
frequency of the EOD and therefore the impedance-related sensory qualities of
an electrolocated object could be related to the change in amplitude and phase
of the sLEOD (Hopkins, 1983;
Dye and Meyer, 1986
). In wave
gymnotids, amplitude is measured by the P-type electroreceptors and phase is
measured by T-type receptors. Scheich et al.
(1973
) showed in the wave
gymnotid Eigenmannia sp. that T- and P-types of electroreceptors
`respond differently in such a manner that information is provided to the
brain adequate to distinguish capacitive from resistive impedance and to
assess the magnitude of the mixture in complex impedance'. Further
support for this argument is provided by the demonstrated neural mechanism in
these fish that allows them to discriminate signals in the phase-amplitude
domain (cf. Heiligenberg,
1991
).
Very little is known about impedance discrimination in pulse gymnotids. The
electric organ of these fish generates a complex spatio-temporal electric
field resulting from the weighted sum of the effects of a series of electric
sources having different time-waveforms and internal resistances
(Caputi, 1999;
Aguilera et al., 2001
). The
sensory side of the system is also complex, in that four types of tuberous
electroreceptors have been described (Bastian,
1976
,
1977
;
Watson and Bastian, 1979
). The
complexity of the system has led us to the hypothesis that these fish have the
ability to discriminate different features in the reafferent signal waveform
and to classify object images in a multidimensional `perceptual domain'.
This paper describes the changes generated by objects of different
impedance on the amplitude and waveform of the self-generated local electric
field (sLEOD) of the pulse gymnotid Gymnotus carapo (L). There is a
zone around the mouth where receptor density is highest and where the variety
of receptor types is highest (the electrosensory fovea;
Castelló et al., 2000).
This anatomo-functional variety is necessary for implementing complex
impedance discrimination. Thus, our study is focused on the electrosensory
fovea. The paper also examines the changes in reafferent electrosensory input
that are able to provoke `novelty responses' (a well-known orienting behavior,
Lissmann; 1958
;
Bullock, 1969
). These
behavioural experiments allowed us to test the hypothesis that these fish are
able to discriminate two types of stimulus parameters, some correlated with
the total energy and others only dependent on waveform.
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Materials and methods |
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Fish were held within a net in the middle of a tank (18 cmx25
cmx10 cm) containing 3 liters of water with a conductivity of 100 µS
cm-1. We used the technique introduced by von der Emde
(1990) to modulate the
reafference, i.e. the transcutaneous current evoked by the fish's own EOD. A
cylindrical `object' (2 mm diameter, 1 cm length) was oriented with its long
axis perpendicular to the skin of the electrosensory fovea
(Castelló et al.,
2000
). The two ends of the cylinder were made of conducting carbon
discs that were inserted into a non-conducting plastic tube. The carbon discs
were connected to a switch by insulated copper wires leaving the tube at its
center. The switch allowed us to apply a capacitive-resistive load
(z) between the bases of the stimulus-object. The component of the
sLEOD perpendicular to the skin was measured as the voltage drop between the
bare tip of a 100 µm diameter insulated copper wire placed against the skin
and the base of the stimulus-object cylinder nearest to the fish (see inset in
Fig. 1). The voltage drop
between the carbon ends of the object was also recorded in most experiments.
Signals were amplified (x100) and filtered (band pass 10-10 000 Hz) for
observation of individual LEOD waveforms using a digital oscilloscope, and
sampled (20 kHz, 12-bit resolution) for off-line processing. To characterise
the waveform generated in the presence of a given stimulus-object impedance,
we averaged 64 consecutive sLEODs for each load in each fish. We compared time
waveforms and their fast Fourier transforms. To compare amplitude and waveform
obtained when applying different loads to the object, sLEODs were plotted
against the sLEOD obtained in the absence of load impedance. The difference in
amplitude was shown by the different inclination of the lines and the
differences in waveform by the deviation of the loop from a straight line.
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To evaluate if G. carapo is able to discriminate between two
different local stimuli we used a `comparative unidimensional judgement'
procedure, a psychophysical method in which a baseline waveform is applied in
alternation with a comparison waveform
(Werner, 1980). We used an
orienting response (the novelty response) as an index of stimulus
discrimination. This response, consisting of a transient acceleration of EOD
frequency, has been used extensively to test the ability of fish to detect
changes in sensory stimuli (Szabo and
Fessard, 1965
; Bullock,
1969
; cf. Hopkins,
1983
; Moller,
1995
).
In each experiment, an external variable impedance z0 was connected to the carbon plugs to set the baseline sLEOD. A timed switch was used to substitute z0 with a second variable impedance z1 to set the comparison sLEOD (Fig. 1, inset). Each trial consisted of ten cycles of 30 s periods (connecting z0 for 29 s and z1 for 1 s). For each trial, the inter-EOD interval was displayed off-line as a function of time to test for the presence of novelty responses. A novelty response was said to occur when duration of the second interval (I2) after presenting the comparison sLEOD was below the inferior confidence limit of the baseline interval (I0, defined as the mean of the preceding 5 baseline inter-EOD intervals, confidence limit=I0-2 S.E.M.) The amplitude of the novelty response was defined as the maximum shortening of the normalised interval [novelty response amplitude = (1-I2/I0)x100].
We explored the novelty responses following transitions between: (i) open circuit (z0) and either a pure resistive or pure capacitive impedance (z1, 5 fish); (ii) pairs of impedance (z0 and z1) causing sLEODs with the same total energy, as measured by their equal root mean square (rms) value (5 fish); (iii) short circuit (z0) and either a pure resistive or pure capacitive impedance (z1, 4 fish). In all cases we studied the amplitude of the novelty response provoked by transitions in both directions (z0 to z1 and vice versa).
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Results |
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The amplitudes of individual wave components were unequivocally determined by a `one to one' function of the rms value, indicating that a single parameter, tightly correlated with the total energy, is enough to characterise the signal when the object impedance is purely resistive. However, small but systematic changes in waveform were also observed. The relative amplitude of sV1 (sV1/sV3) decreased with object resistance, as did the sharpness and relative amplitude of sV4 (sV4/sV3, Fig. 1A). As expected by the good correlation between waveforms obtained with different load resistances, the general shapes of the power spectral density histograms were very similar: a sharp increase, a peak (at 350-500 Hz), and a slow decay vanishing within the noise above 3 kHz. The smooth decay of the power at the high frequency flank was interrupted by a hump (at 700-1000 Hz) of constant amplitude when it was normalised to the peak (arrow, Fig. 1C).
For capacitive loads within the range 0.3-300 nF, sLEOD energy (as measured by rms value) amplitude increased as a function of capacitance. Below 0.3 nF the observed sLEODs were similar to those obtained without connecting any load (open circuit) and above 300 nF the recorded sLEODs were similar to those obtained by short-circuiting the object bases (Fig. 2A). Important changes in sLEOD waveform were provoked by capacitive loads, in contrast to the small ones provoked by resistive loads. Mid-range capacitive loads (color traces, Fig. 2B) produced much larger relative variations of sV1 and sV4 than short circuit and open circuit (grey and black traces Fig. 2B). In addition, important changes in phases were observed by plotting the sLEOD obtained with different capacitive loads versus the sLEOD obtained in the open circuit condition (color traces, Fig. 2C). This contrasts with the tight correlation between the waveforms obtained with open and short circuits (grey trace, Fig. 2C). Striking increments of the high frequency hump in the power spectral density histogram were consistently observed, becoming maximum at intermediate capacitance between 5 nF and 12.3 nF (arrow, Fig. 2D).
|
To analyse changes in the sLEOD waveform systematically we studied the
amplitude of the different peaks as a function of object impedance. In order
to compare fish of different lengths, modulation of each wave component
(defined as its amplitude divided by the amplitude of the same component in
the absence of object) were plotted together as a function of object
longitudinal resistance (Fig.
3). For every waveform component, modulation was a sigmoidal
function of object resistance (Fig.
3A-C). The load resistance value yielding a modulation equivalent
to the 50% of the range was different for each wave component
(68.6±8.23 k; 50.72±5.23 k
; and 33.5±3.99
k
; means ± S.D., for sV1, sV3 and
sV4, respectively), and these differences were significant (Fischer
exact test, P<0.001, N=10). Despite these differences,
both the ratios sV1/sV3 and
sV4/sV3 were well-fitted by monotonic functions of
sV3 with opposite slopes (red symbols,
Fig. 4A,B). Thus, waveform was
predictable from the total energy of the sLEOD.
|
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For capacitive loads, ratios were not predictable from the rms value. The amplitudes of sV1 and sV3 increased following different sigmoidal curves (50% modulation at 20 nF for sV1 and 8 nF for sV3; Fischer exact test, P<0.001, N=10, Fig. 3D,E). The relative amplitude of sV4 sharply increased with capacitance up to a maximum between 10 and 12.3 nF (Fig. 3F). Beyond this maximum the ratio sV4/sV3 decreased up to the short circuit value. To compare between different fish we normalised the amplitude of each wave component by the change observed between open and short circuits. The graph of sV4/sV3 versus sV3 was fitted by an inverted U-shaped curve with its peak at approximately 8 nF (Fig. 4A, blue symbols) and the graph of sV1/sV3 versus sV3 was fitted by a U-shaped function having a minimum generated by capacitors of about 12 nF (Fig. 4B, blue symbols).
For complex impedance having both capacitive and resistive components, connected either in parallel or in series, the data points fell within the surface limited by the curves generated by pure resistive and pure capacitive loads (Fig. 4A,B). Therefore, curves generated by pure resistive and pure capacitive loads define a `reafferent stimulus domain' for complex impedance cylindrical objects placed near the fovea in G. carapo.
A similar analysis was made in the frequency domain, because
electroreceptors in pulse gymnotids have previously been classified according
to their tuning curves (Bastian,
1976,
1977
;
Yager and Hopkins, 1993
). The
rising flank of the spectrum (Fig.
1C) is close to the tuning frequency of low-pass burst duration
coders, and the hump at the descending limb coincides with the tuning
frequency of narrow band burst duration coders
(Watson and Bastian, 1979
). In
order to evaluate the relative changes of these two zones of the spectrum, we
plotted the relative power at 100 Hz and 800 Hz as a function of the total
energy of the sLEOD (see below). The total energy was used because it is a
good measurement of the whole spectrum stimulation potential. For resistive
objects these plots show a flat profile, indicating that the signal is mainly
modulated in amplitude (Fig.
5A,B, red symbols). For capacitive objects, these plots show
U-shaped and inverted U-shaped profiles, respectively
(Fig. 5A,B, blue symbols).
These profiles are not symmetrical, indicating that there is not a `one to
one' correspondence between these parameters and that neither is redundant.
Data points generated by objects having combined capacitive and resistive
loads fell within the domain delimited by the lines corresponding to pure
resistance and pure capacitance loads.
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sLEOD amplitude and waveform discrimination
The total energy of the sLEOD () can be calculated as the integral
over time of the square of current density (J2) times the
specific resistance of the skin (
, Equation 1):
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Since duration of the sLEOD is finite and constant, is proportional
to its effective value or rms value
(Cotton, 1966
), defined as the
direct current intensity that dissipates the same amount of energy per unit of
time. Since cutaneous impedance is mainly resistive
(Caputi and Budelli, 1995
) and
transcutaneous current density is a linear function of the sLEOD, the
amplitude of the local stimulus is proportional to the total energy of the
sLEOD dissipated in the adjacent water. We estimated numerically the rms value
of the local stimulus as the square root of the mean of sLEOD squared values
from the beginning of sV1 to 3 ms after the peak of sV3
(Equation 2). This last limit was arbitrarily set, due to the monotonically-
and asymptotically-to-zero temporal course of sV4:
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We found that peak amplitude of sV3 was highly correlated with the rms value (Fig. 6). This relationship was well fitted by the same linear function for pure resistive, pure capacitive and resistive-capacitive loads in the same fish. This result indicates that the easily measurable sV3 was a good estimator of the total energy dissipated at the skin and consequently a good estimator of the stimulus at the center of the region facing the object. However, since electroreceptor response might be waveform-dependent, the effective stimulating energy eliciting a neural response is equivalent to the total energy attenuated by a factor, depending on receptor responsiveness to the stimulus waveform.
|
Using the novelty response as an index of the perceived change in an electrosensory stimulus, we explored how the amplitude of the novelty response depends on the change in stimulus energy and how changes in waveform modify this relationship. The amplitude of the novelty response caused by a change in the longitudinal resistance of a cylindrical object is not dependent on the pair of load resistors compared but on a logarithmic function of the change in the peak-to-peak amplitude of the sLEOD (a linear regression of the data in Fig. 7A yields r=0.9, N=15, P<0.0001).
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Since for resistive objects the peak-to-peak amplitude is linearly related to sV3 and to the rms value, the amplitude of the novelty response for a change in resistance alone is also a logarithmic function of the changes in sV3 and in rms value. Linear regression analysis confirmed this hypothesis (Fig. 7A, red line; r=0.9, N=15, P<0.0001). The differences between the expected values of the amplitude of the novelty response (according to its regression versus sV3) and the observed values were not significantly correlated with change either in sV1/sV3 or in sV4/sV3 (Fig. 7B,C; r=0.09 and r=0.2, respectively; N=15, P>0.4 in both cases). This lack of correlation with waveform parameters and the good correlation with the rms value suggest that the detection cue in the case of resistive objects is a function of the change in total energy of the signal.
We further tested whether this hypothesis holds for the general case of complex impedance. When the longitudinal load of the cylindrical stimulus-object was changed from open circuit to a given capacitor (Fig. 7, blue symbols), the amplitude of the evoked novelty response was also a function of the change in sLEOD rms value. However, the function with a best fit was not monotonically increasing. Except for the very small and very large rms values, where both curves converge, the novelty responses obtained with capacitors were significantly larger than the expected value predicted by the rms change, as predicted from the experiments with resistors. Differences between this expected value and the measured amplitude of the novelty responses evoked using capacitive loads were significantly correlated with the changes in sV1/sV3 and in sV4/sV3 (Fig. 7B,C; r=0.75 in both cases, N=30, P<0.0001). Decreases in sV1/sV3 and increases in sV4/sV3 account for the difference between the observed and expected amplitudes of the novelty response. This indicated that the amplitude of the novelty response was independently correlated with both energy and waveform parameters.
To test the hypothesis that changes in waveform alone can be detected by G. carapo, we provoked changes in sLEOD waveform maintaining the rms value constant in five fish. Novelty responses were consistently evoked in these experiments, indicating that fish are able to detect a parameter independent of total energy. Responses were asymmetric; large novelty responses (amplitude 6-10%) were always obtained when the change in load produced a decrease of sV1/sV3, an increase of sV4/sV3 or a phase advance of the slope sV3sV4. Conversely, opposite waveform changes did not modify the inter-EOD interval, or elicit changes in the inter-EOD interval that did not fulfil the typical pattern of the novelty response (small reduction of successive intervals; Fig. 8).
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In order to study this phenomenon in more detail, we explored the effect of
waveform changes between selected points in the above-defined stimulus domain
(four points in two fish, six points in one fish). Two of these points were
defined by open and short circuits (black and grey traces, respectively;
Fig. 9). The other selected
points were pairs of points sharing the same total energy but having different
waveform parameters. In each of these pairs, one point was defined by a given
capacitor and the other by the resistor generating a sLEOD of the same rms
value (Fig. 9; 10 nF, blue
traces and 21 k, red traces). As also shown in
Fig. 7, similar changes in
waveform provoked novelty responses of different amplitude depending on the
associated change in rms value (compare
Fig. 9B and C). Decreases of
sV1/sV3 and/or increases of
sV4/sV3 associated with increases in sLEOD rms value
provoked novelty responses of larger amplitude than the same change in rms
value associated with minimum changes in waveform (compare
Fig. 9B and D). Strikingly,
changes in waveform consisting of increasing sV4/sV3 and
a phase advance of the slope sV3sV4 always evoked
novelty responses, even when they were associated with reduction in rms value
(that otherwise did not provoke novelty responses;
Fig. 9E, open symbols). On the
other hand, reductions in sV4/sV3 did not evoke novelty
responses even when they were associated with increases in rms value (that
otherwise did provoke novelty responses;
Fig. 9E, filled symbols). This
suggests that processing of changes in waveform and total energy (estimated by
the rms value) are independent and their effects on the novelty response are
additive.
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Discussion |
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The analysis of the amplitude of the novelty responses provoked by the changes in sLEOD indicates that G. carapo discriminates resistance using a single energy-related parameter. It also discriminates complex impedance by detecting independent changes in intensity and waveform. At present we cannot assess how many independent waveform parameters are coded; nevertheless our findings, together with an analysis of findings reported in previous literature, suggest that these fish evaluate impedance-related sensory qualities of closely located objects integrating at least two parameters of the sLEOD waveform.
Modulation of the sLEOD by nearby objects
Waveform components are the sum of the fields originated in different
regions of the elongated electric organ of G. carapo (Caputi et al.,
1989,
1993
;
Caputi, 1999
;
Assad et al., 1999
). The sum of
effects of the series of different sources that may be used to represent the
electric organ discharge depends on their relative distance to the sLEOD, on
water and object impedance, and on their relative electromotive forces and
internal impedance. At the electrosensory fovea, the three components of the
sLEOD correspond to different sources in the EO: (i) sV1 is
generated by a low electromotive force and low internal resistance source
(V1) located at the abdominal region; (ii) sV3 is
generated all along the fish by a distributed source (V3),
increasing in electromotive force and internal resistance from head to tail;
and (iii) sV4 is generated mainly at the trunk-tail region by a
source (V4) of strong electromotive force and high internal
resistance (Caputi, 1999
). At
the fovea, the field is large, has the same orientation all along the time
course of the EOD and shows identical time waveforms all over the perioral
region (Castelló et al.,
2000
; Aguilera et al.,
2001
). Due to these characteristics the modulation in sLEOD
waveform caused by resistive loads is predictable from the change in total
energy of the sLEOD. Nevertheless, sV1 is less modulated than
sV3, which in turn is less modulated than sV4, as shown
in Figs 1 and
3.
When there is a capacitive load, the base of the cylinder closest to the skin is negatively charged during V1 up to the point that the charge counterbalances the effect of the EOD-generated current; this causes a relative decay of sV1. When the EOD current decays below this limit, opposite current flow originated in the capacitor discharge prevails and the sLEOD, resulting from the addition of both, reverses phase in advance. Thus, sV3 begins and ends earlier than V3. The capacitor discharges and recharges during V3, affecting its time course: the discharge current flows in the same direction causing an advance of phase of sV3, but the recharge current is opposed causing the advance of phase of the following component (sV4). Finally, currents generated at the caudal EO (V4) summate to those driven by the capacitance discharge, enhancing sV4.
In summary, sLEOD waveform components are the sum of effects of currents generated by the EOD and by the charge or discharge of the capacitor. The charge initially accumulated in the capacitor during V1 (generating a virtual source opposing V1 and thus decreasing sV1) is delivered later. Most of this charge is finally delivered at the end of the EOD, causing an increase of sV4. Therefore, the changes in sV1/sV3 and sV4/sV3 associated with capacitive loads are opposite.
Amplitude and waveform discrimination
The amplitude of the novelty response is scaled with the change in rms
value of the reafferent signal. For resistive objects, the constancy of the
shapes of the power spectra and the lack of correlation of the amplitude of
the novelty response with changes in sV1/sV3 and
sV4/sV3, suggest that the change in the total energy of
the signal is the cue for discrimination. Correlation analyses suggest that
other waveform-dependent cues are also important for complex impedance
discrimination. sLEODs having different waveforms but the same rms value are
clearly discriminated, indicating that a waveform-dependent parameter is used
independently of sLEOD amplitude as a discrimination cue. Novelty responses
were only evoked in the direction associated with decreases in
sV1/sV3, increases in sV4/sV3 and
phase advances in the slope sV3sV4, suggesting
that these parameters or their related changes in the power spectra (relative
decrease in the low frequency range and increase in the high frequency range)
are possible cues for waveform discrimination. Since waveform parameters were
significantly correlated among themselves, we could not assess whether a
single waveform parameter or a combination of parameters are sensed.
When changes in waveform were associated with increases in rms value, the
provoked novelty response was much larger than when the waveform changes were
associated with reductions of the rms value. This suggests that cues related
to energy and waveform may have additive effects and therefore that they are
probably sensed in an independent manner (see
Fig. 9). In fact, previous
literature indicates that pulse gymnotids are furnished with the necessary
structural features to evaluate more than a single parameter of the sLEOD
(Bastian, 1976,
1977
,
1986
;
Watson and Bastian, 1979
;
Yager and Hopkins, 1993
).
Watson and Bastian (1979
)
described three subtypes of burst duration coders having different frequency
tuning properties. Receptor variety is particularly important in the foveal
region (Castelló et al.,
2000
), suggesting that pulse fish evaluate a complex spectrum
using different types of receptors encoding different features of the object
associated signals. The similarity with color vision is clear.
Combining our data with those of Watson and Bastian
(1979), we have compared the
power spectral density histograms with typical tuning curves. The threshold of
`low-frequency band' receptors (best frequency below 100 Hz) follows a curve
parallel to the low-frequency flank of the spectra (50% around 200 Hz), the
threshold of `narrow-band receptors' has a minimum (best frequency 500 to 2000
Hz with sharp tuning) in the frequency range where the hump occurs (between
700 and 900 Hz). The independent variation of the relative power observed at
100 Hz and 800 Hz (Fig. 8)
suggests that low-frequency and narrow-band receptors code non-redundant
information. Information about the total sLEOD energy may be provided by
wide-band receptors (broad band tuned between 125-1000 Hz). The various
frequency sensitivities allow the fish to create a multi-dimensional
representation of object impedance-related stimulus features.
It should be noted that not only frequency tuning but also phase dependence
has been demonstrated in electroreceptors of a related species
(Heiligenberg and Altes,
1978). Pulse markers giving raise to the fast electrosensory
pathway in G. carapo (Szabo, 1965;
Castelló et al., 1998
)
have tuning properties similar to narrow-band burst duration coders
(Watson and Bastian, 1979
) and
may provide phase information to integrate the signals conveyed by the three
types of burst duration coders.
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Acknowledgments |
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References |
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