First published online September 5, 2008
Journal of Experimental Biology 211, 2919-2930 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.016154
Multisensory enhancement of electromotor responses to a single moving object
Scott R. Pluta* and
Masashi Kawasaki
Department of Biology, University of Virginia, Charlottesville, VA 22904,
USA
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Fig. 1. A schematic drawing of the experimental setup. The curarized fish is
resting on the platform adjacent to the agar frame. The stimulus object is
connected to the center axis via the arm. The center axis rotates
via a pulley system connected to a speed-controlled DC motor. The
minimum lateral distance between the stimulus object and fish is controlled by
the position of the entire stimulator, which is positioned outside the
aquarium.
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Fig. 2. (A) An example of single novelty response (NR) from a single rotation of
the stimulus object. The dotted line is the instantaneous electric organ
discharge (EOD) rate in pulses s–1. The green line is the
spike density function (SDF) derived from this single sequence of pulses.
Notice how closely the SDF follows the instantaneous EOD rate, except for the
four shortest intervals that compose the `scallop' signature but contribute
very little to the overall area of the NR. The time of minimum object distance
is shown by the broken red line. (B) EOD rate during a session of 10 stimulus
object rotations. The black dots are 10 sequences of instantaneous EOD
frequency, and the green line is their SDF. (C) SDF from the same session,
showing start time and end time as well as the threshold EOD rate and the
baseline period. (D) Spike density derivative (SDD) showing how start time and
end time are determined by zero-crossings relative to maximum. Notice how the
first post-maximum, negative-to-positive zero-crossing does not register as
end time because the corresponding SDF value is above threshold.
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Fig. 3. Electric organ discharge (EOD) activity recorded during the rotation of a
multimodal stimulus object and its unimodal components. Each row of graphs is
from a different fish. The vertical broken arrow represents the time of the
object's minimum lateral distance from the fish. The multisensory novelty
responses (NRs) are larger than their component unisensory NRs (A,C,E).
Moreover, the multisensory NRs are larger than the simple linear sum of their
component unisensory NRs (B,D,F).
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Fig. 4. Area (A) and duration (C) of the novelty response (NR) varies with stimulus
type. The multisensory NRs have a significantly greater area and duration than
each of their component unisensory NRs (except for T+LL vs LL
duration;  P>0.1). Area (B) and duration (D) of
the actual vs calculated multisensory NRs. The actual NRs have a
significantly greater area and duration than the calculated linear sum of
their component unisensory NRs (except for T+LL area; P>0.08).
*P<0.05, error bars equal ± 2 s.e.m. Tub,
tuberous electrosense; Amp, ampullary electrosense; LL, mechanosensory lateral
line.
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Fig. 5. The within-session distribution of novelty response (NR) magnitude in a
single fish among three stimulus types. NR duration (A) and area (B) were
calculated for each of the 10 individual responses. The distribution of NR
magnitude during Tub+Amp stimulation is greater than its unisensory
components. Tub, tuberous electrosense; Amp, ampullary electrosense; LL,
mechanosensory lateral line.
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Fig. 6. An example of multisensory enhancement where the unisensory responses are
marginal but the multisensory responses are evident. A and C are
baseline-subtracted spike density function (SDFs) during each stimulus type. B
and D compare the calculated linear sum of the baseline-subtracted unisensory
responses to the actual baseline-subtracted multisensory response. In each
graph, the vertical broken arrow is the time of the minimum object
distance.
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Fig. 7. Mean amplitude of the novelty response for each stimulus type. Each
multisensory amplitude is significantly greater than its component unisensory
amplitudes. *P<0.05, error bars equal ± 2 s.e.m.
Tub, tuberous electrosense; Amp, ampullary electrosense; LL, mechanosensory
lateral line.
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Fig. 8. (A) Instantaneous electric organ discharge (EOD) rate (black and red) and
the spike density function (SDF) (blue) during a ten rotation session. The six
black lines represent rotations where the novelty response (NR) did occur
(above threshold at time of NR maximum), while the four red lines represent
rotations where the NR did not occur. Therefore, the probability for this
stimulus series is 0.60 or 60%. (B) The effect of stimulus type on
probability. T+A is significantly greater than Tub and Amp
(*P<0.05). Probabilities for all stimulus types
containing LL are approximately equal (P>0.07). Error bars equal
± 2 s.e.m. Tub, tuberous electrosense; Amp, ampullary electrosense; LL,
mechanosensory lateral line.
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Fig. 9. (A) Cumulative distribution functions of scallop production near the
minimum lateral distance (MLD) for each stimulus type. Notice how the
distribution of Amp and LL scalloping is highly skewed, with more than 80% of
the fish scalloping zero times. The distribution of scallop production during
multimodal stimulation is normal (P>0.05). (B) The mean number of
scallops 1 s before and 2 s after the time of the object's minimum lateral
distance. Scallop production elicited from multimodal stimulation was
significantly greater than its component unimodal Tub stimulation. (C,D)
Unfiltered electric organ discharge (EOD) rates during 10 rotations of the
stimulus object. An example of an increase (5 vs 11) in scallop
production near the minimum object distance after the addition of LL
stimulation. *P<0.05, error bars equal ± 2
s.e.m. Tub, tuberous electrosense; Amp, ampullary electrosense; LL,
mechanosensory lateral line.
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Fig. 10. (A,C) Novelty responses (NRs) to stronger stimulus intensities. These
intensities were more effective at eliciting NRs than the stimuli applied in
the main body of evidence of the current paper. The object's minimum lateral
distance was 1.9 cm and the DC field was >600 µVcm –1.
A and C show the baseline-subtracted spike density functions (SDFs) during the
rotation of a multimodal stimulus object and its component unimodal object
stimuli. Some of the unisensory responses have nearly equivalent magnitude as
their multisensory response. B and D show the actual and the calculated
baseline-subtracted SDFs. The calculated NRs have a greater or approximately
equal NR area and duration. Linear and sublinear multisensory integration
probably results from the relatively high intensity of the unimodal stimuli.
(E) Tub+Amp responses can be suppressed by the addition of a LL stimulus.
Therefore, the multisensory response curve is dependent upon the relative
strength of each component. (F) While LL stimulation can cause response
suppression, ampullary responses remained directly related to DC stimulus
strength. The progression in DC field strength is nonlinear. Tub, tuberous
electrosense; Amp, ampullary electrosense; LL, mechanosensory lateral
line.
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© The Company of Biologists Ltd 2008
