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First published online February 15, 2008
Journal of Experimental Biology 211, 773-779 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.009795
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Response properties of electrosensory units in the midbrain tectum of the paddlefish (Polyodon spathula Walbaum)

M. H. Hofmann1,2,*, S. N. Jung2, U. Siebenaller2, M. Preißner2, B. P. Chagnaud1 and L. A. Wilkens1

1 Center for Neurodynamics, Department of Biology, University of Missouri – St Louis, St Louis, MO 63121, USA
2 Institute of Zoology, University of Bonn, Poppelsdorfer Schloss, 53115 Bonn, Germany


Figure 1
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  		Fig. 1. (A) Paddlefish striking at an artificial dipole field. (B) Front part of the rostrum cleared and stained with finger paint to show the electrosensory organs (black dots), which are organized in clusters. (C) Dorsal view of the brain showing the primary electrosensory hindbrain area, the dorsal octavolateral nucleus (DON) and the midbrain tectum (TM). BO, bulbus olfactorius; Cer, cerebellum; nLLd, dorsal root of the anterior lateral line nerve; Tel, telencephalon. Scale bar 1 mm.

 

Figure 2
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  		Fig. 2. (A) Coefficient of variation plotted against spontaneous spike rates of DON (open circles) and tectum (filled diamonds) units. Units in both areas can be clearly separated by these two parameters. (B,C) Fourier transformations of ongoing spike trains in primary afferent fibers (B) and DON (C). In addition to the large peak caused by the spike generator, primary afferents show another smaller peak at 25 Hz (arrow), which is absent in DON units (C).

 

Figure 3
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  		Fig. 3. Responses of a DON unit (A) and two tectal units (B,C) to a 5 Hz sinusoidal electric field (D). Dashes indicate the occurrence of spikes. Five repetitions of the stimulations are shown for each unit. The DON unit had a spontaneous rate that was modulated by the stimulus. Tectal units exhibited little or no spontaneous activity and showed a few spikes phase locked to the stimulus. (C) A few units in the tectum developed spikes a few seconds after stimulus onset.

 

Figure 4
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  		Fig. 4. Phase plots of DON (A–D) and tectal units (E–H). Most DON units showed spikes only at the second half-cycle of the stimulus. In a few units, spikes appeared at the first, positive half-cycle (D). Tectal units showed variable phase angles and sometimes spikes appeared at two different phase angles (E,F).

 

Figure 5
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  		Fig. 5. Mean phase angles of DON (A) and tectal units (B) following 5 Hz stimulation at different amplitudes. Phase angles of individual DON units vary little with amplitude and have mean phase angles of either 310 or 130 degrees. Tectal unit phase angles are mainly between 0 and 180 degrees, but vary considerably. However, phase angles do not depend much on amplitude in most units.

 

Figure 6
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  		Fig. 6. Responses of DON units (A,B) and tectal units (C,D) to sinusoidal stimuli at different amplitudes. A and C show the overall rate change, i.e. the mean spike rate during stimulation minus the spontaneous rate before stimulus onset. B and D show the phase coupling as measured by D (filled circles indicate significant phase coupling, P<0.01). In the DON, the greatest sensitivity is observed from 0.05 to 3 µV cm–1 (B). However, phase coupling is significant only above 0.3 µV cm–1. In this range, there is no change in overall spike rate (A). Tectal units show phase coupling down to 0.05 µV cm–1 (D).

 

Figure 7
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  		Fig. 7. Frequency response curves of DON (A,B) and tectal units (C,D). The response was measured either as the overall spike rate change during stimulation (A,C) or as phase coupling (B,D). Filled circles indicate significant phase coupling (P<0.01). DON units are very uniform in their response and show a linear relationship between the amount of phase coupling and stimulus frequency at lower frequencies (B) with best frequencies of either 10 or 20 Hz. Tectal cells are more variable in their response, but in many units phase coupling does not decrease as much as in DON units with lower frequencies (D).

 




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