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Brainstem lateral line responses to sinusoidal wave stimuli in still and running water

Sophia Kröther^*, Joachim Mogdans and Horst Bleckmann

Institut für Zoologie, Universität Bonn, Poppelsdorfer Schloß, D-53115 Bonn, Germany

View larger version (18K): [in a new window]   Fig. 1 . Measures used to quantify medial octavolateralis nucleus (MON) unit responses. (A) Sensitivity to running water. Discharge rate, mean ± S.D. averaged across 60 s of constant water velocity (ongoing rate subtracted), of a MON unit is plotted as function of flow velocity. A linear regression was fitted to the data (broken line) and was used as a measure of flow sensitivity. (B) Level-response function of a MON unit. Mean discharge rate (ongoing rate subtracted) is plotted as function of sphere displacement in still (filled circles, averaged across 20 stimulus presentations) and running (open circles) water. (C) Strength of phase-locking of a MON unit. Synchronization coefficients R are plotted as function of sphere displacement in still (filled circles) and running (open circles) water. Linear regressions (broken lines) were fitted to the data shown in B and C and compared by analysis of covariance (ANCOVA) to determine whether responses were masked in running water. To determine the degree of masking, the areas under the functions shown in B and C were calculated. Areas under the curves obtained from measurements in running water (dark shaded areas) were expressed as a percentage of the area under the curves obtained from measurements in still water (light shaded areas).

View larger version (21K): [in a new window]   Fig. 2. Sensitivity of medial octavolateralis nucleus (MON) units to running water. Change in discharge rate is plotted as a function of flow velocity. A rate change of zero indicates that discharge rates in still and running water were identical. (A) Data from flow-sensitive units that responded with increasing ongoing discharge rates to increasing flow velocities. Broken lines represent data from units in which discharge rates decreased at flow velocities above approx. 10 cm s-1. (B) Data from flow-sensitive units that responded with decreasing discharge rates to increasing flow velocities. (C) Data from flow-insensitive units. In these units, ongoing discharge rate in still water was not different from the ongoing rates measured at any of the tested flow velocities.

View larger version (17K): [in a new window]   Fig. 3. Peri-stimulus time histograms (bin width 1 s) of the activity of single medial octavolateralis nucleus (MON) units stimulated with a water flow of 15.5 cm s-1. The horizontal bar below each diagram indicates flow duration. (A) Examples of two units that showed sustained increases (top) and decreases (bottom) in discharge rate as long as the water was flowing. (B) Examples of two units that showed increases (top) and decreases (bottom) in discharge rate that were more strongly pronounced during a transient period shortly after flow onset than for the remaining time of water flow. (C) Example of two units that responded to the onset of water flow with a transient increase (top) or decrease (bottom) in discharge rate.

View larger version (37K): [in a new window]   Fig. 4. Responses of a type I medial octavolateralis nucleus (MON) unit to a 50 Hz vibrating sphere stimulus in still (left) and running (15.5 cm s-1; right) water. (Top) Raster plots of the responses to 10 stimulus repetitions for three displacement amplitudes (2717, 890 and 305 µm) (see also Figs 5,6,7,8). The stimulus trace is shown below the rasters. (Bottom) Level-response functions. Evoked discharge rates (means ± S.D.) (filled circles), averaged across 20 stimulus presentations, ongoing discharge rates (open circles) and synchronization coefficients (triangles) are plotted as function of sphere displacement. In still water, this unit responded to the vibrating sphere with an increase in discharge rate. In running water, the unit's ongoing discharge rate was increased. Consequently, the response to the vibrating sphere was masked both in terms of spike rate and phasecoupling.

View larger version (51K): [in a new window]   Fig. 5. Responses of a type I medial octavolateralis nucleus (MON) unit to a 50 Hz vibrating sphere stimulus in still (left) and running (15.5 cm s-1; right) water. Raster plots and level-response functions are as in Fig. 4. Displacement amplitudes were 2720, 305 and 60 µm. In still water, the unit responded to the vibrating sphere with a decrease in discharge rate. In running water, the unit's ongoing discharge rate was decreased (sphere displacements <60 µm were not tested). As a consequence, the response to the vibrating sphere was masked in terms of spike rate. Phase-coupling was comparable in still and running water.

View larger version (38K): [in a new window]   Fig. 6. Responses of a type I medial octavolateralis nucleus (MON) unit to a 50 Hz vibrating sphere stimulus in still (left) and running (15.5 cm s-1; right) water. Raster plots and level-response functions are as in Fig. 4. Displacement amplitudes were 3770, 1660 and 103 µm. In still water, the unit responded to the vibrating sphere with an increase in discharge rate. Ongoing discharge rate was comparable in still and running water. Consequently, the response to the vibrating sphere was not masked.

View larger version (59K): [in a new window]   Fig. 7. Responses of a type I medial octavolateralis nucleus (MON) unit to a 50 Hz vibrating sphere stimulus in still (left) and running (15.5 cm s-1; right) water. Raster plots and level-response functions are as in Fig. 4. Displacement amplitudes were 3770, 2300 and 59 µm. In still water, this unit responded to the vibrating sphere with a decrease in discharge rate. Ongoing discharge rate was comparable in still and running water. Consequently, the response to the vibrating sphere was not masked.

View larger version (54K): [in a new window]   Fig. 8. Responses of a type I medial octavolateralis nucleus (MON) unit to a 50 Hz vibrating sphere stimulus in still (left) and running (15.5 cm s-1; right) water. Raster plots and level-response functions are as in Fig. 4. Displacement amplitudes were 2720, 1660 and 560 µm. In still water, this unit responded to the vibrating sphere with an increase in discharge rate. Ongoing discharge rates in still and running water were comparable. Nevertheless, the response to the vibrating sphere was masked both in terms of spike rate and phase-coupling.

View larger version (17K): [in a new window]   Fig. 9. Summary of the characteristics of type I, type II and type III units. Box-and-whisker plots are shown representing median values and 10, 25, 75 and 90 percentiles as well as data points below the 10th percentile and above the 90th percentile. (A) Slopes of regression lines fitted to flow-response functions (see Fig. 1A). (B) Masking of dipole-evoked discharge rate in running water. Integrals of rate-level functions measured in running water are expressed as percentage integrals of rate-level functions measured in still water (see Fig. 1B). (C) Masking of phase-coupling to the vibrating sphere. Integrals of response functions measured in running water are expressed as percentage integrals of response functions measured in still water (see Fig. 1C). Asterisks indicate statistically significant differences (U-test, P0.05). n.s., no difference (P>0.05).

View larger version (23K): [in a new window]   Fig. 10. Responses of a type I medial octavolateralis nucleus (MON) unit to a 50 Hz vibrating sphere stimulus in still (left) and running (15.5 cm s-1; right) water. Raster plots are as in Fig. 4. Displacement amplitudes were 3700, 890 and 166 µm. In still water, the unit responded to the vibrating sphere with an increase in discharge rate. In running water, the unit exhibited both increased ongoing and increased evoked discharge rates.

View larger version (38K): [in a new window]   Fig. 11. Responses of a type I medial octavolateralis nucleus (MON) unit to a 50 Hz vibrating sphere stimulus in still (left) and running (15.5 cm s-1; right) water. Raster plots are as in Fig. 4. Displacement amplitudes were 3700, 2570 and 1530 µm. In still water, the unit responded to the vibrating sphere with an increase in discharge rate. In running water, ongoing discharge rate was decreased. The responses to the vibrating sphere, however, were comparable in still and running water.

View larger version (64K): [in a new window]   Fig. 12. Responses of three type I medial octavolateralis nucleus (MON) units to a 50 Hz vibrating sphere stimulus. Raster plots are shown of the responses to the sphere presented at two distinct locations along the side of the fish (indicated by open and filled circles). Displacement amplitude was 2717 µm. (A) Example of a unit that responded with a decrease in discharge rate when the sphere was in the head region and with an increase in discharge rate when the sphere was in the trunk region. Both responses were masked in running water. (B) Example of a unit that responded with an increase in discharge rate when the sphere was near the caudal peduncle and when the sphere was in the trunk region. Neither of these responses was masked in running water. (C) Example of a unit that responded with an increase in discharge rate when the sphere was opposite to the operculum and with a decrease in discharge rate when the sphere was near the tip of the snout. The response elicited by the sphere placed near the operculum was not masked, whereas that elicited by the sphere placed near the snout was masked by running water.