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Sensitivity to novel feedback at different phases of a gymnotid electric organ discharge

Stefan Schuster^* and Natalie Otto

Institut für Biologie I, Hauptstrasse 1, Albert-Ludwigs-Universität Freiburg, D-79104 Freiburg, Germany

View larger version (13K): [in a new window]   Fig. 1. Schematic diagram of the experimental arrangement (A) and examples of behavioral responses (B). (A) The experimental animal rested in a cage placed centrally, at half-water level, in a large tank. Two Ag/AgCl pellets, placed at the side of the fish at its trunk/tail region, could be connected, outside the tank, via a fast electronic switch. As a result of switch closure, a part of the electric organ discharge (EOD) current of the fish is redistributed to flow over the low-resistance path of the external circuit rather than through the water. This current could be monitored (cur) by inserting a current amplifier in the external circuit. As a reference of which electrocyte groups produced the current, head—tail EODs were recorded by two silver wires. To start an experiment, a rectangular pulse (en) simultaneously enabled a counter module (CT) and allowed EODs (sig) to be fed into a processor (PC). After 200 inter-EOD intervals were recorded, the counter module issued a reference pulse (ref), that signalled the onset of the 201st recorded EOD. By varying the delay of a command pulse (com) with respect to this reference pulse (DEL), the electronic switch could be closed during a selected phase of this particular EOD, or, as a control, in the silent time after it in which no EOD current is shunted. The processor continued to store 300 inter-EOD intervals that followed after switch closure. Prior to input to the counter module, the EODs were strongly amplified, filtered and converted to rectangular pulses (pulseformer; PF). The inset shows a head-to-tail EOD (sig; V1-V4 denote the major phases of the EOD) and a timing diagram of switch closure with the time course of the signals ref, com and cur. (B) Examples of four novelty responses to shunting of EOD current during particular phases of a single EOD. The switch was closed, for 100 µs, during EOD-phase V4 (upper three traces) or during V3 (bottom trace). The timing of switch closure is indicated by the arrow and the vertical grey line. Note the clear changes in interpulse interval. The abscissa (Time) shows the succession of the 200 pre- and 300 post-stimulus interpulse intervals.

View larger version (18K): [in a new window]   Fig. 2. Precision with which electric organ discharge (EOD) current could be shunted at a selected phase within a single EOD. (A) Oscillogram showing accumulated recordings from 100 successive tests in which the switch was closed for 100 µs in a preassigned phase. The upper trace shows the rectangular reference pulse that marked the `onset' of the EOD and triggered the recordings. The middle trace shows the head—tail EOD with head-positive deflections going upwards. The lower trace shows the current that flowed through the external circuit during switch closure. As a result of a variety of factors (see text) the recorded EODs jitter with respect to the onset-mark and the current signal. However, note that a reasonably small phase jitter could be achieved. (B) To assess the phase stability quantitatively, for each of 1000 successive experiments a precision counter measured the time between the rising flank of the reference pulse and that when the falling phase of the amplified EOD had reached a set threshold (time interval as defined in A). The histogram shows the distribution of recorded values for (relative to a reference value of tr=476.5 µs; bin width 1 µs) as obtained in 1000 successive experiments. Note the small amount of phase jitter (S.D. ±10.4 µs). The structure of this distribution is probably determined by different major resting positions of the fish assumed during the 1000 experiments.

View larger version (10K): [in a new window]   Fig. 3. Analysis of the shunted electric organ discharge (EOD) current. (A) Example illustrating the two effects that contribute to the current measured in the external circuit during maintained switch closure: (i) A slowly decaying current (`offset') that continues even during the silent phases (in the absence of EODs). This component never elicited behavioral responses in the arrangement used in the present study. (ii) The second contribution is from the current that is shunted during the EOD. (B) The two components added linearly within the current range used in the present study. This is illustrated by two recordings of the current in the external circuit, taken at two stages after a maintained switch closure. The first (blue) was taken immediately after the switch was closed and thus when the `offset' was maximal. The second was taken long after the start of switch closure, when the offset had decayed to zero (red). Traces are aligned with respect to the simultaneous recordings of the head—tail EODs, one of which is shown below the two traces (C). Note that the EOD-induced modulation in the absence of the offset is approximately the same as that recorded in the presence of an offset.

View larger version (12K): [in a new window]   Fig. 7. Positioning the shunting electrodes farther away from the side of the fish reduces the current shunted during an electric organ discharge (EOD) but leaves the distribution of responsiveness across phases constant. (A) Recording of EOD current shunted during prolonged switch closure when the shunt electrodes were placed at three lateral displacements, successively farther away from the fish cage (0, 1 or 2 cm). Each point of the waveforms predicts the magnitude of EOD current that is shunted when the electronic switch is closed during an interval that centers at this point. An example of the head—tail EODs, recorded simultaneously, is shown below as a phase reference. (B) Response probabilities obtained as the switch was closed for 100 µs either during one of the extrema V2, V3 or V4, or 4 ms after onset of the EOD (Control). For each of the three distances, 100 responses were recorded at each of the four stimulus conditions (total of 1200 tests on fish 6). The baseline response probability (due to spontaneous rate fluctuations) is given by the dotted horizontal line (pooled from 100 recordings without switch closure at each distance). There was no significant difference between baseline and controls. At all distances, responsiveness in V4 is significantly higher than in V3 (P<0.001) and higher in V3 than in V2 (P<0.05). All response levels are significantly above baseline except for those at distances 1 and 2 cm in phase V2. Decrease in responsiveness with distance is significant in V4 (0-1 cm: P<0.01) and V3 (0-2 cm: P<0.05).

View larger version (17K): [in a new window]   Fig. 4. The electric organ discharge (EOD) current shunted in a particular phase of the EOD can be predicted from the modulation recorded during maintained switch closure. The oscillogram shows accumulated recordings of a set of current signals recorded as the switch was closed briefly for 100 µs during various phases of the EOD and the current recorded during prolonged switch closure lasting for the whole EOD (continuous trace). The peaks of the brief current signals retrace the continuous curve recorded during maintained switch closure.

View larger version (26K): [in a new window]   Fig. 5. Sensitivity to novel feedback during one of the three major phases of a single electric organ discharge (EOD). The diagrams show for each animal the probability of a novelty response when the electronic switch was closed for a brief interval of 100 µs that either centred at one of the extrema of the EOD (V2, V3, V4; see inset) or after the EOD (Control, C; zero for Fish 3 and 6). The horizontal dotted lines indicate the `baseline' of responses (determined from recordings in the absence of switch closure) due to spontaneous fluctuation in the discharge rate of the animal. Each diagram comprises the results of 125 tests (approximately 25 per stimulus condition). Insets indicate the relative position of the two shunting electrodes at the trunk/tail region in relation to the fish's total length (TL).

View larger version (16K): [in a new window]   Fig. 6. Magnitude of the shunted electric organ discharge (EOD) current in the experiments shown in Fig. 5. Values are means ± S.D. derived from approximately 25 measurements each. The inset shows an EOD and indicates the phases (V2, V3, V4) at which feedback changes were triggered for 100 µs.

View larger version (11K): [in a new window]   Fig. 8. Experiments in which electric organ discharge (EOD) current was shunted for 100 µs at various phases of the EOD. The horizontal width and position of the bars indicate the timing of switch closure relative to the head—tail EOD (red trace). The height of each bar indicates the experimentally determined response probability when the switch was closed during the indicated period of the EOD. The dotted horizontal line gives the baseline responsiveness (due to spontaneous fluctuations in discharge rate) determined from 100 recordings without switch closure. Responsiveness in the controls was not above baseline, but switch closure within the EOD led to sigificantly higher than baseline responsiveness (at least P<0.01). Responsiveness in V4 was significantly higher (P<0.001) than in V3 but differed not significantly from that determined in the phase after V4. A total of 1070 tests (100 in each condition, except for phase V4 and for controls after the EOD, in which 200 and 270 tests, respectively, were conducted). Data from fish 6. Electrode positions as indicated in Fig. 5.

View larger version (15K): [in a new window]   Fig. 9. Response probability (A) and phase distribution of response levels within the electric organ discharge (EOD) (B) for EOD shunts of lesser duration than 100 µs. (A) Responsiveness to novel feedback at the trunk/tail region decayed considerably when EOD current was shunted for less than 100 µs. In the experiments the switch was closed for a period that centered at the peak V4 and was either 100 µs, 50 µs or 20 µs in duration. Experiments with fish 6 with electrodes positioned as shown in Fig. 5. Black bars, response probability at V4. For each stimulus duration, response levels were additionally determined in (i) 20 controls in which the switch was closed after the EOD and (ii) in the absence of switch closure (20 tests). As response probabilities obtained under conditions (i) and (ii) showed no significant difference, data from (i) and (ii) were pooled and are displayed as grey bars. Responsiveness to 100 µs and 50 µs shunting, but not to 20 µs shunting, was significantly above the control level (P<0.001 and P<0.05, respectively; N=240 tests). (B) An attempt at a more detailed analysis of the response profile within the EOD using shortings that lasted only for 50 µs. As in Fig. 8, the horizontal position and width of the bars indicate the timing relative to the EOD (red trace). The height of each bar gives the response levels as obtained from a total of 1200 tests with fish 6. The dotted line was determined from pooling (i) 100 controls in which the switch was closed after the EOD and (ii) in the absence of switch closure (100 tests) (no significant difference in responsiveness). Only in the three phases around the final head-negative deflection (V4) was the responsiveness significantly above control level (P<0.01).

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