Timing Actions to Avoid Refractoriness: A Simple Solution for Streaming Sensory Signals

Segmenting self- from allo-generated signals is crucial for active sensory processing. We report a dynamic filter used by South American pulse electric fish to distinguish active electro-sensory signals carried by their own electric discharges from other concomitant electrical stimuli (i.e. communication signals). The filter has a sensory component, consisting of an onset type central electro-sensory neuron, and a motor component, consisting of a change in the fish’s discharge rate when allogenerated electrical events occur in temporal proximity to the fish’s own discharge. We investigated the sensory component of the filter by in vitro mimicking synaptic inputs occurring during behavioral responses to allo-generated interfering signals. We found that active control of the discharge enhances self-generated over allo-generated responses by forcing allo-generated signals into a central refractory period. This hypothesis was confirmed by field potential recordings in freely discharging fish. Similar sensory-motor mechanisms may also contribute to signal segmentation in other sensory systems

Refractoriness and post-stimulus probability explain the pattern dependent differences in responsiveness.

<p>A) Top row: Post-stimulus histograms showing the peri-interference (black arrowhead at 0) histogram of the EOD (red bars) and the shuffled EOD timed stimuli (histograms in heavy black lines). Ordinate: 100× number of EOD per bin divided by the total number of interference timed stimuli. Note the smaller EOD timed stimulus probability at the earlier phases and the slightly larger at longer phases. Bottom: Post-interference histogram of EOD evoked spikes. Ordinate 100× number of EOD evoked spikes per bin divided by the total number of interference-timed stimuli. Because of the absolute refractory period stimuli at earlier phases are similarly blocked in experiments made with natural and shuffled time series. However, since the probability of stimuli at earlier phases is the lowest for EOD timed stimuli, this sequence is the less affected by refractoriness. B) Top row: Post-stimulus histograms showing the post-EOD (red arrowhead at 0) histogram of the interference (black bars) and the shuffled interference timed stimuli (histograms in heavy black lines). Ordinate: 100× number of interference-timed stimuli per bin divided by the total number of EOD-timed stimuli. Note the slightly larger interference timed stimulus probability at the earlier phases and the clearly smaller at longer phases. Bottom: Post-EOD histogram of interference evoked spikes. Ordinate: 100× number of interference evoked spikes per bin divided by the total number of EOD-timed stimuli. Since the probability of stimuli at earlier phases is the highest for interference timed stimuli, this sequence is the most affected by refractoriness. Thus, EOD-timed stimuli were more efficient because natural pacemaker modulations kept them out of the window of refractoriness following responses to interference. In addition, pacemaker modulation kept most of the interference stimuli in the window of refractoriness following the EOD. Therefore, a larger proportion of interference timed stimuli either was blocked or evoked spikes with more variable timing.</p

The time series of EOD in resting and under interference in <i>G. omarorum</i>.

<p><b>A</b>) The experimental setup for behavioral experiments consisted of a small tank where the fish was restrained by a net. Interference stimuli were square pulses delivered to the water at a constant rate. The head to tail EOD and the interference artifact were recorded. In some experiments brain field potential at the magnocellularis mesencephalic nucleus were recorded (represented by the amplifier in red). Inter-EOD intervals sequences are compared in B (resting condition) and C (under interference). B) Top: raw data obtained from the head to tail EOD recordings at rest. Middle: Raster plot of the inter EOD intervals. Bottom: Histogram of the first-order inter-EOD-intervals. C) Top: raw data obtained from the head to tail recordings showing the EOD and the interference artifact (asterisk). Middle: Raster plot showing transient shortenings of the inter EOD intervals under interference condition. Bottom: Histogram of the first-order inter-EOD-intervals showing two modes, one corresponding to longer intervals and the other (red arrow) corresponds to the largest accelerations.</p

Construction of the intracellular stimulus train.

<p>A) Relative timing of EODs and interfering events were selected from behavioral experiments files using a time resolution of 20 microseconds. From these data we constructed a file having three channels (in house software): Channel 1) the intracellular stimulus train, in which EOD-timed stimuli consisted of a rectangular pulse of 1.5 ms duration starting at the timing of the EOD (open rectangles) and the interference-timed stimuli consisted of a rectangular pulse of 1.5 ms duration starting at the timing of the interfering event (filled rectangles); Channel 2) the EOD timing monitor in which only EOD-timed stimuli were represented; and Channel 3) the interference timing monitor in which only interference-timed stimuli were represented. Channel 1 was used to drive the step activation port of the Axoclamp 2B. Intensity was controlled using Axoclamp 2B step activation controls. Channels 2 and 3 were recorded simultaneously with the voltage and current outputs of the Axoclamp 2B.</p

Spherical cell refractoriness.

<p>When the cell received stimulus patterns containing the series of intervals obtained during behavioral experiments the refractory period following any spike (either elicited by the EOD timed stimuli, open symbols, or interference timed stimuli, filled symbols) was identical: A) absolute refractory period (stimulus failure rate vs. previous inter-stimulus interval); B) relative refractory period (coefficient of variation of spike latency vs. previous inter-stimulus interval).</p

Coincidence avoidance behavior of <i>G. omarorum</i>.

<p>The example shows a typical display elicited by artificial electric pulses (1 ms duration) delivered at a constant frequency at about the mean EOD rate. The raster plot is referred to the EOD timing (the vertical series of red dots). A) Raw data showing the EOD and the interfering stimuli artifacts. The following terms are defined: a) inter EOD interval (time between successive EODs), b) interference stimulus interval (time between successive stimuli) which is the sum of c) phase (time between EOD and next interference stimulus) and d) co-phase (time between interference stimulus and next EOD). B) Raster plots showing the time course of the inter-EOD interval (red dots) and the time course of the phase (black dots to the right of the EOD) and the co-phase (black dots to the left of the EOD). The arrow indicates the course of the experiment (total duration 30 s). Series of transient acceleration responses were triggered by interfering stimuli preceding the pacemaker discharge by less than 5 ms <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0022159#pone.0022159-Westby1" target="_blank">[21]</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0022159#pone.0022159-Baker1" target="_blank">[27]</a>. This behavioral display occurred when pacemaker rate is slightly faster than the interference (jamming avoidance, green segments on the side). During these periods the phases of the interfering stimuli progressively increase up to about 20 ms, showing a slight trend to be phase locked at about this latency. Beyond this point the phase increases at a faster rate due to the acceleration of the pacemaker. Pacemaker accelerations were also triggered by coincidence between EODs and allo-generated events when the EOD rate is slower than the rate of interfering pulses (synchronization bout <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0022159#pone.0022159-Capurro1" target="_blank">[24]</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0022159#pone.0022159-Baker1" target="_blank">[27]</a>, blue segments). C) Peri-EOD histogram showing that the probability interfering pulses phases (black bars to the right of the reference point) and co-phases (black bars to the left of the reference point). Note that the probability is smaller than average at a late phases (dashed line), but larger than average at earlier phases.</p

Interference and EOD timed stimuli are not equally effective.

<p>A) Top: Post stimulus raster plot of the spiking responses to EOD-timed pulses applied as in the natural sequence (red dots). Middle: corresponding post stimulus histogram (ordinate: 100× EOD evoked spikes per bin divided total number of EOD-timed stimuli, red bars). Bottom: The responsiveness to the natural patterns was compared with the responsiveness to the same set of intervals applied in a shuffled sequence in a pair wise experiment (histograms in heavy black lines). For the sake of clarity spike latency histograms showing the relative frequency of the latency after the cell spiked (ordinate: 100× EOD evoked spikes per bin divided total number of EOD evoked spikes) were constructed with a different bin size. B) Top: Post stimulus raster plot of the spiking responses to interfering pulses timed as in the natural sequence (black dots). Middle: corresponding post stimulus histogram (ordinate: 100× interference evoked spikes per bin divided total number of interference-timed stimuli, black bars) Bottom: The responsiveness to the natural patterns was compared with the responsiveness to the same set of intervals applied in a shuffled sequence in a pairwise experiment (histograms in heavy black lines). For the sake of clarity spike latency histograms showing the relative frequency of the latency after the cell spiked (ordinate: 100× interference evoked spikes per bin divided total number of interference evoked spikes) were constructed with a different bin size. In the natural sequence experiments the area (stimulus effectiveness) and the sharpness of the spike latency histograms (latency precision) in response to EOD-timed-stimuli (A, red post-stimulus histograms) were larger than those corresponding to interference (B, blue post-stimulus histograms). In the pair wise shuffled-EOD experiment the histograms are similar to each other (superimposed histograms in heavy black lines in the bottom panels of A and B).</p

Field potential responses to self- and allo-generated signals in freely discharging fish.

<p>A) The raster plots shows the time course of phase of interfering stimulus (black) and the next EOD interval (red) in a freely discharging fish chronically implanted at the magnocellularis nucleus. Histograms at the bottom in show the relative frequency of interference after the EOD (black bars, ordinate: 100× number of allo-generated stimuli per bin divided the total number of EODs) and the first order interval histogram (red). B) Raster plot representing the amplitude (abscissa) of the field potentials evoked by the EOD at the magnocellularis nucleus (the time course of the experiment in register with A) and the amplitude histogram. C) Raster plot representing the amplitude (abscissa) of the field potentials evoked by the EOD at the magnocellularis nucleus (the time course of the experiment in register with A and B) and the amplitude histogram. Note that amplitude histogram in B shows a relatively larger dispersion than in C.</p

A low threshold K⁺ current determines the long refractory period.

<p>A) Sub-threshold conditioning stimulus (red trace, 0.8 thresholds) prevent neuron spiking in response to test stimuli. Spike is elicited by a control identical test stimulus in the absence of a conditioning one (black trace). B) Paired pulse stimulation with sub-threshold stimuli show that depolarization leads to an hyperpolarization after the end of the pulse (red arrow)and a decrease of excitability of the neuron evidenced by the reduction depolarization peak in a constant amplitude test pulse (blue arrow). Linear relationships between end conditioning depolarization (end voltage-black arrow), after-hyperpolarization (red arrow) and the amplitude of the hump evoked by the test pulse (blue arrow) are shown in C and D). E) Pair pulses applied at different delays shows a long lasting refractory period. F) As expected for a low threshold K⁺ conductance, refractoriness was shortened after the application of 50 microMolar 4-aminopyridine to the bath.</p