Reactive search strategies and their biological motivation.

<p>(A) MGC recordings for pheromone stimulation: spike times for seven trials of one neuron and the corresponding average firing rate over time (Peri-Stimulus-Time-Histogram): inhibition separates the On from the Off response which smoothly decreases to baseline firing (Bl). (B) Analysis of MGC recordings of multiphasic neurons: Calculating the regularity () and reliability () over time exhibits an Off phase, whereas Off and baseline firing show uniformly low synchrony values (). Dotted black lines represent single neuron trials, the red line gives the averages. (C) Analysis of MGC recordings of monophasic neurons: neither synchrony nor regularity nor reliability over time exhibit any Off phase. Dotted black lines represent single neuron trials, the blue line gives the averages. (Right side: za, ze, sp) Schematic representation of the corresponding movement sequences: Bl spiraling, On upwind surge, and Off zigzagging (if considered) which are combined into three search strategies, <i>sp</i>, <i>za</i>, and <i>ze</i>.</p

Experimental robotic trials.

<p>Experimental robotic trials.</p

Success rates, trajectory lengths, and deviation from the optimal path.

<p>(A) Success rates of reactive search strategies, different colors indicate different strategies (legend in C), grouping indicates different stimulation doses (three doses plus no stimulation). (B) Success rates of cognitive searching with infotaxis (three doses plus no stimulation). (C) Trajectory lengths of reactive search strategies, different colors indicate different strategies, grouping indicates different stimulation doses. (D) Trajectory lengths of cognitive searching with infotaxis (three doses plus no stimulation). (E) Schematic drawing to explain the measure: the average of horizontal deviations (Xi) from trajectory to shortest path between start and source. (F) Deviation from the optimal path () for reactive searching, different colors indicate different reactive strategies for the three groups of pheromone doses. (G) Deviation from the optimal path () for cognitive searching (three doses plus no stimulation). Box plots are explained in the <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003861#s4" target="_blank">Methods</a>, part 2, the numbers indicate mean standard deviation.</p

Reactive search trajectories.

<p>(A) Examples of <i>sp</i> search trajectories (spirals only, i.e., no Off), medium dose. For a better visualization single paths are plotted in distinct colors (cyan and light blue on top of mostly blue trajectories). The dots on the trajectories indicate pheromone detections. The black dashed line indicates the plume contour (see <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003861#s4" target="_blank">Methods</a>). (B) Examples of search trajectories including Off zigzagging, medium dose. Red, yellow and pink trajectories use arithmetic spirals (<i>za</i>), bluish trajectories originate from assuming exponential spirals (<i>ze</i>). Identical conventions as in (A). (C and D) Track-angle histogram of <i>sp</i> and <i>za</i> trajectories, respectively, different colors indicate different pheromone doses. (E) Total number of turns for different stimulations, different colors indicate different reactive strategies for the three groups of pheromone doses, identical conventions as in Fig. 3.</p

Experimental set-up of the cyborg's search task.

<p>(A) Schematic general set-up: the cyborg starts 2 m from the pheromone source in a 2.54 m region. A fan provides a wind blowing from the top (towards the cyborg). (B) Photo of our cyborg: a Khepera III robot with a moth fixed in a styrofoam roll. Zoom-in 1: top of the styrofoam roll with the insect's head and the two antennae on the outside. Zoom-in 2: one antenna enters the tip of a glass electrode. Photographs by H. Raguet — INRIA.</p

Cognitive search trajectories obtained using infotaxis.

<p>(A) Example <i>it</i> trajectories for no stimulation (green, left), minimum (dark green, middle) and medium (cyan, right) stimulation doses. The dots on the trajectories indicate pheromone detections. The black dashed line indicates the plume contour (see <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003861#s4" target="_blank">Methods</a>). (B) Total number of turns in <i>it</i> trajectories for different stimulations. Identical conventions as in Fig. 3. (C) Track-angle histograms of <i>it</i> trajectories, different colors indicate different doses. (D) Total number of detections measured during reactive (<i>sp</i>, <i>za</i>, <i>ze</i>) and cognitive (<i>it</i>) searching using three stimulation doses and no stimulation. Identical conventions as in Fig. 3.</p

Two-step casting.

<p>(<b>A</b>) In two-step casting, <i>Off</i> and baseline activities trigger distinct casting behaviours: <i>Off →</i>crosswind zigzagging, <i>baseline →</i>spiraling. In this view, baseline activity provides information about the absence of the stimulus whereas the <i>Off</i> represents a recent sensory memory, indicating that the pheromone plume had been just lost. (<b>B</b>) Trajectories of the cyborg obtained with the two-step casting strategy. Inset: track angle histogram (p<0.001, Rayleigh circular test of non-uniformity). Same experimental conditions as in 1-step casting (Fig. 5B). To allow real-time processing, the <i>Off</i> detection was not performed explicitly: we simply considered that the <i>On</i> was followed by the <i>Off</i>. (<b>C</b>) Search distance of 2-step versus 1-step strategy (p<0.01, Mann-Whitney test).</p

Pharmacological manipulations and neuron model.

<p>(<b>A</b>) Effects of bicuculline (BIC) and picrotoxin (PTX). Data are shown as raw traces. The stimulus (200 ms) is indicated by a grey bar. With BIC application (100 µM), the inhibitory phase was abolished so that the response to the pheromone changes from multiphasic to monophasic (<i>n</i> = 3 neurons). After wash-out, the multiphasic responses were recovered, as well as the ability to encode pheromone pulses. With PTX (100–250 µM), firing was suppressed during the spontaneous activity and the response to the pheromone (in 6 out of 7 neurons). The multiphasic responses were partially recovered after wash-out. (<b>B</b>) Simulation of the neuron model with SK versus control experimental data (dashed lines). <i>On/Off</i> neurons (<i>n = </i>6) were recorded for different stimulus durations (100 ms to 1 s, a single puff of the pheromone blend at 1 ng). The <i>On</i> duration depended linearly on stimulus duration: <i>On</i> duration = 0.99×(stimulus duration) +18 ms (Pearson correlation r² = 0.97). The inhibitory phase was constant (p = 1, Kruskal–Wallis test): <i>I</i> duration = 399±106 ms. The neuron model (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0061220#pone.0061220.s005" target="_blank">Text S1</a>) was simulated with inputs mimicking a 1 ng pheromone blend stimulus (200, 500 or 1000 ms duration; 10 runs in each condition), resulting from fits of experimental data recorded in olfactory receptor neurons <i>in vivo </i><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0061220#pone.0061220-Jarriault1" target="_blank">[12]</a>. (<b>C</b>) Time dependent effect of BIC. Pheromone responses are shown at different times after BIC application (data as raw traces from the same neuron, stimulus (grey bar) = 200 ms). <i>tmin</i> indicates the start of BIC application (100 µM) and <i>tmax</i> indicates the time right before the inhibitory phase vanishes completely. (<b>D</b>) Simulation of the neuron model with SK blocked <i>versus</i> BIC experimental data. Data points represent <i>On</i> and <i>I</i> duration measured during BIC experiments (<i>n</i> = 3 neurons, stimulus duration = 200 ms), plotted <i>versus</i> the normalized time of BIC application (Δt at bottom axis, with <i>tmin</i> and <i>tmax</i> defined as in panel C). Dashed curves are linear fits of the data where blue and red envelopes show the 50% confidence bands. Plain curves represent <i>On</i> and <i>I</i> durations measured from simulations with partially blocked SK conductance. The durations of the two phases were plotted <i>versus</i> the normalized decrease in SK-like conductance (Δg_SK at top axis, with g ranging from <i>gmin</i> = 0.05 µS to <i>gmax</i> = 0.5 µS).</p

One-step casting.

<p>(<b>A</b>) In one-step casting, <i>Off</i> and baseline activities provide information about the absence of the stimulus and trigger the same casting behaviour: <i>Off</i>, <i>baseline →</i> spiraling. (<b>B</b>) Trajectories of the cyborg controlled by the <i>On/Off</i> neuron model with SK channel intact. The plume contour (red line) is defined as the parabolic region where 90% of all pheromone detections occurred. The target is the source of pheromone (dose = 10 µg). Inset: track angle histogram (p<0.001, Rayleigh circular test of non-uniformity). Track angles were computed as movement vectors with respect to the wind direction. A peak at 0° indicates a tendency to move upwind, as compared to movements perpendicular to the wind direction (±90°). (<b>C</b>) Trajectories with SK channel blocked. Inset: track angle histogram (p = 0.6, Rayleigh circular test of non-uniformity). Same experimental conditions as in panel B. (<b>D</b>) Success rate measured as the percentage of successful trials in the different conditions (SK channel intact and blocked, pheromone dose = 10 and 20 µg; the dose 0 µg stands for no pheromone). (<b>E)</b> Search distance measured from the initial location to the target for all successful trajectories. Conditions having no letters in common are significantly different at <i>p</i><0.05 (Mann-Whitney pairwise comparisons).</p

Precision and reliability of physiological responses.

<p>(<b>A</b>). Responses to single puffs of pheromone. Asterisks indicate significant differences between original and shuffled trials (*p<0.05, **p<0.01, ***p<0.001 and ns = no significant difference, Mann-Whitney test). Left: example of an <i>On/Off</i> neuron (73% of the recordings) over ten repeated trials (ticks depict individual spikes). Figure inset represents a zoom on the <i>On</i> response. The grey bar indicates the 200 ms stimulation period. The spike timing jitter σ (in ms) and the fraction of non-coincident spikes ρ were computed with the SES algorithm by considering all pairs of trials, <i>i.e.</i> 45 in total. Black bars represent σ and ρ obtained on the original spike trains and blue bars indicates σ* and ρ* obtained on shuffled trials (preserving interspike interval distribution). The <i>On/Off</i> neuron was both precise (σ<σ*) and reliable (ρ<ρ*). Middle: example of a monophasic neuron (27% of the recordings). The monophasic neuron was precise (p<0.01) but not reliable (p = 0.5, Mann-Whitney). Right: multiphasic versus monophasic neurons (<i>n = </i>number of neurons). To compensate for differences in firing rates, values were normalized as σ/σ* and ρ/ρ* for each neuron. (<b>B</b>) Responses to pulsed stimulations. Left: example of a multiphasic <i>On/Off</i> neuron exposed to 5 consecutive pheromone pulses of 200 ms separated by air gaps of 300, 500 or 700 ms. Each panel represents the spike trains from two repeated trials, superimposed with the Gaussian-convolved firing rate evolution. The <i>Off</i> phase is present after each pheromone pulses in the 700 and 500 ms air gap conditions and it is absent for higher frequency pulses (air gaps of 300 ms). Middle: precision and reliability across pulses in the different conditions. On average, σ = 3.6 ms and ρ = 0.1 (ns = no significant difference, Kruskal–Wallis test, <i>n = </i>11 neurons). Right: mean autocorrelation functions computed by averaging over monophasic and multiphasic <i>On/Off</i> neurons. Example of a monophasic neuron exposed to five consecutive pheromone pulses of 200 ms separated by air gaps of 300 ms.</p