Figure 1: Stimulus history alters the temporal structure of ensemble responses in the antennal lobe.

(a) Projection neuron (PN) spiking responses evoked by solitary introductions of different odourants are shown for ten consecutive trials. Each row reveals spiking patterns observed during an initial 500 ms prestimulus period (unshaded portions), followed by a four seconds of odour-evoked activity (shaded portions). The mean spike counts as a function of time (peristimulus time histogram (PSTH); 50 ms time bins) are shown in the right panel. Note that the three PNs shown in a column were simultaneously recorded from a single locust. (b) Spiking activities of PNs (pooled across experiments; 10 trials each) with significant responses to the following odourants: hexanol (hex), apple, isoamyl acetate (iaa) and 2-heptanone (2hep) are shown. PNs were sorted based on their average latency to peak response. Arrows along x-axis indicate the time of odour onset. Note that the two different PN ensemble responses shown for hexanol were probed with different odourant sequences. (c) The trial-by-trial spike rasters and mean firing rates of the same set of PNs to the same set of odourants as in a are shown. However, the odourants are now presented in an overlapping sequence with another preceding stimulus with a 2-s delay. The 4-s period of stimulus overlaps are shaded in grey, whereas a blue shade is used to represent a 500-ms time period when the first stimulus in the sequence was present alone. (d) Responses of PNs to the following two-odour overlapping sequences: hexanol–2 s latency–hexanol (hex(hex)), 2octanol–2 s latency–hexanol (hex(2oct)), mint–2 s latency–apple (apple(mint)), benzaldehyde–2 s latency–isoamyl acetate (iaa(bzald)), and cyclohexanone–2 s latency–2heptanone(2hep(chex)). Same PN order as in b. (e) Red traces represent spike counts across all PNs following solitary introductions of an odourant, and the black traces show the spike counts across the same set of PNs following introductions of the same odourant but presented in an overlapping sequence (mean±s.d.; n=10 trials, bin size=10 ms). Asterisks indicate significant reductions in peak spike counts (*P<0.01, paired t-tests, n=10 trials). (f) Spike counts summed over the entire duration of odour presentation (4 s) are shown for all PNs during solitary (in red) and overlapping (in black) stimulus conditions (mean±s.d.; n=10 trials). Correlation coefficients (R) were calculated between PN response profiles shown in black and red. P indicates the significance level of the observed correlation. Insets show the total spike-count across all PNs during solitary and overlapping presentation of the same odour (mean±s.d.). Asterisks indicate significant change in total spike counts (*P<0.01, NS: P>0.01, paired t-tests, n=10 trials).