Article Figures & Data
Figures
- Figure 1.
Bursts of action potentials contain stimulus-specific information. A, Acridid grasshoppers produce species-specific acoustic communication signals by rasping their hindlegs across their forewings. This generates an amplitude-modulated broadband noise signal (AM signal). Shown are two such “songs” from males of the species C. biguttulus. Each song consists of many repetitions of a pair of basic patterns, called “syllable” and “pause,” episodes of high and low amplitude, respectively. B, Conspecific female grasshoppers respond to a certain range of syllable and pause durations, as tested with artificial block stimuli. Within the gray area, a female responded positively in at least 20% of all trials [data were taken from von Helversen and von Helversen (1994) with permission]. C, Enlarged AM signal with song parameters analyzed in the present study. The calculated amplitude threshold for this cell was 55.4 dB SPL. For illustrative purposes, pauses are only marked for threshold crossings that are followed by, on average, at least one spike per trial. Smaller amplitude modulations are neglected; they are also not perceived as pauses by the animal, as shown by behavioral experiments (Wohlgemuth, 2008 A. Einhäupl and B. Ronacher, unpublished results). D, Spike trains of a particular ascending auditory interneuron, the AN12. Syllable onsets trigger burst responses. The response latency of ∼12ms is attributable to acoustic delays, response latencies of the receptor neurons, axonal and synaptic delays, and the intrinsic AN12dynamics. E, IBSC as a function of the burst index (i.e., the position of the burst within the particular calling song). For the two songs in A, the black lines denote the median IBSC, and the gray lines indicate upper and lower quartiles. F, Probability of correctly classifying one of eight songs by IBSC only (chance level, 1/8). Dotted line, Classification using IBSCs from one burst index; solid line, classification by cumulating the information contained in IBSCs from the 10th burst up to the indicated burst. These data demonstrate that songs can be classified successfully in >90% of all trials if IBSCs from 12 or more bursts are used. Hence, the spike count within bursts is sufficient for conspecific song discrimination. Data shown in D–F are from one sample cell.
- Figure 2.
IBSC encodes preceding pause duration. A, Burst-triggered average (i.e., mean stimulus before a burst with given IBSC); for comparison, the spike-triggered average over all action potentials is depicted too. As shown by these data, bursts with large IBSC occur preferentially after long phases of low sound intensity. B, Pause duration is linearly correlated with the IBSC of the following burst [R2 = 0.69 ± 0.15 (SD); p < 10−5; Pearson's]; the gray lines indicate upper and lower quartiles. With a y-intercept of only −1.1 ± 2.1 ms, the best fit agrees with a direct proportionality of pause duration and IBSC. The pause duration was defined individually for each cell by setting a threshold for the sound amplitude level of all songs (Fig. 1C); to account for global amplitude differences between the songs, amplitudes were first normalized with respect to mean and SD. C, Correlation coefficient between pause duration and intraburst spike count, as a function of the amplitude threshold used for defining pauses. The smooth maximum demonstrates that the definition in B is robust against variations of the threshold, here shown relative to the mean song amplitude. D, For artificial block stimuli, IBSC is again linearly correlated with pause duration (R2 = 0.76 ± 0.15; p < 10−5). Different colors correspond to stimuli with different syllable durations. E, For natural songs, the onset amplitude, but not onset slope, is partially correlated with IBSC (R2 = 0.27 ± 0.08; p < 10−5) because of covariations of pause duration and onset amplitude in natural songs. The ordinate depicts amplitude (in decibels) for the solid curve, and amplitude/time (in decibels/millisecond) for the dashed curve. Data in A–C and E are from one sample cell; D refers to a second neuron. The statistics refer to all cells tested.
- Figure 3.
Total spike count is invariant to temporal stimulus rescaling. A, Sketch of two simple auditory sound signals. The lower stimulus is obtained by stretching the upper stimulus by a factor of 2.5. Assuming that the IBSC scales linearly with the preceding pause duration, this rescaling results in 4 instead of 10 bursts, but each burst has now 5 instead of only 2 spikes. Both effects compensate each other so that the total spike count stays constant. B, Measured AN12 responses to two conspecific grasshopper songs. Although the songs differ strongly in their pause durations, the total spike count is approximately equal because the syllable-to-pause ratios of both songs are approximately the same (1.41 for the upper and 1.40 for lower song, respectively), in agreement with the model prediction. C, Total spike count (window length, 500 ms) of responses to the eight tested songs, as a function of the duration of one syllable plus one pause. The duration of this basic song element differs for each song, resulting in the eight different values on the abscissa. Each of the six curves represents data from one neuron, with the species indicated by line type. Error bars indicate upper and lower quartiles of total spike count across eight repetitions. The total spike count varies from cell to cell but without overall correlation to the syllable-plus-pause duration (R2 = 0.00 ± 0.14). Downstream neurons thus have access to the behaviorally relevant syllable-to-pause ratio in a timescale-invariant manner.
