Marsat and Pollack Bursting and ultrasound avoidance FIGURE 1 | Bursting neural responses. Bursting properties of a neuron can be revealed by stimulating it with a Gaussian input and characterizing its output. We show here an example of a response of the AN2 neuron of crickets which displays a characteristic bursting response to ultrasound stimuli. The lower panels are calculated from 45 s of recording; arrows indicate the threshold of the intra-burst-ISI. (A) Stimulus and response. The stimulus is an ultrasound tone (30 kHz) randomly modulated in amplitude. We display here only the amplitude modulation envelope: a low-pass (<200 Hz) ﬁltered Gaussian with a mean amplitude of 90 dB SPL and an SD of 5 dB. The neuron responds with bursts of spikes to the largest peaks in ultrasound amplitude. The short inter-spike intervals (ISI) of spikes within bursts can clearly be identiﬁed (red). The ISI distribution is quantiﬁed in (B–D). (B) ISI distribution histogram (ISIh). The ISIhs of bursting neurons have a characteristic bimodal distribution with a peak (shown in red) at short intervals – the intervals of spikes within bursts – and broad tail of longer intervals (black). The bimodal nature of the ISIh – the output – highlights the non-linearity of the input-output transformation since the input – the stimulus – has a Gaussian distribution. A linear transformation would transform a Gaussian input into a Poisson point process. The ISIh of a model Poisson neuron with identical mean ﬁring rate and refractory period is shown in blue. (C) Autocorrelation function. The autocorrelation function gives the probability of having a spike at different time intervals following each spike in the response. A random process would lead to a probability per bin of (Firing rate) (bin width), in this case 52 Hz 0.001 s, for all intervals (blue line). The peak at short intervals (red) quantiﬁes the higher-than-chance probability of having spikes separated by short intervals in bursting neurons. (D) ISI return map. The return map plots the ISI that precedes each spike as a function of the ISI that follows it. Several clusters can be observed: a cluster along the y -axis consisting of the ﬁrst ISIs of each burst (orange), in the bottom left corner the subsequent intra-burst ISIs (red), a cluster along the x-axis for the ISIs that follow bursts (gray) and a more scattered cluster at longer intervals for ISIs between isolated spikes (black). The ISI return map is a way of visually characterizing the spike-train structure and revealing the bursting tendency of the neuron, as non-bursting neurons will not display clusters along the axes at short intervals. Despite numerous studies on bursting in vertebrate and inver- tebrate sensory systems and the clear understanding of the type of information carried by bursts to higher brain centers, it is only recently that the links between bursts, the information they carry, and behavioral output were ﬁrmly established. We review below recent ﬁndings about bursting and ultrasound avoidance in crick- ets that show how the bursting properties of a few sensory neurons determine the behavioral output. ULTRASOUND AVOIDANCE IN CRICKETS Sound perception and production in crickets co-evolved as a com- munication tool (Stumpner and Von Helversen, 2001). Male crick- ets produce songs with relatively low sound frequencies (3–6 kHz) to communicate with conspeciﬁcs. Crickets are also sensitive to ultrasound, which allows detection of hunting bats. Ultrasound avoidance is widespread among nocturnally ﬂying insects (Hoy et al., 1989; Hoy, 1994). In crickets, the use of hearing for detecting insectivorous bats is thought to have evolved more recently than conspeciﬁc communication; the pre-existing hearing range was most probably widened to encompass the ultrasound of echolo- cating bats (Hoy, 1992; Yager, 1999). Ultrasound stimuli trig- ger a stereotyped, short-latency (30–40 ms), reﬂex-like steering response in tethered ﬂying crickets that, in free ﬂight, would direct them away from the sound source (Moiseff et al., 1978; Nolen and Hoy, 1986). Echolocating bats produce sound pulses that vary in duration (1 to >20 ms) and repetition rates (1–200 Hz) depend- ing on the species of bat and the task they perform (for reviews see Schnitzler and Kalko, 2001; Schnitzler et al., 2003). Typically, bats will increase the repetition rate and decrease the sound-pulse duration when they require better temporal resolution in their auditory representation of space. For example, while searching for insects in open spaces, the bats emit pulses at low rates (a few Hertz) but increase the rate as they approach an insect for cap- ture. In the terminal phases of the bat-insect pursuit, pulses are Frontiers in Neuroscience | Decision Neuroscience July 2012 | Volume 6 | Article 95 | 2