Marsat and Pollack Bursting and ultrasound avoidance A B FIGURE 3 | Studying the neural basis of ultrasound avoidance. (A) Ultrasound pulses such as the ones produced by foraging insectivorous bats cause avoidance steering in ﬂying crickets directed away from the sound source. The steering movements consist of changes in the wingbeat pattern but also of movements of the abdomen, hindlegs and antennae in the direction of the intended turn. (B) Tethered crickets continue to produce the motor pattern of ﬂight and can thus be stimulated with ultrasound while the neural and/or motor output is recorded (Nolen and Hoy, 1984, 1986; Marsat and Pollack, 2006, 2010). were not part of a burst had no detectable inﬂuence on avoidance behavior (Marsat and Pollack, 2006). Although bursts are often considered as all-or-none units, they may vary in the number of spikes they contain or in the intra- burst ﬁring rate. The role of burst structure in temporal coding has been discussed in several systems (Kepecs et al., 2002; Oswald et al., 2007; Butts et al., 2010), because this can carry ﬁne-scale information about the feature that the bursts encode. For exam- ple, the intra-burst ﬁring rate of AN2 varies with the duration of the peak in ultrasound amplitude, and number of spikes per burst is correlated with the intensity of the ultrasound peak. Marsat and Pollack (2010) examined the inﬂuence of these parameters on behavior. They found that the amplitude of steering responses varied with the number of spikes per burst, but that the intra-burst ﬁring rate had no effect on behavior. These results make the point that even though information about a stimulus can be retrieved by the experimenter by analyzing a speciﬁc aspect of the spike train, this does not necessarily mean that the nervous system uses that information. Together these results show that the bursting properties of AN2 have a key impact on ultrasound avoidance behavior. A stimulus that does not trigger spiking in AN2, or that triggers only non-burst spikes, will not cause ultrasound avoidance. Therefore the burst- ing mechanism is a key component of the decision mechanism. Although it is not yet known whether AN2’s intrinsic properties contribute to bursting, Sabourin and Pollack (2009) showed that ultrasound receptors also burst and that these bursts have a high probability of causing a burst in AN2. NEURAL DYNAMIC UNDERLYING ULTRASOUND AVOIDANCE When neurons are part of a population, as is the case for the 15 or so ultrasound-sensitive receptors, the relationship between the spike trains of the different neurons shapes the population response. Ultrasound receptors tend to burst synchronously, caus- ing a large input to AN2 (Sabourin and Pollack, 2009). Redun- dancy in response patterns across a population has the well known consequence of limiting the amount of information carried by the population, since different neurons do not carry entirely differ- ent information (Sompolinsky et al., 2001; Puchalla et al., 2005; Sabourin and Pollack, 2010). A useful comparison can be estab- lished with the population of low-frequency receptors, the spike trains of which show much less redundancy than those of ultra- sound receptors. Consequently the accuracy of stimulus coding is greatly improved by pooling the information from many of these low-frequency receptors. It is tempting to relate these two cod- ing strategies to behavior: low-frequency communication sounds need to be encoded with much detail to permit, for example, the ﬁne discrimination of male quality during mate choice, whereas bat-like ultrasound need only be detected to trigger a stereotyped avoidance response. Interestingly, it is not sufﬁcient that the population response of ultrasound receptors be composed of many synchronous spikes to elicit bursting in AN2; rather, individual receptors must burst (Sabourin and Pollack, 2009). A “distributed burst” of eight spikes (eight single spikes coming in close temporal proximity from eight different receptors) has much less chance of triggering a burst in AN2 than eight spikes produced by four cells, each producing a two-spike burst. These results suggest that synaptic properties contribute to processing the signal, speciﬁcally, that facilitation might enhance the response to true bursts, acting as a ﬁlter to pre- vent responses to coincident non-burst spikes in different recep- tors. This mechanism helps to maintain the separation between burst-encoded information and that encoded by isolated spikes. Recent studies have also revealed that adaptation in AN2 can lead to ﬁltering out weaker signals (Wimmer et al., 2008; Hilde- brandt et al., 2011). Based on experiments combining current injection and pharmacological manipulation Hildebrandt et al. (2011) suggested that both cell-intrinsic currents and pre-synaptic inhibition contribute to adaptation of AN2. The former mech- anism has a subtractive effect on the input-output relationship whereas the latter has a divisive effect. Both mechanisms inﬂuence www.frontiersin.org July 2012 | Volume 6 | Article 95 | 5