Marsat and Pollack Bursting and ultrasound avoidance the response to ultrasound and presumably help to scale AN2’s response so that only peaks in ultrasound amplitude larger than the recent average amplitude elicit bursts. Another factor con- tributing to adaptation of AN2 is the very pronounced adaptation of ultrasound-tuned receptors (Sabourin and Pollack, 2009). Bursts appear as a distinct medium of information transmis- sion and the computations performed by network interactions can operate selectively on the burst-component of the response. This is apparent, as described above, when only bursts in receptors can cause bursts in AN2. It is also apparent in the synaptic interaction between ON1 and AN2. ON1 provides contralateral inhibition to AN2, and its response pattern to ultrasound – including bursting – is very similar to that of AN2. Interestingly, ON1 does not produce bursts when stimulated with low, cricket-like sound frequencies (Marsat and Pollack, 2004, 2005). By using dichotic stimulation and judicious choice of stimuli, it was shown that ON1’s coding properties – in particular its bursting tendency – play an impor- tant role in shaping the inﬂuence of ON1 on AN2. Speciﬁcally, bursts in AN2 are inhibited most efﬁciently by coincident bursts in ON1 (Marsat and Pollack, 2005, 2007). The role of ON1 is to enhance the bilateral contrast that serves as a basis for sound local- ization. As a consequence, stimuli that trigger bursts in AN2 and ON1 will be represented with larger bilateral contrast, and thus sound localization will be selectively enhanced for these stimuli. These observations support the idea that neural codes and the architecture of neural computations are mutually determined. MODULATION OF NEURAL PROPERTIES Crickets are most at risk of predation by bats while ﬂying. Although some bats are able to glean prey from the tops of vegetation, this is unlikely to be a problem for crickets, which spend their time on the ground in burrows, under the leaf litter, etc. Not all crick- ets can ﬂy. In many species, individuals may develop either with stunted wings and weak ﬂight muscles, or with full-length wings and strong muscles; the former cannot ﬂy, and the latter can. As crickets age their ﬂight muscles undergo histolysis, so that even though the wings remain long, the ﬂight muscles can no longer generate sufﬁcient force for ﬂight. Thus, there are three ﬂight- classes in the same species; individuals that could never ﬂy, those that can, and those that could at one time but can no longer. Inter- estingly, sensitivity to ultrasound varies with ﬂight capability: the minimum sound level required to elicit avoidance steering is lower in ﬂight-capable than in ﬂight-incapable individuals; and this is paralleled by a lower threshold of AN2 for ultrasound stimuli (Pol- lack and Martins, 2007) and by an increased rate of bursting in AN2 compared with that of ﬂight-incapable crickets (Pollack, unpub- lished observations). These ﬂight-related changes in sensitivity are speciﬁc to ultrasound; neither behavioral threshold – in this case for attraction to a model of the species’ communication sig- nal – nor threshold sound level to stimulate AN2, differs between ﬂight-classes for low-frequency, cricket-like sounds (Pollack and Martins, 2007). The “decision” to develop with short or long wings may be mediated in part by the titer of Juvenile Hormone (JH) during late larval stages (Zera and Tiebel, 1988), with high JH-level lead- ing to short-winged phenotype. An increase in JH-level during adulthood also triggers wing-muscle histolysis (Shiga et al., 1991). Experiments suggest that JH may also be involved in setting sen- sitivity to ultrasound. Treatment with a JH analog of late-stage larvae of a species in which all individuals are normally long- winged and ﬂight-capable, results in adults with poor sensitivity to ultrasound. As in wing-dimorphic species, the loss of sensitiv- ity is speciﬁc to high sound frequencies (Narbonne and Pollack, 2008). The possible role of JH in the loss of ultrasound sensitivity that accompanies wing-muscle histolysis remains to be tested. Another factor affecting the risk of predation by bats is the presence of bats themselves. Bat-free islands in French Polyne- sia have been invaded by a cricket species that also occurs in bat-rich areas, including Australia. Behavioral threshold for ultra- sound avoidance, and AN2 threshold for ultrasound stimulation, are both higher for crickets in the bat-free environment (Fullard et al., 2010), suggesting that sensitivity to ultrasound has consider- able evolutionary, as well as developmental, plasticity. The fact that this plasticity inﬂuences the sensitivity of sensory neurons and is reﬂected in the behavioral output argues for the importance of these neurons’ properties in the decision process. BURSTS, A CODE FOR RAPID SENSORY PROCESSING OF IMPORTANT SIGNALS? The preceding paragraphs describe the relationship between the bursting properties of the cricket auditory neurons and ultrasound processing. The results summarized in these paragraphs also show that beyond determining the information available to the nervous system, the response properties of the sensory neurons relate to the motor output. Speciﬁcally, the encoding of different parts of the ultrasound stimulus is multiplexed onto two channels within the neuron’s response: some features of the stimulus are encoded by bursts and others by isolated spike; only the “burst channel” inﬂu- ences avoidance behavior. Consequently, the neuronal properties that generate bursts in response to speciﬁc stimulus features also determine when avoidance steering is triggered and can therefore be considered part of the decision mechanism. Although some aspects of ultrasound avoidance, such as its reliability and short- latency, are reﬂex-like in nature, the fact that responses occur only following speciﬁc patterns of sensory input is indicative that a decision has been made. More speciﬁcally, the neural network gen- erating the response must decide whether an ultrasound stimulus is sufﬁciently intense, relative to recent experience, to represent a potential threat and, if so, in which direction to steer. The inﬂuence of a sensory neuron’s coding properties on behav- ioral decisions has been demonstrated in many systems and is particularly clear in reﬂex-like behaviors such as the escape behav- ior of ﬁsh triggered by the activity of Mauthner cells (Korn and Faber, 2005), the crayﬁsh escape response (Edwards et al., 1999), or the avoidance of looming stimuli in locust (Fotowat and Gabbiani, 2011). The inﬂuence of sensory neuron properties on decisions has also been examined in much more complex behaviors. For exam- ple, the decisions of a monkey asked to perform a discrimination task seem to be correlated with the response patterns of popula- tions of sensory neurons, although the causal nature of the rela- tionship is difﬁcult to establish (Nienborg and Cumming, 2010). Indeed, a clear, causal link between the bursting properties of sensory neurons and behavior has not yet been demonstrated in other systems. Nevertheless, the central role of burst codes Frontiers in Neuroscience | Decision Neuroscience July 2012 | Volume 6 | Article 95 | 6