Our lab works on insect communication systems, with a focus on the katydid genus Neoconocephalus. Most projects fall into one of two overlapping research areas – neurobiology and evolution.
Neoconocephalus includes over 30 species distributed throughout temperate and tropical areas of the western hemisphere, about half of which live in North America or the Caribbean. Morphologically the species are highly similar and often difficult to distinguish, but each species is recognizable by the male call. Females use the spectral and temporal patterns of male calls to recognize and localize conspecific males. Calls vary in frequency, pulse rate, pulse patten (e.g. single versus double pulses), and verse structure. This great diversity in male calls across species raises interesting questions about the evolution of communication systems and the role of call diversification in speciation. Given that a communication system can only function if the sender and receiver are matched (i.e. if the male produces a call that females are able to recognize), it is perhaps surprising that communication systems are so variable across species. We are interested in understanding how changes in senders and receivers transpire over evolutionary time, e.g. Do changes in the male call precede changes in the female preference, or vice versa? Why do closely related species tend to assess different parameters of the call (e.g. pulse rate versus pulse duration), rather than simply being tuned to different values of the same parameter? What are the selection pressures that lead to call synchronization in species that produce calls with verses? What role does the communication system play in speciation events? How do new call trait or new call preferences evolve?
Addressing evolutionary questions entails recording, analyzing, and describing male calls of each species, identifying the parameters of interest to females, and determining the phylogenetic relationships among species within the genus. We are focusing on the North American and Caribbean species and are making significant progress in answering many of these questions.
One task that every nervous system must perform is to separate relevant from irrelevant information. An example of this phenomenon in the auditory system is ‘the cocktail party effect,’ in which a person can listen to a single conversation in a crowded room, ignoring all other conversations within earshot until a highly relevant signal – such as one’s name – suddenly becomes salient out of the background. Katydids must perform a similar task when engaged in phonotaxis toward a potential mate while listening in the background for the echolocation cries of bats, which are an important predator of katydids. Auditory Stream Segregation is the separation of different types of acoustic input into separate ‘tracks’ according to relevance or information content. Our lab was the first to demonstrate Auditory Stream Segregation in an invertebrate, and in 2007, we received a new NSF-award to study how bat signals within a complex auditory scene are detected and processed at cellular and behavioral levels. We use a variety of methods (e.g. extra- and intracellular recordings, electrical stimulation, calcium imaging) to study how one interneuron segregates bat cries from the acoustic background to test our hypothesis that dendritic rather than presynaptic processes are involved. In psychoacoustic experiments, we test how masking influences the detection and processing of bat cries. In behavioral experiments, we test how the context (e.g. reproductive state, or presence versus aabsence of attrative males) changes bat avoidance responses by flying katydids. Defects in Auditory Stream Segregation and other processes involved in separating sensory information have been implicated in various human disorders, including schizophrenia. If we can understand how Audiotry Stream Segregation occurs in the simple nervous system of an insect, we may gain a better understanding of how this process functions in the more complicated brains of vertebrates, including humans.
We are also embarking on a project to understand how temporal selectivity is generated in the nervous system. What are the neuronal differences between a species whose females are selective for a particular pulse rate, and a species whose females are selective for (or against) particular durations of inter-pulse intervals? We have been exploring a model in which temporal selectivity for pulse rate is generated by intrinsic resonances of the plasma membrane of neurons within the CNS. We propose that membrane potential oscillates with a frequency that corresponds to the preferred pulse rate of the male calls. Using psychoacoustic experiments, we have obtained behavioral evidence for resonance in three different katydid species representing a wide range of preferred pulse rates, and we have collaborated with electrical engineers to model the combinations of ion channels that could generate this resonance. Future work will be directed at 1) identifying the brain neurons that are involved in this process, and 2) determining how these membrane processes could generate selectivity for temporal properties other than pulse rate (e.g. pulse or interval duration).