Auditory processing: The cellular and synaptic mechanisms of a delay-line and coincidence-detector circuit

Animals across a wide range of species communicate with acoustic signals composed of simple repetitive patterns of sound pulses like frogs and many insects do. Such simple processes of pattern recognition are essential elements to more complex auditory communication including human speech and also music.
Sound however, has a very transient nature. The nervous system must be able to process sequences of signals spread across time to detect their temporal pattern in order to identify a meaning. The four pulses, that form the letter "H" in Morse code, are very similar to the 4 pulses that female crickets orient to, when they are attracted by the calling song of a male. How can the very different nervous systems process and detect such patterns? The computational challenge is very similar across species. When nervous systems of different animals perform a similar task, the nerve cells and neural networks show very similar adaptations. For the processing of temporal sound sequences, researchers analysing different animals came up with very similar concepts. Their idea is that delay-lines and coincidence-detectors would be ideal neuronal circuits to address the question of temporal processing. What does this mean? Let's look at the 4 pulses of the letter "H" in Morse code, which may be separated by a time interval of 50 ms. In the nervous system the auditory signal is split into two pathways. One pathway forwards the information directly to a coincidence detector, the other pathway uses a neuron that imposes a delay of 50 ms on the signal. The consequence is very simple: When the first pulse is processed, it will arrive at the coincidence-detector with no other matching signal; the detector will not be activated. When the second pulse of the Morse signal is processed, the delayed signal from the first pulse and the direct signal to the second pulse will coincide at the detector and elicit a strong response. Summing the coincidence-detector responses over time, can drive feature detectors with activity representing signals of a very specific pattern.
In complex brains like ours, thousands of such delay-lines and coincidence-detectors are likely arranged to detect sound signals of different intervals and patterns and form the basis for the perception of rhythms in language and music. These neural circuits are notoriously difficult to study, and no one neuron can be identified in one animal to the next. Some invertebrates, with simpler nervous systems such as the cricket, offer the chance to deeper understand neural processing because delay-line and coincidence-detector circuits can be analysed at the level of identified neurons. We can therefore thoroughly analyse spike and synaptic activity and their network properties across many individuals. We have already characterised the key components of the circuit in the cricket brain, like the delay-line neuron, the coincidence-detector neuron and the feature detector.
This circuit is tuned towards the pulse intervals of the cricket song, but how is the duration of pulses detected and how is the overall chirp pattern of the song processed? These are questions that we will address by recording the activity of the circuit neurons and testing them with specific sound patterns that we have already used to characterise the auditory preferences of the females. If we find activity patterns that match the behavioural preferences, then these will allow us to explain how the tuning of the circuit to pulse durations and chirp patterns is established. This will complete our understanding of the processing of temporal patterns at two very different time scales. We will then explore how the pattern recognition circuit leads to the control of female auditory behaviour by identifying and analysing descending neurons that initiate and maintain auditory orientation behaviour.

Female crickets will be positioned on a trackball to record their auditory walking and orientation behaviour. Computer generated sound patterns will be presented to obtain behavioural tuning curves for changes in pulse duration, chirp duration or chirp intervals. The same crickets will be used for neurophysiological experiments. The head capsule will be dissected to allow stable intracellular recordings from central brain neurons while the same test patterns are presented to the animals. The spiking and synaptic activity of the delay-line and coincidence-detector neurons will be analysed in relation to the sound stimuli, and neuronal tuning curves will be obtained. Data will be analysed with custom written software. We will compare the tuning of the behaviour and of the neuronal activity to reveal how the auditory filter properties emerge from the properties of individual neurons and/or from processing within the network. Neurons will be labelled with fluorescent tracers. Their structure in the brain will be captured with confocal imaging, and reconstructed in 3D.
1. THE NEURAL FILTER FOR PULSE DURATION, will be analysed by recording the central auditory neurons and stimulating them with pulse patterns in which the duration of single sound pulses is systematically altered.
2. THE NEURAL FILTER FOR CHIRP PATTERNS will be demonstrated by testing the response properties of the delay-line and coincidence detector neurons to pulse patterns with different chirp durations and different inter-chirp intervals.
3. The CONTROL OF PHONOTACTIC WALKING will be analysed by systematic intracellular probing of a well-known neuropil area to finally identify neurons that are implicated in the control of phonotactic walking. The OUTPUT OF THE CIRCUIT for auditory steering will be revealed indirectly, by analysing if the bilateral auditory activity that is forwarded to the brain is sufficient to explain hyper-acute steering.

1. Neuroscience community: In 2011 the PI gave the prestigious Florey Lecture of the German Neuroscience Society. In 2012 the PI was invited to conferences on the Central Complex of the Insect Brain and on the Evolution of Acoustic Communication Systems at the HHMI Janelia Farm. The PI has given a plenary lecture at the 2012 Conference on Crickets organised by Prof. S. Noji (Japan) and presented the data on pattern recognition at the SfN meeting in Chicago in 2015. The PI and the named post-doc will use future meetings to share research methods, to discuss research with other scientists and to build up collaborations. The PI has contributed chapters for textbooks on auditory processing in insects and edited a book on "Insect Hearing and Acoustic Communication" in 2014. The data of the planned experiments will be of interest to the wider community of researchers working on auditory processing.
2. Research collaboration: The PI collaborates with Dr. K. Kostarakos/Graz on central auditory brain neurons in bush-crickets, with Dr. Manuela Nowotny/Frankfurt and Fernando Montalegre-Z on auditory processing in auditory afferents. I also have a collaboration with N. Bayley (St Andrews) on the behaviour of silently singing crickets, which have a flat-wing mutation. As a draft sequence of the cricket genome has been worked out, the PI is in close contact with the group of Prof. S. Noji (Tokushima/Japan) and with Dr. H. Horch (Bowdoin/US), with the aim to generate specific knock-out crickets and transgenic crickets expressing calcium reporters like GCaMP3 in their central nervous system.
3. Engage with the public: Two events in Cambridge are suited to give the public insight to our research: the Cambridge Science Festival and the Conversazione of the Natural History Society. Some minor costs for poster printing and a suitable large video monitor may occur. We also will use opportunities to present our research in radio or TV programs. The PI has lectured at the IBRO course on Insect Neuroscience and Drosophila Neurogenetics in Ishaka/Uganda in 2013.
4. Industrial exploitation: An industrial exploitation of our research is not yet obvious. Principles of auditory processing encountered in brains may be incorporated into the electronic circuits of hearing aids. The design of bioinspired robots orienting to acoustic signals may benefit from our research.
5. Capacity and involvement: The RA will be further trained in neurophysiological research techniques, and will be strongly involved the management of project objectives, time and resources, and in the publication process. The RA will present data at scientific conferences and contribute to any public science events. An essential part of the training will deal with job and grant applications to secure the next position for the RA. The PI will provide substantial support of these RA activities in form of discussions of research results, proof reading of manuscripts and applications, and with advice on career development. Cambridge University runs an excellent program career development and transferable skills. The RA will be strongly encouraged to attend such seminars.