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

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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.