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The detection of specific temporal patterns in communication signals may be of vital importance for certain organisms. In crickets, for instance, a female will move towards a singing male only if she can recognize the appropriate pulse rate characteristic to its own species' song. Additionally, in order to evade predatory insectivorous bats, flying crickets must be able to track the predator's ultrasonic echolocation signals, which are emitted at a variety of pulse rates. In this thesis, the temporal processing, or the integration of stimulus through time, in the peripheral1 auditory system of the cricket will be investigated. / The ON1 interneuron temporal processing was first examined and compared at high (bat-like) and low carrier (cricket-like) frequencies in three different experimental paradigms. First, integration time, which corresponds to the time it takes for a neuron to reach threshold when stimulated at the minimum effective intensity, was found to be significantly shorter at high carrier frequency than at low carrier frequency. Second, phase locking to sinusoidally amplitude modulated (SAM) signals was more efficient at high frequency, especially at high modulation rates and low modulation depths. Finally, we examined the efficiency with which ON1 detects gaps in a constant tone. As reflected by the decrease in firing rate in the vicinity of the gap, ON1 is better at detecting gaps at low carrier frequency. Following a gap, firing rate increases beyond the pre-gap level. This "rebound" phenomenon is similar for low and high carrier frequencies. / To determine the source of this differential temporal processing, the sensory afferents making synapses with ON1 were investigated. Low frequency (MT-type) and ultrasound auditory receptors were compared on the basis of latency, maximum firing rate, adaptation, information transmission, bursting and feature detection. Ultrasound receptors (HFs) were found to have a shorter latency, a higher maximum firing rate and stronger adaptation than low-frequency receptors (LFs). Individual HFs transmitted more linear (lower-bound) information than LFs. However, HFs' responses were more correlated than LFs' (i.e. they had larger mutual information), so that when superposing the spike trains of LFs, information transmission in the lowest amplitude modulation rates was greatly improved, and, in some cases, reached the level of HFs. Feature detection by spike in HFs was better than in LFs. Feature detection by bursts was better than for spikes, but equivalent in both types of receptors. The level of bursting in HFs, however, was much higher than in LFs, making them better feature detectors in general. / 1Because it lies in the prothoracic ganglion, ON1 is technically part of the central nervous system. For the purpose of this thesis, however, because ON1 receives direct input from the receptors, it will be considered to be part of the peripheral auditory systems.

We used the auditory system of crickets as a model system to examine the importance of temporal coding in sensory processing. The bilaterally paired Ascending Neurons 1 and 2 (AN1 and AN2) of crickets receive inputs from the auditory receptors on one side and carry the information to the brain. We used stimuli with either conspecific-like or predator-like (i.e. bats) carrier frequency to quantify the accuracy with which the interneurons code the information contained within the amplitude modulation (AM) envelope of the stimulus. AN1, which is tuned to the dominant carrier frequency of cricket songs, selectively codes the limited range of amplitude-modulation frequencies that occur in these signals. AN2, which is most sensitive to ultrasound, serves as a "bat-detector" and codes a broader range of AM frequencies, as occur in bat calls. / A striking characteristic in AN2's responses to ultrasound is the presence of bursts of high-frequency spiking separated by relatively sparse spikes. We examined the relative importance of isolated spikes and bursts in the processing of ultrasound. We showed that bursts reliably signal the occurrence of salient amplitude increases. Furthermore, we showed that burst, but not isolated spikes, reliably predict behavioural responses. We suggest AN2 encodes behaviourally important information with bursts. / The Omega Neuron 1 (ON1) responds to conspecific signals and to the ultrasonic echolocation sounds. ON1's temporal coding properties vary with carrier frequency, allowing it to encode both of these behaviourally important signals. Furthermore, the temporal coding properties of ON1 in response to cricket-like sound and bat-like sound match those of AN1 and AN2 respectively. / ON1 is a source of contralateral inhibition to AN1 and AN2, enhancing binaural contrast and facilitating sound localization. We used dichotic stimulation to examine the importance of the temporal structure of contralateral inhibition for enhancing binaural contrast. Contralateral inhibition degrades the accuracy with which amplitude modulation is encoded by AN 1 and AN2, but only if the temporal pattern of inhibitory input matches that of excitation. Our results show that the CF-specific coding properties of ON1 allow this single neuron to enhance localization cues most effectively for both cricket-like and bat-like acoustic signals.

In the field cricket, Teleogryllus oceanicus, two types of auditory interneurons, AN2 and ON1, have been studied. AN2 responds best to ultrasound frequencies (≥20kHz) produced by echolocating bats and initiates negative phonotaxis by the cricket. ON1 responds to both low (4-5kHz) and high frequency sounds and encodes the temporal information present in sounds of both frequencies. ON1 also provides lateral inhibition to contralateral interneurons, such as AN2, thereby enhancing binaural contrast. The pharmacology associated with these two interneurons is investigated here. ON1's response to 4.5kHz sound is increased following picrotoxin application, while it's ability to encode temporal information present in low and high frequency sounds is reduced. An increase in spiking response and a decrease in response latency to low frequency sound is produced by the addition of atropine, while d-tubocurarine application increased ON1's response latency to both 4.5kHz and 30kHz sound. The neuromodulator octopamine reduced AN2's response to ultrasound.

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The phonotactic steering behavior of tethered flying crickets (Teleogryllus oceanicus) was examined as a measure of the insect's attraction to temporal patterns of calling song at different frequencies and intensities. A stimulus with a 5 kHz carrier becomes less attractive the further its pulse repetition rate deviates from 16 pulses/s. Increasing the intensity increases selectivity for temporal pattern. At sufficiently high intensity level crickets cease to respond to stimuli with altered temporal patterns. / High frequency neurons were suspected to be behind cessation of responsiveness to stimuli with altered temporal features. This hypothesis predicts that the effect on selectivity of increasing the intensity of the 5 kHz stimulus might be mimicked by adding a high frequency to the stimulus. My results contradict this hypothesis. / The response to a 30 kHz carrier demonstrates a dependency on the duration and pulse repetition rate of the stimulus.

Crickets rely on binaural comparisons of intensity to locate sound. Intensity can be encoded by response magnitude as well as response latency. The effects of sound intensity and pulse repetition rate on the auditory responses of the tympanal nerve were investigated. Adaptation, a decline in the response due to repeated stimulation, is greater for higher pulse rates and higher intensities. Since sound intensity is louder at the ear closer to the sound source, adaptation is more pronounced in the ipsilateral ear. As a result, the interaural difference in response magnitude decreases. Therefore response magnitude cannot be a reliable cue for sound location. I found that response latency also adapts: it increases over time. However, this change is not intensity dependent. So interaural latency difference is stable over time. The results show that interaural latency difference is a more reliable cue than interaural magnitude difference to locate sound.