At the bench: The power of song
By Andrew N Chen
What’s in a song? For humans, it may be something to bond over with a new acquaintance or enjoyed after a long day’s work. For songbirds, songs are weapons used in a soundscape battlefield where the stakes are passing down genes to the next generation. However different the perception of songs may be, they share a common complexity in their spectrotemporal properties.
In a few short synapses, sound waves in the air are transformed into information for our brains to interpret. Neurons in the cochlea are sensitive to different frequencies of sound. This emerges as a tonotopic map where low frequency sounds are represented at the base of the cochlea, while high frequency sounds are represented near the apex of the cochlea. Representation of sounds becomes more complex as you ascend the auditory pathway. The caudal mesopallium (CM), a central auditory area in songbirds, is thought to be one of the first areas in the avian auditory pathway where invariant representations of conspecific song emerges, and shows greater bias towards song over other sounds such as pure tones.
We postulated that there may be some physiological properties that allow CM neurons to be responsive to song over other sounds. Evidence of selectivity and tolerance in CM neurons had already been demonstrated in extracellular recordings [1]. In search of an intrinsic intracellular correlate, we conducted whole-cell current clamp electrophysiology on CM neurons, using Scientifica's SliceScope upright microscope and PatchStar micromanipulator. We found that there were three distinct modes of firing: phasic, tonic, and intermediate [2]. Phasic cells respond to suprathreshold stimulation with few spikes at onset. Tonic cells show sustained firing at low current stimulation, but display depolarization block when stimulated with higher current levels. Intermediate cells fall in between phasic and tonic cells, firing for up to a few hundred milliseconds.
Phasic cells showed pronounced outward rectification starting at lower voltages than tonic cells, leading us to speculate that a low threshold current is involved in regulating the degree of phasicness. We teased out what kind of currents were responsible for mediating phasic activity by recording with various channel blockers. There was no change in phasic activity in the presence of fast synaptic blockers for NMDA, AMPA, and GABA receptors, indicating that phasicness was most likely an intrinsic property. When the nonspecific voltage-gated potassium channel blocker 4-Aminopyridine (4-AP) was used in conjunction with the fast synaptic blockers, phasic firing changed to tonic firing.
We analysed response coherence to a broadband current stimuli to answer the question of how phasicness functionally contributes to auditory processing in CM. The current stimuli contained small, fast oscillations punctuated with larger depolarisations and hyperpolarisations, increasing in frequency over the duration of the 15 second stimuli. Increasing the frequency allowed us to probe the maximum frequency at which spiking responses could follow the injected currents.
Overall, phasic neurons showed maximum coherence to modulation frequencies around 20-30 Hz, tonic neurons were responsive to 5 Hz, and intermediate neurons fell in between. Coincidentally, zebra finch songs contain modulation frequencies up to 30 Hz. In addition to being responsive to higher modulation frequencies, phasic neurons also showed strong lowpass attenuation, dampening their response to low frequencies. We interpreted this behaviour as phasic neurons acting more as bandpass filters, while tonic neurons acted more as lowpass filters. There is still much to learn about intrinsic physiology in the avian auditory system, but this work provides foundational groundwork in exploring how individual neurons contribute to a complex, circuit-based process.
References
1. Meliza, C.D., and Margoliash, D. (2012). Emergence of Selectivity and Tolerance in the Avian Auditory Cortex. Journal of Neuroscience 32 , 15158–15168.
2. Chen, A.N., and Meliza, C.D. (2017). Phasic and Tonic Cell Types in the Zebra Finch Auditory Caudal Mesopallium. J. Neurophysiol.
Banner image: adapted from Chen, A.N., and Meliza, C.D. (2017). Phasic and Tonic Cell Types in the Zebra Finch Auditory Caudal Mesopallium. J. Neurophysiol.
About the author
Andrew N Chen is a recently graduated PhD student from the Meliza lab at the University of Virginia Neuroscience Graduate Program. His work focused on studying the effects of early auditory experience on the intrinsic physiology of central auditory neurons in songbirds.
Scientifica PatchStar Micromanipulator
The most versatile motorised manipulator for electrophysiological studies. The PatchStar is ultra-stable for long-term experiments, electrically quiet for recording extremely small signals and has super-smooth movement for absolute positioning.
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