A good night's sleep helps young birds master the art of singing, but only after a rather groggy start, Nature magazine has reported. Based on neuronal recordings in the HVc, this study investigated differences between reproductively active male and sexually receptive female canaries. It is the first study to describe auditory responses and cell characteristics of HVc neurons in female songbirds and to compare them with the responses and characteristics obtained in males.
Extracellular single unit recordings showed that in males HVc cells exhibited two types of auditory responses to conspecific and heterospecific song playbacks: tonic and phasic responses.
The major finding of the present study is the absence of tonic responses in females. Neurons in the HVc of females only responded phasically to song playbacks. In both sexes, neurons exhibiting auditory responses had thinner action potentials than the others. In both sexes, two categories of nonresponsive cells were found: neurons that did not fire at song onset and had the lowest spontaneous firing rate; and neurons that did not exhibit changes in activity in response to song playbacks.
Analyses of physiological characteristics of HVc neurons revealed that the rate of spontaneous activity was higher in males than in females. This study is a first step towards identifying [corrected] the cellular bases of the sexual dimorphism in HVc function and highlights the pivotal role of interneurons in HVc auditory processing. In songbirds, anatomical attributes of song nuclei exhibit sexual and seasonal differences.
Surprisingly, a particular type of action potential waveforms was observed more frequently in breeding than in non-breeding birds and in males than in females. Such characteristics are usually associated with interneurons in the songbird HVc as well as in the mammalian neocortex.
