Figure 7: Babbling statistics are similar across different vocal learners and in the model.
From: A canonical neural mechanism for behavioral variability
(a) Statistics of the babbling behaviour generated by the model circuit depicted in Fig. 2a–e when coupled to a minimal model of the vocal organ (see Methods section). Top: spectrogram of the vocal output signal 
. Bottom: probability density function (pdf) of vocal gesture durations (left) and averaged autocovariance of the envelope (ACE; right). Inset: distribution of gesture durations when the y axis is in log-scale. The distribution of gesture durations is well approximated by an exponential with a ‘scale parameter’, 
(see Methods section). ACE decorrelates over a time duration of 
. Slow synaptic dynamics in the premotor-to-motor projections (red:
; blue: 
) results in slowly fluctuating vocal output (red: 
and 
; blue:
and 
). (b–i) Statistics of the babbling behaviour in four species of vocal learners (ages of the subjects (‘babbling period’) are given in Methods section). Blue: Zebra finches (Zf); Red: Swamp sparrows (Sw); Green: Canaries (Ca); Black: Human infants (Bab). Different lines of the same colour correspond to different subjects from the same species. (b–e) Same as in (a), but for the Zf (b: compare to the blue line in a), Sw (c: compare to the red line in a), Ca (d) and Bab (e). Gesture duration distributions lack any clear peak and are well fit with exponential decaying function with scale parameters (mean±s.e.m.): 
; 
; 
; 
. The ACE decay time is specie-dependent: 
; 
; 
; 
. (f,g) Cumulative distribution functions (cdf) of gesture duration for the four species before (f) and after (g) normalizing the gesture durations by 
. (h) Top: Interspecies differences in cdfs are much smaller than intraspecies differences (Kolmogorov–Smirnov statistic as a distance measure between cdfs). Bottom: Differences of cdfs in pairs of learners within (left to right: Zf–Zf, Sw–Sw,Ca–Ca, Bab–Bab) and between species (left to right: Zf–Sw, Zf–Ca, Zf–Bab, Sw–Ca, Sw–Bab, Ca–Bab). (i) Most of the interspecies differences in (h) are accounted for by normalizing the gesture durations to the scale parameter of the exponential fit of their distributions (see Results and Methods sections for statistical comparisons).
