Daily and developmental modulation of "premotor" activity in the birdsong system

Abstract

Human speech and birdsong are shaped during a sensorimotor sensitive period in which auditory feedback guides vocal learning. To study brain activity as song learning occurred, we recorded longitudinally from developing zebra finches during the sensorimotor phase. Learned sequences of vocalizations (motifs) were examined along with contemporaneous neural population activity in the song nucleus HVC, which is necessary for the production of learned song (Nottebohm et al. 1976: J Comp Neurol 165:457-486; Simpson and Vicario 1990: J Neurosci 10:1541-1556). During singing, HVC activity levels increased as the day progressed and decreased after a night of sleep in juveniles and adults. In contrast, the pattern of HVC activity changed on a daily basis only in juveniles: activity bursts became more pronounced during the day. The HVC of adults was significantly burstier than that of juveniles. HVC bursting was relevant to song behavior because the degree of burstiness inversely correlated with the variance of song features in juveniles. The song of juveniles degrades overnight (Deregnaucourt et al. 2005: Nature 433:710-716). Consistent with a relationship between HVC activity and song plasticity (Day et al. 2008: J Neurophys 100:2956-2965), HVC burstiness degraded overnight in young juveniles and the amount of overnight degradation declined with developmental song learning. Nocturnal changes in HVC activity strongly and inversely correlated with the next day's change, suggesting that sleep-dependent degradation of HVC activity may facilitate or enable subsequent diurnal changes. Collectively, these data show that HVC activity levels exhibit daily cycles in adults and juveniles, whereas HVC burstiness and song stereotypy change daily in juveniles only. In addition, the data indicate that HVC burstiness increases with development and inversely correlates with song variability, which is necessary for trial and error vocal learning.

Figure 1

The complex nature of HVC multiunit activity during singing renders thresholding methods inadequate. (A) For reference, a sonogram of recorded vocalizations is temporally aligned with B. (B) HVC population activity typically increases during singing when compared with nonsinging. In juveniles, there is often some activity immediately preceding and/or following song (Crandall et al., 2007b). Gray bars indicate time windows that are shown at higher resolution below. (C1, C2) Higher temporal resolution of activity during nonsinging reveals relatively distinct spiking events. (D1, D2) Higher resolution plotting of singing activity reveals near continuous overlap and interference of multiple units. All data are from finch Blue-46, age 66 days. Scale bar: 100 μV; A, B: 1 s; C1, D1: 55 ms; C2, D2: 3 ms. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Figure 2

HVC activity levels increase each day and decrease overnight in juvenile and adult zebra finches. A motif produced early in the day (A, red) was accompanied by less HVC activity than a motif produced later in the same day (B, green). In A and B, the top panel is a sonogram of a stereotyped learned motif. The colored outlines correspond with the colored points in C. The lower panel shows HVC population activity. Scale bar: 100 ms; 100 μV. (C) The RMS of HVC population activity during singing increases during the day. The RMSs during the motifs shown in A and B are indicated by their respective colors. Exemplar data in A–C are from bird Blue-81, age 87 days. (D, E) Comparing the first 20 motifs with the last 20 motifs of each day revealed that HVC activity significantly increases in juveniles (D) and adults (E; *p < 0.03, paired t-test; day Ns in parentheses). By finch, the percent change in singing activity levels over the day (F, p = 0.63) and overnight (G, p = 0.86; finch Ns in parentheses) was not different between juveniles and adults. In general, HVC activity during singing increased during the day (F) and decreased overnight (G).

Figure 3

Apparent cycling of HVC activity during song motifs was noted in many longitudinal recordings. Mean HVC activity during each motif for 7 consecutive days is plotted versus the time of day. HVC activity increased until approximately noon each day. Occasionally activity decreased during the day, as on Day 78. HVC activity in the morning trended to be lower than the evening before. The inset shows a sonogram of the canonical motif from bird Blue-70 that was used to extract all other motifs. All data in this figure are from Blue-70. Scale bar: 100 ms. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Figure 4

HVC singing activity becomes burstier each day in juveniles, but not in adults. In juveniles (A, C, E), peaks in neural activity during motif production typically increased more than valleys, such that the burstiness increased. In adults (B, D, F), HVC bursting activity was generally stable or declined during the day. (A) Sonograms of the first and last song motif reveal that the juvenile song (A) became more structured and complex during the day, whereas the adult song (B) was relatively stable. (C, D) HVC neural activity also changed more in the juvenile (C) than in the adult (D). (C) In the juvenile, comparison of the mean rectified and smoothed HVC activity of the first 20 motifs (red) to that of the last 20 motifs (black) reveals that the increase in neural activity during the day was focused in the peaks, with relatively little increase in activity during troughs. (D) An adult recording with an overall decrease in burstiness highlights the differences between adults and juveniles. (E, F) The change in HVC activity was directly proportional to the starting HVC activity (the first 20 motifs) during the same millisecond (relative to the motif) in the juvenile (E) and inversely proportional in the adult (F). Data are from Blue-57, age 58 days, and Orange-331, age 100 days. Two other examples similar to C and D are shown in Figure 8(A,B).

Figure 5

As measured by peak:valley comparison, HVC singing activity becomes burstier each day in juveniles, but not in adults. Juveniles (A; **p < 0.02, paired t-test), but not in adults (B, p = 0.75), exhibit significant changes in HVC burstiness (peak:valley %) each day. (C) The percent daily change in burstiness was significantly greater in juveniles (*p < 0.04, unpaired t-test). (D) The percent overnight change in burstiness was not significantly different in juveniles versus adults (p = 0.77).

Figure 6

HVC burstiness increases with development, whereas the degree of diurnal change in burstiness decreases with development. (A) HVC burstiness, as measured by peak:valley % across days, was greater in adults than juveniles (*p < 0.0005, unpaired t-test). (B) Analysis of data by bird yielded the same trend as the daily analysis, but the difference was not significant (p = 0.12). (C) The increase in HVC burstiness was observed day-by-day in longitudinal recordings of juveniles. The peak:valley % for each motif is shown in gray dots, and the median per day is indicated by the line. (D) Burstiness increased with age in juveniles and was most clearly observed in the first 20 motifs of each day. (E) HVC burstiness during the last 20 motifs of each day also correlated with age, but the relationship appeared more complex. (F) The percent daily change in burstiness decreased with age in juveniles. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Figure 7

HVC burstiness inversely correlates with behavioral variability. A day during which behavior was relatively more variable and HVC activity was less bursty is shown in the left column (A, C, E; bird Blue-70, age 77 days) compared with a day during which behavior was more stable and HVC activity was more bursty (B, D, F; bird Blue-70, age 89 days). The top panel of each example shows spectral derivatives, which are similar to sonograms, with parts of the song spectrotemporal field labeled with red and yellow for clarity. These song segments were relatively variable in the left column when compared with the right. For example, note the leftmost red mark in each panel. Note how this sound is variable in A and C when compared with E. Note how this sound is relatively simple when compared with the corresponding and stable sound in B, D, and F. The bottom panel of each example shows rectified and smoothed HVC activity. Note the shallow peaks and valleys in the left column when compared with the right. (G) Day-by-day comparison of HVC burstiness (peak:valley %) and FM variance reveals a weak but significant correlation in juveniles. Data are from 10 birds, with multiple values reflecting multiple days from the same finch (Supporting Information Fig. 6). Exclusion of the rightmost outlier results in R = –0.39, p = 0.006. Inset: Adult HVC burstiness and song variance are not well correlated.

Figure 8

Nocturnal decrements in burstiness decrease with development and inversely correlate with the next day's change in burstiness. (A) Overnight decrements in HVC burstiness decreased with age in juveniles. (B) The overnight change in burstiness strongly and inversely correlated with the degree of change in burstiness the next day. (C) The change in burstiness during the day correlated with the overnight change in bursting the following night. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Figure 9

A model of daily cycling of motor control during song learning. Nightly decrement of HVC activity levels and bursting may allow song variability and subsequent rearrangement and/or strengthening of HVC activity pattern via activity dependent plasticity. In turn, activity-dependent plasticity may result in daily increment in HVC activity levels and bursting. (A) Mean juvenile HVC activity for the first 20 motifs (red) was greater than that of the last 20 (black) motifs on the same day. (B) Mean adult HVC activity for the first 20 also increased relative to the last 20 motifs on the same day. HVC activity levels cycled in both juveniles (A) and adults (B). In contrast, the relative burstiness increased during the day in juveniles (A), but was relatively stable in adults (B). For A and B, y-axis: relative voltage; x-axis: time relative to the motif; scale bar: 200 ms. (C) We propose that increasing HVC activity increases the entrainment of projection neurons and stabilizes behavior. In the morning, juvenile HVC population activity is low in amplitude with low oscillatory power (red). Lack of correlated interneuron bursting in juveniles may allow variability in the firing of projection neurons and consequent variability in song behavior (the three motif sonograms in the upper right). Later in the day, HVC interneuron population activity is higher amplitude (black) and concentrated in more compact bursts. The hypothesis predicts that putative interneuron bursts entrain the activity of projection neurons and thereby decrease behavioral variability (the three motif sonograms in the lower right).