, 2006, Matsuzaki et al , 2004, Okamoto et al , 2004, Roberts et 

, 2006, Matsuzaki et al., 2004, Okamoto et al., 2004, Roberts et al., 2010 and Zhou et al., 2004). Size measurements were made from spines that were maintained across two nights of imaging (over a 24 hr interval),

and a size index was calculated for each measured spine (time 24 size/time 0 size), with values greater than check details 1 indicating an increase in size and values less than 1 indicating a decrease in size. Prior to deafening, spines in HVCX neurons tended to increase slightly in size, while spines in HVCRA neurons tended not to change in size over 24 hr (size index = 1.07 ± 0.03 for HVCX neurons: 106 spines, 10 cells, 9 birds; size index = 1.00 ± 0.02 for HVCRA neurons: 94 spines, 9 cells, 8 birds; p = 0.05 for difference between cell types, Mann-Whitney U test). Interestingly, comparing spine size measurements made in a subset of these cells during the first 24 hr time window to those obtained in the last 24 hr time window following

deafening (7-8 nights postdeafening on average) revealed that spine size index decreased significantly following deafening in HVCX but not HVCRA neurons (example images in Figure 1B; group data in Figure 1C; HVCX: average of 10.8 ± 0.3 spines scored per cell in each 24 hr comparison, total of 152 spines from 7 Z-VAD-FMK cells in 6 birds, p = 0.03, Wilcoxon signed-ranks test; HVCRA: average of 11.3 ± 0.4 spines scored per cell in each 24 hr comparison, total of 146 spines from 8 cells in 6 birds, p = 0.67). Thus, deafening causes a cell-type-specific decrease in the size of spines of HVCX neurons. Establishing when these structural changes occur relative to deafening-induced song degradation depends on detecting initially subtle vocal changes following deafening. To this end, we analyzed two spectral features, Wiener entropy and entropy variance (EV), of each syllable in a bird’s song over time (see Experimental Procedures). These parameters

respectively measure the uniformity of a sound’s Mephenoxalone power spectrum and intrasyllabic transitions from tonal to broadband sounds (Tchernichovski et al., 2000) and were chosen because they remain stable in hearing adults (Figure S2A), change in predictable directions following deafening (Figure S2B), and were found to be the earliest spectral features that changed following deafening (data not shown). This analysis detected subtle but significant effects of deafening on syllable spectral features in nearly all birds (18/19) within the first 4 days that they sang following deafening, with ∼50% (10/19) of birds showing significant degradation over the first day of singing after deafening (Figures 2A and 2C). Notably, the changes we detected occur days to weeks earlier than those reported in previous studies (Brainard and Doupe, 2000, Horita et al.

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