Total synapse number

The total synapse numbers in HVC in each bird and group are listed in Table 3. 15 and illustrated in Figure 3. 16, Figure 3. 17, and Figure 3. 18.

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Table 3. 15: The total synapse number estimations in HVC. The mean ± SD HVC total synapse number is shown for each bird in the 3^rd column. The group means ± SE were calculated by averaging the values across all of the birds belonging to the group, and shown in the last column.

Figure 3. 16: The total synapse numbers in HVC (for data, see Table 3. 15). See Figure 3.

11 legend for additional information. Wilcoxon rank-sum tests were performed between the groups. The significant differences between the groups are not shown on the figure for simplicity (for the test results, see Table 3. 16).

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In HVC, the total synapse numbers were calculated from the HVC size and HVC synapse density results (see Results sections 3.3.2 and 3.3.3), with Equation 3. 8.

The resulting variations of the data are represented with SD, which was computed with Equation 3. 9 and Equation 3. 10 for each bird.

The ISO_30 group had an average total synapse number, 1.57 × 10⁸ synapses in HVC, and the total synapse number increased at 60 dph (ISO_60) to 2.92 × 10⁸ synapses and then decreased at 90 dph (ISO_90) to 1.84 × 10⁸ synapses in the song-isolated groups. In the song-tutored groups, the HVC total synapse number decreased at 60 dph (TUT_60) to 1.28 × 10⁸ synapses and then increased at 90 dph (TUT_90) to 1.61 × 10⁸ synapses.

Table 3. 16: The results of the Wilcoxon rank-sum tests of the total synapse numbers in HVC. P values less than 0.05, which indicate significance, are written in bold font.

Significant differences were found between the ISO_30 and ISO_60, ISO_60 and TUT_60, and ISO_60 and ISO_90 experimental groups.

The group-wise rank-sum test results are listed in Table 3. 16. Significant differences were found between several groups, which will be discussed.

In summary, when the juveniles were deprived of sensory exposure to the tutor song, the total synapse number in HVC increased significantly by 86% from 30 dph to 60 dph and then decreased significantly by 37% from 60 dph to 90 dph. Both the initial increase and later decrease in the total synapse number were significant. When the juveniles were provided proper sensory exposure to the tutor song, the total synapse number decreased by 18% from 30 dph to 60 dph and then increased by 26% from 60 dph to 90 dph. Both the initial decrease and later increase in the total synapse number were not significant for the song-tutored groups. For the age-matched groups, the tutored group showed significantly decreased total synapses in HVC

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compared with the isolated group at 60 dph. At 90 dph, this difference was smaller and became insignificant between the song-isolated and song-tutored groups.

Figure 3. 17: The total asymmetric synapse numbers in HVC (for data, see Table 3. 15).

See Figure 3. 11 legend for additional information.

In HVC, the total asymmetric synapse number results showed that the ISO_30 group had an average total asymmetric synapse number, 1.46 × 10⁸ synapses, and the total asymmetric synapse number increased at 60 dph (ISO_60) to reach 2.33 × 10⁸ synapses and then decreased at 90 dph (ISO_90) to 1.48 × 10⁸ synapses in the song-isolated groups. In the song-tutored groups, the total asymmetric synapse number decreased at 60 dph (TUT_60) to 0.98 × 10⁸ synapses and then increased at 90 dph (TUT_90) to 1.39 × 10⁸ synapses.

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Table 3. 17: The results of the Wilcoxon rank-sum tests of the total synapse numbers in HVC. P values less than 0.05, which indicate significance, are written in bold font.

Significant differences were found in the comparisons between the ISO_30 and ISO_60, ISO_60 and TUT_60, ISO_60 and ISO_90, ISO_60 and TUT_90, TUT_60 and ISO_90, and TUT_60 and TUT_90 experimental groups.

In summary, when the juveniles were deprived of sensory exposure to the tutor song, the total asymmetric synapse number in HVC significantly increased by 60% from 30 dph to 60 dph and then significantly decreased by 36% from 60 dph to 90 dph.

Both the initial increase and later decrease in the total asymmetric synapse number were significant. When the juvenile was provided proper sensory exposure to the tutor song, the total asymmetric synapse number in HVC decreased nonsignificantly by 33% from 30 dph to 60 dph and then increased significantly by 42% from 60 dph to 90 dph. When the age-matched groups were compared, the tutored group exhibited significantly decreased (more than one-fold) total asymmetric synapse number in HVC compared with the isolated group at 60 dph. At 90 dph, this difference was much smaller and became insignificant between the song-isolated and song-tutored groups.

Figure 3. 18: The total symmetric synapse numbers in HVC (for data, see Table 3. 15).

See Figure 3. 11 legend for additional information.

In HVC, the total symmetric synapse number results showed that the ISO_30 group had an average total symmetric synapse number, 0.12 × 10⁸ synapses, and the total symmetric synapse number increased drastically at 60 dph (ISO_60) to reach 0.58 × 10⁸ synapses and then decreased at 90 dph (ISO_90) to 10.36 × 10⁸ synapses in the song-isolated groups. In the song-tutored groups, the total symmetric synapse

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number in HVC increased at 60 dph (TUT_60) to reach 0.29 × 10⁸ synapses and then decreased at 90 dph (TUT_90) to 0.22 × 10⁸ synapses.

Table 3. 18: The results of the Wilcoxon rank-sum tests of the total symmetric synapse numbers in HVC. P values less than 0.05, which indicate significance, are written in bold font. Significant differences were found between the ISO_30 and ISO_60, ISO_30 and TUT_60, ISO_30 and ISO_90, ISO_60 and TUT_60, ISO_60 and ISO_90, and ISO_60 and TUT_90 experimental groups.

In summary, when the juveniles were deprived from sensory exposure to the tutor song, the total symmetric synapse number in HVC increased almost 4 times from 30 dph to 60 dph and then decreased by 38% from 60 dph to 90 dph. Both the initial increase and later decrease in the total symmetric synapse number were significant.

When the juvenile was provided tutoring, the total symmetric synapse number in HVC increased significantly by 1.4 times from 30 dph to 60 dph and then decreased nonsignificantly by 24% from 60 dph to 90 dph. When the age-matched song-isolated and song-tutored groups were compared, the tutored group showed significantly less (one-fold) total symmetric synapse number in HVC compared with the isolated group at 60 dph. At 90 dph, this difference was much smaller and became nonsignificant between the song-isolated and song-tutored groups.

3.4. Discussion

In this section, the experimental techniques and findings of Experiment II will be discussed. The discussion will be organized in the same order that the results were presented. To broaden the biological significance of the findings, some results will be discussed together with the results of Experiment I and/or related studies. The major goals of the discussion are to arrive at a conclusion and explain the

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experimental findings of Experiment II, confirm reported, and propose new general principles of the neural ultrastructural changes that occur during early development with or without critical sensory experiences.