Table 2

Correlation coefficients for sleep spindle/σ activity in the cross-sectional sample (above diagonal) and the longitudinal sample (below diagonal)

	Spindle density	Spindle frequency	Fast spindle percent	Spindle power	σ power
Spindle density	---	0.205	0.263	0.553	0.311
Spindle frequency	0.286	---	0.931	−0.120	−0.104
Fast spindle percent	0.349	0.564	---	−0.061	−0.106
Spindle power	0.471	0.191	0.248	---	0.628
σ power	0.553	0.262	0.225	0.671	---

Data are Pearson correlation coefficients. For the cross-sectional sample, the correlation coefficients for spindle density (#/min), spindle frequency (Hz), fast spindle percent (%), and spindle and σ power (μV²) were determined. For the longitudinal sample, the correlation coefficients for the percent change in each spindle metric from baseline (ages 5–12) to follow-up (ages 12–22) were calculated.