Memory consolidation in fragmented sleep. N2 sleep spindles for verbalmemory in patients with obstructive sleep apnea

Background. N2 sleep spindles have been frequently linked to sleep-dependent memory consolidation in healthy adults. Sleep spindles (9-15 Hz) increase during sleep after learning and correlate positively with the memory retention rate. Objectives. As memory consolidation is impaired, but not completely absent, in patients suffering from obstructive sleep apnea (OSA), we tested whether learning affects N2 spindles in OSA patients. Materials and Methods. We presented 91 word pairs to 18 patients with severe OSA (52 +/- 2.28 years), asking one group (n = 9) to memorize them, while another group (n = 9) was prevented from learning. After a first recall in the evening (learning level) and a polysomnographicallymonitored night's sleep, participants were tested again in the morning (recall level). N2 sleep spindle count and density were assessed for the whole night and for four consecutive sleep intervals of 90 min. Results. N2 spindles did not significantly increase after learning in OSA patients. On a descriptive level, the spindle count at all electrode positions was higher after learning compared with the non-learning group for the whole night and for the first three 90 min intervals. Moreover, spindle scores only correlated with the memory retention rate in the learning group. Conclusion. Despite the small sample size, these results provide a first hint toward the involvement of sleep spindles in memory consolidation in patients suffering from severe OSA.