Many ambiguous patterns elicit spontaneous alternations of phenomenal appearance. Attention is known to influence these phenomenal reversals, as do several other factors. We asked whether a shift of attention individually prompts each reversal of phenomenal appearance. By combining intermittent presentation with a proven method of attention control, we monitored phenomenal alternations in the complete absence of attention shifts. We found that reversals become less frequent but continue even when observers neither report on nor shift attention to an ambiguous pattern. The statistical variability of reversals remains unaffected. We conclude that reversals of phenomenal appearance are not prompted externally by attention shifts, but internally by an intrinsic instability of the neural representation of ambiguous patterns.