Figure 15.

A, Concept of ApEn statistic, a measure of relative process randomness (irregularity). ApEn distinguishes subtle differences in regularity, orderliness, or reproducibility of subpatterns in sequential measurements. Low ApEn denotes more orderly and less random (irregular) contributions to the signaling pattern. Conversely, high ApEn signifies more irregular or apparently “random” inputs. High ApEn characterizes hormone secretion patterns associated with autonomous endocrine tumors, low feedback states, aging individuals, and fixed exogenous stimulation of hormone output. B and C, Exogenously imposed negative feedback by Te onto LH secretion and by glucose onto insulin secretion. The fall in ApEn with higher feedback-signal concentrations denotes greater regularity of LH and insulin secretion. [Adapted with permission from P. Y. Liu et al.: J Clin Endocrinol Metab 91:4077–4084, 2006 (240).]