Figure 1
Model for detection and validation of phase resetting responses. (A,B) Phase resetting of a single, stable oscillating variable (shown in red) to an advanced position (A) or a delayed position (B) (shown in green) relative to an external time cue or Zeitgeber. Changes in phase angle (in entrained systems) or phase shifts in freerunning systems is detected by comparing values at two Zeitgeber (ZT) or circadian time points. Measurements at single time points may correlate with phase but are imprecise due to variability, and are confounded in that more than one point may be associated with the same value. The state of an oscillation is represented more accurately by a combination of corroborating values at two Zeitgeber phases (e.g. ZT3 and ZT11 in the model). Additional points may be necessary to account to include to control for acute amplitude changes that may give false positive results or for when measurements occur near the nadir or acrophase that may give false negatives. (C) Assessment of phase resetting using multiple state variables. A stable circadian cycle, whether entrained or freerunning, predicts that reliable state variables in the molecular cycle will attain relatively stable phase relationships, and will return to these relationships after a resetting of the overall cycle. Assessment of a stable component (shown in red) is accompanied by a second clock component (shown in purple). Resetting of the oscillator system is indicated by predictable changes in the ratios of the two components when measured at different phase reference points. Therefore measurements taken at ZT3 (b, as shown), should reflect the ratios at the points indicated by the vertical dashed lines (a or c), if a phase shift has occurred.
