Heart Rate and Heart Rate Variability Assessment Identifies Individual Differences in Fear Response Magnitudes to Earthquake, Free Fall, and Air Puff in Mice
(A) Illustration of three fearful stimuli paradigms: namely, air puff, a sudden air blow (10 psi, 0.5 s) to the back of the animal; free fall, an abrupt drop of the animal inside a small chamber (13 cm high); and earthquake, an unexpected earthquake-like shaking to the animal (3000 rpm, 0.5 s). (B) Examples of ECG recording during air puff, free fall and earthquake stimuli. The red dots indicate the peaks of the R-wave. The blue triangles indicate the onsets of each stimulus. Please note the significant shortened R-R intervals after the onset of all the three types of stimuli. Although noise was introduced during the presence of the stimuli, the custom-written Matlab program can still precisely extract the timestamps of the R-wave peaks. Scales: 0.5 mV. (C) Schematic representation of the three distinct phases of HR responses to fearful stimuli: first (t1, the purple triangle), the rising phase; second (t2, the red rectangle), the maximum plateau phase; and third (t3, the cyan triangle), the recovery phase.