Over hundreds of trials, Soltysik and Jelen trained 16 rats to expect an electric shock to their tail after they heard an auditory tone, but not to expect a shock if a light came on after the tone. In this way, fear could be induced in the rats, followed by relief if they saw the light come on. All the while, the researchers monitored the rats’ breathing by recording their diaphragm muscles. Sighs are easily recognisable, the researchers explained, because they appear as a “deep ‘additional’ inhalation that starts at or around the peak of a normal respiratory cycle”.
Three hundred and eleven sighs were recorded across the course of the experiment, the vast majority of them during the ‘relief phase’ that followed a light coming on, indicating a shock would not occur. Averaged across all the rats, 7.4 times more sighs occurred during this ‘relief phase’ than during the equivalent period when the light didn’t come on – a fear phase – that occurred between the tone sounding and a shock being given.
The researchers said it’s possible “This respiratory act was recruited during evolution to signal reduced perception of danger, and/or to synchronise the emotional state of the group (collective sighs of relief?)…” They added that “the sigh could be a signal opposite to the alarm cry”.
To test this theory further they plan experiments to see if sighing is more prevalent in the company of other rats, and to test whether sighing is impaired in rats raised in social isolation.
Soltysik, S. & Jelen, P. (2005). In rats, sighs correlate with relief. Physiology and behaviour, 85, 598-602.