Research ReportTransient changes in frontal alpha asymmetry as a measure of emotional and physical distress during sleep

The aim of this project was to identify measurable physiological responses linked to the human experience of distress in a non-communicative state. Since sleep is a state of sensory-motor detachment in which subjects cannot report how they feel or what they experience, it can be considered a suitable model of non communicative states. A transitory distress-related response has consistently been reported in healthy awake subjects in the form of power spectrum alpha band changes in frontal brain areas. Hence, changes in frontal alpha asymmetry (FAA) were analysed in 16 volunteers before and after administration of distressful aversive stimuli during sleep. Transient physical or emotional distress was achieved by presentation of conditioned (to electroshocks or affective images) or directly-applied aversive stimuli (electroshocks). Frontal beta asymmetry (FBA) was also evaluated for comparisons. Power spectrum data were assessed from frontal electroencephalographic (EEG) leads (F3/F4). Galvanic skin response (GSR) and heart rate (HR) were also simultaneously monitored. To test the possibility that the short lasting stimulations would have long-term physiological effects, we included saliva cortisol sampling, evaluation of dream emotional valence and a standard assessment of sleep quality measurements. Aversive stimulation of diverse nature, compared to neutral valence stimuli, produced measurable changes in FAA (left > right) and in GSR evoked responses in both stage 2 (S2) and rapid eye movement (REM) sleep suggesting implicit processing of experiences with negative valence. Noticeably, dream content evaluation showed a trend towards increased negative emotionality. No FBA was observed, confirming the specificity of FAA responses. Saliva cortisol, and sleep parameters were not significantly affected by the protocol. This is, to our knowledge, the first time that an aversive imaging conditioning protocol has been successfully applied during sleep.

Research Highlights► This study aimed at identifying measurable physiological responses linked to the human experience of distress in a non-communicative state, using sleep as a model of such states. ► Changes to aversive stimuli compared to neutral stimuli were expressed as changes in frontal alpha asymmetry and as GSR responses. ► Such findings may have clinical implications for other non communicative states, such as coma and anaesthesia, and investigations of these patient groups are warranted.