Predictability of Painful Stimulation Modulates Subjective and Physiological Responses

Clinical observations suggest that the perceived intensity of a painful event increases as the unpredictability of its occurrence increases. We examined the effect of varying stimulus predictability on the Somatosensory Evoked Potential (SEP), Pupil Diameter Response (PDR), Pain Report (PR), and Fear Report (FR) in 25 healthy female volunteers experiencing repeated noxious fingertip shocks. Each volunteer underwent high- and low-stimulus intensities in 4 stimulus patterns defined by stimulus sequence (SEQ) and interstimulus interval (ISI) as follows: A) serial stimulus intensity SEQ with fixed ISI; B) serial stimulus intensity SEQ with varied ISI; C) random stimulus intensity SEQ with fixed ISI; and D) random stimulus intensity SEQ with varied ISI. Results revealed that: (1) lower stimulus predictability led to higher PR and FR, greater PDR magnitude, and greater SEP amplitude; and (2) the 4 dependent measures showed the same response pattern across levels of stimulus predictability. These findings support the hypothesis that lower stimulus predictability is associated with higher reported pain and fear as well as greater physiological arousal.PerspectivePatients undergoing painful procedures experience more distress when the occurrence of a painful event is unpredictable. Poor predictability increases pain, fear, and associated physiological arousal. Maximizing the predictability of painful events may improve the quality of patient care by minimizing associated levels of pain and fear.