Enhanced dimensional complexity of the EEG during memory for personal pain in chronic pain patients.

Associative connections between cortical cell assemblies representing pain-related memories should be stronger and more extensive in subjects with chronic pain. To test this hypothesis, the dimensional complexity of the electroencephalograph (EEG) was examined during the actual experience as well as during memory for pain. Nine chronic pain patients and nine matched healthy controls participated in the study. During acute pain induction, acute pain recall, personal stress and pain recall, the EEG was recorded from 15 scalp sites. Non-linear analysis, based on the theory of deterministic chaos, revealed higher and more widespread EEG complexity in the patients compared to the healthy controls only during the recall of the personal pain scene. The personal stress scene was rated equally aversive but did not induce more EEG complexity. These more extensive and more readily accessible pain memories may be instrumental for the persistence of chronic pain.