Cortical potentials related to assessment of pain intensity with visual analogue scale (VAS).
OBJECTIVE
To elucidate brain mechanisms underlying the psychophysical processes to measure pain intensity, pain-related somatosensory evoked potentials (pain SEPs) following painful CO(2) laser stimulation were studied while employing a task to measure intensity of pain on a visual analogue scale (VAS).


METHODS
In 12 healthy subjects, 3 kinds of CO(2) laser stimuli, different in intensity as determined by irradiation duration of 40, 60 and 80ms, were randomly delivered to the left hand dorsum at an irregular interval of 4-6s. The subject was requested to assess the intensity of each pain stimulus and point to the VAS scale by moving a pointer held with the right hand according to the subjective feeling of pain sensation (pain intensity assessment (PIA) condition). For the control condition, the subject moved the pointer to the midpoint of the VAS line irrespective of the pain intensity (control motor task condition). Electroencephalograms were recorded from 21 scalp electrodes, referenced to the linked earlobes, and were averaged time-locked to the stimulus onset for each stimulus duration as well as for each task condition.


RESULTS
The VAS scores were 2.8+/-0.5/10 for the stimulus of 40ms duration, 4.8+/-0.8/10 for 60ms and 6.1+/-0.9/10 for 80ms, and showed a highly significant positive correlation with the stimulus duration. Following the early components of pain SEPs which were affected by stimulus duration but not modulated by task conditions, a surface-positive peak at latency of 612-642ms was identified exclusively under the PIA condition regardless of the stimulus intensity and was called 'intensity assessment-related potential (IAP)'. The IAP was maximal at the midline parietal area and symmetrically distributed over the scalp. Neither latency nor amplitude of the IAP was significantly different among the 3 different stimulus intensities.


CONCLUSIONS
IAP is an event-related potential (ERP) associated with assessment of pain intensity but not influenced by pain intensity itself. From its scalp distribution, it can be assumed that the assessment of pain intensity involves multiple areas in both hemispheres.
Kanda
Masutaro
M
Department of Brain Pathophysiology, Human Brain Research Center, Kyoto University Graduate School of Medicine and Faculty of Medicine, Shogoin, Sakyo-ku, Japan.
Matsuhashi
Masao
M
Sawamoto
Nobukatsu
N
Oga
Tatsuhide
T
Mima
Tatsuya
T
Nagamine
Takashi
T
Shibasaki
Hiroshi
H
D000328:Adult; D002540:Cerebral Cortex / Q000502:physiology*; D004569:Electroencephalography; D004576:Electromyography; D005073:Evoked Potentials, Somatosensory / Q000502:physiology*; D005260:Female; D006801:Humans; D007834:Lasers; D008297:Male; D009048:Motor Skills / Q000502:physiology; D010146:Pain / Q000503:physiopathology*; D010147:Pain Measurement; D011601:Psychophysics