S189 THERMAL (LASER) ERBP: RELATIONSHIP OF WAVEFORM COMPONENTS 228 Slide TO STIMULUS AND SUBJECTIVE "PAIN" RESPONSE. R. Coger, Thu 15:00 B. Kenton, J. Pinsky, B. Crue, New Hope Research Foundation Theatre C i00 South Raymond Avenue, Alhambra, CA 91801 USA

The waveform of the event-related brain potential ERBP associated with painful laser stimulation of the finger includes five dominant peaks: a positive peak at about 200 msec (P200), a negative peak at about 300 msec (N300), alarge positive peak at about 400 msec (P400), a negative peak at about 600 msec (N600), and a positive peak at about 750 msec (P7S0). Prev- ious work has indicated that the large P400 component is strongly related to the subjective report of pain, and may be a reflection of activity in

A-delta fibers. The present report concerns the strength of the relationships between

each of the peaks and both stimulus intensity and subjective "pain" re- sponse. Two techniques were used: correlation of the RMS amplitude of each peak with the stimulus and response parameters; and correlation of the voltage at each time point along the waveform with stimulus and re- sponse. Both were done on a trial-by-trial basis using i0 male and i0

female volunteer subjects. Although all five peaks were correlated with the stimulus and response,

the P400 had the strongest relationships and the RMS amplitude of the en- tire waveform had stronger correlations than any peak or specific time

point along the ERBP wave.

EFFECTS OF COMPRESSION BLOCK ON CEREBRAL EVOKED POTENTIALS I 229 S l ide IN RELATION TO FIRST AND SECOND PAIN. S. W. Harkins & D. D.I Thu 15:15 Price, Medical College of V i rg in ia , V i rg in ia Commonwealth Theatre C Univers i ty , Richmond, V i rg in ia 23284, USA Aim of Invest igat ion: Long latency components ( ) I sec) of the cerebral evoked potent ia l (CEP) to i s t and 2nd pain and the ef fects of select ive compression block of i s t pain on both early and late CEP components were evaluated in human volunteers. Methods: A series of I0 experiments using trained observers was performed with and without se lect ive block of f ibers necessary for I s t pain. Block was achieved by constant pressure applied to the ulnar nerve. Noxious shocks were presented via electrodes inserted into the horny layer of skin of the dorsal surface of the 5th f inger . CEPs were measured before and a f te r block of Is t pain as well as during progression of the block. Results: Preblock, a series of indiv idual peaks in the CEP were conslstent ly ~ i e d . These included N70 (scalp negative at 70 msec), PIO0, NI4C, P240, and N350. Upon complete block of Is t pain these waves disappeared. That is , in the presence of a complete block of Is t pain the CEP was f l a t up to I sec fol lowing stimulus onset. When only 2nd pain was reported, a slow posi t ive wave, peaking at 1.3 sec, was observed when stimulus pr o- sentation rates were greater that I per 3 sec. Conclusions: Early components of the CEP up to 1 sec fol lowing noxious st imulat ion r e f l ec t combined act ivat ion of f ibers subserving the cutaneous senses including but not res t r i c ted to 1st pain. Complete block of sensi- b i l i t y except for 2nd pain el iminates a l l components of the CEP wi th in i sec fo l lowing stimulus onset. A late posi t ive component of the CEP appears to r e f l e c t propert ies of 2nd pain sens ib i l i t y but th is wave is highly var iable and is observed only in the absence of Is t pain.