Frequency-dependent changes in cerebral blood flow and evoked potentials during somatosensory stimulation in the rat

Contrary to the concept of neuronal-vascular coupling, cortical evoked pote ntials do not always correlate with blood flow responses during somatosenso ry stimulation at changing stimulus rates. The goal of this study is to cla rify the effects of stimulus frequency on the relationship between somatose nsory evoked potentials (SEPs) and cerebral blood flow. In rats anesthetize d with alpha-chloralose, we measured SEPs by signal-averaging field potenti als recorded with an electrode placed on dura overlying the hindlimb somato sensory cortex. Regional blood flow was simultaneously assessed in the same region with a laser-Doppler flow (LDF) probe. The contralateral sciatic ne rve was stimulated with 0.1 A pulses at the frequencies of 1, 2, 5, 10 and 20 Hz. SEPs (both P1 and N1 components) declined with increasing frequency regardless whether stimulus duration (20 s) or number (100) were kept const ant, suggesting that frequency is an important determinant of neuronal acti vity. In contrast, LDF responses increased to a maximum at 5 Hz, and do not correlate with SEPs. Because CBF should reflect integrated neuronal activi ty, we computed the sum of SEPS (Sigma SEP = SEP x stimulus frequency) as a n index of total neuronal activity at each frequency. Sigma SEP indeed corr elates positively (P < 0.001) with LDF responses. Thus, during somatosensor y stimulation at various frequencies, cerebral blood flow is coupled to int egrated neuronal activity but not to averaged evoked potentials. (C) 1999 E lsevier Science B.V. All rights reserved.