EFFECTS OF INTERSTIMULUS-INTERVAL ON SOMATOSENSORY-EVOKED MAGNETIC-FIELDS (SEFS) - A HYPOTHESIS CONCERNING SEF GENERATION AT THE PRIMARY SENSORIMOTOR CORTEX

Cerebral responses evoked by peripheral stimuli are known to depend cr itically on the interstimulus interval (ISI). Here we report on the ef fects of ISI on somatosensory evoked magnetic fields (SEFs) to right m edian nerve stimulation, obtained in 9 healthy adults with ISIs of 0.1 5, 0.3, 1, 3 and 5 s. At the contralateral (left) primary sensorimotor cortex (SMI), the first cortical response, N20m, was stable between t he ISIs 0.3 and 5 s, but slightly attenuated at the shortest ISI of 0. 15 s. In contrast, the P35m and P60m deflections were very sensitive t o changes of the ISI, declining steadily with shortening of the ISI th roughout the entire range. These deflections were frequently undetecta ble at the shortest ISI of 0.15 s. Concomitant with the reductions of P35m and P60m, an N45m deflection was enhanced toward the short ISIs. Responses from second somatosensory cortex (SII) and posterior parieta l cortex (PPC) were seen only with ISIs of 1 s or greater, being stron gest at the 5 s ISI. Based on known effects of the ISI on intracellula r evoked potentials, we present the following tentative model for the generation mechanism of the SMI response: N20m represents early excita tory postsynaptic potentials (EPSPs), P35m early inhibitory postsynapt ic potentials (IPSPs), N45m secondary EPSPs and P60m late IPSPs in pyr amidal neurones of area 3b. For practical purposes, SEFs from SMI can be obtained with short ISIs, while responses from SII and PPC require an ISI of at least 1 s.