Effects of variations in interstimulus interval on activation-flow coupling response and somatosensory evoked potentials with forepaw stimulation in the rat

In functional neuroimaging studies, the hemody- namic response to functiona l activation is used as a surrogate marker for neuronal activity, typically in response to task paradigms that use periodic stimuli. With use of a mod el system of electrical forepaw stimulation in rats (n = 14) with laser-Dop pler (LD) monitoring of cerebral blood now (CBF) changes in the somatosenso ry cortex, the effects of variations in the interstimulus interval (ISI) on the hemodynamic response to periodic stimuli were examined. A characterist ic peak flow response was seen for 4-second stimuli and a peak and plateau response were seen for all 8-second stimuli regardless of LSI. However, bot h the amplitude of the LDCBF response and the integrated response were sign ificantly reduced fur shorter ISIs, whereas the baseline now was not altere d. Somatosensory evoked potential responses were also recorded in some rats (n = 8) and remained unchanged for the various ISIs for a particular stimu lus duration. These results suggest that the decrease in the LDCBF response s observed with shorter ISIs likely represents a refractoriness of the hemo dynamic response and not neuronal function. These results may have importan t implications for the optimization and interpretation of functional activa tion paradigms that use periodic stimuli.