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Using microstimulation to imprint meaningful activity patterns into intrinsically highly interconnected neuronal substrates is hampered by activation of fibers of passage leading to a spatiotemporal "blur" of activity. The focus of the present study was to characterize the shape of this blur in the neocortex to arrive at an estimate of the resolution with which signals can be transmitted by multielectrode stimulation. The horizontal spread of significant unit activity evoked by near-threshold focal electrical stimulation (charge transfer 0.8-4.8 nC) and multielectrode recording in the face representation of the primary somatosensory cortex of ketamine anesthetized rats was determined to be about 1,350 microm. The evoked activity inside this range consisted in a sequence of fast excitatory response followed by an inhibition lasting >100 ms. These 2 responses could not be separated by varying the intensity of stimulation while a slow excitatory rebound after the inhibitory response was restricted to higher stimulus intensities (>2.4 nC). Stimulation frequencies of 20 and 40 Hz evoked repetitive excitatory response standing out against a continuous background of inhibition. At 5- and 10-Hz stimulation, the inhibitory response showed a complex interaction pattern attributed to highly sublinear superposition of individual inhibitory responses. The present data help to elucidate the neuronal underpinnings of behavioral effects of microstimulation. Furthermore, they provide essential information to determine spatiotemporal constraints for purposeful multielectrode stimulation in the neocortex.
Butovas et al. (Sat,) studied this question.