Butovas, Sergejus and Cornelius Schwarz. Spatiotemporal effects of microstimulation in rat neocortex: a parametric study using multielectrode recordings. J Neurophysiol 90: 3024 -3039, 2003. First published July 23, 2003 10.1152/jn.00245.2003. 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 m. The evoked activity inside this range consisted in a sequence of fast excitatory response followed by an inhibition lasting Ͼ100 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 (Ͼ2.4 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.
This paper describes experimental techniques with head-fixed, operantly conditioned rodents that allow the control of stimulus presentation and tracking of motor output at hitherto unprecedented levels of spatio-temporal precision. Experimental procedures for the surgery and behavioral training are presented. We place particular emphasis on potential pitfalls using these procedures in order to assist investigators who intend to engage in this type of experiment. We argue that head-fixed rodent models, by allowing the combination of methodologies from molecular manipulations, intracellular electrophysiology, and imaging to behavioral measurements, will be instrumental in combining insights into the functional neuronal organization at different levels of observation. Provided viable behavioral methods are implemented, model systems based on rodents will be complementary to current primate models—the latter providing highest comparability with the human brain, while the former offer hugely advanced methodologies on the lower levels of organization, for example, genetic alterations, intracellular electrophysiology, and imaging.
Butovas, Sergejus, Sheriar G. Hormuzdi, Hannah Monyer, and Cornelius Schwarz. Effects of electrically coupled inhibitory networks on local neuronal responses to intracortical microstimulation. J Neurophysiol 96: 1227-1236, 2006; doi:10.1152/jn.01170.2005. Using in vivo multielectrode electrophysiology in mice, we investigated the underpinnings of a local, long-lasting firing rate suppression evoked by intracortical microstimulation. Synaptic inhibition contributes to this suppression as it was reduced by pharmacological blockade of ␥-aminobutyric acid type B (GABA B ) receptors. Blockade of GABA B receptors also abolished the known sublinear addition of inhibitory response duration after repetitive electrical stimulation. Furthermore, evoked inhibition was weaker and longer in connexin 36 knockout (KO) mice that feature decoupled cortical inhibitory networks. In supragranular layers of KO mice even an unusually long excitatory response (Յ50 ms) appeared that was never observed in wild-type (WT) mice. Furthermore, the spread and duration of very fast oscillations (Ͼ200 Hz) evoked by microstimulation at a short latency were strongly enhanced in KO mice. In the spatial domain, lack of connexin 36 unmasked a strong anisotropy of inhibitory spread. Although its reach along layers was almost the same as that in WT mice, the spread across cortical depth was severely hampered. In summary, the present data suggest that connexin 36 -coupled networks significantly shape the electrically evoked cortical inhibitory response. Electrical coupling renders evoked cortical inhibition more precise and strong and ensures a uniform spread along the two cardinal axes of neocortical geometry.
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