In vivo microstimulation with cathodic and anodic asymmetric waveforms modulates spatio-temporal calcium dynamics in cortical neuropil and pyramidal neurons of male mice.

Electrical stimulation has been critical in the development of an understanding of brain function and disease. Despite its widespread use and obvious clinical potential, the mechanisms governing stimulation in the cortex remain largely unexplored in the context of pulse parameters. Modeling studies have suggested that modulation of stimulation pulse waveform may be able to control the probability of neuronal activation to selectively stimulate either cell bodies or passing fibers depending on the leading polarity. Thus, asymmetric waveforms with equal charge per phase (i.e. increasing the leading phase duration and proportionately decreasing the amplitude) may be able to activate a more spatially localized or distributed population of neurons if the leading phase is cathodic or anodic, respectively. Here, we use two-photon and mesoscale calcium imaging of GCaMP6s expressed in excitatory pyramidal neurons of male mice to investigate the role of pulse polarity and waveform asymmetry on the spatiotemporal properties of direct neuronal activation with 10 Hz electrical stimulation. We demonstrate that increasing cathodic asymmetry effectively reduces neuronal activation and results in a more spatially localized subpopulation of activated neurons without sacrificing the density of activated neurons around the electrode. Conversely, increasing anodic asymmetry increases the spatial spread of activation and highly resembles spatiotemporal calcium activity induced by conventional symmetric cathodic stimulation. These results suggest that stimulation polarity and asymmetry can be used to modulate the spatiotemporal dynamics of neuronal activity thus increasing the effective parameter space of electrical stimulation to restore sensation and study circuit dynamics. Significance StatementElectrical stimulation has promise to restore sensation and motor function, as well as treat the symptoms of several neurological disorders. However, the mechanisms responsible for the beneficial effects of stimulation are not fully understood. This work supports modeling predictions by demonstrating that modulation of the stimulation waveform dramatically affects the spatial recruitment and activity level of neurons in vivo. These findings suggest that stimulation waveform symmetry represents a parameter that may be able to increase the dynamic range of stimulation applications. Further characterization of these parameters with frequency, and amplitude could provide further insight into the mechanisms of electrical stimulation.