Somatosensory Cortex Microstimulation: Behavioral Effects of Phase Duration and Asymmetric Waveforms

Intracortical microstimulation has proven to be effective in a variety of sensory applications, such as returning touch percepts to paralyzed patients. The parameters of microstimulation play an important role in the perception quality of the stimulus. Eliciting naturalistic percepts is essential for the adaptability and functionality of this technology. Compared to the typical biphasic symmetric waveforms, asymmetric waveforms enhance activation selectivity by preferentially activating cell bodies. Behavioral studies have shown that asymmetric waveforms can elicit behavioral responses, but these require higher charges than typical symmetric waveforms. Here, we investigated the effects of phase duration and waveform asymmetry for somatosensory cortex intracortical microstimulation of freely-behaving rats. Detection thresholds were obtained using a conditioned avoidance behavioral paradigm. Our results indicate that phase duration has significant effects on threshold regardless of symmetry, polarity, and phase order of the waveform. Specifically, shorter phase durations tend to elicit lower behavioral thresholds. Analogous to studies in the auditory cortex, asymmetric waveforms in which the short pulse was cathodic were more effective than those with short-anodic pulses. With short phase durations, these short-cathodic waveforms are capable of evoking behavioral responses at low charges (<5 nC/Phase). Altogether, these results suggest the possibility of cell body selective microstimulation at safe thresholds, as well as the potential translatability of neuromodulation parameters across distinct sensory cortical areas.