Effects of spatial and temporal parameters of primary afferent microstimulation on neural responses evoked in primary somatosensory cortex of an anesthetized cat

Patterned microstimulation of muscle and cutaneous afferent neurons may provide tactile and proprioceptive feedback to users of advanced prosthetic limbs. However, it is unclear what types of stimulation patterns will be effective, and the parameter space for creating these patterns is prohibitively large to explore systematically using only psychophysics paradigms. In this study, we used an array of microelectrodes in primary somatosensory cortex (S1) of an isoflurane anesthetized cat to measure responses in a population of neurons evoked by various patterns of primary afferent microstimulation delivered to the L6 and L7 dorsal root ganglia (DRG). Each pattern consisted of a 300 ms train of microstimulation pulses having a fixed amplitude, pulse rate, and location in the array of DRG electrodes. Evoked responses were detectable on many S1 channels at the lowest amplitude tested (5 μA) and pulse rate (10 pulses per second). Increasing the pulse rate lowered the threshold amplitude for evoking a response on some S1 channels. Location effects were also observed. Adjacent stimulation sites evoked discriminable responses at low but not high (20 μA) amplitudes. In summary, we observed interactions between stimulation pulse rate, pulse amplitude, and location. Such interactions must be considered when designing stimulation patterns for transmitting sensory feedback by primary afferent microstimulation.