Ultra-micro electrodes support low-current amplitude discrimination in intracortical microstimulation

Abstract Restoring somatosensory feedback is essential for achieving natural, embodied control of prosthetic limbs. Intracortical microstimulation (ICMS) of the somatosensory cortex can provide artificial touch by activating cortical neurons directly. Here, we systematically investigate how electrode contact size influences amplitude discrimination performance during ICMS. Six rats were chronically implanted with custom multielectrode shanks in the somatosensory cortex. Over twelve weeks, behavioral just-noticeable differences (JNDs) were quantified across eight electrode diameters (5–50 µm) using an adaptive discrimination paradigm. Discrimination thresholds were significantly lower for 5 µm and 10 µm sites than for larger contacts because these electrodes operated at lower reference stimulation amplitudes within their established charge injection safety limits. Weber fractions remained stable across electrode sizes, demonstrating proportional scaling of perceptual sensitivity with stimulus intensity. We used a biophysically realistic modeling framework to identify the underlying neural correlates of ICMS-induced amplitude discrimination. Both the magnitude of neural responses (changes in the number of activated neurons and the total population spike count) and the population mean spike count contributed to perceptual discrimination. Together, these findings show that ultra-microelectrodes produce smaller absolute JNDs by enabling operation at lower currents, rather than by improving discrimination sensitivity at matched currents. Their preservation of Weber-like scaling within safe charge-injection limits may be advantageous for high-density sensory neuroprostheses.