39Intracortical microstimulation (ICMS) of the primary somatosensory cortex (S1) can 40 produce percepts that mimic somatic sensation and thus has potential as an approach to 41 sensorize prosthetic limbs. However, it is not known whether ICMS could recreate active texture 42 exploration-the ability to infer information about object texture by using one's fingertips to scan 43 a surface. Here we show that ICMS of S1 can convey information about the spatial frequencies 44 of invisible virtual gratings through a process of active tactile exploration. Two rhesus monkeys 45 scanned pairs of visually identical screen objects with the fingertip of a hand avatar, controlled 46 via a joystick and later via a brain-machine interface, to find the one with denser virtual gratings.
47The gratings consisted of evenly spaced ridges that were signaled through ICMS pulses 48 generated when the avatar's fingertip crossed each ridge. The monkeys learned to interpret 49 these ICMS patterns evoked by the interplay of their voluntary movements and the virtual 50 textures of each object. Discrimination accuracy across a range of grating densities followed
51Weber's law of just-noticeable differences (JND), a finding that matches normal cutaneous 52 sensation. Moreover, one monkey developed an active scanning strategy where avatar velocity 53 was integrated with the ICMS pulses to interpret the texture information. We propose that this 54 approach could equip upper-limb neuroprostheses with direct access to texture features 55 acquired during active exploration of natural objects.
57Sensory neuroprostheses offer the promise of restoring perceptual function to people 59 with impaired sensation 1,2 . In such devices, diminished sensory modalities (e.g., hearing 3 , 60 vision 4,5 , or cutaneous touch 6-8 ) are reenacted through streams of artificial input to the nervous 61 system, typically using electrical stimulation of nerve fibers in the periphery or neurons in the 62 central nervous system. Restored cutaneous touch, in particular, would be of great benefit for 63 the users of upper-limb prostheses, who place a high priority on the ability to perform functions 64 without the necessity to constantly engage visual attention 9 . This could be achieved through the 65 addition of artificial somatosensory channels to the prosthetic device 1 . Such an approach would 66 endow persons suffering from limb loss 10-12 , paralysis 1,13 or somatosensory deficits with the 67 ability to perform active tactile exploration of their physical environment and aid in dexterous 68 object manipulation [14][15][16][17] .
69Previously we demonstrated that motor and sensory functions could be simultaneously 70 enacted though a bidirectional neuroprosthetic system, called a brain-machine-brain interface 71 (BMBI) 18 . In that demonstration, the active exploration enabled by our BMBI-driven 72 neuroprosthesis used a limited and fixed set of ICMS temporal patterns to generate artificial 73 sensory inputs that mimicked the sense of flutter-vibration. However, it r...