Designing a thalamic somatosensory neural prosthesis: consistency and persistence of percepts evoked by electrical stimulation.

Intuitive somatosensory feedback is required for fine motor control. Here we explored whether thalamic electrical stimulation could provide the necessary durations and consistency of percepts for a human somatosensory neural prosthetic. Continuous and cycling high-frequency (185 Hz, 0.21 ms pulse duration charge balanced square wave) electrical pulses with the cycling patterns varying between 7% and 67% of duty cycle were applied in five patients with chronically implanted deep brain stimulators. Stimulation produced similar percepts to those elicited immediately after surgery. While consecutive continuous stimuli produced decreasing durations of sensation, the amplitude and type of percept did not change. Cycling stimulation with shorter duty cycles produced more persisting percepts. These features suggest that the thalamus could provide a site for stable and enduring sensations necessary for a long term somatosensory neural prosthesis.

Designing a somatosensory neural prosthesis: percepts evoked by different patterns of thalamic stimulation.

Although major advances have been made in the development of motor prostheses, fine motor control requires intuitive somatosensory feedback. Here we explored whether a thalamic site for a somatosensory neural prosthetic could provide natural somatic sensation to humans. Different patterns of electrical stimulation (obtained from thalamic spike trains) were applied in patients undergoing deep brain stimulation surgery. Changes in pattern produced different sensations, while preserving somatotopic representation. While most percepts were reported as 'unnatural', some stimulations produced more 'natural' sensations than others. However, the additional patterns did not elicit more 'natural' percepts than high-frequency (333 Hz) electrical stimulation. These features suggest that despite some limitations, the thalamus may be a feasible site for a somatosensory neural prosthesis and different stimulation patterns may be useful in its development.