Shapes traced on the surface of the visual cortex by stimulating electrodes in dynamic sequence have enabled accurate recognition of letter shapes, according to researchers at Baylor College of Medicine and the University of California.
Two blind people and three sighted people were implanted with a visual cortical prosthesis (VCP) and found to be able to interpret the letters when traced on the visual cortex. The researchers likened the approach to tracing letters on the palm of a hand.
subjects were able to reliably discriminate letter forms from a small number of electrodes without training
According to the study authors, a VCP has long been proposed as a strategy for restoring useful vision to the blind. The assumption being “that visual percepts of small spots of light produced with electrical stimulation of visual cortex (phosphenes) will combine into coherent percepts of visual forms, like pixels on a video screen”.
Using a current steering technique that involves passing current through two adjacent electrodes to create a virtual electrode between them, researchers were able to stimulate the visual cortex in a continuous trajectory. Varying the amount of current delivered to adjacent electrodes allowed the virtual electrode to be positioned at different locations along the path.
The study participants were able to reliably discriminate letter forms from a small number of electrodes without training. In both sighted and blind participants, dynamic stimulation enabled accurate recognition of letter shapes predicted by the brain’s spatial map of the visual world. Blind participants were able to recognise up to 86 forms per minute.
The authors of the study noted previous instances where retinal prostheses used electrical stimulation delivered in a regular sequence of stimulated electrodes, rather than activation of all electrodes at once, to reduce the likelihood of epileptic seizures. Alternatively, in the case of dynamic current steering, the pattern of sequential stimulation is related to the form that is being conveyed to the participant, rather than a fixed order.
Michael S. Beauchamp, Denise Oswalt, Ping Sun, Brett L. Foster, John F. Magnotti, Soroush Niketeghad,2 Nader Pouratian,3 William H. Bosking,1,4 and Daniel Yoshor. Dynamic Stimulation of Visual Cortex Produces Form Vision in Sighted and Blind Humans. Cell. 181, 774–783 May 14, 2020