Postmortem investigation of a human cortical visual prosthesis that was implanted for 36 years.

Postmortem analysis of the brain from a blind human subject that had a cortical visual prosthesis implanted for 36 years (Dobelle, 2000) provides insight into the optimal design characteristics of a successful human cortical visual prosthesis, by revealing, (a) unexpected rotation of the electrode array about 40 degrees away from the midsagittal plan, thought to be due to the torque of the connecting cable, (b) degeneration of the platinum electrodes, and (c) only partial coverage of the primary visual cortex by the rectangular array. The electrode array only overlapped with the anterior 45% of primary visual cortex (identified by the line of Gennari), largely missing the posterior foveal representation of visual cortex. Histology did not reveal appreciable loss of neurons in cortex that surrounded the migrated array, perhaps due to the very slow rotation of this implant. The proportion of stimulated electrodes that elicited phosphenes was higher over extrastriate cortex, compared to striate cortex (p < .05). This pioneering effort to develop a cortical visual prosthesis suggests that to maximize efficacy, the long-term effects of implanted alien materials on nervous tissue, and vice versa, need to be considered in detail, and that electrode array design considerations need to optimally match the electrodes to the patient's cortical anatomy. Modern pre-implant imaging can help optimize future implants by identifying the location and extent of bridging veins with MRI and even map the location of the V1/V2 border in vivo with PET.