Performance and Biocompatibility of Multiple Surface Electrodes in Cat Primary Visual Cortex

Abstract

Abstract: : Purpose: To test the electrophysiological performance and the biocompatibility of a chronic cortical microelectrode implant. Methods: Flexible electrode arrays (Adtech) consisting of 6 Platinum surface electrodes spaced equally within a 15mm x 8mm x 0.5mm silicone platform were implanted unilaterally under Sevoflurane anesthesia onto the medial bank of the cerebral hemisphere to cover nearly all of the primary visual cortex in adult cats. Connecting wires were tunneled subcutaneously to the scapular region, and the implants were left in place for 2-5 months. Electrophysiology (evoked potentials and retinotopic mapping) was performed periodically throughout the duration of the experiment. After sacrifice, cortical tissue adjacent to the implant was evaluated for histopathology using immunohistochemical markers for neurons, astrocytes, and microglia. Results: Implants remained in place with no signs of infection. The dura mater had healed completely but some deformation of the cortical surface was apparent on gross examination. Portions of the implant were encapsulated with a thin layer (<500 um) of gliotic tissue but most of the implant remained in close contact with the cortical surface. In one cat with a 4-month implant, electrode impedance increased about 20-fold in four of the six electrode leads. This lead to decreased measured visual evoked potential. We suspect this problem resulted from the slight encapsulation of the electrode contact points within the visual cortex. Normal visual evoked potentials and retinotopic maps continued to be obtained repeatedly following stimulation of the ipsilateral retina. Cortical gray and white matter appeared normal on gross examination. Ongoing immunohistochemical evaluation of neuronal and glial alterations subjacent to the implant will be reported. Conclusion: Preliminary observations suggest that arrays of Platinum surface electrodes implanted onto the primary visual cortex remain functional for long periods in adult cats, and may be tolerated at the cellular level. Thus, the cat model is well-suited for further development of visual cortical prostheses.

Keywords: 315 anatomy • 621 visual cortex • 485 neuro-ophthalmology: cortical function/rehabilitation