Cortical Implant



The basic structure of these implants is shown on fig. 2. The image is captured by a camera integrated into eyeglass frames, which follows the eye movements. Then the image is transmitted through a transcutaneous link that stimulates the microelectrodes implanted in the visual cortex and results into the creation of an image (Troyk, P., et al. 2003).

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Fig.2 Structure of a cortical implant device

(Troyk, P., et al. "A Model for Intracortical Visual Prosthesis Research."

Artificial Organs 27.11 (2003): 1005-15)


There are two categories of cortical implants:

  • Surface Cortical Stimulation

The earliest experiments with stimulation of the visual cortex were done by Brindley and Dobelle in the late sixties and early seventies. They stimulated the visual cortex by placing electrodes over its surface, because this area was supposed to process the visual signals from the eyes (fig.3). Using this method they were able to evoke phosphenes, but it was observed that multiple phosphenes were created by one and the same electrode and that the created phosphenes were inconsistent. Also large electrodes were used with currents of 13 mA and the electrodes were spaced 3mm apart. Dobelle and his colleagues designed a prosthesis that allowed a blind person to recognize 6-inch characters at 5 feet, which is approximately 20/1200 visual acuity (Margalit, Eyal, et al. 2002). Dobelle also concluded that the brightness of a phosphene is a logarithmic function of the current amplitude of the stimulating devices. The above-mentioned experiments were not able to provide information on how the electrical stimulation on the visual cortex could be used to communicate images to the brain. But they confirmed that electrical stimulation of the visual cortex could create visual perceptions (Margalit, Eyal, et al. 2002).

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Fig. 3 X-rays of cortical visual prosthesis after implantation

(Margalit, Eyal, et al. Retinal Prosthesis for the Blind. Survey of Ophthalmology 47.4 (2002): 335-56)


  • Intracortical Microstimulation

The first studies with Intracortical Implants were done by the National Institutes of Health in the early seventies. They implanted microelectrodes directly in the visual cortex. The tip sizes of the microelectrodes were close to the sizes of the neurons, in this way the control of the neuron function could be achieved by more selective stimulation. Using this kind of implant, arrays of smaller electrodes were created, stimulating a smaller surface area on the visual cortex and using lower current thresholds. Thus, predictable forms of phosphenes were observed and the interactions between the different phosphenes were reduced (Troyk, P., et al. 2003). One of the most known Intracortical Implant, called Utah electrode array (UEA) Fig.4, was developed by the University of Utah. They used silicon, which is highly biocompatible, as electrode material. In UEA, large number of 1.5mm long electrodes was separated from each other by 0.4mm space. Their tips (fig. 5), 80-100 microns in diameter at their basis, were covered with platinum. One of the most important design considerations was that the array had to be very thin since it had to stay on the surface of the brain and not interact with the skull. Because of its smaller size, a special surgical techniques and tools had to be developed that facilitate the implantation of the intracortical implant in the visual cortex. Thus, a pneumatically activated insertion tool was created, fig. 6. It was able to insert the array into the cortical tissue in 200 s and not damage the tissue (Normann, R. A., et al. 1999). However, the technology is still unable to create comprehensive visual pictures in a blind person. Dealing with the visual cortex is not a simple thing; the special organization is very complex at cortical level. Two adjacent neurons in the cortical tissue do not necessarily map two adjacent areas in space. Thus pattern stimulation may not evoke pattern perception. Also the neurons in the visual cortex control color, orientation, direction, and depth of the visual perception. Because of that the electrode stimulation pattern that creates a comprehensive visual pictures in the visual cortex is hard be mapped (Normann, R. A., et al. 1999).

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Fig. 4 A scanning electron micrograph of the 100 microelectrode,

Utah electrode array (UEA).

(Normann, R. A., et al. "A Neural Interface for a Cortical Vision Prosthesis."

Vision research 39.15 (1999): 2577-87)


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Fig. 5 A scanning electron micrograph of the platinum

coated tips of the UEA electrodes.

(Normann, R. A., et al. "A Neural Interface for a Cortical Vision Prosthesis."

Vision research 39.15 (1999): 2577-87)



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Fig. 6 A drawing of the high velocity, impact insertion tool

used to implant the UEA into cortical tissues.

(Normann, R. A., et al. "A Neural Interface for a Cortical Vision Prosthesis."

Vision research 39.15 (1999): 2577-87)


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Implanting intracortical prostheses.

(Troyk, P., et al. "A Model for Intracortical Visual Prosthesis Research."

Artificial Organs 27.11 (2003): 1005-15)


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A typical Utah Electrode Array.

(Maynard, E. M. "Visual Prostheses." Annual Review of Biomedical Engineering 3 (2001): 145-68).


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BMES 212
Summer Term 2007
School of Biomedical Engineering, Science and Health Systems