In Vitro Laboratory

In vitro experiments (i.e. carried out in an artificial environment outside the living organism) include the study of methods to minimize the amount of electricity needed to stimulate retinal nerve cells to create visual sensations of light that ultimately will be perceived as visual images. Furthermore, it is important to realize that these ideal stimulation parameters in retinas undergoing degeneration such as in retinitis pigmentosa and age-related macular degeneration may be very different from what is determined from normal healthy retinas.

Responses of a retinal ganglion cell to cathodal and anodal stimuli applied through a subretinal stimulating electrode. Note that in both cases a response was generated but the timing of the response differed.

Understanding the effects of various parameters of electrical stimulation on the responses of retinal ganglion cells is crucial to the success to implementing a retinal prosthesis strategy to restore functional vision. Recently, we have been exploring the effects of electrode polarity on stimulation thresholds and responses of ganglion cells in the normal (non-diseased) retina. Many ganglion cells can be subdivided into ON and OFF cells. ON cells detect spots that are brighter than the background while OFF cells respond to the opposite, leading to the general consensus that ON cells give rise to the perception of brightness and OFF cell system to the perception of darkness. Selective stimulation of either ON or OFF type cells is therefore critical in order to create meaningful patterns of visual stimulation. We have found that the polarity ("cathodal" and "anodal") of the delivered current pulses is critical. The currents for activation for ON and OFF retinal ganglion cells are different and so are the timing of the responses. The findings suggest that it may be possible to selectively activate specific neuronal populations in the retina leading to more effective strategies of electrical stimulation.