1Institute of Biomedical Engineering, Imperial College, London, UK
2Division of Neuroscience, Imperial College, London, UK
The field of neural coding has relied on electrical techniques such as patch clamping and microelectrodes for both stimulation and recording. There are of course other methods such as sensory interaction in animals. But until recently it has not been possible to target the stimulation of specific neurons with light in the way that calcium imaging has lit up field of neural recording. However, it is now possible to encode specific cells with light sensitive ion channels and stimulate action potentials at will.
The dominant light sensitive channel used at present is the channelrhodopsin-2 which originates from the eye of a motile swamp microalgae called Chlamydomonas reinhardtii. It can be readily transfectable into neurons via standard techniques and AAV viri are now available which can express it in vivo with ample potential for cell targeting promoters. To date, it has been expressed in cell culture, C. Elegans, rodents, and there is now discussion about its potential for prosthetics and clinical trials in humans.
In cells expressing sufficient levels of ChR2, and stimulated with sufficient light, it is possible to generate action potentials at will. This can be done by stimulating individual action potentials with individual pulses of light, or by generating responses as a result of a broad temporal illumination. Light requirement is high, and it takes around 2-5 nJ of light stimulation per action potential. This response is poor compared to rhodopsin due to the lack of dedicated optical architectures found in dedicated cells such as rods and cones. In addition, as the channel is basically a passive system, there is no amplification such as the G-protein linked cascade in most visual systems.
In this presentation we highlight our own work in developing prosthetic retina strategies using this technique. We will describe the equipment we have developed to stimulate multiple neurons and neuron substructures simultaneously. This can be seen in Figure 1 where a neuron has being stimulated with 3 targeted spots of light. Individual pulses create individual action potentials. We will discuss the efficacy of signal transfer to the neurons, and spatial and temporal frequency limitations. Finally we will discuss the future directions of the technique.