The current state that its in is rather early. Currently I'm using an Analog to digital converter (ADC) to convert signals to binary. In time I'll switch the analog input to a photo transistor or photo diode, of which I have experience from optically controlled DC pulse motors.
The binary data is fed into a USBee. It has a small buffer that can be read from the computer so the data from the ADC can be sampled and processed on the computer. The processing will come when I connect it to some thing more complex then a simple voltage sensor.
The computer can send signals to the out buffer of the USBee, and these signals can be sent to a Digital to Analog Converter DAC.
The plan will be to have the DAC send signals to an appropriately tuned set of amplifiers, which will then control some LEDs or lower power lasers. The photons from the light source will trigger optically controlled ion channels in a subset of cortical neurons. These neurons will then influence the population codes of the neural networks. With the correct set of inputs it is hoped that meaningful responses can be induced in the brain.