Berger, Hampson, Song, Goonawardena, Marmarelis, and Deadwyler created a system for recording from and stimulating up to 32 neurons at once. The system learned a model to predict firing of some hippocampal CA1 neurons given some inputs from CA3, and could be “played back” later.
In a delayed-nonmatch-to-sample task, a rat was shown one of two levers, then there was a delay during which the rat was distracted, then the rat was shown both levers and was supposed to press the one it hadn’t been shown at first. The model of CA1 was trained on the most difficult, successful trials, then replayed later to stimulate CA1.
Stimulation occurred in two conditions: normal, and when glutamate transmission was blocked. In both conditions, the prosthesis augmented performance by about 20%. I couldn’t tell from the paper whether they had different models depending on which lever was about to be pressed, and chose to play the correct model to stimulate recall; if they did, then this is really just showing that the prosthesis can affect which memory is recalled, not that it can actually substitute for CA1.
Theodore W Berger, Robert E Hampson, Dong Song, Anushka Goonawardena, Vasilis Z Marmarelis and Sam A Deadwyler. A cortical neural prosthesis for restoring and enhancing memory. 2011 J. Neural Eng. 8 046017
We blogged about this project a few years ago: http://neurodudes.com/2007/04/04/interview-on-usc-hippocampal-prosthetic/