I took a brief look at the current state of such connections for our Coalescing Minds paper:
3.1. Direct brain-to-brain connections. The easiest approach seems to be to connect human
brains directly in much the same way as the two brain hemispheres are connected. The corpus
callosum, which connects the hemispheres comprises 200–250 million axons crossing from
one hemisphere to the other. It seems likely that to coalesce minds, the number of connections
should be of a similar order of magnitude, probably at least millions.
The technology exists today for creating hundreds of connections: e.g. Hochberg et al. [2006]
used a 96-microelectrode array which allowed a human to control devices and a robotic
hand by thought alone. Cochlear implants generally stimulate the auditory nerve with 16-22
electrodes, and allow the many recipients to understand speech in every day environments
without needing visual cues [Peterson et al. 2010]. Various visual neuroprostheses are currently
under development. Optic nerve stimulation has allowed subjects to recognize simple patterns
and localize and discriminate objects. Retinal implants provide better results, but rely on
existing residual cells in the retina. [Ong & da Cruz, 2011] Some cortical prostheses have also
been recently implanted in subjects. [Normann et al. 2009] We are still likely to be below the
threshold of coalescing minds by several orders of magnitude. Nevertheless, the question is
merely one of scaling up and improving current techniques.
I took a brief look at the current state of such connections for our Coalescing Minds paper: