IEEE Spectrum Online points to interesting research lead by William J. Tyler. The research group used low-power, low-frequency (440 kHz to 670 kHz) ultrasound to stimulate areas within the brain.

This technique, called “ultrasonic neuromodulation“, works by toggling the sodium channels of a neuron, possibly triggering the release of neurotransmitters – in other words, making the neuron “fire”.

You can listen to Mr. Tyler explain it himself.

The technology is still in early stages and may yet develop towards interesting mass-market applications in a time-scale of 4-5 years. Maybe a similar technique could even be used to “read”, instead of just “write”.