Tuesday, January 19, 2010

A thought to speech machine.

When I was a post-doctoral fellow in the Neurobiology Dept. at Harvard Medical School in 1967-68, I regularly attended tea time discussions in the Hubel and Wiesel laboratory (These guys got a Nobel Prize a few years later for their work on how the visual cortex works). I recall being astounded by their discussions about experiments with microelectrodes implanted in a monkey brain that were finding that the activity of almost any nerve cell reported by an electrode could be trained to fire on demand by operant conditioning (for example, a cell being trained that a certain pattern of its activity could produce a reward stimulus like fruit juice). This memory came back to me when Mindblog reader Tristan emailed me excited about work (which turns out to have been in my queue of potential post topics) that is a logical extension of those experiments over 40 years ago.

This work by Guenther et al. implanted in the motor cortex a long-term cone electrode that records from neurites that grow onto its recording surface. The subject was a 26 year old male suffering from locked-in syndrome due to a brain stem stroke incurred at age 16, leaving the brain areas responsible for consciousness, cognition, and higher-level aspects of movement control intact while eliminating nearly all voluntary movement. They were able to effect some speech restoration by decoding continuous auditory parameters for a real-time speech synthesizer from neuronal activity during attempted speech reported by implanted motor cortex electrode. The paper has interesting figures and a video. Here is a central summary figure:

Black circles and curved arrows represent neurons and axonal projections, respectively, in the neural circuitry for speech motor output. The volunteer's stroke-induced lesion in the efferent motor pathways (red X) disconnects motor plans represented in the cerebral cortex from the speech motoneurons, thus disabling speech output while sparing somatic, auditory, and visual sensation as well as speech motor planning centers in cerebral cortex. Signals collected from an electrode implanted in the subject's speech motor cortex are amplified and sent wirelessly across the scalp as FM radio signals. The signals are then routed to an electrophysiology recording system for further amplification, analog-to-digital conversion, and spike sorting. The sorted spikes are sent to a Neural Decoder which translates them into commands for a Speech Synthesizer. Audio signals from the synthesizer are fed back to the subject in real time. [Abbreviation: PrCG = precentral gyrus.]

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