Who needs eyes when you have a tongue?

After reading Haley’s blog post about the monkey that could control a robotic hand with nothing more than neural activity, it reminded me of a phenomenon I had learned about in undergrad.  Using the same principle of a human-machine interface, but utilizing the sensory system, researchers have been able to restore some level of sight in blind individuals.  We all remember learning in Physiology that as long as downstream neurons are being stimulated, you will still perceive senses, even if the primary transducing neuron is damaged or absent entirely – A behavior exemplified by phantom limb pain.  Interestingly enough, the current standard for vision replacement does not attempt to recreate visual pathways, but instead relies on crossmodal activation of the primary visual cortex. 

The technique involves placing a camera in a pair of glasses worn by the blind individual.  This camera is attached to a grid of electrodes that is placed somewhere on the body, most frequently on the tongue (for its high level of acuity).  The live feed from the camera causes the electrodes to stimulate mechanoreceptors on the tongue.  

As with any physical stimulation of the tongue, this signal is then sent to the thalamus, where the amazing happens – the signal gets routed to the primary visual cortex!  This physical stimulation is actually allowing the blind people to “see”!  A recent study used fMRI to determine whether pathways downstream of the primary visual cortex are maintained as well, and they found that both the dorsal and ventral stream (the two primary pathways leading from the occipital lobe responsible for location and identification respectively) are being recruited just as it would be in an individual with normal vision (Ptito et al., 2012).  Unfortunately, the Tongue Display Unit (TDU) is not powerful enough to replicate vision entirely.  Functionally, utilization of the TDU allows blind individuals to see large shapes and it helps them navigate around obstacles, but not too much beyond that so further research is necessary.  Additionally, only blindness resulting from damage upstream of the occipital lobe and thalamus can be treated by such a device.

Alternatively, a recent review suggested bypassing the tongue entirely.  They propose that implantation of electrodes into the lateral geniculate nucleus of the thalamus could do the trick (Pezaris et al., 2009).  This technique, known as deep brain stimulation, is used to treat other problems like Parkinson’s and major depression.  Preliminary animal models for the technique show promising results! 

1. Ptito, M., Matteau, I., Wang, A. Z., Paulson, O. B., Slebner, H. R., Kupers, R. (2012).  "Crossmodal Recruitment of the Ventral Visual Stream in Congenital Blindness." Neural Plasticity, 2012, 1-9.  doi 10.1155/2012/304045.
2. Pezaris, J. S. and Eskandar, E. N. (2009).  "Getting signals into the brain: visual prosthetics through thalamic microstimulation." Neurosurg Focus, 27(1) 1-20. doi:10.3171/2009.4.FOCUS0986.
3. Scott, R.  (2011).  "The injured eye."  Philosophical transactions of the royal society, 366, 251-260. doi:10.1098/rstb.2010.0234