A major concern in the emerging field of neuroprosthetics is the limited plasticity of the adult mammalian brain. The developing brain is considered to be the most plastic, while plasticity is thought to decrease over time (imagine a more technical version of “it’s hard to teach an old dog new tricks”). Given that neuroprosthesis requires the sensory processing of an entirely new external input, limits in plasticity would presumably hamper the applicability of these new technologies to more-developed adults.
A New Study at The Nicolelis Lab
During the month of February 2016, The Nicolelis Lab at Duke published a study in the Journal of Neuroscience in which adult rats were made to discriminate between infrared light sources via microstimulation-based neuroprosthesis.
The research group acknowledged the difficultly inherent in working with adult mammals, as “to work properly, cortical sensory neuroprosthetics will require that the adult brain be plastic enough to continuously process real-time streams of synthetic information sources, and then use the extracted information to guide appropriate behavioral responses.” While they had previously done work in which the microstimulation transmitted data from a single infrared sensor to the somatosensory cortex (S1), in this experiment the researchers used four different infrared (IR) sensors to give “a panoramic representation of the surrounding IR environment.”
For the neuroprosthesis, thirty-two electrodes were implanted in the S1 (sixteen in each hemisphere) of adult rats. A cap was then placed on the electrodes that consisted of four evenly spaced IR sensors. To show that the rats were integrating this new information, the rats were placed in a circular chamber with four different “reward ports.” For the rats to receive any reward they were required to track an IR light beam all of the way to the reward port from where it was emitted.
Interestingly, the trials showed that when one of the four sensors was oriented toward one of the IR sources it “evoked higher-frequency microstimulation in its corresponding S1 stimulating channel.” Such indicated that the neuroprosthesis was successfully able to stimulate the adult rat brain, and that the rat was able to translate that stimulation to a somatosensory response. In the trial, the rats with four infrared sensors found the reward ports more easily than those that had only one sensor “likely because the four IR sensors provided them with a more complete perspective of the IR sources.”
The researchers then performed an ablation study to see if the placement and orientation of any of the IR sensors was more important than the others. The study showed that the sensors each played a roughly equal role in the completion of the trial, though the ablation of the back-facing sensor was surprisingly shown to have the most deleterious effect on trial completion time. The control for the experiment came with the deactivation of all four sensors (i.e. the neuroprosthetics transmitting no information to the rats), and resulted in the performance in the trial dropping below chance.
Significance for Brain-Machine Interface
In assessing the significance of their work, the researchers noted that the research “is quite promising clinically, as the largest demand for sensory prosthetic devices is in adults whose brains already fully developed.” This work was not the first to establish neuroplasticity of the adult brain—the 2016 Kavli Prize, for example, was award to a trio who independently studied the “discovery of mechanism that allow experience and neural activity to remodel brain function.”
The Nicolelis Lab’s study, however, did further establish that not only could an adult rat brain integrate information from a neuroprosthesis, but that it actually performed better in trials when it was provided with more information from more prosthetics.
The most popular sensory prosthetic system is the cochlear implant that directly transmits signals to electrodes placed in the cochlea. This study by a research group that is among the world’s leaders in the brain-machine interface would appear to be a promising first step toward the eventual production of a neuroprosthetic that could be developed for sensory-deprived human adults (or children for that matter). Electrodes could theoretically be placed in the appropriate brain region and attached to a sensor that would transmit sensory information that otherwise would have been lost.
Nevertheless, we should not be expecting this research to translate into a usable prosthetic in the near future. No form of this technology has been tested in human subjects, and there is no information as to whether this technology (i.e. the implanting of many electrodes directly into the brain) could be altered into a more feasible wearable device. The research is exciting, should likely change how we think about the plasticity of the brain, but the hype of the potential applications should be taken with a grain of salt—at least for now.
- A. L. Nicolelis et al. “Embedding a Panoramic Representation of Infrared Light in the Adult Rat Somatosensory Cortex through a Sensory Neuroprosthesis.” Journal of Neuroscience36.8 (2016): 2406-424.