Thus, the first factor in voluntarily reaching your hand to grasp a cup of coffee is your ability to visualize the cup, or even to smell the coffee. Unfortunately, different pathological conditions, such as stroke and cancer, result in the loss of our ability to receive sensory information such as sight and smell. Scientists are trying to find ways to bypass our reliance on sensory input for learning about our environment and navigating through the world. Recently, Kevin Mazurek and Marc Schieber at Rochester University, publishing in the journal Neuron, described a study in which monkeys learned to perform a specific action simply by receiving electrical brain stimulation.
“… monkeys learned to make the right choice merely by responding to the electrical signals… “
Soon after monkeys mastered the training, researchers placed an array of 16 electrodes in the premotor cortex of each monkeys’ brain. Through these electrodes, different frequencies of electrical pulses of durations could be transmitted to the premotor cortex while monkeys were performing the assigned task. First, each visual cue was accompanied by a synchronous electrical stimulation with a certain configuration of pulse frequencies and durations, unique to the specific handle that is supposed to be manipulated. Later, researchers gradually decreased the brightness of the LED cues, and finally turned the lights off. As predicted, the monkeys’ performance declined slightly, because they no longer could rely on the visual cue to tell them which handle to manipulate.
However, after many repetitions of the same task, monkeys learned to associate each electrical stimulus with a certain pattern of behavior, such that they ultimately did not need to rely on the visual cue to choose the right handle and manipulate it correctly. Indeed, monkeys learned to make the right choice merely by responding to the electrical signals from the electrodes when no LED light was shining, bypassing the sensory visual cue they tend to rely on to perform the same expected action.
“… direct stimulation of the premotor cortex in these patients can help them relearn the precise motions they relied on daily.”
Restarting the training process, researchers next assigned a different set of electrodes to each handle to more precisely investigate the nature of decision making in the same monkeys. For example, if stimulation through electrode 1 was previously supposed to be accompanied with handle B manipulation, this time researchers trained monkeys to manipulate handle D every time the electrical pulses were delivered through electrode 1. Curiously, monkeys could still play the game with a high performance, indicating the underlying learning process in the monkeys’ brains.
Previous studies indicated that delivering electrical signals to the sensory areas of the brain could lead to specific motor actions in primates, including humans. However, Mazurek and Schieber ruled out this possibility by showing that an electrical pulse, as short as 200ms that cannot theoretically cause any muscle movement, could still push the monkeys to perform the expected action.
The significance of these findings arises from the fact that researchers did not stimulate the sensory areas of the brain, the typical method to make damaged sensory areas responsive to external stimuli. Instead, by directly stimulating the premotor cortex, which is downstream to sensory brain areas such as visual, auditory, and somatosensory cortices, they demonstrated the possibility of bypassing these sensory areas.
Once again, imagine a cup of coffee. Every time you decide to grasp the cup, you need to rely on your eyes to comprehend the shape of the cup and its location so that you can decide the best possible movement of your hand to grasp it. If you decided to challenge yourself to reach and grasp the cup with your eyes closed, you can still rely on your hearing and ask your friend to tell you the exact shape and location of the cup. In the more sophisticated case related to this research, you may want to rely on the implant inside your brain that stimulates some parts of your premotor cortex and makes you comprehend the shape and location of the cup without relying on any external visual or auditory cue.
Different neuroprosthetics rely on stimulation of sensory organs, yet these findings open new avenues to make shortcuts on the damaged upstream sensory areas and still manifest the same expected motor actions. This is of particular interest for stroke patients suffering from malfunctions in sensory regions of the brain. Damaged brain areas in these patients hinder their attempts to properly receive or comprehend the external sensory signals. Thanks to current research, we now have better ideas on how to bypass damaged areas through direct delivery of required instructions to the premotor cortex. Indeed, direct delivery of stimulation to the premotor cortex could potentially connect noncommunicative brain areas to other parts of the brain in these patients. For example, direct stimulation of the premotor cortex may help patients relearn the precise motions they relied on daily.
How such electrical stimulation can produce the perception that leads to making a proper decision remains unanswered. Obviously, researchers did not have the opportunity to ask monkeys what they were feeling inside their brains or anywhere else that pushed them to make the right move. Could this be an unconscious decision that their brains were making without their own will?