Grasping, reaching, climbing – these are just some of the basic instinctive behaviors that we see even in babies. While these movements are not uniquely human, how they unfold neurologically is unique to primates.
All primates have these motor behaviors, or “motor primitives,” and they all occur in similar ways across different primates. “It’s part of our ancestry,” said Jon Kaas of Vanderbilt University, speaking at the CNS annual meeting in Boston last month.
On the occasion of receiving the George A. Miller award from CNS, Kaas took the audience through study after study that have mapped the highly organized motor cortex and how it is sub-divided for specific motor behaviors. Kaas, whose work spans many areas of cognitive neuroscience, including pioneering work on the plasticity of the human brain, first became interested in motor primitives when he visited the Yale University Psychology Department.
There, he saw the experiments of Mike Graziano, in which researchers stimulated the motor cortex in a monkey to produce hand-to-mouth movements. “Also, at a visit to a lab at the National Institutes of Health, my motor cortex was magnetically stimulated through the skull to move my thumb, and that was an impressive experience,” he recalls.
Kaas spoke with CNS about this body of work, including some yet-published research using a new technique for disabling motor primatives by cooling cortical regions. Speaking of the new work at the CNS meeting, he said: “It’s a wonderful technique for looking at what happens when you take something out of the system.”
As Kaas describes in this CNS Q&A, his and colleagues’ work is uncovering the vital role of the posterior parietal cortex in producing motor behaviors across primates – and it could ultimately inform treatments for brain injuries and stroke.
CNS: Would you please describe “motor primitives”?
Kaas: Motor primitives are the basic movement sequences that are found across members of the species, so that they appear to be “built in” to brain circuits, and not dependent on much or any learning. They are also called “ecologically relevant movements.”
CNS: Would you please describe how you experimentally induce and study these behaviors in primates?
Kaas: Such complex behaviors can be evoked by electrical stimulation of parts of the brain or spinal cord. Thus, neural circuits seem to be in place for these movement patterns. We evoke them from posterior parietal cortex, motor cortex, and premotor cortex. But others have developed such behavior from the midbrain, brainstem, and spinal cord.
CNS: What distinguishes primates from other mammals when it comes to motor behaviors?
Kaas: It appears that only primates have a large amount of posterior parietal cortex devoted to motor behavior. This region of cortex is small in tree shrews, which are closely related to primates, and in squirrels and other rodents that are less closely related to primates.
CNS: What has been the most exciting finding in this research area for you?
Kaas: It was very surprising to me to find that we could evoke these complex behaviors from anesthetized primates, and all primates studied including prosimian galagos, New World monkeys, and Old World macaques, and that we could even evoke these behaviors from three cortical regions, posterior parietal cortex, premotor cortex, and primary motor cortex. This was exciting because it meant that there were all sorts of experiments we could do to try to find out how the brain does this.
CNS: In your CNS talk, you mentioned some new work in your lab in which cortical areas are cooled to study motor behaviors. Would you please describe how this works?
Kaas: If you place something cold on the surface of cortex, you can rapidly cool cortex under that object to the point that it will not work anymore – that is, the neurons don’t fire. When the cortex is warmed, functions return. It is like a small lesion, only it is temporary. (See photo at right.)
CNS: What are the implications of your work for the public?
Kaas: People might want to understand how the brain works, especially when they reach, or when other behaviors seem automatic and with little thought. At another level, the level of functional organization of cortex we are aiming to understand, it will allow better predictions of patient’s outcomes after cortical damage due to injury or stroke. In addition, we are also looking at mechanisms of recovery from brain damage, and treatments that will promote recoveries.
CNS: What are you hoping to ultimately accomplish with your work?
Kaas: I am hoping, ultimately, but over the next few years, to understand much more fully the neural mechanisms of motor behavior, especially the mechanism that determines how it is that one behavioral outcome occurs over others. We feel that many different parts of the brain are involved in this decision process at the same time. So our goal will be to determine the different contributions of the different brain parts.
CNS: What are you most excited to learn in the future?
Kaas: I hope we are soon to learn how the decision to do something specific is made in one region of cortex, that is, posterior parietal cortex. We now know that eight or more different behaviors can be evoked from different regions of posterior parietal cortex. We hope to find out how the small regions are normally activated, and how they relate to each other to assure that only one motor command goes out to motor and premotor cortex. If we can understand that, other parts of the more extensive motor system may contribute to behavioral outcomes in a similar manner, but to produce other outcomes.
-Lisa M.P. Munoz
Kaas delivered the George A. Miller Award address, “Sensorimotor processing streams involving posterior parietal, premotor and motor cortex of primates: Comparative studies,” on April 5, 2014, at the CNS annual meeting in Boston.