Welcome to part two of On the brain, a Conversation series by people whose job it is to know as much as there is to know about the body’s most complex organ. Here, Malcolm Horne, deputy director of the Florey Neuroscience Institutes, explores the brain’s role in our capacity to move and, through our understanding of this, the way we might treat brain disorders.
The 2003 Rugby World Cup final between Australia and England was a tense match with scores level through each team scoring penalty goals.
Under the scrutiny of a 100,000 strong crowd and the millions around the world watching on TV, the players had to step up and strike the kick accurately to keep their team in the game.
Kicking (well) is a highly learned and practiced skill but one that easily disappears under cognitive pressure.
It also exemplifies the way human movement differs from that of other animals by showing the capacity of humans to learn and modify their movement to achieve specific outcomes.
The capacity to move so as to capture food or engage in reproduction was a very early step in human evolutionary history. Traces of this ancient history are reflected in the structure and workings of the human spinal cord, which controls movement in the same way as almost all other animals including reptiles.
With time, the mammalian brain has evolved extra neural equipment, layered over these basic spinal reflex paths, to achieve more complex control of the spinal systems.
These neural systems coordinate the four limbs and trunk with information from the eyes, ears and balance system: this function is well exemplified in the silky movements of the cat.
Our understanding of how these systems work has advanced to the degree that active research is directed at tapping into and recording from these brain structures to control prosthesis for aiding people with brain injury in standing, walking and grasping objects.
Although cats and other animals move gracefully and are perfectly adapted to survive and function in their natural habitat, humans more than any other animal can learn or modify movements, adapting them to carry out new tasks.
No other animal has the capacity to learn the piano, to play tennis, putt in golf, drive a car or kick goals in rugby.
This capacity results from frontal lobes (see below) forming a further layer of control of the older movement systems. The frontal lobes lie behind the frontal bones and above the eyes, and have expanded greatly in humans.
Their growth is a major reason for the increased brain size of homo sapiens and importantly, for their capacity to reason.
Understanding how conscious, goal-directed behavior drives skilled movement is a major goal. The issues can be exemplified by reflecting on learning to drive.
The young driver initially devotes all of his or her attention on coordinating gears, clutch, steering wheel, brakes and accelerator, with almost no capacity for attending to pedestrians or other traffic.
Slowly the basic skills are learned and are carried out almost effortlessly and, in effect, subconsciously, so attention can be directed at negotiating traffic and talking to passengers. This process is a fundamental action of the frontal lobes.
By using a mechanism called “working memory” the frontal lobes can attend to a handful of items: one example might be remembering a telephone number for long enough to dial it.
Depending on the complexity of the task, most people can hold five or so items in their attention.
When learning to drive, working memory/attention is fully engaged in coordinating the mechanics of driving. At the same time, the frontal lobes, including a structure called the basal ganglia, is busy “learning” the components of driving, such as changing gears.