Movement and Space
Movement is the most fundamental feature of animals. Plants can be stationary and enjoy a long and healthy life rooted to the earth. Animals move. If you had to limit your study of humans to two essential features of existence, they would be eating and movement. The brain is the organ of the mind and the organ of movement. The brain is a matrix of meaningful connections between the body inside and the environment outside.
Humans have an innate sense of spacetime. We live inside a virtual spacetime frame, created by neuronal networks in the brain. At several levels of interaction the brain creates spacetime and body maps that intersect. Sensory information flows into these spacetime maps and motor output flows out of these spacetime maps. Each human has a sense of personal extension defined by the boundary of skin and the extension of arms and legs.
We are creatures of gravity and work hard to lift our bodies and burdens upward. We are alert and agile enough to maneuver down rocky slopes and avoid falls that would injure or kill. Our speech grows out of body spacetime and communication with sounds. There is rhythm in our motion, in the sounds we hear and the sounds we make. Our languages emerge from spacetime maps and rhythmic sounds. We speak in terms of movement through spacetime, and of journeys both literal and metaphoric. We project our minds into the world and merge with the world of continuous changes and constant motion.
Humans act on the world through praxis or skilled movements. A complete set of movement patterns such as walking and running are innate but must be practiced to develop skills. The root adaptive task is to learn what movements are required for survival today. Ten thousand years age, if you were male, you learned to throw a spear, catch a fish or carry a deer carcass on your back. Today, you learn to learn to throw a football, move a pen across a paper surface, push keys on a keyboard and control movement with a mouse or joystick.
Humans learn by imitating what they see and hear. Learning movement skills is so implicit in life experiences that most of the lessons are not recognized as such and most of the practice is built into the daily experiences of life. The central feature of intelligence is the ability to understand what is really going on out there and to respond to events with successful and adaptive behavior. Praxis is skillful movement and is central to intelligent behavior. If you add mimesis to praxis, you can start building a meaningful model of intelligence. To learn, you copy the skillful movements of others and practice these movements until you match or surpass the teacher’s skills. The movements that are copied and learned extend into speech.The construction of language, both in form and content is based on movement in space. While you can admire the complexity of the best examples written language, most human communication is a mixture of sounds and gestures in the tradition of our primate relatives and ancestors. Mimesis is the ability to imitate and copy the movements of others. Humans learn by imitating what they see and hear.
Humans create neuronal models of their own behavior and the behavior of others, remember and communicate these models. We can simulate experience and anticipate what we are going to do in the future. We can practice skills in advance so that can improve our performance. We can expand this modeling capacity into verbal and body communication, invent language and substitute words for objects and action. We can learn to handle words much like objects and do symbolic transactions with each other.
Movement originates in several areas of the brain. The final signals to muscles to contract emerge from the thalamus and motor cortex and travel along the spinal cord to the motor neurons in the anterior horn of the spinal cord grey matter. The spinal motor neuron sends a signal along a peripheral nerve to the muscle cells. The cerebellum does the fine-tuning of coordinated movements by adding to the signals emerging from the motor cortex. The parietal cortex stores maps that connect body movements with spacetime and recall learned patterns of movement. If the motor cortex is damaged, you are paralyzed. If the cerebellum is damaged, movement coordination is peculiar or lost. If the parietal cortex is damaged, you retain movements but learned motor skills may be missing and you may ignore part of your body as if it did not exist. A typical parietal deficit is that you cannot perform learned movements such as dancing or using a screwdriver.
Three cortical regions control voluntary movement
1. the primary motor cortex M1
The simplest idea of the brain begins with a sensory input entering a processor that then decides what to move and sends motor outputs to motors (muscle cells). The first complication in this model is that the motor cortex has sensory input and the sensory cortex has motor output. The second complication is that some movement is generated in response to real-time sensory input and other movement is generated from memory that operates like internal sensory input.
The body is mapped onto the sensory and motor cortex. Smaller body parts such as the fingers, lips and tongue that are used for fine manipulative movement occupy larger areas of the motor cortex than larger body parts such as arms and legs that are involved in more vigorous movements such as throwing and walking. A smaller cortical area (SMA) in front of the motor cortex contains a separate body map and at least 4 other regions of the brain contain body maps.
While cortical maps exist, the regions in the map are not as discrete as once thought. If the map is displayed as pieces of a jig saw puzzle, the pieces are not placed side by side to form the map, but overlap. The arrangement would be easier to understand if motor neurons were assigned to one body part and made point-to-point connections that were stable over time. However, the real cortex appears to have a dynamic map and a scheme of connections based on fields of activity that converge and diverge in complex patterns. Over time, the pieces of the map change with learning and practice, so that the construction of cortical connections is in flux.
Neuroscientists now make distinctions among many components of movement. For example, the preparation to make a movement is regarded separately from the volley of signals sent to implement the movement. Scheiber stated: "Neurons in M1, SMA and PM discharge at the highest rate while a subjects waits to move in particular direction… during the delay between instruction and movement triggering, PM and SMA appear to store information on the direction of the impending movement… this represents (the retrieval of) stored information...To pick up a pencil, for example, you may glance at the pencil and then move your hand to the same place. Insight into how cue direction is transformed into movement direction has come from tasks in which these two features were experimentally dissociated, similar to glancing at your pencil in a mirror and then reaching to pick up the real pencil instead of a mirror image…The cue direction is transformed into movement direction in the area principalis (of the frontal lobe) during the delay period… information is sent to M1 at the time of execution."
The cerebellum lies below the cerebral hemispheres and is connected to the rest of the brain and spinal cord. Like the cerebrum, the cerebellum has two hemispheres and a cortex with gyri and sulci. The cerebellum is involved in producing smooth, coordinated movements by regulating muscle tone and the rate, range, and force of muscle contraction. Dysfunction is expressed as disorders of movement and equilibrium.