• Home
• Table of Contents
• Further Reading

Encoding of Movement by Motor Cortex (continued)

Premotor Cortex

The premotor cortex sends axons to the primary motor cortex as well as to the spinal cord directly. It performs more complex, task-related processing than primary motor cortex. Stimulation of premotor areas in the monkey at a high level of current produces more complex postures than stimulation of the primary motor cortex. The premotor cortex appears to be involved in the selection of appropriate motor plans for voluntary movements, whereas the primary motor cortex is involved in the execution of these voluntary movements.

1. Premotor cortex neurons signal the preparation for movement. Monkeys were trained to make a particular movement in response to a visual signal, with a variable delay between the onset of the signal and the onset of the movement (Figure 3.10). Recordings from premotor cortex have shown that many neurons fire selectively in the delay interval, for many seconds before the onset of the movement. A particular neuron will fire when the monkey is preparing to make a movement to the left, for example, but will be silent when the monkey is preparing to make a movement to the right. Thus, the firing of this type of neuron does not cause the movement itself, but appears to be involved in preparing the monkey to make the correct movement when the “Go” signal is given. This type of neuron is called a motor-set neuron, as it fires when the monkey is preparing, or getting set, to make a movement.
   
   

   Figure 3.10 Premotor cortex neurons signal preparation for movement. A monkey is trained to prepare to make a movement to the right or left depending on a cue instruction, but to delay the movement until a “Move” signal is given (Weinrich and Wise 1982). Some neurons will fire selectively when the animal is preparing to make a movement to the right (Play Prepare right cell). Other neurons will fire selectively when the animal is preparing to make a movement to the left (Play Prepare left cell). Note that the cells fire in the interval between the Prepare instruction and the Move instruction, but they do not fire during the movement itself.

   
   
2. Premotor cortex neurons signal various sensory aspects associated with particular motor acts. Some premotor neurons fire when the animal is performing a particular action, such as breaking a peanut (Figure 3.11). Interestingly, the same neuron fires selectively when the animal sees another monkey or person breaking the peanut. It also fires selectively to the sound of a peanut shell being broke, even without any visual or motor activity. These neurons are called “mirror” neurons, because they respond not only to a particular action of the monkey but also to the sight (or sound) of another individual performing the same action. (For an interesting PBS video on mirror neurons, go to http://www.pbs.org/wgbh/nova/sciencenow/3204/01.html.)
   
   

   Figure 3.11 Mirror neuron in premotor cortex fires to the monkey’s action as well as the monkey’s perception of a person performing the same action (Kohler et al., 2002).

   
   
3. Premotor cortex is sensitive to the behavioral context of a particular movement. The premotor cortex of human subjects was imaged with functional MRI as they observed video of a hand grasping a cup (Figure 3.12). In one condition, the cup was full and surrounded by full plates of food; the implication was that the person was grasping the cup to take a drink. In the other condition, the cup was empty and surrounded by dirty dishes; the implication was that the person was grasping the cup to clear the table. In this experiment, the premotor cortex was more active when subjects viewed the former video than the latter, even though the movements were the same. Thus, premotor cortex neurons are sensitive to the inferred intentions of a movement, not just the movement itself, as deduce from the behavioral context in which the movement occurred.
   
   

   Figure 3.12 Premotor cortex activity distinguishes the same movement based on the behavioral context of the movement (Iacoboni et al., 2005). When the subject viewed an arm moving to pick up a cup to drink (PLAY top), the activity in premotor cortex was greater than when the subject viewed an arm moving to pick up a cup to clear the table after a meal (PLAY bottom). Note that the strength of activity in the cortex (denoted by the brightness of the activated cortical region) is greater in the top than in the bottom animations.

   
   
4. Premotor cortex signals correct and incorrect actions. Human subjects were studied in an fMRI experiment as they observed video clips of various correct and incorrect motor acts. A correct action was one in which the movement and the associated object was performed correctly, such as setting the time on a clock. An object error was one in which the action was correct, but the object was incorrect, such as polishing a brown shoe with black shoe polish. A movement error was one in which the object was correct, but the movement was incorrect, such as attempting to put a coin into a piggy bank when the coin was oriented perpendicular to the coin slot. In this experiment, the premotor cortex was activated bilaterally during the correct actions trials and the movement error trials; for the object error trials, only the premotor cortex of the left hemisphere was activated preferentially.

Supplementary Motor Area

The supplementary motor area (SMA) is involved in programming complex sequences of movements and coordinating bilateral movements. Whereas the premotor cortex appears to be involved in selecting motor programs based on visual stimuli or on abstract associations, the supplementary motor area appears to be involved in selecting movements based on remembered sequences of movements.

1. SMA responds to sequences of movements and to mental rehearsal of sequences of movements (Figure 3.13). Brain activity was measured in a PET scanner while subjects made simple and complex sequences of movement. When the movements were simple, such as a repetitive movement of a single digit, the primary motor cortex and the primary somatosensory cortex were activated on the contralateral hemisphere. When the subject was asked to perform a complex sequence of finger movements, the SMA was activated bilaterally, in addition to the contralateral primary motor and somatosensory cortex activation. Finally, when the subject was asked to remain still but to mentally rehearse the complex sequence of activity, the SMA was still active, even though the primary motor and somatosensory cortex areas were silent. Thus, the SMA appears to be involved in bilateral movements and in the mental rehearsal of these movements.
   
   

   Figure 3.13 Positron emission tomography (PET) study of simple vs. complex finger movements (Roland et al.,1980). The SMA is activated bilaterally when subjects perform complex movements, and even when they only imagine performing the movements.

   
   
2. SMA is involved in the transformation of kinematic to dynamic information. Movements can be defined in terms of dynamics (the amount of force necessary to make a movement) and kinematics (the distance and angles that define a particular movement in space). Many movement plans are represented in kinematic terms (e.g., Move the hand to the left). However, the motor system must eventually translate this to a representation based on dynamics, in order to instruct the appropriate muscles to contract with the appropriate force. Recordings from monkeys have shown that during the preparatory delay before a monkey makes an instructed movement, some SMA neurons change their firing correlates from a kinematic-based representation to a dynamics-based representation, suggesting that SMA plays a vital role in this transformation.