Premovement neuronal activity

Research of pre-movement neuronal activity generally involves studying two different kinds of movement, movement in natural settings versus movement triggered by a sensory stimulus. These two types of movements are referred to with different nomenclature throughout different studies and literature on the topic of premovement neuronal activity. Voluntary movements are also known as self-timed, self-initiated, self-paced, and non-triggered movements. This type of movement is what generally occurs in natural settings, carried out independently of a sensory cue or external signal which would trigger or cause the movement to be performed.^[1] In contrast, movements that are carried out as a result of a sensory cue or stimulus, or reflex-reactions to external conditions or changes are called reactive movements, but also known as cued movements, stimulated movements, and externally triggered movements depending on the choice of a particular study. In one such study by Lee and Assad (2003), rhesus monkeys were trained to execute arm movement in response to a visual cue versus the same arm movement performed without any correlation to this external (visual) cue. This is one example of reactive movements in contrast to self-initiated movements.^[1] Subsequent studies of rates of neuronal firing in the respective types of movements are recorded in different areas of the brain in order to develop a more thorough understanding of premovement neuronal activity.

Functions in:

• Decision making
• Response selection with move
• Timing of movement
• Initiation/suppression of action^[2]

Functions in:

• Preparatory processes^[2]

Functions in:

• Initiation of movement
• Execution of movement^[2]

In 1964, two movement related cortical potentials were discovered by Kornhuber and Deecke. Using both the electroencephalography (EEG) and the electromyogram (EMG) recordings, Kornhuber and Deecke were able to identify two components prior to movement onset.^[3] These components are the Bereitschaftspotential (abbreviated BP, and also known as readiness potential, abbreviated RP) and the Contingent Negative Variation (CNV). The difference between these two potentials is that the BP is involved in self-paced, or voluntary movements, whereas the CNV is involved with cued movements, movements performed as reactions to an environmental signal.

The Bereitschaftspotential is a movement related potential. The initiation of the BP occurs approximately 2 seconds prior to movement onset. The BP is an index of motor preparation and is therefore also referred to as the "readiness potential", as it is the potential for movement to occur. The initial stage of the BP, or readiness potential, is an unconscious intention of, and preparation for movement. After this initial stage, the preparation of movement becomes a conscious thought.^[2]

The BP, more specifically, is composed of movement related cortical potentials (MRCPs)^[4] the peak being the MP or Motor potential. MRCPs tend to resemble a "set of plans" used by the cortex for the generation and control of movement. The BP is activated by voluntary movements involving the SMA and the somatosensory cortex in movement preparation and initiation. Initially only the late BP was considered to be specific for the site of movement and the early BP was thought to be characterized by more general preparation for upcoming movements. However, over the past couple of decades the early BP is considered to perhaps also be site specific within the supplementary motor area (SMA) and the lateral premotor cortex.^[3] Using principal component analysis and functional magnetic resonance imaging (fMRI) the main source of early BP was determined to be Area 6 of the precentral gyrus bilaterally, and the main sources of late BP were determined to be Area 4 (also known as the Primary Motor Cortex) and Area 6. The current consensus is that the early BP starts first in the SMA, including pre-SMA and SMA proper, and then approximately 400ms later in the lateral premotor cortices bilaterally prior to the movement onset, and the late BP starts in the M1 and premotor cortex contralaterally.^[3]

The two factors that most greatly influence the BP are the effect of discreteness and complexity of movement. A study conducted in 1993 compared isolated extensions of the middle finger with simultaneous extensions of the middle and index fingers. The results showed that the isolated movement of the middle finger produced a larger amplitude in the late BP, but not the early BP. The amplitude difference in the late BP was seen over the central region contralateral to the movement, which suggests an important role of M1.^[5] Complex movements cause greater amplitudes of the BP, which reflects the fact that there is greater activation of the SMA. Further experiments also suggest that the bilateral sensorimotor cortices play a role in the preparation of complex movements, along with the SMA.^[3]

Some of the first relevant experimentation and subsequent findings about the organization of the primary motor cortex were observed by Wilder Penfield. Penfield, a neurosurgeon in Montreal began his experimentation in the 1950s, to better serve his epileptic patients. Penfield understood that his epileptic patients experience a warning sign before the seizures occur. This knowledge started the beginning of his stimulation experimentations. Here, Penfield tried to induce this warning sign in an attempt to specifically pinpoint the source of epilepsy. Penfield confirmed the presence of a spatial map of the contralateral body of the brain. He noted the location of muscle contractions with the site of electro-stimulation on the surface of the motor cortex and subsequently mapped the motor representation of the pre-central gyrus. This follows the same trends and disproportions in the somatic sensory maps in the post central gyrus.

Experimentation via intra-cortical micro-stimulation brought about a more detailed understanding of motor maps. By injecting current via a sharpened tip of a microelectrode into the cortex, the upper motor neurons in layer 5, which project to lower motor neurons can be stimulated. These neurons are associated with the neurons in the spinal cord, and thus stimulating specific movements which occur in specified muscular regions rather than stimulating specific muscles which produce those movements. Neuron connections in the motor map are linked for the purpose of generating specific movements. These connections are not linked for the purpose of generating specific muscles movements or contractions.^[6]

Spike-triggered averaging is a way to measure the activity of one cortical motor neuron, on a group of lower motor neurons in the spinal cord. Experimentation confirmed that single upper motor neurons are connected to multiple lower motor neurons. This supports the general conclusion that movements and not individual muscles are controlled by the activity of upper motor neurons.^[7]

Individual motor neurons were recorded using implanted microelectrodes to record their activity in awake and behaving monkeys. This experimentation provided a way to figure out the correlation between neuronal activity and voluntary movement. It was found that the force generated by contracting muscles changed as a function of the firing rate of upper motor neurons. The firing rates of the active neurons often change prior to movements involving very small forces. This suggests that, the primary motor cortex contributes to the initial phase of the recruitment of lower motor neurons, involved in the generation of finely controlled movements.^[6]

Approximately 65% of the neurons in the pre-motor cortex are responsible for conditional "closed-loop" motor tasks. In experimentation using monkeys, when they were trained to reach in different directions, depending on the specified visual cue, the approximately coordinated lateral pre-motor neurons began to fire at the appearance of that specified cue, but before the actual signal to perform the movement. As learning takes place, to associate a new visual cue with a particular movement, the approximately coordinated neurons increase their rate of fire during the time between the initial specified cue and the actual signal for the initiation of the movement. It now seems that these specific neurons do not command the initiation of the movements but the intention to perform the movements. Thus these pre-motor neurons are especially involved in the selection of movements based on external events.

More evidence that the lateral pre-motor area is involved in movement selection comes from observations of the effects of cortical damage on motor behavior. Lesions to this area severely impair the ability of monkeys to perform visually cued conditional tasks. Meaning that on command, it becomes extremely difficult for the monkey to perform the trained movement. But, when placed in another setting, the monkey is perfectly capable of performing that movement in a spontaneous, self initiated manner, as a response to the same visual stimulus.

The medial pre-motor cortex seems to be specialized for initiating movements specified by internal rather than external cues. These movements based on internal events are called "open-loop" conditions. In contrast to lesions in the lateral pre-motor area, removal of this medial pre-motor area reduces the number of self initiated or spontaneous movements that the animal makes. Conversely, the ability to move in response to an external cue is largely intact.^[7]

The parietal cortex plays a role in the internal command of actions. Most specifically, parietal area 5 is responsible for the actions which precede movement. Area 5 neurons exhibit pre-movement activity in response to self initiated movements. The neurons in area 5 play a role in the initiation and execution of movement and respond at enormously quick speeds. An EMG (electromyogram) is a test of electrical activity in muscles. The neurons in area 5 respond at least 100ms faster than EMG detectable activity allows. The cerebral cortex forms a series of loops with the basal ganglia and the cerebellum which drive the initiation of movements, via these positive feedback loops. The neurons on the parietal associative cortex are most strongly involved in programming and execution of voluntary movements.^[8]

A learned act is the movement which is produced when the starting sensory signal launches the programmed execution. This action requires the neurons of the parietal associative cortex. There are two phases of the readiness potential, the early phase and the late phase. The early phase is responsible for the "planning of programmed movements". The late phase is responsible for the "stimulation of the movement’s direct implementation." The early phase of the readiness potential occurs in the supplementary motor region and is involved in the generation of voluntary movement. The late phase of premovement occurs in the cortical regions and is involved in definite voluntary movements. The two formal stages of premovement are planning and initiation.^[9]

Mirror motor neurons are found in the ventrolateral portion of the pre-motor cortex. These mirror motor neurons respond not only to the preparation for movement execution, but also to observation of the same movements by others. But, these mirror motor neurons do not respond as well when an action is being pantomimed without the presence of a motor goal. Additionally, in observations of goal oriented movements, these neurons fire even when the result is blocked from view. The mirror motor neuron system is responsible for encoding intention and relevant behaviors of others. Additionally, these neurons may play a role with the frontal and parietal lobes in imitation learning.^[6]

A study by Daniel Glaser involved dancers trained in ballet and those trained in capoeira (Brazilian martial art form). The dancers were shown a short video of both ballet and capoeira dance moves. The research indicated that the mirror motor neurons showed increased activity when the dancers watched the video for the style they have been trained in. Additionally, the control, non-dancers, showed significantly less brain activation in the mirror motor neurons when watching either type of dance.^[10]

This research provides insight into how the brain responds to movements you have personally learned to do. This may provide a way to allow professionals to maintain a skill without actually performing the movement. Or it may provide an outlet for mental rehabilitation to those with impaired motor skills. Simple observation of the movement allows the same type of brain stimulation as the actual physical movement.^[10]

Execution of certain motor tasks requires an instructed delay. This delay period occurs in between the instructed cue and the subsequently triggered movement. During these delay periods preparatory changes occur in neuronal activity. The primary motor cortex, the pre-motor cortex, the supplementary motor area, parietal cortex,^[11] and the basal ganglia all may experience these preparatory delay periods. These activities coordinate during the delay periods and reflect movement planning in accordance with the instructional cue and the subsequent movement but occur prior to muscle activity.^{[12][13][14][11]} The movement planning may be anything from the direction of the movement to the extent of the movement.^[15]

Premovement neuronal activity has been widely experimented upon in three major motor fields of the frontal cortex. The goal of this experimentation is to compare the neuronal activity which comes from visual signals, versus neuronal activity which comes from non-triggered or self-paced movements. From this comparison, two changes were identified, occurring at different time scales in relation to the onset of movement. These changes are the short lead and long lead changes. The short lead changes are observed about 480ms before the movement, whereas the long lead changes occur about 1–2 seconds earlier. The short lead changes are exhibited in the SMA (supplementary motor area) and the PM (pre-motor area) during both the visual signal trials and the non-triggered/self-paced trials. The pre-central motor cortex was also identified in this study as having similar neuronal activities as in the PM and SMA. Experimentation found that approximately 61% of the neurons in the PM were preferentially related to the triggered (visual) movements. The long lead neuronal changes were more frequently active during the self paced stimuli than before the triggered movements. These long lead changes are particularly abundant among the SMA neurons. In summation, these experiments challenged the idea that the SMA primarily takes part in self-paced movements and the PM is only involved in visually triggered movements. Although the PM neurons showed more preference for the visual trigger signals and the SMA neurons are intimately related to initiation of self paced movements, both are involved with premovement for both types of stimuli.^[16]