How do motor control and action work? - Chapter 12

Why are action and motor control relevant functions for a neuropsychologist?

The motor skills are important for the neuropsychologist, because:

• Refined motor skills are indispensable for neuropsychological examinations and motor impairments can negatively influence test performance.

• Motor impairments often occur after brain injury.

• Motor control is a cognitive process.

Although there is an enormous amount of motor movements, this chapter is limited in particular to the most relevant movements for the neuropsychologist: hand movements and arm movements.

How is the motor system organized?

Three hierarchical functional levels are known: reflexes, automated movements (such as the posture), and voluntary movements. There are also three hierarchical neuroanatomical levels: activation of the muscles by neurons in the spinal cord; the activity of these neurons is influenced by brainstem nuclei; in turn, they are influenced by the motor areas of the cerebral cortex.

Activated muscles contract (contraction) in order to perform movements. Muscles usually work in pairs (agonist-antagonist). The muscles receive signals via motor neurons on the ventral side (facing toward the stomach) of the spinal cord. First, these motor neurons receive sensory information from muscles via muscle spindles that provide information about the contraction state. Second, they receive input from other motor neurons and third, they receive input from the cerebral cortex, and in particular the brainstem. The main projections from the brainstem to the spinal cord are the 'reticulospinal', 'tectospinal', and 'vestibulospinal' pathways. These neural pathways are related to postural control and the activation of proximal arm and leg muscles. The most important cortical area involved in motor control is the primary motor cortex, but the premotor cortex and the supplementary motor area in the frontal lobe are also important. In humans, 60% of corticospinal projections originate from the primary motor cortex. This area contains a representation of the body (homunculus). In this representation of the body, the lips, hands, and fingers cover a relatively large area, since these functions require more sophisticated control. Two types of corticospinal projections can be distinguished:

There are neural pathways that terminate on interneurons in the spinal cord. The interneurons pass information to the motor neurons. These projections are mainly (but not exclusively) contralateral. This route mainly controls the muscles involved in movements of the torso, shoulders, and upper arm.

Corticomotoneuronal pathways: these pathways project directly onto the motor neurons and mainly control distal movements such as individual finger movements. These neurons are more vulnerable after brain injury.

How are movements being represented?

There are two important aspects in controlling movements. There must be a representation of movement in the central nervous system. On the basis of sensory information, we gain knowledge about the movement and the objects that the movement has to deal with. Propriocepsis is essential for movement: receptors in our muscles, skin and joints provide information about our positions, changes in positions, and bodily movements.

Mirror neurons

Mirror neurons are not only activated when we perform an action ourselves, but also when we see the same action being performed by someone else (Di Pellegrino and colleagues, 1992). Some of these neurons are more sensitive to a precision grip (a grip with the thumb and forefinger); others are more sensitive to a "power grip" (a grip with the whole hand). The above information has been demonstrated in monkey research; people do not have such neurons, but they do have homologous areas. For example, the posterior parietal cortex is nowadays seen as an important part of this ''mirror neuron system": neurons in this area have both visual and motor properties. Perhaps the most obvious suggestion about the functional importance of this system, is that it is a vocabulary of movement representation (Rizzolatti & Arbib, 1998). It also has a functional importance in the context of social cognition and empathy (Keysers & Gazzola, 2006).

Forward models

According to the 'forward' model, the (sensory) consequences of an action are predicted by an internal model within the motor system. This predicts, for example, how quickly you must perform the movement. According to Frith, Blakemore and Wolpert (2000), the delusions of control in schizophrenia are at least partly caused by a disruption in the forward model.

What is motor planning?

Cognitive motor plans are prepared prior to movements. It is still unclear what exactly is being planned and at what level this is taking place. Some models assume a schedule of movement execution; other models do not focus on the movement, but on their ultimate goal. The 'end-state comfort' principle belongs to the latter category. Within this principle, the purpose of the movement is first determined and subsequently the movement towards this purpose.

Motor imagery

According to the concept of 'motor imagery', the imagined movements are the same as the underlying motor representations of the movements. 'Fitt's law' states that it takes longer to point to a circle if the circle is smaller than if the circle is larger. This effect is also observed when the pointing movement is carried out only in the imagination (Jeannerod, 1997). Although motor disorders affect both output and imagination movements to the same extent, there are also dissociations: sometimes a patient can execute movements but cannot imagine it or vice versa.

How are targets (and movement targets) represented?

The posterior parietal lobe is strongly involved in linking the sensory information of our environment to the motor system in order to be able to interact effectively with the environment. The visual system is the most important sense that provides information about our environment. The ventral route (what route) ends in the inferior part of the temporal cortex; the dorsal route (how route) ends in the superior part of the posterior parietal lobe. The dorsal route is therefore involved in executing visually controlled movements. This route can be divided in:

The dorso-dorsal route processes somatosensory and visual information to guide targeted reach and grip movements;

The ventro-dorsal route (in the inferior posterior parietal lobe) understands and recognizes movements, understands and recognizes the function of objects and plays a major role in spatial perception (think of "neglect"). It is based on semantic and conceptual input of both movements and objects.

The posterior parietal lobe is also related to proprioception and the integration of proprioceptive and tactile information. The ventral route also plays a role in visuomotor activity: this route is involved in the control of movements if the goal of the movement can only be remembered because it is no longer visible (Goodale, Jakobson & Keillor, 1994).

What impairments are possible within action and motor skills?

The body schema (a body representation focused on action) is often distinguished from the body image (a body representation focused on perception). The body scheme processes sensory information in a bottom-up way, and is not sufficient for the conscious sensation (as demonstrated by body-related illusions such as the "rubber hand illusion"). The areas of the brain that are related to the body schema are the superior posterior parietal lobe and premotor areas.

Cerebral paresis and hemiplegia

An umbrella term for a group of chronic, non-progressive impairments in motor control and muscle coordinations is 'cerebral paresis' (CP). Patients with a CP experience many problems in everyday motor skills. Damage in the motor areas is already present prenatal, perinatal or in the first year of life. CP is purely motor, but has a high comorbidity with other kinds of disorders such as epilepsy or vision disorders. About 75% of patients have spastic CP; 20% have dyskinetic CP (constant movements as a result of continuous changes in muscle tension); 5% suffer from ataxic CP (irregular movements). The group with spastic CP can be divided into two categories: unilateral spastic CP (hemiparesis) or a bilateral spastic CP (both arms or legs are affected). 'Developmental coordination disorder' (DCD; formerly also known as 'minimal cerebral palsy') is another motor disorder in which the patient experiences motor impairments without any demonstrable neurological abnormality (in contrast to CP). The motor impairments in DCD are less severe than in CP.

Apraxia

Patients with apraxia have an inability to carry out purposeful behaviour in the absence of a paralysis or paresis. They have problems executing movements on command, imitating movements, using items such as stools, making gestures, and planning and implementing various movements one after another (movement sequences). Apraxia is associated with lesions in different neural areas, but usually there is a lesion in the left parietal lobe. The two best-known types are 'ideomotor' and 'ideational' apraxia. In ideomotor apraxia the patient cannot execute movements based on instruction or imitation. In the case of ideational apraxia, the patient cannot execute movement sequences. Other types of apraxia are: conceptual apraxia, conduct apoxia, constructive apraxia, limb apraxia, and tactile apraxia. Various models of sensorimotor information processing with regard to apraxia suggest that there are two routes to the motor system that, if damaged, can lead to apraxia: a direct route visual route and an indirect auditory and visual route. According to Goldenberg (2009), damage to the left posterior parietal cortex is related to a disorder in the imitation of meaningless gestures and disturbed use of objects.

Optic ataxia and alien hand syndrome

Optic ataxia - the inability to make saccades based on verbal instructions - is related to injury to the visual dorsal (or dorsodorsal) route. Patients with 'alien hand syndrome' (AHS) feel as if someone else is in control of their hand movements (Kikkert, RIbbers & Koudstaal, 2006). After selective damage to the corpus callosum, the left hand is almost always affected. A distinction is made between two types of AHS:

The anterior frontal AHS is caused by damage to the supplementary motor area (SMA). The core symptoms are the compulsive tactile exploration and the grasp reflex. Probably there is a disorder in the top-down control (the bottom-up movements, or movements provoked by external stimuli, are still intact).

The posterior AHS can occur after a corticobasal degeneration or with a subcortical (thalamus) lesion. The core symptoms here are a feeling of alienation, less complex movements and hostile movements.