Sensory-motor coupling is the coupling or integration of the sensory system and motor system. Sensorimotor integration is not a static process. For a given stimulus, there is no one single motor command. "Neural responses at almost every stage of a sensorimotor pathway are modified at short and long timescales by

biophysical Biophysics is an interdisciplinary science that applies approaches and methods traditionally used in physics to study biological phenomena. Biophysics covers all scales of biological organization, from molecular to organismic and populations. Bi ...

and synaptic processes, recurrent and

feedback Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...

connections, and

learning Learning is the process of acquiring new understanding, knowledge, behaviors, skills, value (personal and cultural), values, attitudes, and preferences. The ability to learn is possessed by humans, animals, and some machine learning, machines ...

, as well as many other internal and external variables".

Overview

The integration of the sensory and motor systems allows an animal to take sensory information and use it to make useful motor actions. Additionally, outputs from the motor system can be used to modify the sensory system's response to future stimuli. To be useful it is necessary that sensory-motor integration be a flexible process because the properties of the world and ourselves change over time. Flexible sensorimotor integration would allow an animal the ability to correct for errors and be useful in multiple situations. To produce the desired flexibility it's probable that

nervous system In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes th ...

s employ the use of internal models and efference copies.

Transform sensory coordinates to motor coordinates

Prior to movement, an animal's current sensory state is used to generate a motor command. To generate a motor command, first, the current sensory state is compared to the desired or target state. Then, the nervous system transforms the sensory coordinates into the motor system's coordinates, and the motor system generates the necessary commands to move the muscles so that the target state is reached.

Efference copy

An important aspect of sensorimotor integration is the efference copy. The efference copy is a copy of a motor command that is used in internal models to predict what the new sensory state will be after the motor command has been completed. The efference copy can be used by the nervous system to distinguish self-generated environmental changes, compare an expected response to what actually occurs in the environment, and to increase the rate at which a command can be issued by predicting an organism's state prior to receiving sensory input.

Internal model

An internal model is a theoretical model used by a nervous system to predict the environmental changes that result from a motor action. The assumption is that the nervous system has an internal representation of how a motor apparatus, the part of the body that will be moved, behaves in an environment. Internal models can be classified as either a forward model or an inverse model.

Forward model

A forward model is a model used by the nervous system to predict the new state of the motor apparatus and the sensory stimuli that result from a motion. The forward model takes the efference copy as an input and outputs the expected sensory changes. Forward models offer several advantages to an organism. Advantages: * The estimated future state can be used to coordinate movement before sensory feedback is returned. * The output of a forward model can be used to differentiate between self-generated stimuli and non-self-generated stimuli. * The estimated sensory feedback can be used to alter an animal's

perception Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous system ...

related to self-generated motion. * The difference between the expected sensory state and sensory feedback can be used to correct errors in movement and the model. *

Inverse model

An inverse model behaves oppositely of a forward model. Inverse models are used by nervous systems to estimate either the motor command that caused a change in sensory information or to determine the motor command that will reach the target state.

Examples

Gaze stabilization

During flight, it is important for a fly to maintain a level gaze; however, it is possible for a fly to rotate. The rotation is detected visually as a rotation of the environment termed optical flow. The input of the optical flow is then converted into a motor command to the fly's neck muscles so that the fly will maintain a level gaze. This

reflex In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus. Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...

is diminished in a stationary fly compared to when it is flying or walking.

Singing crickets

Male crickets sing by rubbing their forewings together. The sounds produced are loud enough to reduce the cricket's

auditory system The auditory system is the sensory system for the sense of hearing. It includes both the sensory organs (the ears) and the auditory parts of the sensory system. System overview The outer ear funnels sound vibrations to the eardrum, increasin ...

's response to other sounds. This desensitization is caused by the hyperpolarization of the Omega 1 neuron (ON1), an auditory

interneuron Interneurons (also called internuncial neurons, relay neurons, association neurons, connector neurons, intermediate neurons or local circuit neurons) are neurons that connect two brain regions, i.e. not direct motor neurons or sensory neurons. I ...

, due to

activation Activation, in chemistry and biology, is the process whereby something is prepared or excited for a subsequent reaction. Chemistry In chemistry, "activation" refers to the reversible transition of a molecule into a nearly identical chemical or ...

by auditory stimulation. To reduce self-desensitization, the cricket's thoracic

central pattern generator Central pattern generators (CPGs) are self-organizing biological neural circuits that produce rhythmic outputs in the absence of rhythmic input. They are the source of the tightly-coupled patterns of neural activity that drive rhythmic and stereot ...

sends a

corollary discharge In physiology, an efference copy or efferent copy is an internal copy of an outflowing ('' efferent''), movement-producing signal generated by an organism's motor system.Jeannerod, Marc (2003): "Action Monitoring and Forward Control of Movements". ...

, an efference copy that is used to inhibit an organism's response to self-generated stimuli, to the auditory system. The corollary discharge is used to inhibit the auditory system's response to the cricket's own song and prevent desensitization. This inhibition allows the cricket to remain responsive to external sounds such as a competing male's song.

Speech

Sensorimotor integration is involved in the development, production, and perception of speech.

Speech development

Two key elements of speech development are babbling and audition. The linking of a motor action to a heard sound is thought to be learned. One reason for this is that deaf infants do not canonically babble. Another is that an infant's

is known to be affected by his babbling. One model of speech development proposes that the sounds produced by babbling are compared to the sounds produced in the language used around the infant and that association of a motor command to a sound is learned.

Speech production

Audition plays a critical role in the production and maintenance of speech. As an example, people who experience adult-onset deafness become less able to produce accurate speech. This decline is because they lack auditory feedback. Another example is acquisition of a new accent as a result of living in an area with a different accent. These changes can be explained through the use of a forward model. In this forward model, the motor cortex sends a motor command to the

vocal tract The vocal tract is the cavity in human bodies and in animals where the sound produced at the sound source (larynx in mammals; syrinx (biology), syrinx in birds) is filtered. In birds it consists of the Vertebrate trachea, trachea, the Syrinx (bio ...

and an efference copy to the internal model of the vocal tract. The internal model predicts what sounds will be produced. This prediction is used to check that the motor command will produce the goal sound so that corrections may be made. The internal model's estimate is also compared to the produced sound to generate an error estimate. The error estimate is used to correct the internal model. The updated internal model will then be used to generate future motor commands.

Speech perception

Sensorimotor integration is not critical to the perception of speech; however, it does perform a modulatory function. This is supported by the fact that people who either have impaired speech production or lack the ability to speak are still capable of perceiving speech. Furthermore, experiments in which motor areas related to speech were stimulated altered but did not prevent the perception of speech.

Patient R.W.

Patient R.W. was a man who suffered damage in his parietal and occipital lobes, areas of the brain related to processing visual information, due to a

stroke A stroke is a medical condition in which poor blood flow to the brain causes cell death. There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. Both cause parts of the brain to stop functionin ...

. As a result of his stroke, he experienced vertigo when he tried to track a moving object with his eyes. The vertigo was caused by his brain interpreting the world as moving. In normal people, the world is not perceived as in moving when tracking an object despite the fact that the image of the world is moved across the

retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...

as the eye moves. The reason for this is that the brain predicts the movement of the world across the retina as a consequence of moving the eyes. R.W., however, was unable to make this prediction.

Disorders

Parkinson's

Patients with

Parkinson's disease Parkinson's disease (PD), or simply Parkinson's, is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly, and as the disease worsens, non-motor symptoms becom ...

often show symptoms of bradykinesia and

hypometria Dysmetria ( en, wrong length) is a lack of coordination of movement typified by the undershoot or overshoot of intended position with the hand, arm, leg, or eye. It is a type of ataxia. It can also include an inability to judge distance or scale. ...

. These patients are more dependent on external cues rather than proprioception and

kinesthesia Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense". Proprioception is mediated by proprioceptors, mechanosensory neurons ...

when compared to other people. In fact, studies using external vibrations to create proprioceptive errors in movement show that Parkinson's patients perform better than healthy people. Patients have also been shown to underestimate the movement of limb when it was moved by researchers. Additionally, studies on

somatosensory evoked potentials An evoked potential or evoked response is an electrical potential in a specific pattern recorded from a specific part of the nervous system, especially the brain, of a human or other animals following presentation of a stimulus such as a light fla ...

have evidenced that the motor problems are likely related to an inability to properly process the sensory information and not in the generation of the information.

Huntington's

Huntington's Huntington's disease (HD), also known as Huntington's chorea, is a neurodegenerative disease that is mostly inherited. The earliest symptoms are often subtle problems with mood or mental abilities. A general lack of coordination and an unst ...

patients often have trouble with

motor control Motor control is the regulation of movement in organisms that possess a nervous system. Motor control includes reflexes as well as directed movement. To control movement, the nervous system must integrate multimodal sensory information (both f ...

. In both quinolinic models and patients, it has been shown that people with Huntington's have abnormal sensory input. Additionally, patients have been shown to have a decrease in the inhibition of the

startle reflex In animals, including humans, the startle response is a largely unconscious defensive response to sudden or threatening stimuli, such as sudden noise or sharp movement, and is associated with negative affect.Rammirez-Moreno, David. "A computation ...

. This decrease indicates a problem with proper sensorimotor integration. The " various problems in integrating sensory information explain why patients with HD are unable to control voluntary movements accurately."

Dystonia

Dystonia Dystonia is a neurological hyperkinetic movement disorder in which sustained or repetitive muscle contractions result in twisting and repetitive movements or abnormal fixed postures. The movements may resemble a tremor. Dystonia is often inten ...

is another motor disorder that presents sensorimotor integration abnormalities. There are multiple pieces of evidence that indicate

focal dystonia Focal dystonia, or focal task specific dystonia, is a neurological condition, a type of ''dystonia'', that affects a muscle or group of muscles in a specific part of the body during specific activities, causing involuntary muscular contractions and ...

is related to improper linking or processing of afferent sensory information in the motor regions of the brain. For example, dystonia can be partially relieved through the use of a

sensory trick Dystonia is a neurological hyperkinetic movement disorder in which sustained or repetitive muscle contractions result in twisting and repetitive movements or abnormal fixed postures. The movements may resemble a tremor. Dystonia is often intens ...

. A sensory trick is the application of a stimulus to an area near to the location affected by dystonia that provides relief.

Positron emission tomography Positron emission tomography (PET) is a functional imaging technique that uses radioactive substances known as radiotracers to visualize and measure changes in Metabolism, metabolic processes, and in other physiological activities including bl ...

studies have shown that the activity in both the supplementary motor area and

primary motor cortex The primary motor cortex (Brodmann area 4) is a brain region that in humans is located in the dorsal portion of the frontal lobe. It is the primary region of the motor system and works in association with other motor areas including premotor co ...

are reduced by the sensory trick. More research is necessary on sensorimotor integration dysfunction as it relates to non-focal dystonia.

Restless leg syndrome

Restless leg syndrome (RLS) is a sensorimotor disorder. People with RLS are plagued with feelings of discomfort and the urge to move in the legs. These symptoms occur most frequently at rest. Research has shown that the motor cortex has increased excitability in RLS patients compared to healthy people. Somatosensory evoked potentials from the stimulation of both posterior nerve and

median nerve The median nerve is a nerve in humans and other animals in the upper limb. It is one of the five main nerves originating from the brachial plexus. The median nerve originates from the lateral and medial cords of the brachial plexus, and has contr ...

are normal. The normal SEPs indicate that the RLS is related to abnormal sensorimotor integration. In 2010, Vincenzo Rizzo et al. provided evidence that RLS sufferers have lower than normal short latency afferent inhibition (SAI), inhibition of the motor cortex by afferent sensory signals. The decrease of SAI indicates the presence of abnormal sensory-motor integration in RLS patients.

References

{{DEFAULTSORT:Sensory-motor coupling Motor cognition Neurology Motor control Nervous system Sensory systems