The bi-directional hypothesis of language and action proposes that the sensorimotor and language comprehension areas of the brain exert reciprocal influence over one another. This hypothesis argues that areas of the brain involved in movement and sensation, as well as movement itself, influence cognitive processes such as language comprehension. In addition, the reverse effect is argued, where it is proposed that language comprehension influences movement and sensation. Proponents of the bi-directional hypothesis of language and action conduct and interpret linguistic, cognitive, and movement studies within the framework of embodied cognition and embodied language processing. Embodied language developed from embodied cognition, and proposes that sensorimotor systems are not only involved in the comprehension of language, but that they are necessary for understanding the semantic meaning of words.
Development of the bi-directional hypothesis
The theory that sensory and motor processes are coupled to cognitive processes stems from action-oriented models of cognition. These theories, such as the embodied and situated cognitive theories, propose that cognitive processes are rooted in areas of the brain involved in movement planning and execution, as well as areas responsible for processing sensory input, termed sensorimotor areas or areas of action and perception. According to action-oriented models, higher cognitive processes evolved from sensorimotor brain regions, thereby necessitating sensorimotor areas for cognition and language comprehension. With this organization, it was then hypothesized that action and cognitive processes exert influence on one another in a bi-directional manner: action and perception influence language comprehension, and language comprehension influences sensorimotor processes.
Although studied in a unidirectional manner for many years, the bi-directional hypothesis was first described and tested in detail by Aravena et al. These authors utilized the Action-Sentence Compatibility Effect (ACE), a task commonly used to study the relationship between action and language, to test the effects of performing simultaneous language comprehension and motor tasks on neural and behavioral signatures of movement and language comprehension. These authors proposed that these two tasks cooperate bi-directionally when compatible, and interfere bi-directionally when incompatible. For example, when the movement implied by the action language stimuli is compatible with the movement being performed by the subject, it was hypothesized that performance of both tasks would be enhanced. Neural evidence of the bi-directional hypothesis was demonstrated by this study, and the development of this hypothesis is ongoing.
Effects of language comprehension on systems of action Language comprehension tasks can exert influence over systems of action, both at the neural and behavioral level. This means that language stimuli influence both electrical activity in sensorimotor areas of the brain, as well as actual movement.
Neural activation Language stimuli influence electrical activity in sensorimotor areas of the brain that are specific to the bodily association of the words presented. This is referred to as semantic somatotopy, which indicates activation of sensorimotor areas that are specific to the bodily association implied by the word. For example, when processing the meaning of the word "kick," the regions in the motor and somatosensory cortices that represent the legs will become more active. Boulenger et al. demonstrated this effect by presenting subjects with action-related language while measuring neural activity using fMRI. Subjects were presented with action sentences that were either associated with the legs (e.g. "John kicked the object") or with the arms (e.g. "Jane grasped the object"). The medial region of the motor cortex, known to represent the legs, was more active when subjects were processing leg-related sentences, whereas the lateral region of the motor cortex, known to represent the arms, was more active with arm-related sentences. This body-part-specific increase in activation was exhibited about 3 seconds after presentation of the word, a time window that is thought to indicate semantic processing.Humphries, C., Binder, J.R., Medler, D.A., Liebenthal, E. (2007). "Time-course of semantic processes during sentence comprehension: an fMRI study." Neuroimage. 36: 924-932. In other words, this activation was associated with subjects comprehending the meaning of the word. This effect held true, and was even intensified, when subjects were presented with idiomatic sentences. Abstract language that implied more figurative actions were used, either associated with the legs (e.g. "John kicked the habit") or the arms (e.g. "Jane grasped the idea"). Increased neural activation of leg motor regions were demonstrated with leg-related idiomatic sentences, whereas arm-related idiomatic sentences were associated with increased activation of arm motor regions. This activation was larger than that demonstrated by more literal sentences (e.g. "John kicked the object"), and was also present in the time window associated with semantic processing.
Action language not only activates body-part-specific areas of the motor cortex, but also influences neural activity associated with movement. This has been demonstrated during an Action-Sentence Compatibility Effect (ACE) task, a common test used to study the relationship between language comprehension and motor behavior. This task requires the subject to perform movements to indicate understanding of a sentence, such as moving to press a button or pressing a button with a specific hand posture, that are either compatible or incompatible with movement implied by the sentence. For example, pressing a button with an open hand to indicate understanding of the sentence "Jane high-fived Jack" would be considered a compatible movement, as the sentence implies an open-handed posture. Motor potentials (MP) are an Event Related Potentials (ERPs) stemming from the motor cortex, and are associated with execution of movement. Enhanced amplitudes of MPs have been associated with precision and quickness of movements. Re-afferent potentials (RAPs) are another form of ERP, and are used as a marker of sensory feedback and attention. Both MP and RAP have been demonstrated to be enhanced during compatible ACE conditions. These results indicate that language can have a facilitory effect on the excitability of neural sensorimotor systems. This has been referred to as semantic priming, indicating that language primes neural sensorimotor systems, altering excitability and movement.
Movement The ability of language to influence neural activity of motor systems also manifests itself behaviorally by altering movement. Semantic priming has been implicated in these behavioral changes, and has been used as evidence for the involvement of the motor system in language comprehension. The Action-Sentence Compatibility Effect (ACE) is indicative of these semantic priming effects. Understanding language that implies action may invoke motor facilitation, or prime the motor system, when the action or posture being performed to indicate language comprehension is compatible with action or posture implied by the language. Compatible ACE tasks have been shown to lead to shorter reaction times. This effect has been demonstrated on various types of movements, including hand posture during button pressing, reaching, and manual rotation.
Language stimuli can also prime the motor system simply by describing objects that are commonly manipulated. In a study performed by Masson et al., subjects were presented with sentences that implied non-physical, abstract action with an object (e.g. "John thought about the calculator" or "Jane remembered the thumbtack"). After presentation of language stimuli, subjects were cued to perform either functional gestures, gestures typically made when using the object described in the sentence (e.g. poking for calculator sentences), or a volumetric gesture, gestures that are more indicative of whole hand posture (e.g. horizontal grasp for calculator sentences). Target gestures were either compatible or incompatible with the described object, and were cued at two different time points, early and late. Response latencies for performing compatible functional gestures significantly decreased at both time points, whereas latencies were significantly lower for compatible volumetric gestures in the late cue condition. These results indicate that descriptions of abstract interactions with objects automatically (early time point) generate motor representations of functional gestures, priming the motor system and increasing response speed. The specificity of enhanced motor responses to the gesture-object interaction also highlights the importance of the motor system in semantic processing, as this enhanced motor response was dependent on the meaning of the word.
A study performed by Dr. Olmstead et al., described in detail elsewhere, demonstrates more concretely the influence that the semantics of action language can have on movement coordination. Briefly, this study investigated the effects of action language on the coordination of rhythmic bimanual hand movements. Subjects were instructed to move two pendulums, one with each hand, either in-phase (pendulums are at the same point in their cycle, phase difference of roughly 0 degrees) or anti-phase (pendulums are at the opposite point in their cycle, phase difference of roughly 180 degrees). Robust behavioral studies have revealed that these two phase states, with phase differences 180 and 0 degrees, are the two stable relative phase states, or the two coordination patterns that produce stable movement. This pendulum swinging task was performed as subjects judged sentences for their plausibility; subjects were asked to indicate whether or not each presented sentence made logical sense. Plausible sentences described actions that could be performed by a human using the arms, hands, and/or fingers ("He is swinging the bat"), or actions that could not be performed ("The barn is housing the goat"). Implausible sentences also used similar action verbs ("He is swinging the hope"). Plausible, performable sentences lead to a significant change in the relative phase shift of the bimanual pendulum task. The coordination of the movement was altered by action language stimuli, as the relative phase shift that produced stable movement was significantly different than in the non-performable sentence and no language stimuli conditions. This development of new stable states has been used to imply a reorganization of the motor system utilized to plan and execute this movement, and supports the bi-directional hypothesis by demonstrating an effect of action language on movement.
Effects of systems of action on language comprehension The bi-directional hypothesis of action and language proposes that altering the activity of motor systems, either through altered neural activity or actual movement, influences language comprehension. Neural activity in specific areas of the brain can be altered using transcranial magnetic stimulation (TMS), or by studying patients with neuropathologies leading to specific sensory and/or motor deficits. Movement is also used to alter the activity of neural motor systems, increasing overall excitability of motor and pre-motor areas.
Neural activation