Chapter 16 Apraxia

Some historical notes orienting towards brain mechanisms that could underlie hemispheric asymmetries

The first minor aim of this synthetical historical survey consisted in showing that the discovery of the internal organization of language within the left hemisphere has been mainly determined by theoretical models and cultural factors, whereas the discovery of the left lateralisation of language and of the right lateralization of emotions and of other cognitive and perceptual functions has been mainly determined by empirical observations. A second more relevant aim of the survey consisted in discussing historical and more recent data suggesting that the different lateralisation of language and emotions has influenced not only the asymmetrical representation of other cognitive, affective and perceptual functions, but also (thank to the shaping influence of language on human cognition) of asymmetries regarding more general aspects of thought (such as the distinctions between 'propositional vs automatic' and 'conscious vs unconscious' ways of functioning). In the last part of the review, these data will be included in a more general discussion, concerning the brain functions that could be subsumed by the right hemisphere for three main reasons: (a) to avoid conflicts with the language mediated activities of the left hemisphere; (b) because of unconscious and automatic aspects of its non-verbal organisation or (c) due to the competition for cortical space determined by the development of language within the left hemisphere.

Comprehension of Co-Speech Gestures in Aphasic Patients: An Eye Movement Study

BACKGROUND

Co-speech gestures are omnipresent and a crucial element of human interaction by facilitating language comprehension. However, it is unclear whether gestures also support language comprehension in aphasic patients. Using visual exploration behavior analysis, the present study aimed to investigate the influence of congruence between speech and co-speech gestures on comprehension in terms of accuracy in a decision task.

METHOD

Twenty aphasic patients and 30 healthy controls watched videos in which speech was either combined with meaningless (baseline condition), congruent, or incongruent gestures. Comprehension was assessed with a decision task, while remote eye-tracking allowed analysis of visual exploration.

RESULTS

In aphasic patients, the incongruent condition resulted in a significant decrease of accuracy, while the congruent condition led to a significant increase in accuracy compared to baseline accuracy. In the control group, the incongruent condition resulted in a decrease in accuracy, while the congruent condition did not significantly increase the accuracy. Visual exploration analysis showed that patients fixated significantly less on the face and tended to fixate more on the gesturing hands compared to controls.

CONCLUSION

Co-speech gestures play an important role for aphasic patients as they modulate comprehension. Incongruent gestures evoke significant interference and deteriorate patients' comprehension. In contrast, congruent gestures enhance comprehension in aphasic patients, which might be valuable for clinical and therapeutic purposes.

Different visual exploration of tool-related gestures in left hemisphere brain damaged patients is associated with poor gestural imitation

According to the direct matching hypothesis, perceived movements automatically activate existing motor components through matching of the perceived gesture and its execution. The aim of the present study was to test the direct matching hypothesis by assessing whether visual exploration behavior correlate with deficits in gestural imitation in left hemisphere damaged (LHD) patients. Eighteen LHD patients and twenty healthy control subjects took part in the study. Gesture imitation performance was measured by the test for upper limb apraxia (TULIA). Visual exploration behavior was measured by an infrared eye-tracking system. Short videos including forty gestures (20 meaningless and 20 communicative gestures) were presented. Cumulative fixation duration was measured in different regions of interest (ROIs), namely the face, the gesturing hand, the body, and the surrounding environment. Compared to healthy subjects, patients fixated significantly less the ROIs comprising the face and the gesturing hand during the exploration of emblematic and tool-related gestures. Moreover, visual exploration of tool-related gestures significantly correlated with tool-related imitation as measured by TULIA in LHD patients. Patients and controls did not differ in the visual exploration of meaningless gestures, and no significant relationships were found between visual exploration behavior and the imitation of emblematic and meaningless gestures in TULIA. The present study thus suggests that altered visual exploration may lead to disturbed imitation of tool related gestures, however not of emblematic and meaningless gestures. Consequently, our findings partially support the direct matching hypothesis.

Contribution of the posterior parietal cortex in reaching, grasping, and using objects and tools

Neuropsychological and neuroimaging data suggest a differential contribution of posterior parietal regions during the different components of a transitive gesture. Reaching requires the integration of object location and body position coordinates and reaching tasks elicit bilateral activation in different foci along the intraparietal sulcus. Grasping requires a visuomotor match between the object's shape and the hand's posture. Lesion studies and neuroimaging confirm the importance of the anterior part of the intraparietal sulcus for human grasping. Reaching and grasping reveal bilateral activation that is generally more prominent on the side contralateral to the hand used or the hemifield stimulated. Purposeful behavior with objects and tools can be assessed in a variety of ways, including actual use, pantomimed use, and pure imagery of manipulation. All tasks have been shown to elicit robust activation over the left parietal cortex in neuroimaging, but lesion studies have not always confirmed these findings. Compared to pantomimed or imagined gestures, actual object and tool use typically produces activation over the left primary somatosensory region. Neuroimaging studies on pantomiming or imagery of tool use in healthy volunteers revealed neural responses in possibly separate foci in the left supramarginal gyrus. In sum, the parietal contribution of reaching and grasping of objects seems to depend on a bilateral network of intraparietal foci that appear organized along gradients of sensory and effector preferences. Dorsal and medial parietal cortex appears to contribute to the online monitoring/adjusting of the ongoing prehensile action, whereas the functional use of objects and tools seems to involve the inferior lateral parietal cortex. This functional input reveals a clear left lateralized activation pattern that may be tuned to the integration of acquired knowledge in the planning and guidance of the transitive movement.

Mastication dyspraxia: a neurodevelopmental disorder reflecting disruption of the cerebellocerebral network involved in planned actions

This paper reports the longitudinal clinical, neurocognitive, and neuroradiological findings in an adolescent patient with nonprogressive motor and cognitive disturbances consistent with a diagnosis of developmental coordination disorder (DCD). In addition to prototypical DCD, the development of mastication was severely impaired, while no evidence of swallowing apraxia, dysphagia, sensorimotor disturbances, abnormal tone, or impaired general cognition was found. He suffered from bronchopulmonary dysplasia and was ventilated as a newborn for 1.5 months. At the age of 3 months, a ventriculoperitoneal shunt was surgically installed because of obstructive hydrocephalus secondary to perinatal intraventricular bleeding. At the age of 5 years, the patient's attempts to masticate were characterized by rough, effortful, and laborious biting movements confined to the vertical plane. Solid food particles had a tendency to get struck in his mouth and there was constant spillage. As a substitute for mastication, he moved the unground food with his fingers in a lateral direction to the mandibular and maxillary vestibule to externally manipulate and squeeze the food between cheek and teeth with the palm of his hand. Once the food was sufficiently soft, the bolus was correctly transported by the tongue in posterior direction and normal deglutition took place. Repeat magnetic resonance imaging (MRI) during follow-up disclosed mild structural abnormalities as the sequelae of the perinatal intraventricular bleeding, but this could not explain impaired mastication behavior. Quantified Tc-99m-ethylcysteinate dimer single-photon emission computed tomography (Tc-99m-ECD SPECT), however, revealed decreased perfusion in the left cerebellar hemisphere, as well as in both inferior lateral frontal regions, both motor cortices, and the right anterior and lateral temporal areas. Anatomoclinical findings in this patient with DCD not only indicate that the functional integrity of the cerebellocerebral network is crucially important in the planning and execution of skilled actions, but also seem to show for the first time that mastication deficits may be of true apraxic origin. As a result, it is hypothesized that "mastication dyspraxia" may have to be considered as a distinct nosological entity within the group of the developmental dyspraxias following a disruption of the cerebellocerebral network involved in planned actions.

Tool manipulation knowledge is retrieved by way of the ventral visual object processing pathway

Using functional Magnetic Resonance Imaging (fMRI), we find that object manipulation knowledge is accessed by way of the ventral object processing pathway. We exploit the fact that parvocellular channels project to the ventral but not the dorsal stream, and show that increased neural responses for tool stimuli are observed in the inferior parietal lobule when those stimuli are visible only to the ventral object processing stream. In a control condition, tool-preferences were observed in a superior and posterior parietal region for stimuli titrated so as to be visible by the dorsal visual pathway. Functional connectivity analyses confirm the dissociation between sub-regions of parietal cortex according to whether their principal afferent input is via the ventral or dorsal visual pathway. These results challenge the 'Embodied Hypothesis of Tool Recognition', according to which tool identification critically depends on simulation of object manipulation knowledge. Instead, these data indicate that retrieval of object-associated manipulation knowledge is contingent on accessing the identity of the object, a process that is subserved by the ventral visual pathway.

Diagnosis and treatment of upper limb apraxia

Upper limb apraxia, a disorder of higher motor cognition, is a common consequence of left-hemispheric stroke. Contrary to common assumption, apraxic deficits not only manifest themselves during clinical testing but also have delirious effects on the patients' everyday life and rehabilitation. Thus, a reliable diagnosis and efficient treatment of upper limb apraxia is important to improve the patients' prognosis after stroke. Nevertheless, to date, upper limb apraxia is still an underdiagnosed and ill-treated entity. Based on a systematic literature search, this review summarizes the current tools of diagnosis and treatment strategies for upper limb apraxia. It furthermore provides clinicians with graded recommendations. In particular, a short screening test for apraxia, and a more comprehensive diagnostic apraxia test for clinical use are recommended. Although currently only a few randomized controlled studies investigate the efficacy of different apraxia treatments, the gesture training suggested by Smania and colleagues can be recommended for the therapy of apraxia, the effects of which were shown to extend to activities of daily living and to persist for at least 2 months after completion of the training. This review aims at directing the reader's attention to the ecological relevance of apraxia. Moreover, it provides clinicians with appropriate tools for the reliable diagnosis and effective treatment of apraxia. Nevertheless, this review also highlights the need for further research into how to improve diagnosis of apraxia based on neuropsychological models and to develop new therapeutic strategies.

Size-weight illusion and anticipatory grip force scaling following unilateral cortical brain lesion

The prediction of object weight from its size is an important prerequisite of skillful object manipulation. Grip and load forces anticipate object size during early phases of lifting an object. A mismatch between predicted and actual weight when two different sized objects have the same weight results in the size-weight illusion (SWI), the small object feeling heavier. This study explores whether lateralized brain lesions in patients with or without apraxia alter the size-weight illusion and impair anticipatory finger force scaling. Twenty patients with left brain damage (LBD, 10 with apraxia, 10 without apraxia), ten patients with right brain damage (RBD), and matched control subjects lifted two different-sized boxes in alternation. All subjects experienced a similar size-weight illusion. The anticipatory force scaling of all groups was in correspondence with the size cue: higher forces and force rates were applied to the big box and lower forces and force rates to the small box during the first lifts. Within few lifts, forces were scaled to actual object weight. Despite the lack of significant differences at group level, 5 out of 20 LBD patients showed abnormal predictive scaling of grip forces. They differed from the LBD patients with normal predictive scaling by a greater incidence of posterior occipito-parietal lesions but not by a greater incidence of apraxia. The findings do not support a more general role for the motor-dominant left hemisphere, or an influence of apraxia per se, in the scaling of finger force according to object properties. However, damage in the vicinity of the parietal-occipital junction may be critical for deriving predictions of weight from size.