Tongue reflex for speech posture control.

Although there is no doubt from an empirical viewpoint that reflex mechanisms can contribute to tongue motor control in humans, there is limited neurophysiological evidence to support this idea. Previous results failing to observe any tonic stretch reflex in the tongue had reduced the likelihood of a reflex contribution in tongue motor control. The current study presents experimental evidence of a human tongue reflex in response to a sudden stretch while holding a posture for speech. The latency was relatively long (50 ms), which is possibly mediated through cortical-arc. The activation peak in a speech task was greater than in a non-speech task while background activation levels were similar in both tasks, and the peak amplitude in a speech task was not modulated by the additional task to react voluntarily to the perturbation. Computer simulations with a simplified linear mass-spring-damper model showed that the recorded muscle activation response is suited for the generation of tongue movement responses that were observed in a previous study with the appropriate timing when taking into account a possible physiological delay between reflex muscle activation and the corresponding force. Our results evidenced clearly that reflex mechanisms contribute to tongue posture stabilization for speech production.

Predicting primate tongue morphology based on geometrical skull matching. A first step towards an application on fossil hominins.

As part of a long-term research project aiming at generating a biomechanical model of a fossil human tongue from a carefully designed 3D Finite Element mesh of a living human tongue, we present a computer-based method that optimally registers 3D CT images of the head and neck of the living human into similar images of another primate. We quantitatively evaluate the method on a baboon. The method generates a geometric deformation field which is used to build up a 3D Finite Element mesh of the baboon tongue. In order to assess the method's ability to generate a realistic tongue from bony structure information alone, as would be the case for fossil humans, its performance is evaluated and compared under two conditions in which different anatomical information is available: (1) combined information from soft-tissue and bony structures; (2) information from bony structures alone. An Uncertainty Quantification method is used to evaluate the sensitivity of the transformation to two crucial parameters, namely the resolution of the transformation grid and the weight of a smoothness constraint applied to the transformation, and to determine the best possible meshes. In both conditions the baboon tongue morphology is realistically predicted, evidencing that bony structures alone provide enough relevant information to generate soft tissue.

Pressure sensor calibration using a water-filled latex finger to account for the mechanical interaction between the hard palate and the deformable human tongue.

We present a calibration system called Dried Water Column (DWC). It applies pressure on a sensor with a latex finger filled with water, which pressure is controlled with a water column. This is intended to mimic the way the deformable tongue mechanically interacts with the hard palate. We show that, once some specificities of the elastic/plastic behavior of the latex finger are taken into account, namely the softening due to Mullins Effect and the non-elastic deformation occurring above a certain pressure level, the DWC provides a reliable measure of the linear relation between the pressure and the output voltage of the sensor within the limited pressure range [0, 2.5 kPa]. Such a precise calibration would not be possible with a rigid actuator, which position on the sensor can dramatically influence the measures. Extrapolating the linear relationship thus determined to a larger pressure range compatible with speech production and swallowing ([0, 35 kPa]), is possible once it has been verified that the behavior of the sensor is linear over this pressure range. This can be done with any rigid or semi rigid actuator. This reliable calibration procedure can be easily reproduced in any laboratory, and can be applied to any pressure sensor.

FIM: A fatigued-injured muscle model based on the sliding filament theory.

Skeletal muscle modeling has a vital role in movement studies and the development of therapeutic approaches. In the current study, a Huxley-based model for skeletal muscle is proposed, which demonstrates the impact of impairments in muscle characteristics. This model focuses on three identified ions: H+, inorganic phosphate Pi, and Ca2+. Modifications are made to actin-myosin attachment and detachment rates to study the effects of H+ and Pi. Additionally, an activation coefficient is included to represent the role of calcium ions interacting with troponin, highlighting the importance of Ca2+. It is found that maximum isometric muscle force decreases by 9.5% due to a reduction in pH from 7.4 to 6.5 and by 47.5% in case of the combination of a reduction in pH and an increase of Pi concentration up to 30 mM, respectively. Then the force decline caused by a fall in the active calcium ions is studied. When only 15% of the total calcium in the myofibrillar space is able to interact with troponin, up to 80% force drop is anticipated by the model. The proposed fatigued-injured muscle model is useful to study the effect of various shortening velocities and initial muscle-tendon lengths on muscle force; in addition, the benefits of the model go beyond predicting the force in different conditions as it can also predict muscle stiffness and power. The power and stiffness decrease by 40% and 6.5%, respectively, due to the pH reduction, and the simultaneous accumulation of H+ and Pi leads to a 50% and 18% drop in power and stiffness.

Finite element analysis of biomechanical interactions of a subcutaneous suspension suture and human face soft-tissue: a cadaver study.

In order to study the local interactions between facial soft-tissues and a Silhouette Soft® suspension suture, a CE marked medical device designed for the repositioning of soft tissues in the face and the neck, Finite element simulations were run, in which a model of the suture was embedded in a three-layer Finite Element structure that accounts for the local mechanical organization of human facial soft tissues. A 2D axisymmetric model of the local interactions was designed in ANSYS, in which the geometry of the tissue, the boundary conditions and the applied loadings were considered to locally mimic those of human face soft tissue constrained by the suture in facial tissue repositioning. The Silhouette Soft suture is composed of a knotted thread and sliding cones that are anchored in the tissue. Hence, simulating these interactions requires special attention for an accurate modelling of contact mechanics. As tissue is modelled as a hyper-elastic material, the displacement of the facial soft tissue changes in a nonlinear way with the intensity of stress induced by the suture and the number of the cones. Our simulations show that for a 4-cone suture a displacement of 4.35 mm for a 2.0 N external loading and of 7.6 mm for 4.0 N. Increasing the number of cones led to the decrease in the equivalent local strain (around 20%) and stress (around 60%) applied to the tissue. The simulated displacements are in general agreement with experimental observations.

Effects of somatosensory perturbation on the perception of French /u/.

In a study of whether somatosensory feedback related to articulatory configuration is involved in speech perception, 30 French-speaking adults performed a speech discrimination task in which vowel pairs along the French /u/ (rounded vowel requiring a small lip area) to /œ/ (rounded vowel associated with larger lip area) continuum were used as stimuli. Listeners had to perform the test in two conditions: with a 2-cm-diameter lip-tube in place (mimicking /œ/) and without the lip-tube (neutral lip position). Results show that, in the lip-tube condition, listeners perceived more stimuli as /œ/, in line with the proposal that an auditory-somatosensory interaction exists.

How the conception of control influences our understanding of actions.

Wilful movement requires neural control. Commonly, neural computations are thought to generate motor commands that bring the musculoskeletal system - that is, the plant - from its current physical state into a desired physical state. The current state can be estimated from past motor commands and from sensory information. Modelling movement on the basis of this concept of plant control strives to explain behaviour by identifying the computational principles for control signals that can reproduce the observed features of movements. From an alternative perspective, movements emerge in a dynamically coupled agent-environment system from the pursuit of subjective perceptual goals. Modelling movement on the basis of this concept of perceptual control aims to identify the controlled percepts and their coupling rules that can give rise to the observed characteristics of behaviour. In this Perspective, we discuss a broad spectrum of approaches to modelling human motor control and their notions of control signals, internal models, handling of sensory feedback delays and learning. We focus on the influence that the plant control and the perceptual control perspective may have on decisions when modelling empirical data, which may in turn shape our understanding of actions.

Rhythmic tapping difficulties in adults who stutter: A deficit in beat perception, motor execution, or sensorimotor integration?

OBJECTIVES: The study aims to better understand the rhythmic abilities of people who stutter and to identify which processes potentially are impaired in this population: (1) beat perception and reproduction; (2) the execution of movements, in particular their initiation; (3) sensorimotor integration. MATERIAL AND METHOD: Finger tapping behavior of 16 adults who stutter (PWS) was compared with that of 16 matching controls (PNS) in five rhythmic tasks of various complexity: three synchronization tasks - a simple 1:1 isochronous pattern, a complex non-isochronous pattern, and a 4 tap:1 beat isochronous pattern -, a reaction task to an aperiodic and unpredictable pattern, and a reproduction task of an isochronous pattern after passively listening. RESULTS: PWS were able to reproduce an isochronous pattern on their own, without external auditory stimuli, with similar accuracy as PNS, but with increased variability. This group difference in variability was observed immediately after passive listening, without prior motor engagement, and was not enhanced or reduced after several seconds of tapping. Although PWS showed increased tapping variability in the reproduction task as well as in synchronization tasks, this timing variability did not correlate significantly with the variability in reaction times or tapping force. Compared to PNS, PWS exhibited larger negative mean asynchronies, and increased synchronization variability in synchronization tasks. These group differences were not affected by beat hierarchy (i.e., "strong" vs. "weak" beats), pattern complexity (non-isochronous vs. isochronous) or presence versus absence of external auditory stimulus (1:1 vs. 1:4 isochronous pattern). Differences between PWS and PNS were not enhanced or reduced with sensorimotor learning, over the first taps of a synchronization task. CONCLUSION: Our observations support the hypothesis of a deficit in neuronal oscillators coupling in production, but not in perception, of rhythmic patterns, and a larger delay in multi-modal feedback processing for PWS.

Accurate Tongue-Palate Pressure Sensing Device to Study Speech Production and Swallowing in Patients with Complete Denture.

OBJECTIVES: The mechanical interactions between tongue and palate are crucial for speech production and swallowing. In this study, we presented examples of pressure signals that can be recorded with our PRESLA system (PRESLA holds for the French expression "PRESsion de la LAngue" [Pressure from the tongue]) to assess these motor functions, and we illustrate which issues can be tackled with such a system. MATERIALS AND METHODS: A single French-speaking edentulous subject, old wearer of a complete denture, with no speech production and swallowing disorders, was recorded during the production of nonsense words including French alveolar fricatives, and during dry and water swallowing. The PRESLA system used strain-gauge transducers that were inserted into holes drilled in the palatal surface of a duplicate of the prosthesis at six locations that were relevant for speech production and swallowing. Pressure signals were postsynchronized with the motor tasks based on audio signals. RESULTS: Patterns of temporal variations of the pressure exerted by the tongue on the palate are shown for the two studied motor tasks. It is shown for our single subject that patterns for fricative /s/ are essentially bell shaped, whereas pressure signals observed for water swallow begin with a maximum followed by a slow decrease during the rest of the positive pressure phase. Pressure magnitude is almost 20 times larger for water swallow than for /s/ production. CONCLUSIONS: This study illustrates the usefulness of our PRESLA system for studying speech production and swallowing motor control under normal and pathological conditions.

Machine-Learning based model order reduction of a biomechanical model of the human tongue.

BACKGROUND AND OBJECTIVES: This paper presents the results of a Machine-Learning based Model Order Reduction (MOR) method applied to a complex 3D Finite Element (FE) biomechanical model of the human tongue, in order to create a Digital Twin Model (DTM) that enables real-time simulations. The DTM is designed for future inclusion in a computer assisted protocol for tongue surgery planning. METHODS: The proposed method uses an "a posteriori" MOR that allows, from a limited number of simulations with the FE model, to predict in real time mechanical responses of the human tongue to muscle activations. RESULTS: The MOR method is evaluated for simulations associated with separate single tongue muscle activations. It is shown to be able to account with a sub-millimetric spatial accuracy for the non-linear dynamical behavior of the tongue model observed in these simulations. CONCLUSION: Further evaluations of the MOR method will include tongue movements induced by multiple muscle activations. At this stage our MOR method offers promising perspectives for the use of the tongue model in a clinical context to predict the impact of tongue surgery on tongue mobility. As a long term application, this DTM of the tongue could be used to predict the functional consequences of the surgery in terms of speech production and swallowing.

Speech rehabilitation in post-stroke aphasia using visual illustration of speech articulators: A case report study.

Recent studies on the remediation of speech disorders suggest that providing visual information of speech articulators may contribute to improve speech production. In this study, we evaluate the effectiveness of an illustration-based rehabilitation method on speech recovery of a patient with non-fluent chronic aphasia. The Ultraspeech-player software allowed visualization by the patient of reference tongue and lip movements recorded using ultrasound and video imaging. This method can improve the patient's awareness of their own lingual and labial movements, which can increase the ability to coordinate and combine articulatory gestures. The effects of this method were assessed by analyzing performance during speech tasks, the phonological processes identified in the errors made during the phoneme repetition task and the acoustic parameters derived from the speech signal. We also evaluated cognitive performance before and after rehabilitation. The integrity of visuospatial ability, short-term and working memory and some executive functions supports the effectiveness of the rehabilitation method. Our results showed that illustration-based rehabilitation technique had a beneficial effect on the patient's speech production, especially for stop and fricative consonants which are targeted (high visibility of speech articulator configurations) by the software, but also on reading abilities. Acoustic parameters indicated an improvement in the distinction between consonant categories: voiced and voiceless stops or alveolar, post-alveolar and labiodental fricatives. However, the patient showed little improvement for vowels. These results confirmed the advantage of using illustration-based rehabilitation technique and the necessity of detailed subjective and objective intra-speaker evaluation in speech production to fully evaluate speech abilities.

Quick compensatory mechanisms for tongue posture stabilization during speech production.

The human tongue is atypical as a motor system since its movement is determined by deforming its soft tissues via muscles that are in large part embedded in it (muscular hydrostats). However, the neurophysiological mechanisms enabling fine tongue motor control are not well understood. We investigated sensorimotor control mechanisms of the tongue through a perturbation experiment. A mechanical perturbation was applied to the tongue during the articulation of three vowels (/i/, /e/, /ε/) under conditions of voicing, whispering, and posturing. Tongue movements were measured at three surface locations in the sagittal plane using electromagnetic articulography. We found that the displacement induced by the external force was quickly compensated for. Individual sensors did not return to their original positions but went toward a position on the original tongue contour for that vowel. The amplitude of compensatory response at each tongue site varied systematically according to the articulatory condition. A mathematical simulation that included reflex mechanisms suggested that the observed compensatory response can be attributed to a reflex mechanism, rather than passive tissue properties. The results provide evidence for the existence of quick compensatory mechanisms in the tongue that may be dependent on tunable reflexes. The tongue posture for vowels could be regulated in relation to the shape of the tongue contour, rather than to specific positions for individual tissue points.NEW & NOTEWORTHY This study presents evidence of quick compensatory mechanisms in tongue motor control for speech production. The tongue posture is controlled not in relation to a specific tongue position, but to the shape of the tongue contour to achieve specific speech sounds. Modulation of compensatory responses due to task demands and mathematical simulations support the idea that the quick compensatory response is driven by a reflex mechanism.

What anticipatory coarticulation in children tells us about speech motor control maturity.

PURPOSE: This study aimed to evaluate the role of motor control immaturity in the speech production characteristics of 4-year-old children, compared to adults. Specifically, two indices were examined: trial-to-trial variability, which is assumed to be linked to motor control accuracy, and anticipatory extra-syllabic vowel-to-vowel coarticulation, which is assumed to be linked to the comprehensiveness, maturity and efficiency of sensorimotor representations in the central nervous system. METHOD: Acoustic and articulatory (ultrasound) data were recorded for 20 children and 10 adults, all native speakers of Canadian French, during the production of isolated vowels and vowel-consonant-vowel (V1-C-V2) sequences. Trial-to-trial variability was measured in isolated vowels. Extra-syllabic anticipatory coarticulation was assessed in V1-C-V2 sequences by measuring the patterns of variability of V1 associated with variations in V2. Acoustic data were reported for all subjects and articulatory data, for a subset of 6 children and 2 adults. RESULTS: Trial-to-trial variability was significantly larger in children. Systematic and significant anticipation of V2 in V1 was always found in adults, but was rare in children. Significant anticipation was observed in children only when V1 was /a/, and only along the antero-posterior dimension, with a much smaller magnitude than in adults. A closer analysis of individual speakers revealed that some children showed adult-like anticipation along this dimension, whereas the majority did not. CONCLUSION: The larger trial-to-trial variability and the lack of anticipatory behavior in most children-two phenomena that have been observed in several non-speech motor tasks-support the hypothesis that motor control immaturity may explain a large part of the differences observed between speech production in adults and 4-year-old children, apart from other causes that may be linked with language development.

Speakers are able to categorize vowels based on tongue somatosensation.

Auditory speech perception enables listeners to access phonological categories from speech sounds. During speech production and speech motor learning, speakers' experience matched auditory and somatosensory input. Accordingly, access to phonetic units might also be provided by somatosensory information. The present study assessed whether humans can identify vowels using somatosensory feedback, without auditory feedback. A tongue-positioning task was used in which participants were required to achieve different tongue postures within the /e, ε, a/ articulatory range, in a procedure that was totally nonspeech like, involving distorted visual feedback of tongue shape. Tongue postures were measured using electromagnetic articulography. At the end of each tongue-positioning trial, subjects were required to whisper the corresponding vocal tract configuration with masked auditory feedback and to identify the vowel associated with the reached tongue posture. Masked auditory feedback ensured that vowel categorization was based on somatosensory feedback rather than auditory feedback. A separate group of subjects was required to auditorily classify the whispered sounds. In addition, we modeled the link between vowel categories and tongue postures in normal speech production with a Bayesian classifier based on the tongue postures recorded from the same speakers for several repetitions of the /e, ε, a/ vowels during a separate speech production task. Overall, our results indicate that vowel categorization is possible with somatosensory feedback alone, with an accuracy that is similar to the accuracy of the auditory perception of whispered sounds, and in congruence with normal speech articulation, as accounted for by the Bayesian classifier.

Study of tongue-palate pressure patterns during the hold phase in the production of French denti-alveolar and velar stops.

The hold phase of the stop consonants is crucial for a successful production of the release and the acoustic burst. Concurrently, it is also associated with weak acoustic energy and minimal movement, so that conventional acoustic and kinematic approaches are not relevant to investigate motor control. This paper presents an innovative experimental method to study speech motor control during this phase, based on meticulous measurement of the time variation of the mechanical pressure exerted by the tongue against the palate and also characterizing tongue-palate interaction. The concept is based on using miniature transducers with enhanced response characteristics inserted in different locations of the complete denture of edentulous subjects without perturbing the articulation. The study was done with a French-speaking adult whose maxillary denture was duplicated and mounted with six strain gauge transducers. The experiment was done with denti-alveolar and velar stop consonants with two vowel contexts. The results illustrate the potential of such device to analyse speech motor control when contact constrains tongue movements.

Modeling Sensory Preference in Speech Motor Planning: A Bayesian Modeling Framework.

Experimental studies of speech production involving compensations for auditory and somatosensory perturbations and adaptation after training suggest that both types of sensory information are considered to plan and monitor speech production. Interestingly, individual sensory preferences have been observed in this context: subjects who compensate less for somatosensory perturbations compensate more for auditory perturbations, and vice versa. We propose to integrate this sensory preference phenomenon in a model of speech motor planning using a probabilistic model in which speech units are characterized both in auditory and somatosensory terms. Sensory preference is implemented in the model according to two approaches. In the first approach, which is often used in motor control models accounting for sensory integration, sensory preference is attributed to the relative precision (i.e., inverse of the variance) of the sensory characterization of the speech motor goals associated with phonological units (which are phonemes in the context of this paper). In the second, "more original" variant, sensory preference is implemented by modulating the sensitivity of the comparison between the predicted sensory consequences of motor commands and the sensory characterizations of the phonemes. We present simulation results using these two variants, in the context of the adaptation to an auditory perturbation, implemented in a 2-dimensional biomechanical model of the tongue. Simulation results show that both variants lead to qualitatively similar results. Distinguishing them experimentally would require precise analyses of partial compensation patterns. However, the second proposed variant implements sensory preference without changing the sensory characterizations of the phonemes. This dissociates sensory preference and sensory characterizations of the phonemes, and makes the account of sensory preference more flexible. Indeed, in the second variant the sensory characterizations of the phonemes can remain stable, when sensory preference varies as a response to cognitive or attentional control. This opens new perspectives for capturing speech production variability associated with aging, disorders and speaking conditions.

The ConDialInt Model: Condensation, Dialogality, and Intentionality Dimensions of Inner Speech Within a Hierarchical Predictive Control Framework.

Inner speech has been shown to vary in form along several dimensions. Along condensation, condensed inner speech forms have been described, that are supposed to be deprived of acoustic, phonological and even syntactic qualities. Expanded forms, on the other extreme, display articulatory and auditory properties. Along dialogality, inner speech can be monologal, when we engage in internal soliloquy, or dialogal, when we recall past conversations or imagine future dialogs involving our own voice as well as that of others addressing us. Along intentionality, it can be intentional (when we deliberately rehearse material in short-term memory) or it can arise unintentionally (during mind wandering). We introduce the ConDialInt model, a neurocognitive predictive control model of inner speech that accounts for its varieties along these three dimensions. ConDialInt spells out the condensation dimension by including inhibitory control at the conceptualization, formulation or articulatory planning stage. It accounts for dialogality, by assuming internal model adaptations and by speculating on neural processes underlying perspective switching. It explains the differences between intentional and spontaneous varieties in terms of monitoring. We present an fMRI study in which we probed varieties of inner speech along dialogality and intentionality, to examine the validity of the neuroanatomical correlates posited in ConDialInt. Condensation was also informally tackled. Our data support the hypothesis that expanded inner speech recruits speech production processes down to articulatory planning, resulting in a predicted signal, the inner voice, with auditory qualities. Along dialogality, covertly using an avatar's voice resulted in the activation of right hemisphere homologs of the regions involved in internal own-voice soliloquy and in reduced cerebellar activation, consistent with internal model adaptation. Switching from first-person to third-person perspective resulted in activations in precuneus and parietal lobules. Along intentionality, compared with intentional inner speech, mind wandering with inner speech episodes was associated with greater bilateral inferior frontal activation and decreased activation in left temporal regions. This is consistent with the reported subjective evanescence and presumably reflects condensation processes. Our results provide neuroanatomical evidence compatible with predictive control and in favor of the assumptions made in the ConDialInt model.

Transfer of sensorimotor learning reveals phoneme representations in preliterate children.

Reading acquisition is strongly intertwined with phoneme awareness that relies on implicit phoneme representations. We asked whether phoneme representations emerge before literacy. We recruited two groups of children, 4 to 5-year-old preschoolers (N = 29) and 7 to 8-year-old schoolchildren (N = 24), whose phonological awareness was evaluated, and one adult control group (N = 17). We altered speakers' auditory feedback in real time to elicit persisting pronunciation changes, referred to as auditory-motor adaptation or learning. Assessing the transfer of learning at phoneme level enabled us to investigate the developmental time-course of phoneme representations. Significant transfer at phoneme level occurred in preschoolers, as well as schoolchildren and adults. In addition, we found a relationship between auditory-motor adaptation and phonological awareness in both groups of children. Overall, these results suggest that phoneme representations emerge before literacy acquisition, and that these sensorimotor representations may set the ground for phonological awareness.

Vowel Reduction in Coratino (South Italy): Phonological and Phonetic Perspectives.

Vowel reduction may involve phonetic reduction processes, with nonreached targets, and/or phonological processes in which a vowel target is changed for another target, possibly schwa. Coratino, a dialect of southern Italy, displays complex vowel reduction processes assumed to be phonological. We analyzed a corpus representative of vowel reduction in Coratino, based on a set of a hundred pairs of words contrasting a stressed and an unstressed version of a given vowel in a given consonant environment, produced by 10 speakers. We report vowelformants together with consonant-to-vowel formant trajectories and durations, and show that these data are rather in agreement with a change in vowel target from /i e ɛ·É” u/ to schwa when the vowel is a non-word-initial unstressed utterance, unless the vowel shares a place-of-articulation feature with the preceding or following consonant. Interestingly, it also appears that there are 2 targets for phonological reduction, differing in F1 values. A "higher schwa" - which could be considered as /ɨ/ - corresponds to reduction for high vowels /i u/ while a "lower schwa" - which could be considered as /ə/ - corresponds to reduction for midhigh.

Adaptation to visual feedback delays on touchscreens with hand vision.

Direct touch finger interaction on a smartphone or a tablet is now ubiquitous. However, the latency inherent in digital computation produces an average feedback delay of ~ 75 ms between the action of the hand and its visible effect on digital content. This delay has been shown to affect users' performance, but it is unclear whether users adapt to this delay and whether it influences skill learning. Previous work studied adaptation to feedback delays but only for longer delays, with hidden hand or indirect devices. This paper addresses adaptation to touchscreen delay in two empirical studies involving the tracking of a target moving along an elliptical path. Participants were trained for the task either at the minimal delay the system allows (~ 9 ms) or at a longer delay equivalent to commercialized touch devices latencies (75 ms). After 10 training sessions over a minimum of 2 weeks (Experiment 1), participants adapt to the delay. They also display long-term retention 7 weeks after the last training session. This adaptation generalizes to a similar tracking path (e.g., infinity symbol). We also observed generalization of learning from the longer delay to the minimal-delay condition (Experiment 2). The delay thus does not prevent the learning of tracking skill, which suggests that delay adaptation and tracking skill could be two separate components of learning.