Date Published: January 19, 2017
Publisher: Public Library of Science
Author(s): Lari Vainio, Aleksi Rantala, Mikko Tiainen, Kaisa Tiippana, Naeem Komeilipoor, Martti Vainio, Niels O. Schiller.
Previous research has shown that precision and power grip performance is consistently influenced by simultaneous articulation. For example, power grip responses are performed relatively fast with the open-back vowel [a], whereas precision grip responses are performed relatively fast with the close-front vowel [i]. In the present study, the participants were presented with a picture of a hand shaped to the precision or power grip. They were required to pronounce speech sounds according to the front/above perspective of the hand. The results showed that not only the grip performance is affected by simultaneously pronouncing the speech sound but also the production of speech sound can be affected by viewing an image of a grip. The precision grip stimulus triggered relatively rapid production of the front-close vowel [i]. In contrast, the effect related to the power grip stimulus was mostly linked to the vertical dimension of the pronounced vowel since this stimulus triggered relatively rapid production of the back-open vowel [a] and back-mid-open vowel [o] while production of the back-close vowel [u] was not influenced by it. The fact that production of the dorsal consonant [k] or coronal consonant [t] were not influenced by these stimuli suggests that the effect was not associated with a relative front-back tongue shape of the articulation in the absence of changes in any vertical articulatory components. These findings provide evidence for an intimate interaction between certain articulatory gestures and grip types, suggesting that an overlapping visuomotor network operates for planning articulatory gestures and grasp actions.
The present study uses behavioural techniques to investigate interaction between processes that plan articulatory gestures and hand movements. Given that body parts are somatotopically represented in the primary motor cortex , at the first glance, this research topic might appear to be rather surprising. However, it is also known that the motor representations of one body part can be involved in planning and executing actions with another body part. In particular, this has been shown to be the case in relation to hand and mouth actions [2,3]. For example, magnetoencephalography (MEG) evidence have shown increased activation in the hand motor area of the primary motor cortex during lip and tongue protrusion task and during silent phoneme production . Correspondingly, electromyographic (EMG) responses of hand muscles are increased by the transcranial magnetic stimulation (TMS) applied to hand motor area if participants are reading, speaking or listening to speech spontaneously during the stimulation [5–7].
In the paradigm used by Vainio et al.  the participants were required to respond using the precision or power grip while they were presented with the precision or power grasp stimuli. In the current task, the manual responses were replaced by vocal responses. The participants were presented with an image of the precision or power grip and they were required to vocalize as fast as possible one of the two vocal response choices (Block 1: [i] or [α]; Block 2: [ti] or [kα]) according to whether the hand stimulus was shown from the front or above perspective. Similarly to the traditional stimulus-response compatibility (SRC) tasks , we were interested in measuring how a task-irrelevant stimulus property (i.e., a grip type) influences responses. We assumed that in this task, articulatory motor responses would be primed by visual properties in the same way as grasp motor responses are primed in the task reported by Vainio et al. . That is, the perceived grip type could result in automatic activation of the articulatory gesture associated with the grip type leading to relatively rapid production of this articulatory gesture in comparison to the opposite response. Based on our previous observations of motor interactions , we predicted that the visual precision grip stimuli would be associated with relatively rapid vocal responses of [i] and [ti] whereas the visual power grip stimuli would be associated with relatively rapid vocal responses of [α] and [kα].
The results of Experiment 1 leave open couple of questions: Firstly, they do not conclusively show whether viewing the precision or power grip is associated with facilitated pronunciation of consonants [t] and [k], respectively. That is, because the effect is mostly linked to pronouncing the vowels alone, it remains unclear whether the consonant content contributed to the effect in the [ti]-[kα] block. As a consequence, Experiment 2 investigates whether the effect can be associated with pronouncing the consonants [t] and [k] by asking the participants to pronounce the syllables [te] or [ke] in the task used in Experiment 1. The consonants were coupled with the vowel [e] because it is difficult to pronounce them alone. The vowel [e] was selected because it is unrounded and relatively neutral in Finnish language in terms of tongue position.
It has been previously found that when participants pronounce a syllable and simultaneously perform either a precision or power grip response, the manual responses are made faster when there is a congruency between the syllable and the grip response (e.g., [ti]-precision grip) . The current study shows that this articulation-grip congruency effect can be also observed in reaction times of vocal responses when the participants are presented with a hand that is shaped to the precision or power grip. These findings support the account that the overlapping visuomotor network operates for perceiving and planning articulatory gestures and grasp actions, and there are systematic interactions between the precision and power grip representations and specific articulatory gestures within this network. That is, the interaction between certain articulatory and manual grasp processes manifest itself not only in behavioural level when articulation and grasping has to be performed simultaneously, but also viewing an image of a hand grip similarly primes articulatory processes that are congruent with the perceived grip type.
The present study shows that not only are manual grasp responses influenced by observed grasp actions  but similar SRC effects can also be observed in relation to vocal responses. Reaction times of those vocal responses that are associated with precision grip execution (articulation of the vowel [i]) are relatively short when participants see a hand shaped to the precision grip. In contrast, reaction times of those responses that are associated with the power grip execution (e.g., articulation of the vowel [α]) are relatively short if the seen stimulus is a hand shaped to the power grip. The effect appears to be mostly linked to processes responsible for programming vertical mouth movements related to vowel production. In addition to showing evidence that there is intimate and systematic interaction between certain articulatory gestures and grip types, our results also suggest that the overlapping visuomotor network operates for planning articulatory gestures and grasp actions as well as perceiving grasp actions.