There is a tight interplay that exists between the planning and execution of hand and mouth movements. For instance, athletes often perform verbalizations that can be overt (e.g., grunting when swinging a tennis racquet or even the pronunciation of meaningless words [e.g., “kiap”] when performing a punching movement; see Tedeschi, 2000). As well, some athletes are encouraged to employ covert speech (e.g., self-talk via inner speech; see Van Raalte et al., 2016). From an applied or performance perspective, it is thought that such verbalizations enhance performance. And from a scientific perspective, manual reaction time (RT) responses have been proposed to be shortened by meaningless verbalizations (e.g., Vainio et al., 2013; Vainio et al., 2014) as well as meaningful verbalizations (Liepelt et al., 2012). However, due to the lack of control conditions, there is still no compelling evidence to support the idea of a vocalization facilitating/ interfering with the planning and initiation of hand movements. Further, it is also unclear if manual movements can be influenced differently by overt and covert syllable production. Thus, of particular interest to the current study is how meaningless syllables, whether silently read (i.e., covertly read) or verbalized (i.e., overtly read), influence the initiation of manual movements.
There is a body of literature proposing that meaningless sounds can modulate the reaction time (RT) of different manual movements. For example, Vainio et al., (2013) reported that the overt verbalization of the syllable /ti/ in comparison to /ka/ decreased the initiation latency of a precision grip, while the verbalization of the syllable /ka/ in comparison to /ti/ decreased the initiation latency of a power grip. This influence of verbalization on hand grip initiation was coined the articulation-grip correspondence (AGC) effect. In a subsequent study (Vainio et al., 2014), the AGC effect was also shown to be elicited by silently reading a syllable prior to a hand grip response (Vainio et al., 2014). Specifically, the initiation latency of a precision grip was decreased by silently reading the syllable /ti/ in comparison to /ka/, while the initiation latency of a power grip was decreased by silently reading the syllable /ka/ in comparison to /ti/. Notably, these empirical studies are also consistent with well-established theoretical constructs of perception-action coupling.
From the ideomotor perspective (e.g., Hommel et al., 2001), the perceptual consequences of an action (i.e., the effect) and the motor codes that bring about that action (i.e., the response) are tightly coupled. Based on this tenet of tightly coupled response-effect codes, it is predicted that the activation of one code will lead to the activation of the related codes and features. The spreading activation of coupled or related perception or action codes can modulate subsequent processes. For instance, a study by Leighton and Heyes (2010) showed that participants could initiate a movement (e.g., open mouth or hand) with a shorter latency if they were also presented with a picture of a movement that shared a compatible effect (e.g., open mouth or hand). On the other hand, longer initiation latencies were observed when the effect presented in the picture was incompatible (e.g., close mouth or hand; see also Liepelt et al., 2012) with the response they were required to perform (e.g., open mouth or hand). Thus, it is possible that the AGC effect is the result of partially overlapping motor representations between phonation structures and hand grips that share similar response-effect coding. For example, both the production of the phoneme /t/ and a precision grip produce perceptible tactile sensation (i.e., shared effect) at the distal portion of the effectors (i.e., the tip of the tongue and the fingertips). In contrast, both the production of the phoneme /k/ and a power grip produce perceptible tactile sensation at proximal portions of the effector (i.e., the back of the tongue and base of the palm). With response-effect coupling (i.e., the main tenet of ideomotor theory), the anticipation or mere presentation of an action’s effect can activate motor representations that have elicited said effect in the past (e.g., Elsner and Hommel, 2001). Importantly, if multiple actions can elicit similar effects (e.g., a syllable comprised of the consonant /t/ and precision grips), then the activation of the effect representation will activate other motor representations associated with similar effects (e.g., tactile sensation at the tip of the effector; see response-effect compatibility effect, Kunde et al., 2002). As such, if the syllable /ti/ is prepared and the required manual response is a precision grip, then the initiation of a precision grip should be facilitated because the tactile sensations involved with the pronunciation of /ti/ and precision grip are compatible – RTs to initiate the precision grip would be shorter because a response associated with a “compatible” response has already been pre-activated, or primed (see Liepelt et al., 2008 for more on motor priming). However, if the syllable /ti/ is prepared but the required manual response is a power grip, then the motor representations associated with an “incompatible” effect should be pre-activated (i.e., precision grip). The pre-activation of this response associated with an incompatible effect should result in a delayed initiation of the correct response (e.g., power grip) due to interference between the two different grip representations. Although the data of Vainio et al (2013, 2014) are generally consistent with the ideomotor perspective accounts, determining whether compatible/ incompatible syllables can facilitate/ interfere with hand grip RTs still requires a control condition.
Given that all the previous work investigating the AGC effect have shown that precision and power grip RT was relatively shorter when the syllable /ti/ vs. /ka/ was produced, the relationship between syllable and grip was said to be facilitatory. However, none of the above-mentioned studies included a condition whereby hand grips were performed alone (i.e., without the presentation of syllables). As such, it remains impossible to unequivocally determine whether syllable production can indeed facilitate and/ or interfere with the initiation of a hand grip because that hand grip RTs has not been tested alone.
This lack of control condition initially prompted this study. Specifically, in a separate baseline condition, participants initiated either a precision or a power grip, without the presentation of syllables (i.e., grip alone conditions). The baseline condition was included so as to assess whether facilitation (i.e., RTs shorter than baseline) or interference (RTs longer than baseline) can be elicited by the verbalization of syllables. Further, the AGC effect has been shown to be elicited by both overt (Vainio et al., 2013) and covert (Vainio et al., 2014) verbalizations in separate experiments, so it could be proposed that overt verbalizations suffice to elicit any compatibility or interference effects.
In Vainio et al.’s (2013) study, participants silently read a syllable prior to overtly verbalizing it aloud. And in Vainio et al.’s (2014), participants silently read a syllable without subsequently verbalizing aloud. In other words, in both cases (covert vs. overt reading), the silent reading of a syllable was performed prior to initiating a hand grip. Thus, it is possible and likely that covert reading suffices to elicit the said facilitatory (or interference) effects. However, that stance could be strengthened if said effects were reproduced in a within-subject design. Thus, in the current experiment, participants were required to initiate a precision or power grip, while verbalizing or silently reading the syllables /ti/ or /ka/.
The purpose of the present study was to investigate whether hand grip initiation could be facilitated/ interfered with by the covert/ overt reading of a syllable. If the syllables /ka/ and /ti/ indeed interfere with and facilitate precision grip RTs, respectively, then the syllable /ka/ should increase precision grip RTs relative to when a precision grip is performed alone (i.e., mean delta RTs should be greater than zero), whereas the syllable /ti/ should decrease precision grip RTs relative to when a precision grip is performed alone (i.e., mean delta RTs should be smaller than zero). Moreover, if the syllables /ti/ and /ka/ indeed interfere with and facilitate power grip RTs, respectively, then the syllable /ti/ should increase power grip RTs relative to when a power grip is initiated alone, while the syllable /ka/ should decrease power grip RTs relative to when a power grip is initiated alone. Finally, it was hypothesized that the magnitude of facilitation/ interference should be no different between covert and overt reading conditions, given that the anticipation of the same sensory consequences (i.e., reading) is presumably taking place in both conditions. That is, given that silent reading is thought to activate inner speech (e.g., Abramson et al., 1997), and inner speech is said to produce corollary discharge (i.e., the anticipation of sensory consequences of a movement such as tactile sensation; e.g., Scott, 2012), then it can be proposed that the anticipation/ presentation of the effect of a syllable should be sufficient at interfering/ facilitating a hand grip response.