Besides the well-known contribution of the basal ganglia (BG) to motor behaviour, the numerous language deficits reported in patients with Parkinson's disease (PD), or with other BG lesions, suggest they participate in language production, as Ackermann et al. discuss in the target article. However, most inferences about the contribution of BG to motor control based on deficits observed in patients have proven to be flawed, and, despite decades of investigation, the actual role of BG remains debated. For example, recent studies in humans and monkeys have shown that a lesion of the internal part of the globus pallidus – one of the main BG outputs – leads to rather subtle motor deficits, mostly unrelated to PD motor symptoms (Desmurget & Turner Reference Desmurget and Turner2008; Reference Desmurget and Turner2010; Obeso et al. Reference Obeso, Jahanshahi, Alvarez, Macias, Pedroso, Wilkinson, Pavon, Day, Pinto, Rodríguez-Oroz, Tejeiro, Artieda, Talelli, Swayne, Rodríguez, Bhatia, Rodriguez-Dias, Lopez, Guridi and Rothwell2009). This suggests that many of the symptoms resulting from BG lesions, including language deficits, are likely to result from the perturbation, by noisy BG signals, of a larger network of cortical and subcortical structures.

Nonetheless, two motor functions have emerged recently as being distinctively imputable to BG. First, it appears almost undisputable that BG contribute to motor sequence learning (Desmurget & Turner Reference Desmurget and Turner2010; Obeso et al. Reference Obeso, Jahanshahi, Alvarez, Macias, Pedroso, Wilkinson, Pavon, Day, Pinto, Rodríguez-Oroz, Tejeiro, Artieda, Talelli, Swayne, Rodríguez, Bhatia, Rodriguez-Dias, Lopez, Guridi and Rothwell2009; Turner & Desmurget Reference Turner and Desmurget2010). This is in sharp contrast to the idea that BG are involved in the storage and execution of overlearned movements, or “habits” (Graybiel Reference Graybiel2008). Interestingly, a recent study has shown that BG are involved in motor chunking (Wymbs et al. Reference Wymbs, Bassett, Mucha, Porter and Grafton2012), as already suggested by the finding that this process is dopamine-dependent (Boyd et al. Reference Boyd, Edwards, Siengsukon, Vidoni, Wessel and Linsdell2009; Tremblay et al. Reference Tremblay, Bedard, Langlois, Blanchet, Lemay and Parent2010; Reference Tremblay, Bedard, Levesque, Chebli, Parent, Courtemanche and Blanchet2009). Chunking is a key mechanism in sequence learning, and it comprises two distinct processes occurring at different stages: a first operation, called “segmentation,” consists of parsing sequences into shorter clusters (Clerget et al. Reference Clerget, Poncin, Fadiga and Olivier2012; Sakai et al. Reference Sakai, Kitaguchi and Hikosaka2003; Verwey Reference Verwey2001; Verwey & Eikelboom Reference Verwey and Eikelboom2003), and this is followed by “concatenation,” which consists of assembling these short chunks into longer clusters (Sakai et al. Reference Sakai, Kitaguchi and Hikosaka2003; Verwey Reference Verwey1996). Whereas BG would be involved in the concatenation process, the initial segmentation would rely on a left fronto-parietal network, including Broca's area (Wymbs et al. Reference Wymbs, Bassett, Mucha, Porter and Grafton2012). The second function currently assigned to BG is the control of movement “vigor” according to motivational factors (Kurniawan et al. Reference Kurniawan, Seymour, Talmi, Yoshida, Chater and Dolan2010; Mazzoni et al. Reference Mazzoni, Hristova and Krakauer2007; Turner & Desmurget Reference Turner and Desmurget2010). Indeed, a loss of dopaminergic neurones in the substantia nigra affects the reward and decision-making processes (Mimura et al. Reference Mimura, Oeda and Kawamura2006). Bigger rewards typically lead to higher efforts (Berridge Reference Berridge2004; Schmidt et al. Reference Schmidt, Forgeot d'Arc, Lafargue, Galanaud, Czernecki, Grabli, Schübach, Hartmann, Lévy, Dubois and Pessiglione2008; Takikawa et al. Reference Takikawa, Kawagoe, Itoh, Nakahara and Hikosaka2002) and, when selecting an action among multiple candidates, the choice critically depends on a comparison of the cost/benefit ratio for each option (Niv et al. Reference Niv, Joel and Dayan2006). The motivational effect that an anticipated reward has on the execution of an action is known as the “incentive salience,” which is then translated into movement “vigor” to optimize the balance between the effort invested and the outcome value. An impairment of this process would lead to an inability to adjust the level of effort invested in movements, explaining the occurrence of bradykinesia in PD patients (Mazzoni et al. Reference Mazzoni, Hristova and Krakauer2007).

It is remarkable that two of the main roles assigned to BG in spoken language by Ackermann and colleagues match very well with the contribution of BG to motor behaviour. First, following perinatal BG lesions or in patients with inherited language disorders, one major spoken language impairment is “a significant disruption of simultaneous or sequential sets of motor activities to command, in spite of a preserved motility of single vocal tract organs” (target article, sect. 4.2.1, para. 2) (Alcock et al. Reference Alcock, Passingham, Watkins and Vargha-Khadem2000a; Watkins et al. Reference Watkins, Dronkers and Vargha-Khadem2002a), which is consistent with the role of BG in processing structured sequences. Intriguingly, these patients also present a deficit in grammatical rules acquisition (Alcock et al. Reference Alcock, Passingham, Watkins and Vargha-Khadem2000a; Gopnik Reference Gopnik1990a), as also reported in PD patients (Chan et al. Reference Chan, Ryan and Bever2013; Pell & Monetta Reference Pell and Monetta2008). It is noteworthy that the other brain region critically involved in syntax and sequence processing is Broca's area (Clerget et al. Reference Clerget, Winderickx, Fadiga and Olivier2009; Reference Clerget, Badets, Duque and Olivier2011; Reference Clerget, Andres and Olivier2013; Fadiga et al. Reference Fadiga, Craighero and D'Ausilio2009; Tettamanti & Weniger Reference Tettamanti and Weniger2006), which is tightly interconnected with BG (Ullman Reference Ullman2006). It is remarkable that the two brain regions involved in chunking (Wymbs et al. Reference Wymbs, Bassett, Mucha, Porter and Grafton2012) are also those in which a lesion yields a syntax deficit suggesting that the chunking process might provide the basis for hierarchical processing and represent the common denominator of BG contribution to motor and language production (Kotz et al. Reference Kotz, Schwartze and Schmidt-Kassow2009).

Second, the “hypokinetic dysarthria” or “hypophonia” reported in PD, and viewed by Ackermann et al. as a consequence of “a diminished impact of motivational, affective/emotional, and attitudinal states on the execution of speech movements, leading to sparse motor activity” and interpreted as a “general loss of ‘motor drive’ at the level of the speech motor system” (sect. 4.2.2, para. 3) fits quite well with the view that BG play a central role in controlling the movement “vigor” according to motivational factors (Turner & Desmurget Reference Turner and Desmurget2010). According to this view, bradykinesia and hypophonia would not arise from a mere impairment of the motor command execution but from a loss of influence of the motivational drive on the motor output. Ackermann and colleagues go one step further in proposing that, in the case of language, this impairment would not only result in a reduced amplitude of the motor output but also in a decreased motivational and emotional modulation of speech. This could be paralleled, in the context of motor behaviour, to the relative lack of spontaneous facial expression exhibited by PD patients despite their preserved ability to produce posed facial expression (Smith et al. Reference Smith, Smith and Ellgring1996). Even though there are currently very little data supporting this hypothesis, this is an interesting idea that deserves further investigation.

The similitude between the putative functions of BG to motor behaviour and language provides an additional example of how the motor system has served the emergence of high-level cognitive functions, by minimal transformation, from ancestral structures already present in nonhuman primates (Andres et al. Reference Andres, Olivier and Badets2008; Dehaene & Cohen Reference Dehaene and Cohen2007; Olivier et al. Reference Olivier, Davare, Andres and Fadiga2007).