Role of Broca's Area in Implicit Motor Skill Learning: Evidence from Continuous Theta-burst Magnetic Stimulation

Abstract

Complex actions can be regarded as a concatenation of simple motor acts, arranged according to specific rules. Because the caudal part of the Broca's region (left Brodmann's area 44, BA 44) is involved in processing hierarchically organized behaviors, we aimed to test the hypothesis that this area may also play a role in learning structured motor sequences. To address this issue, we investigated the inhibitory effects of a continuous theta-burst TMS (cTBS) applied over left BA 44 in healthy subjects, just before they performed a serial RT task (SRTT). SRTT has been widely used to study motor skill learning and is also of interest because, for complex structured sequences, subjects spontaneously organize them into smaller subsequences, referred to as chunks. As a control, cTBS was applied over the vertex in another group, which underwent the same experiment. Control subjects showed both a general practice learning effect, evidenced by a progressive decrease in RT across blocks and a sequence-specific learning effect, demonstrated by a significant RT increase in a pseudorandom sequence. In contrast, when cTBS was applied over left BA 44, subjects lacked both the general practice and sequence-specific learning effects. However, surprisingly, their chunking pattern was preserved and remained indistinguishable from controls. The present study indicates that left BA 44 plays a role in motor sequence learning, but without being involved in elementary chunking. This dissociation between chunking and sequence learning could be explained if we postulate that left BA 44 intervenes in high hierarchical level processing, possibly to integrate elementary chunks together.

Role of Broca’s area in motor sequence programming

“…Because the contribution of Broca's area to language is inevitably more recent in the course of evolution than to movements, it has been proposed that the foundation for the involvement of Broca's area in language syntax should be found in motor behavior [8]. Accordingly, the role of Broca's area in syntax processing for nonlinguistic sequences has been demonstrated [9][10][11] and recently, we found that altering the functioning of left Brodmann area (BA) 44, the posterior part of Broca's area, in healthy individuals before they performed a serial reaction time task impaired the implicit learning of a complex structured sequence, suggesting a role of Broca's area in high-hierarchical level processing [12].…”

“…The absence of effect of left BA44 cTBS on learning cannot be explained by a lack of efficacy of this TMS protocol, as we have already used it successfully in a previous experiment to alter the function of the same area [12]. The most likely explanation for this absence of TMS effects on sequence learning is that, in our experimental conditions, it did not rely on processes controlled by left BA44, possibly because the relative timings between key-presses was not treated as syntax.…”

Chunking improves symbolic sequence processing and relies on working memory gating mechanisms

“…measures changed congruently with the improvement in RT occurring during training, a finding interpreted as evidence that chunking is not causally involved in sequence learning (Song and Cohen 2014). Along the same line, another recent study reported that Broca's area disruption by means of transcranial magnetic stimulation impedes learning of a motor sequence but without altering the chunking strategy, supporting also the lack of connection between chunking and learning performance (Clerget et al 2012). The novelty of the present study lies in accounting for between-subject variability of chunking features, which allowed us to show that chunking is an effective strategy to improve symbolic sequence-learning performance, supposedly by alleviating cognitive workload.…”

A Proposed Neurological Interpretation of Language Evolution

“…Many other authors have presented a similar point of view [183–187]. Some recent research seemingly supports this interpretation [188]. …”