Abstract
Although the contribution of Broca's area to motor cognition is generally accepted, its exact role remains controversial. A previous functional imaging study has suggested that Broca's area implements hierarchically organised motor behaviours and, in particular, that its anterior (Brodmann area 45, BA45) and posterior (BA44) parts process, respectively, higher and lower-level hierarchical elements. This function of Broca's area could generalize to other cognitive functions, including language. However, because of the correlative nature of functional imaging data, the causal relationship between Broca's region activation and its behavioural significance cannot be ascertained. To circumvent this limitation, we used on-line repetitive transcranial magnetic stimulation to disrupt neuronal processing in left BA45, left BA44 or left dorsal premotor cortex, three areas that have been shown to exhibit a phasic activation when participants performed hierarchically organised motor behaviours. The experiment was conducted in healthy volunteers performing the same two key-press sequences as those used in a previous imaging study, and which differed in terms of hierarchical organisation. The performance of the lower-order hierarchical task (Experiment #1) was unaffected by magnetic stimulation. In contrast, in the higher-order hierarchical task (Experiment #2, “superordinate” task), we found that a virtual lesion of the anterior part of Broca's area (left BA45) delayed the processing of the cue initiating the sequence in an effector-independent way. Interestingly, in this task, the initiation cue only informed the subjects about the rules to be applied to produce the appropriate response but did not allow them to anticipate the entire motor sequence. A second important finding was a RT decrease following left PMd virtual lesions in the superordinate task, a result compatible with the view that PMd plays a critical role in impulse control. The present study therefore demonstrates the role of left BA45 in planning the higher-order hierarchical levels of motor sequences.
Highlights
While a large variety of cognitive tasks, including motor tasks, leads to an activation of Broca’s region, the exact meaning of this possible overlap remains debated (e.g., [1])
Experiment #1: Simple task An ANOVARM performed on no-TMS trials with CUE as a within factor revealed a main effect of the CUE on reaction time (RT) (F3,18 = 6.78, p,0.003) (Figure 3A)
We applied TMS over AB (BA45) and PB (BA44), in which a singular phasic activation was reported in subjects executing distinct motor sequences characterized by various levels of hierarchical organization [17]
Summary
While a large variety of cognitive tasks, including motor tasks, leads to an activation of Broca’s region, the exact meaning of this possible overlap remains debated (e.g., [1]). In a so-called ‘‘superordinate task’’, subjects had to select amongst three possible ‘‘simple action chunks’’ corresponding to three distinct pre-learned rules associating a given cue (a letter) to a given button-press. This is reminiscent of the three hierarchical levels proposed by Dehaene and Changeux [18] for sequence planning, namely (i) elementary gestures, (ii) an operation combining several elementary gestures and (iii) a planning system. Based on distinct phasic activations of BA6, BA44 and BA45 during the performance of these two motor tasks, Koechlin and Jubault [17] concluded that Broca’s area process hierarchically structured behaviour, a view applicable to other cognitive functions, including language
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