ausblenden:
Schlagwörter:
-
Zusammenfassung:
It is widely assumed that the generation of hierarchies in the human brain depends on activity in inferior frontal gyrus (IFG), particularly in area BA44 (Friederici et al., 2011; Fitch & Martins, 2014, for reviews). We tested this assumption for the generation of motor hierarchies using fMRI. We devised an innovative procedure in which the execution of hierarchical sequences could be elicited using three different strategies (‘fractal’, ‘iterative’ and ‘repetition’; Fig. 1). We then compared the cognitive and neural mechanisms underlying these strategies.
Method
The task is based on the execution of finger movement sequences following a rhythmic structure (Fig. 1 left D) using the thumb, index and middle fingers (denoted in red, green and blue in Fig. 1) on a 16-key MR compatible keyboard.
We tested 20 participants (9 females, aged 21-35) across 4 experimental sessions, each composed of 20 trials - 8 ‘fractal’, 8 ‘iterative’ and 4 ‘repetition’ trials. On each trial, participants had first to execute the first two steps (I and II, Fig. 1) of the given generating strategy (‘fractal’, ’iterative’ or ’repetition’), as shown on a computer screen. Then, we asked them to ‘plan’ the next step for 6 seconds (without pressing any key), and finally, to ‘execute’ step III on the keyboard without any visual support, but following an auditory metronome. The keyboard keys did not generate any tonal sound.
Functional images were acquired on a 3 Tesla Skyra scanner (Siemens, Erlangen, Germany). We focused on the activity during step III ‘planning’ and ‘execution’ phases. Image processing was performed using SPM8 and included slice-time correction, correction for motion, spatial normalization and smoothing. Only voxels that had a probability of being grey matter exceeding 25% were used. We compared the brain activity elicited by different generating strategies (‘fractal’, ‘iterative’, or ‘repetition’) using a within-subject flexible factorial ANOVA (in SPM8), and including both ‘strategy’ and ‘session’ as regressors.
Results
We found that the generation (“planning”) of motor hierarchical structures via the application of “fractal” rules was supported by a neural system used for motor learning, planning and imaging in general (Elsinger et al., 2006; Hardwick et al., 2013; Hétu et al., 2013), which included the Somato-Motor and Premotor cortices, Cerebellum, Lateral Occipital Cortex, and the left Pallidum (all clusters p cluster < .05 FWE corrected; p voxel < .001) (Fig. 2).
In the ‘execution’ phase, the ‘fractal’ strategy (vs. ‘iterative’) activated a cluster which included portions of the right Putamen and Orbitofrontal cortex (p cluster < .05 FWE corrected; p voxel < .001).
Discussion
The generation of hierarchical levels in the motor domain did not require the recruitment of multi-domain systems hypothesized to play a specific role in the processing of hierarchies (e.g. BA44). While these other neural systems might be important to parse hierarchical structures, they do not seem to be specific for the contrast tapping into the generation of new levels, which is at the core of generative procedures.
This finding is consistent with previous work in the visual-spatial domain (Martins et al., 2014). In these previous experiments, we showed that the generation of new hierarchical levels is represented by brain networks that bind information from the visual ventral and visual dorsal streams (Kravitz et al., 2011). Again, we found no specific activations within IFG, suggesting that the generation of new hierarchical levels, at least in the motor and visual domains, might also be achieved using domain-specific mechanisms.
