Functional MRI recordings were conducted using a 3.0 T Siemens Trio with a 12-channel phased-array head coil. For each epoch, a single-shot echo planar imaging sequence that is sensitive to BOLD contrast was used to acquire 33 slices per repetition time (TR = 2000 ms; 3 mm thickness; 0.5 mm gap), echo time (TE) of 30 ms, flip angle of 90°, field of view of 192 mm, and 64 × 64 acquisition matrix. Before the collection of the first epoch, a high-resolution

T1-weighted sagittal image of the whole brain was acquired (TR = 15.0 ms; TE = 4.2 ms; flip angle = 9°, 3D acquisition, field of view of 256 mm; slice thickness = 0.89 mm; and acquisition matrix = 256 × 256). We collected three behavioral variables during training: the time between key presses (i.e., the vector of interkey intervals), movement time (MT), and error. MT is the time elapsed selleck inhibitor from the initial to final key press. Error was scored as any trial not produced in the correct order, as well as those trials not completed within the 8 s time limit. To test for learning,

we entered the MT data for each subject, sequence, and session into a repeated-measures ANOVA (with subject treated as a random factor). To test for differences in error over training, we combined error for each frequent sequence and entered them for each subject and session using a repeated-measures ANOVA. For all statistical tests, we set a probability threshold of p < 0.05 for the rejection of the null hypothesis. We collected IKI data for

all correct frequent-sequence trials. Each trial consisted of 11 IKI data points (Figure 1A). CB-839 We excluded the first key press in the sequence isothipendyl from the IKIs because it contained the time elapsed from initial cue presentation to the completion of the first button press. We calculated the mean for each frequent-sequence IKI (giving a total of 11 mean IKIs/sequence) for each participant. We then excluded trials containing IKIs greater than 3 SDs from each mean IKI. To facilitate the examination of chunking behavior, we constructed a sequence network to encode the relationship between IKIs for each trial. We defined the nodes for each sequence network as the 11 IKIs for a trial (Figure 1B). We defined motor chunks as specific groups of movements that occur serially in time. Consecutive nodes are therefore connected to one another using undirected edges; the node representing IKI1 is connected to the node representing IKI2, and the node representing IKI2 is also connected to the node representing IKI1 (Figure 1C). Furthermore, intrachunk movements occur in rapid succession relative to interchunk movements. We therefore defined the similarity in IKIs as (d¯ij−dij)/d¯ij, where dij   is defined as the absolute difference in IKIs, (i.e., dij   = |IKIi – IKIj|) and d¯ij is defined as the maximum of dij over the entire trial.