During fMRI scanning, all subjects were able to make controlled exertions of the lower limb to match the target torque at the requested joint, as shown in . Positive torque values indicate extension of the joint (plantarflexion for the ankle joint) whereas negative values indicate joint flexion (dorsiflexion for the ankle). For the ankle dorsiflexion task, the mean amplitude of the torque at the ankle joint was −5.39 Nm (0.15) (mean of means ± standard error mean, for all subjects, data combined across both sessions). The mean amplitude of the torques at the knee and hip joints during the ankle dorsiflexion task were 0.91 Nm (0.08) and 2.47 Nm (0.39), respectively.
Reliability of behavioral performance and brain activation across sessions presented for individual subjects.
The mean amplitude of the torque at the ankle during ankle plantarflexion was 5.06 Nm (0.16), whereas the mean torques at the knee and hip joints were −0.93 Nm (0.06) and 0.57 Nm (0.31), respectively.
For knee extension, the mean amplitude of the torque at the knee joint was 5.02 Nm (0.09). The mean amplitude of the torques at the ankle and hip joints were −0.53 Nm (0.10) and −4.51 Nm (0.87), respectively.
Repeated measures ANOVAs performed on the torques generated at each joint for each movement did not identify significant session-related differences, as reported in . For the ankle tasks, between-session differences in secondary, associated torques were approaching significance (p<0.1, at hip for ankle dorsiflexion, at knee and hip for ankle plantarflexion).
Mean torque amplitudes measured at each joint for each contraction type for each session.
Brain Activation Patterns
All three lower limb tasks resulted in significant BOLD signal increases in the left paracentral lobule, corresponding to SM1, as well as in left dorsolateral premotor cortex (PMd), and bilaterally in supplementary motor area (SMA) and secondary somatosensory cortex (Sii). Significant subcortical activation was observed bilaterally in the putamen, thalamus and cerebellum. (See and )
Regions showing significant task-related BOLD signal increases
Figure 3 Brain regions showing significant positive BOLD signal changes for the main effects of ankle dorsiflexion (A), ankle plantarflexion (B) and knee extension (C) tasks across both sessions. Contrast maps show voxels at p < 0.001, uncorrected. Lowest (more ...)
Within the sensorimotor network, there was substantial overlap of activation associated with each of the contraction types. However, some somatotopic organization was observed within the paracentral lobule. For example, activation extended more inferiorly during knee extension than during either of the ankle movements, as shown in .
As presented in and , significant BOLD signal decreases were observed for all lower limb tasks bilaterally within SM1 in the region of the hand knob, though for the knee extension task this effect was only seen without correction for multiple comparisons (p<0.001 uncorrected voxel threshold only). In addition, both ankle tasks resulted in significant BOLD signal decreases in anterior cingulate cortex, precuneus and right angular gyrus.
Regions showing significant task-related BOLD signal decreases
Figure 4 Brain regions showing significant negative BOLD signal changes for the main effect of ankle dorsiflexion (A), ankle plantarflexion (B) and knee extension (C) tasks across both sessions. Contrast maps show voxels at p < 0.001 and p<0.005, (more ...)
Differential Activity between Movement Types
Significant differential activity was observed between the contraction types, as reported in and . The most striking differences were observed when comparing the knee extension task to either of the ankle tasks. When compared with ankle dorsiflexion, knee extension resulted in significantly greater activation of SMA, right PMv and contralateral M1 adjacent to the hand knob. (voxels significant at p<0.001 (uncorrected) and clusters significant at p<0.05 (corrected)) Further clusters of voxels within sensorimotor areas showed greater activity for knee extension than ankle dorsiflexion, but did not survive correction for multiple comparsions. These included portions of left SM1 at inferior levels within the paracentral lobule, as well as right M1, adjacent to the hand knob. (voxels significant at p<0.001 (uncorrected), see )
Regions showing significant differences in activation between tasks
Figure 5 Brain regions showing significant differential activation between movements: Regions showing greater response during knee extension compared with ankle dorsiflexion (A) and ankle planatarflexion (B). Regions showing greater activity during ankle plantarflexion (more ...)
Comparison of knee extension with ankle plantarflexion also identified significantly greater knee-related BOLD signal changes in sensorimotor areas including clusters in left M1 within the paracentral lobule and extending into SMA, as well as in bilateral S1, right M1 at the hand knob, right PMd, right superior frontal sulcus and right parieto-occipital sulcus. (, voxels significant at p<0.001 (uncorrected), clusters significant at p<0.05 (corrected))
The region of the left paracentral lobule showing greater activation during the knee task compared with either of the ankle tasks corresponds to the inferior portion of SM1 that shows specific activation to knee extension only in the maps of overlap of the main effect of task type. ()
When contrasted with the knee extension task, the ankle tasks resulted in marginally greater activation in right anterior cerebellum (for both ankle tasks) and left SM1 within the paracentral lobule (z=69, ankle plantar flexion only) as shown in . However, these clusters only survived voxel-level thresholding of p<0.005 uncorrected.
Similarly, as shown in , only small differences were observed when contrasting ankle dorsiflexion with ankle plantarflexion. Dorsiflexion resulted in marginally greater activation of right cerebellum and left SM1 at the level of the hand knob when compared with plantarflexion (voxel-level threshold of p<0.005 uncorrected) There were no regions showing significantly greater activation for ankle plantarflexion when compared with dorsiflexion.
Reliability of fMRI measurements
Contrast of the activation between different sessions for each task did not reveal widespread differences in sensorimotor areas. Comparison of the two sessions for the ankle dorsiflexion task showed greater activation in right superior temporal sulcus, right supramarginal gyrus, right cuneus and right superior parietal lobule for the first session compared with the second session. No regions showed greater activity of the second session compared with the first. No significant differences were detected between sessions for the ankle plantarflexion task, even with searches at the uncorrected threshold of p<0.005. Contrast of the sessions for the knee extension task revealed greater activation in left hippocampus, right S1 within the paracentral lobule and right PMd for the second session compared with the first session, though the latter was only observed at an uncorrected voxel threshold of p<0.005.
The lack of major session related effects at the group level was supported by high intraclass correlation coefficients for t-values of voxels in contralateral SM1 and bilateral Area 6 for each of the contraction types, as shown in .
For individual subjects, the ICC of t-values between the sessions for voxels within contralateral SM1 ranged from 0.37 to 0.78 for ankle dorsiflexion, from 0.44 to 0.82 for ankle planarflexion and from 0.29 to 0.74 for knee extension. For voxels within bilateral Area 6, ICC values for individual subjects ranged from 0.35 to 0.84 for ankle dorsiflexion, from 0.54 to 0.80 for ankle planarflexion and from 0.38 to 0.83 for knee extension.
In , t-values of voxels in each anatomical ROI for the ankle dorsiflexion task in the first session are plotted against those of the second session for the individuals at each extreme of the reported ICC range.
Figure 6 Scatterplots of the t-values for ankle dorsiflexion in the first session plotted against the t-values of the second session for the two subjects with the highest (A–B) and lowest (C–D) ICC values for voxels within SM1 (A,C) and bilateral (more ...)