Figure 9.
Comparison of dual-task and single-task responses. A, Regions differentially involved in the number and sound tasks (positive t values, number > sound; negative t values, sound > number). Activation in motor cortex as well as in SMA was contralateral to the response hand, whereas activation in the cerebellum was ipsilateral. A massive activation is observed in superior temporal cortex, including primary auditory cortex, for the sound relative to the number task. In the converse direction, lateral occipital/fusiform cortex was more active in the number task, although no significant difference was seen in medial occipital cortex (area 17/18). B, To investigate the relation of each of the five clusters obtained in the dual-task analysis to these single-task activations, the voxels from each dual-task cluster were projected onto a scatter-plot plane in which the x-coordinate represents the single-task activation to the sound task (T2) and the y-coordinate represents the single-task activation to the number task (T1). C, Each scatter plot was collapsed to a histogram, counting the fraction of voxels within each individual cluster as a function of the t value of the number–sound contrast on the single-task runs. The vertical line indicates the mean of each distribution. The first cluster type (blue, no phase variation) had a major overlap with the T1 task (number task), including a subset of voxels strongly activated in the number task but virtually inactive in the sound task. The second cluster type (yellow, purely linear) involved exclusively regions strongly activated by the sound task but inactive during the number task. The third cluster type (red, linear and nonlinear variation corresponding to T2-only bottleneck and postbottleneck areas) also involved voxels with dominant activation for the T2 task (sound task). The fourth (cyan, slope ½ nonlinear corresponding to shared bottleneck areas) and fifth (green, purely nonlinear PRP effect) cluster types showed positive and positively correlated activations in both tasks.